Wind Turbine System Design Volume 1 Nacelles Drivetrains And Verification Jan Wenske

Wind Turbine System Design Volume 1 Nacelles Drivetrains And Verification Jan Wenske
Wind Turbine System Design Volume 1 Nacelles Drivetrains And Verification Jan Wenske
Wind Turbine System Design Volume 1 Nacelles Drivetrains And Verification Jan Wenske

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5. Wind Turbine System Design
Volume 1: Nacelles, drivetrains and verification
Edited by Jan Wenske
Wenske
Wind energy is a pillar of the strategy to mitigate greenhouse gas emissions and stave off 
catastrophic climate change, but the market is under tremendous pressure to reduce costs. 
This results in the need for optimising any new wind turbine to maximise the return on 
investment and keep the technology profitable and the sector thriving. Optimisation involves 
selecting the best component out of many, and then optimising the system as a whole. Key 
components are the nacelles and drivetrains, and the verification of their work as a system.
Wind Turbine System Design: Volume 1: Nacelles, drivetrains and verification is a valuable 
reference for scientists, engineers and advanced students engaged in the design of wind 
turbines offering a systematic guide to these components. Chapters written by industry 
experts cover load calculation and validation, models and simulation, pitch and yaw system 
concepts and designs, drivetrain concepts and developments, gearboxes, hydraulic systems, 
lubrication, and validation. The book aims to enable readers to make informed and systematic 
choices in designing the best turbine for a given situation.
About the Editor
Jan Wenske is a professor at the University of Bremen and deputy director of the Fraunhofer-
Institute for Wind Energy Systems (IWES), Germany
Wind Turbine System Design
Volume 1: Nacelles, drivetrains and verification
Wind Turbine System
Design
Volume 1: Nacelles, drivetrains and 
verification
Edited by 
The Institution of Engineering and Technology
theiet.org
978-1-78561-856-7

6. Wind Turbine System
Design
IET ENERGY ENGINEERING SERIES 142A

7. Other volumes in this series:
Volume 1 Power Circuit Breaker Theory and Design C.H. Flurscheim (Editor)
Volume 4 Industrial Microwave Heating A.C. Metaxas and R.J. Meredith
Volume 7 Insulators for High Voltages J.S.T. Looms
Volume 8 Variable Frequency AC Motor Drive Systems D. Finney
Volume 10 SF
6
Switchgear H.M. Ryan and G.R. Jones
Volume 11 Conduction and Induction Heating E.J. Davies
Volume 13 Statistical Techniques for High Voltage Engineering W. Hauschild and W. Mosch
Volume 14 Uninterruptible Power Supplies J. Platts and J.D. St Aubyn (Editors)
Volume 15 Digital Protection for Power Systems A.T. Johns and S.K. Salman
Volume 16 Electricity Economics and Planning T.W. Berrie
Volume 18 Vacuum Switchgear A. Greenwood
Volume 19 Electrical Safety: a guide to causes and prevention of hazards J. Maxwell Adams
Volume 21 Electricity Distribution Network Design, 2nd Edition E. Lakervi and E.J. Holmes
Volume 22 Artificial Intelligence Techniques in Power Systems K. Warwick, A.O. Ekwue and R. Aggarwal (Editors)
Volume 24 Power System Commissioning and Maintenance Practice K. Harker
Volume 25 Engineers’ Handbook of Industrial Microwave Heating R.J. Meredith
Volume 26 Small Electric Motors H. Moczala et al.
Volume 27 AC-DC Power System Analysis J. Arrillaga and B.C. Smith
Volume 29 High Voltage Direct Current Transmission, 2nd Edition J. Arrillaga
Volume 30 Flexible AC Transmission Systems (FACTS) Y-H. Song (Editor)
Volume 31 Embedded generation N. Jenkins et al.
Volume 32 High Voltage Engineering and Testing, 2nd Edition H.M. Ryan (Editor)
Volume 33 Overvoltage Protection of Low-Voltage Systems, Revised Edition P. Hasse
Volume 36 Voltage Quality in Electrical Power Systems J. Schlabbach et al.
Volume 37 Electrical Steels for Rotating Machines P. Beckley
Volume 38 The Electric Car: Development and future of battery, hybrid and fuel-cell cars M. Westbrook
Volume 39 Power Systems Electromagnetic Transients Simulation J. Arrillaga and N. Watson
Volume 40 Advances in High Voltage Engineering M. Haddad and D. Warne
Volume 41 Electrical Operation of Electrostatic Precipitators K. Parker
Volume 43 Thermal Power Plant Simulation and Control D. Flynn
Volume 44 Economic Evaluation of Projects in the Electricity Supply Industry H. Khatib
Volume 45 Propulsion Systems for Hybrid Vehicles J. Miller
Volume 46 Distribution Switchgear S. Stewart
Volume 47 Protection of Electricity Distribution Networks, 2nd Edition J. Gers and E. Holmes
Volume 48 Wood Pole Overhead Lines B. Wareing
Volume 49 Electric Fuses, 3rd Edition A. Wright and G. Newbery
Volume 50 Wind Power Integration: Connection and system operational aspects B. Fox et al.
Volume 51 Short Circuit Currents J. Schlabbach
Volume 52 Nuclear Power J. Wood
Volume 53 Condition Assessment of High Voltage Insulation in Power System Equipment R.E. James and Q. Su
Volume 55 Local Energy: Distributed generation of heat and power J. Wood
Volume 56 Condition Monitoring of Rotating Electrical Machines P. Tavner, L. Ran, J. Penman and H. Sedding
Volume 57 The Control Techniques Drives and Controls Handbook, 2nd Edition B. Drury
Volume 58 Lightning Protection V. Cooray (Editor)
Volume 59 Ultracapacitor Applications J.M. Miller
Volume 62 Lightning Electromagnetics V. Cooray
Volume 63 Energy Storage for Power Systems, 2nd Edition A. Ter-Gazarian
Volume 65 Protection of Electricity Distribution Networks, 3rd Edition J. Gers
Volume 66 High Voltage Engineering Testing, 3rd Edition H. Ryan (Editor)
Volume 67 Multicore Simulation of Power System Transients F.M. Uriate
Volume 68 Distribution System Analysis and Automation J. Gers
Volume 69 The Lightening Flash, 2nd Edition V. Cooray (Editor)
Volume 70 Economic Evaluation of Projects in the Electricity Supply Industry, 3rd Edition H. Khatib
Volume 72 Control Circuits in Power Electronics: Practical issues in design and implementation M. Castilla (Editor)
Volume 73 Wide Area Monitoring, Protection and Control Systems: The enabler for Smarter Grids A. Vaccaro and
A. Zobaa (Editors)
Volume 74 Power Electronic Converters and Systems: Frontiers and applications A. M. Trzynadlowski (Editor)
Volume 75 Power Distribution Automation B. Das (Editor)
Volume 76 Power System Stability: Modelling, analysis and control A.A. Sallam and B. Om P. Malik
Volume 78 Numerical Analysis of Power System Transients and Dynamics A. Ametani (Editor)
Volume 79 Vehicle-to-Grid: Linking electric vehicles to the smart grid J. Lu and J. Hossain (Editors)
Volume 81 Cyber-Physical-Social Systems and Constructs in Electric Power Engineering S. Suryanarayanan, R. Roche and T.M. Hansen (Editors)
Volume 82 Periodic Control of Power Electronic Converters F. Blaabjerg, K.Zhou, D. Wang and Y. Yang

8. Volume 86 Advances in Power System Modelling, Control and Stability Analysis F. Milano (Editor)
Volume 87 Cogeneration: Technologies, Optimisation and Implentation C. A. Frangopoulos (Editor)
Volume 88 Smarter Energy: from Smart Metering to the Smart Grid H. Sun, N. Hatziargyriou, H. V. Poor, L. Carpanini
and M. A. Sánchez Fornié (Editors)
Volume 89 Hydrogen Production, Separation and Purification for Energy A. Basile, F. Dalena, J. Tong and T.N.Vezirog˘lu (Editors)
Volume 90 Clean Energy Microgrids S. Obara and J. Morel (Editors)
Volume 91 Fuzzy Logic Control in Energy Systems with Design Applications in Matlab/Simulink
®
I˙. H. Altas¸
Volume 92 Power Quality in Future Electrical Power Systems A. F. Zobaa and S. H. E. A. Aleem (Editors)
Volume 93 Cogeneration and District Energy Systems: Modelling, Analysis and Optimization M. A. Rosen and S. Koohi-Fayegh
Volume 94 Introduction to the Smart Grid: Concepts, technologies and evolution S.K. Salman
Volume 95 Communication, Control and Security Challenges for the Smart Grid S.M. Muyeen and S. Rahman (Editors)
Volume 96 Industrial Power Systems with Distributed and Embedded Generation R Belu
Volume 97 Synchronized Phasor Measurements for Smart Grids M.J.B. Reddy and D.K. Mohanta (Editors)
Volume 98 Large Scale Grid Integration of Renewable Energy Sources A. Moreno-Munoz (Editor)
Volume 100 Modeling and Dynamic Behaviour of Hydropower Plants N. Kishor and J. Fraile-Ardanuy (Editors)
Volume 101 Methane and Hydrogen for Energy Storage R. Carriveau and D. S-K. Ting
Volume 104 Power Transformer Condition Monitoring and Diagnosis A. Abu-Siada (Editor)
Volume 106 Surface Passivation of Industrial Crystalline Silicon Solar Cells J. John (Editor)
Volume 107 Bifacial Photovoltaics: Technology, applications and economics J. Libal and R. Kopecek (Editors)
Volume 108 Fault Diagnosis of Induction Motors J. Faiz, V. Ghorbanian and G. Joksimovic ’
Volume 109 Cooling of Rotating Electrical Machines: Fundamentals, modelling, testing and design D. Staton, E. Chong, S. Pickering and A. Boglietti
Volume 110 High Voltage Power Network Construction K. Harker
Volume 111 Energy Storage at Different Voltage Levels: Technology, integration, and market aspects A.F. Zobaa, P.F. Ribeiro, S.H.A. Aleem and S.N. Afifi (Editors)
Volume 112 Wireless Power Transfer: Theory, Technology and Application N.Shinohara
Volume 114 Lightning-Induced Effects in Electrical and Telecommunication Systems Y. Baba and V. A. Rakov
Volume 115 DC Distribution Systems and Microgrids T. Dragicˇevic ’, F.Blaabjerg and P. Wheeler
Volume 116 Modelling and Simulation of HVDC Transmission M. Han (Editor)
Volume 117 Structural Control and Fault Detection of Wind Turbine Systems H.R. Karimi
Volume 119 Thermal Power Plant Control and Instrumentation: The control of boilers and HRSGs, 2
nd
Edition
D. Lindsley, J. Grist and D. Parker
Volume 120 Fault Diagnosis for Robust Inverter Power Drives A. Ginart (Editor)
Volume 121 Monitoring and Control using Synchrophasors in Power Systems with Renewables I. Kamwa and C. Lu (Editors)
Volume 123 Power Systems Electromagnetic Transients Simulation, 2
nd
Edition N. Watson and J. Arrillaga
Volume 124 Power Market Transformation B. Murray
Volume 125 Wind Energy Modeling and Simulation Volume 1: Atmosphere and plant P. Veers (Editor)
Volume 126 Diagnosis and Fault Tolerance of Electrical Machines, Power Electronics and Drives A.J. M. Cardoso
Volume 128 Characterization of Wide Bandgap Power Semiconductor Devices F. Wang, Z. Zhang and E.A. Jones
Volume 129 Renewable Energy from the Oceans: From wave, tidal and gradient systems to offshore wind and solar D. Coiro and T. Sant (Editors)
Volume 130 Wind and Solar Based Energy Systems for Communities R. Carriveau and D. S-K. Ting (Editors)
Volume 131 Metaheuristic Optimization in Power Engineering J. Radosavljevic ’
Volume 132 Power Line Communication Systems for Smart Grids I.R.S Casella and A. Anpalagan
Volume 134 Hydrogen Passivation and Laser Doping for Silicon Solar Cells B. Hallam and C. Chan (Editors)
Volume 139 Variability, Scalability and Stability of Microgrids S. M. Muyeen, S. M. Islam and F. Blaabjerg (Editors)
Volume 143 Medium Voltage DC System Architectures B. Grainger and R. D. Doncker (Editors)
Volume 145 Condition Monitoring of Rotating Electrical Machines P. Tavner, L. Ran, C. Crabtree
Volume 146 Energy Storage for Power Systems, 3
rd
Edition A.G. Ter-Gazarian
Volume 147 Distribution Systems Analysis and Automation 2
nd
Edition J. Gers
Volume 151 SiC Power Module Design: Performance, robustness and reliability A. Castellazzi and A. Irace (Editors)
Volume 152 Power Electronic Devices: Applications, failure mechanisms and reliability F Iannuzzo (Editor)
Volume 153 Signal Processing for Fault Detection and Diagnosis in Electric Machines and Systems M. Benbouzid (Editor)
Volume 155 Energy Generation and Efficiency Technologies for Green Residential Buildings D. Ting and R. Carriveau (Editors)
Volume 156 Lithium-ion Batteries Enabled by Silicon Anodes C. Ban and K. Xu (Editors)
Volume 157 Electrical Steels, 2 Volumes A. Moses, K. Jenkins, Philip Anderson and H. Stanbury
Volume 158 Advanced Dielectric Materials for Electrostatic Capacitors Q Li (Editor)
Volume 159 Transforming the Grid Towards Fully Renewable Energy O. Probst, S. Castellanos and R. Palacios (Editors)
Volume 160 Microgrids for Rural Areas: Research and case studies R.K. Chauhan, K. Chauhan and S.N. Singh (Editors)
Volume 161 Artificial Intelligence for Smarter Power Systems: Fuzzy Logic and Neural Networks M. G. Simoes
Volume 165 Digital Protection for Power Systems 2nd Edition Salman K Salman

9. Volume 166 Advanced Characterization of Thin Film Solar Cells N. Haegel and M Al-Jassim (Editors)
Volume 167 Power Grids with Renewable Energy Storage, integration and digitalization A. A. Sallam and B. OM P. Malik
Volume 169 Small Wind and Hydrokinetic Turbines P. Clausen, J. Whale and D. Wood (Editors)
Volume 170 Reliability of Power Electronics Converters for Solar Photovoltaic Applications F. Blaabjerg, A.l Haque,
H. Wang, Z. Abdin Jaffery and Y. Yang (Editors)
Volume 171 Utility-scale Wind Turbines and Wind Farms A. Vasel-Be-Hagh and D. S.-K. Ting
Volume 172 Lighting interaction with Power Systems, 2 volumes A. Piantini (Editor)
Volume 174 Silicon Solar Cell Metallization and Module Technology T. Dullweber (Editor)
Volume 180 Protection of Electricity Distribution Networks, 4
th
Edition J. Gers and E. Holmes
Volume 181 Modelling and Simulation of Complex Power Systems A. Monti and A. Benigni
Volume 182 Surge Protection for Low Voltage Systems A. Rousseau (Editor)
Volume 184 Compressed Air Energy Storage: Types, systems and applications D. Ting and J. Stagner
Volume 186 Synchronous Reluctance Machines: Analysis, optimization and applications N. Bianchi, C. Babetto and G. Bacco
Volume 191 Electric Fuses: Fundamentals and new applications 4
th
Edition N. Nurse, A. Wright and P. G. Newbery
Volume 193 Overhead Electric Power Lines: Theory and practice S. Chattopadhyay and A. Das
Volume 194 Offshore Wind Power Reliability, availability and maintenance, 2nd edition P. Tavner
Volume 196 Cyber Security for Microgrids S. Sahoo, F. Blaajberg and T. Dragicevic
Volume 198 Battery Management Systems and Inductive Balancing A. Van den Bossche and A. Farzan Moghaddam
Volume 199 Model Predictive Control for Microgrids: From power electronic converters to energy management J. Hu, J. M. Guerrero and S. Islam
Volume 204 Electromagnetic Transients in Large HV Cable Networks: Modeling and calculations Ametani, Xue, Ohno and Khalilnezhad
Volume 208 Nanogrids and Picogrids and their Integration with Electric Vehicles S. Chattopadhyay
Volume 211 Blockchain Technology for Smart Grids: Implementation, management and security Gururaj H L, Ravi K V, F. Flammini, H. Lin, Goutham B, Sunil K. B R and C Sivapragash
Volume 212 Battery State Estimation: Methods and Models S. Wang
Volume 215 Industrial Demand Response: Methods, best practices, case studies, and applications H. H. Alhelou, A. Moreno-Muñoz and P. Siano (Editors)
Volume 213 Wide Area Monitoring of Interconnected Power Systems 2
nd
Edition A. R. Messina
Volume 217 Advances in Power System Modelling, Control and Stability Analysis 2
nd
Edition F. Milano (Editor)
Volume 225 Fusion-Fission Hybrid Nuclear Reactors: For enhanced nuclear fuel utilization and radioactive waste reduction W. M. Stacey
Volume 238 AI for Status Monitoring of Utility Scale Batteries Shunli Wang, Kailong Liu, Yujie Wang, Daniel-Ioan Stroe, Carlos Fernandez and Josep M. Guerrero
Volume 905 Power system protection, 4 volumes

10. Wind Turbine System
Design
Volume 1: Nacelles, drivetrains and
verification
Edited by
Jan Wenske
The Institution of Engineering and Technology

11. Published by The Institution of Engineering and Technology, London, United Kingdom
The Institution of Engineering and Technology is registered as a Charity in England &
Wales (no. 211014) and Scotland (no. SC038698).
© The Institution of Engineering and Technology 2022
First published 2022
This publication is copyright under the Berne Convention and the Universal Copyright
Convention. All rights reserved. Apart from any fair dealing for the purposes of research or
private study, or criticism or review, as permitted under the Copyright, Designs and Patents
Act 1988, this publication may be reproduced, stored or transmitted, in any form or by
any means, only with the prior permission in writing of the publishers, or in the case of
reprographic reproduction in accordance with the terms of licences issued by the Copyright
Licensing Agency. Enquiries concerning reproduction outside those terms should be sent to
the publisher at the undermentioned address:
The Institution of Engineering and Technology
Futures Place
Kings Way, Stevenage
Herts, SG1 2UA, United Kingdom
www.theiet.org
While the authors and publisher believe that the information and guidance given in this
work are correct, all parties must rely upon their own skill and judgement when making use
of them. Neither the author nor publisher assumes any liability to anyone for any loss or
damage caused by any error or omission in the work, whether such an error or omission is
the result of negligence or any other cause. Any and all such liability is disclaimed.
The moral rights of the author to be identified as author of this work have been asserted by
him in accordance with the Copyright, Designs and Patents Act 1988.
British Library Cataloguing in Publication Data
A catalogue record for this product is available from the British Library
ISBN 978-1-78561-856-7 (hardback)
ISBN 978-1-78561-857-4 (PDF)
Typeset in India by Exeter Premedia Services Private Limited
Printed in the UK by CPI Group (UK) Ltd, Croydon
Cover Image: Vinzo via Getty Images

12. Contents
1 Load calculation and load validation 1
Philipp Thomas
 
, Mareike Leimeister
 
, Anna Wegner
 
, and Matthias L. Huhn
 
1.1 Design loads of wind turbines 3
1.1.1 Standard load calculation 4
1.1.2 Use cases and exemplary loads 11
1.2 Design load validation 16
1.2.1 Standard load measurements 16
1.2.2 Data evaluation process 21
1.2.3 Standard load validation 21
Acknowledgements 25
References 25
2 Models and simulation 27
Paul Robert Feja
 
, Mareike Leimeister
 
, and Muhammad Omer Siddiqui
 
2.1 Introduction 27
2.1.1 Overview of modelling at different levels of fidelity 28
2.1.2 Requirements of standards for model fidelity 29
2.2 Modelling of environmental conditions 31
2.2.1 Modelling of wind conditions 32
2.2.2 Modelling of sea conditions 36
2.2.3 Modelling of soil conditions 39
2.3 Fully coupled wind turbine modelling 39
2.3.1 Aeroelasticity and standard tools 40
2.3.2 Aerodynamic models 40
2.3.3 Hydrodynamic models 53
2.3.4 Modelling of structural components 54
2.3.5 Modelling of other components 58
2.4 Detailed modelling of wind turbine drivetrains 58
2.4.1 General modelling approaches, methods and tools 59
2.4.2 Different approaches of modelling a wind turbine drivetrain61
2.4.3 Modelling recommendations and best practices 66
2.5 Conclusion and summary 68
References 68
About the Editor xiii
Preface xv
Abbreviations and Terminologies xxv

13. viii Wind turbine system design
3 Pitch system concepts and design 75
Karsten Behnke
 
, Arne Bartschat
 
, Eike Blechschmidt
 
, Matthis Graßmann
 
,
Florian Schleich
 
, Oliver Menck
 
, and Heiko Jungermann
 
3.1 Blade bearing 78
3.1.1 Preliminary outer bearing design 79
3.1.2 Preliminary inner bearing design 84
3.1.3 Preliminary design of the bolted connections 89
3.1.4 FE blade bearing model 93
3.1.5 FE simulation of internal blade bearing loads 98
3.1.6 Calculation and dimensioning 101
3.1.7 Lubrication system 110
3.1.8 Coating 113
3.2 Pitch actuator 114
3.2.1 Electrical actuator 114
3.2.2 Operating conditions 118
3.2.3 Calculation and dimensioning 118
References 121
4 Yaw system concepts and designs 125
Christian Bulligk
 
and Daniel von dem Berge
 
4.1 Fundamentals 125
4.1.1 Introduction 125
4.1.2 Wind direction and yaw misalignment 129
4.1.3 Typical key data 131
4.2 Design loads 133
4.2.1 Introduction 133
4.2.2 Yaw bearing loads 135
4.2.3 Yaw drivetrain aerodynamic loads 139
4.2.4 Loads acting on the yaw drivetrain 143
4.2.5 Modification of yaw drivetrain aerodynamic loads 146
4.2.6 Yaw slippage events during non-yawing operation 148
4.2.7 Overload events during yawing operation 150
4.2.8 Yaw start and stop events 152
4.3 System concepts and components 153
4.3.1 Differentiating features at system level 153
4.3.2 Yaw bearing 156
4.3.3 Yaw brake system 164
4.3.4 Yaw gearbox 167
4.3.5 Yaw motor and yaw motor brake 172
4.3.6 Auxiliary systems 176
4.3.7 Evaluation criteria 178
4.3.8 Common system concepts 180
4.4 System dimensioning and design aspects 181
4.4.1 Introduction and general requirements 182
4.4.2 Step 1: yaw system, holding torque and driving torque 185
4.4.3 Step 2a: yaw bearing, yaw brake and yaw drive 188

14. Contents ix
4.4.4 Step 2b: dimensioning of the yaw brake system 190
4.4.5 Step 2c: dimensioning of the yaw bearing 192
4.4.6 Step 2d: dimensioning of the yaw drive system 197
4.4.7 Step 3: auxiliary systems 202
4.4.8 Summary 202
References 205
5 Drivetrain concepts and developments 207
Jan Wenske
 
5.1 Fundamentals 207
5.2 Drivetrain concepts 210
5.2.1 Drivetrain diversification and classification 210
5.2.2 Drivetrain concepts and design principles 216
5.3 General design rules and procedures 237
5.3.1 Safety, protection, reliability and control 238
5.3.2 Loads and load cases 244
5.3.3 Loads analysis and strength verification 249
5.3.4 Modularization, standardization, and platform concepts 258
5.3.5 Scalability of designs and performance indicators 263
5.4 Onshore wind turbines and drivetrain developments 270
5.4.1 ENERCON 271
5.4.2 Nordex 272
5.4.3 General Electric wind energy (GE) 274
5.4.4 Vestas 275
5.4.5 Siemens Gamesa Renewable Energy 277
5.5 Offshore wind turbines and drivetrain developments 279
5.6 Outlook and potential development trends 289
References 292
6 Gearbox concepts and design 297
Urs Giger
 
6.1 Introduction 297
6.2 Challenge for load gearboxes in wind turbines 298
6.3 Historical drivetrains in wind turbines 300
6.3.1 Hybrid systems 309
6.3.2 Exceptional developments in the drivetrain 310
6.3.3 A Swiss geared wind turbine 311
6.3.4 State of the art 311
6.4 Basic gear tooth design 312
6.4.1 PGT planetary stage in detail 318
6.4.2 PGTs have a number of advantages and applications 319
6.4.3 Difficulties in using PGTs 320
6.4.4 Increasing the power sharing 320
6.4.5 The problem of load distribution and its control 322
6.4.6 The load-sharing measurement 323

15. x Wind turbine system design
6.4.7 Microgeometry 324
6.4.8 Absolute, coupling, and relative (rolling) power 326
6.5 Bearings 326
6.5.1 Bearing failure mechanisms 329
6.6 Coupling 329
6.7 Mechanical brakes 330
6.8 Lubrication system and its design principles 330
6.9 Bolted joints 332
6.10 Pitch tube 333
6.11 Repair work 334
6.12 Standards for load gear units in the drivetrain 335
6.13 Gearbox design methodology 336
6.13.1 Oil quantities and power losses 342
6.13.2 Calculation of gearing according to ISO 6336 standard
(Part 1–6) 342
6.14 Future prospects 346
6.15 Conclusion 347
References 348
7 Hydraulic systems and lubrication systems 351
Andreas Nocker
 
, Arved Hildebrandt
 
, Christian Bulligk
 
, and
Daniel von dem Berge
 
7.1 Hydraulic systems 351
7.1.1 Main Components 352
7.1.2 Hydraulic auxiliaries 356
7.1.3 Manifold / control block 358
7.1.4 Centralized and decentralized systems 359
7.1.5 How to engineer a hydraulic power pack 360
7.2 Hydraulic pitch systems 365
7.2.1 History 365
7.2.2 Pitch control 365
7.2.3 Hydraulic pitch adjustment systems 370
7.2.4 How to engineer a hydraulic pitch system 375
7.2.5 Outlook 379
7.3 Automatic lubrication system for bearings 380
7.3.1 Fundamentals 380
7.3.2 Components of an automatic lubrication system 382
7.3.3 Simplified exemplary design of an automatic lubrication
system 387
7.3.4 Schematic overview and final clarifications 389
References 391
8 Cooling systems concepts and designs 393
Ernst-­Wilhelm Langhoff
 
8.1 Introduction 393

16. Contents xi
8.2 Gearbox 394
8.2.1 Filtration 397
8.3 Generator 398
8.4 Main converter 400
8.5 Main transformer 403
8.6 Essential questions for cooling system design 404
8.7 Example – cooling design for IWT-7.5-164 variant 405
8.8 Experiences 412
References 415
9 Validation, verification, and full-­scale testing 417
Hans Kyling
 
, Anna Wegner
 
, Karsten Behnke
 
, Malo Rosemeier
 
, and
Alexandros Antoniou
 
9.1 Introduction 417
9.2 Validation and verification strategy 417
9.3 Purpose of testing 420
9.4 Product development using the V-Model 421
9.5 Full-system testing 422
9.5.1 Certification measurements 422
9.5.2 Measurements on the yaw system 424
9.6 Integration testing 426
9.6.1 System test benches 426
9.6.2 Test requirements 428
9.6.3 Projecting a nacelle test campaign 429
9.7 Sub-system testing 432
9.7.1 Gearbox 432
9.7.2 Brake system 435
9.8 Component testing 436
9.8.1 Main shaft 436
9.8.2 Pitch bearing 439
9.8.3 Rotor blade 442
9.9 Material testing 444
9.9.1 Leading edge protection 445
9.9.2 Polymer and composite testing 445
9.10 Outlook 446
References 447
10 Main shaft suspension system 451
Marc Reichhart
 
, Tobias Baumgratz
 
, and Clemens Brachmann
 
10.1 Introduction and bearing arrangement selection 451
10.1.1 Cylindrical roller bearings 452
10.1.2 Spherical roller bearings 452
10.1.3 Toroidal roller bearings 453
10.1.4 Tapered roller bearings 453
10.1.5 Moment bearings 453

17. xii Wind turbine system design
10.1.6 Bearing type and bearing arrangement selection 454
10.1.7 Bearing type selection in relation to the drivetrain concept457
10.1.8 Influence of turbine size on rotor bearing size and type 459
10.2 General design and bearing calculation process 461
10.2.1 Drivetrain for calculation example 461
10.2.2 Calculations according to applicable standards and
guidelines 462
10.2.3 Rated life calculation 462
10.2.4 Contact stress 465
10.2.5 Static safety 467
10.2.6 Loads for rotor bearing calculation 468
10.2.7 Extreme loads for rotor bearing calculation 469
10.2.8 Fatigue load cases for bearing calculation 470
10.2.9 Bearing calculation models and software 473
10.2.10 Rigid calculation model 473
10.2.11 Calculation with the stiffness matrix 474
10.2.12 Calculation with non-linear stiffness (FE calculation) 476
10.3 Example for rotor bearing calculation 477
10.3.1 Influence of calculation model and boundary conditions479
10.3.2 Definition and influence of bearing system preload 481
10.4 Reliability, failures and root causes 483
10.5 Development trends 486
References 487
Index 489

18. About the Editor
Jan Wenske is a professor at the University of Bremen and deputy director of the
Fraunhofer-Institute for Wind Energy Systems (IWES), Germany. He received
his PhD in 1999 at the Institute of Electrical Engineering at the TU Clausthal.
Professional assignments from 2000-2010 included advanced development for elec-
trical drives at the STILL GmbH and the Power Electronics Development at Jenoptik
Defense & Civil Systems. He works intensively with major industry players in wind
energy technology.

19. This page intentionally left blank

20. Preface
Synopsis
The work in the field of wind energy use and the development of modern on- as well
as offshore wind turbines has long since arrived in its own very special, complex
and multidisciplinary research universe. Due to the enormous progress, in model-
ling, simulation, experimental validation and measurements, it is now possible to
model the subsystems of a wind power turbine in detail, to analyse it in the multi-
domain realm. On the other hand, to devise a real turbine design based upon the
model, study the construction about the highly complex interaction between the
subsystems, and feed lessons learned back to the construction process. The turbine
manufacturers have established and optimised their internal processes, however
sometimes adjusted them primarily to formally meeting the requirements of the
certifying authorities. Currently, at least onshore turbines as a still highly complex
product are in permanently danger of becoming a commodity. Means a product,
which perceived by the customers as nearly identical regardless of the manufacturer,
with the price as the only easily distinguishable attribute, even though substantial
differences in detail do exist. There is already an enormous pressure on manufactur-
ers to reduce costs. Evidence for this is the rapid consolidation movement on the
side of the manufacturers as well as of the equipment suppliers and introduction of
new platforms, modifications and variants in always-shorter periods. For the wind
industry, continuous innovation is essential to escape from this situation. Research
institutions and universities try to support with specific expertise in various field of
research from wind physics (explaining the character of wind with its turbulences
as a complex, stochastic process and the atmosphere), about turbine technology
and controls, aerodynamics, civil and ocean engineering, grid integration down to
material science.
The offshore industry has also developed rapidly and optimized all parts of the
value chain within a few years, especially in the areas of logistics, founding and
construction. Reducing costs to achieve even lower LCoE is still the main driver
for technical developments. A few years ago, operators of new offshore wind farms
still expected that the rated output of next-generation offshore wind turbines would
almost double to around 15 MW within the next five years in order to be able to
operate in a free electricity generation market without subsidies, and it really hap-
pens. In the face of these rapid developments in the market industrial, application-
oriented research on the one hand, and academic research in wind power on the other
hand should be coupled. Appropriate specialist literature available to students, PhD

21. xvi Wind turbine system design
candidates, industry experts and interested laypeople or carrier changes is always
necessary and one key enabler for the motivation to dig deeper in the fantastic world
of wind turbines. There is a lot of standard literature available for wind energy utili-
zation and turbine technology. Mostly they aim at providing the readers with a first
general introduction and broad overview about the topic of wind energy. Also in
the specific areas of simulation, modelling, control, aerodynamics, hydro-and aero-
elastic, fibre based materials, and structural dynamics in general a broad range of
specialised literature is available as well.
So why to write a small book series on turbine technology, particularly with a
focus on power mechatronics of the drivetrain and the nacelle systems? Because
the authors and the editor are of the opinion that there are still gaps in the literature
for application oriented readers from engineering, mechatronics and electronics, i.e.
written for novices, engineers, scientists and students engaged in applied research in
wind power. With the goal to establish, the link between foundations and practical
application under consideration of boundary conditions (such as construction related,
system related, or economical), this book shall offer answers to questions like:
••How to configure a main bearing arrangement?
••Which loads are design relevant for different subsystems?
••Why especially the pitch system is such a critical turbine subsystem?
••What is essential for designing a gearbox for wind turbines?
••How to deal with the losses within the turbine?
••Which are boundary conditions to configure a Direct-Drive generator?
••What are the secrets of a proper DFIG system design?
••What is a modern, generic physical controller design for a wind turbine?
••What is the status of current CMS and SHM technologies and how to integrate them?
••Is an extensive test and validation program reasonable?
••Is the next challenge still the turbine or its grid integration?
••…
The reader shall develop a sense for the concrete, practical design work. Of course, also this book cannot cover each design process to the minute detail. The goal is rather to describe the processes using examples, referencing, where applicable and necessary, to further literature or existing requirements and standards. Having read this book, the reader will be capable of understanding the designing of specific com-
ponents and subsystems of modern wind turbines or to specifying them in more detail, and have a sound understanding of boundary conditions, dos and don’ts, system interaction and requirements in the design process of nacelle systems and drivetrains. Some chapters describe less the practical design process, but providing rather an application-oriented overview of the state of technology and research, such as CMS/SHM, controls and signals and drivetrains concepts.
A small group of experts wrote therefore each, individual chapter. The teams are
professionals from industry, experienced researcher or a mixed team always with a significant share of authors with industry expertise to ensure the practical relevance

22. Preface xvii
of the contents. Each chapter will convey briefly the necessary basics, sometime a
historical overview, examples for design processes, requirements, challenges, opti-
misation potentials as well as future research issues.
Readership, who might be interested
Scientists and engineers interested or engaged in the design of wind turbines in
general, and the involved drivetrain and mechatronics in particular. For students of
the subjects of wind energy systems, in particular for those focusing on mechanical
design, simulation, mechatronics or electrical engineering, this book series (Vol.1,
Vol.2) hopefully is of special interest in terms of applied research and a deeper
understanding of the entire wind turbine as a complex system. Especially in combi-
nation with the established fundamental literature on wind energy systems but also
other books of the IET wind energy series (e.g. Modeling and Simulation), this book
shall be a practical addition.
Thus, engineers and practitioners in wind power industry, for them to obtain
a detailed overview of this topic area. In particular, when their day-to-day work
involves adjacent subsystems, such as tower or blades, drivetrain specialists from
other industries, open for inspiration and perspectives from wind industry; students
specializing on different topics, in order to comprehend the requirements for turbine
and subsystem designs. This series of books is only intended to provide a contribu-
tion to effectively dealing with the necessary prerequisites, the complex challenges
in the areas of multi-body simulation, FEM calculation or advanced fatigue strength
calculation for machine components, control and generator design and power elec-
tronics development for advanced wind turbines.
Wind Turbine System Design and its authors
Volume 1: Nacelles, drivetrains and verification
The first Chapter deals with the condensed, fundamental topic of load calculation
and validation of wind turbines, explaining where loads are coming from and how to
calculate, briefly explain processes to calculate them respectively, for the entire tur-
bine. Within this chapter also the generic 7.5MW wind turbine IWT-7.5-164 is intro-
duced, which serves in the following chapters of the book as a source for design data
that are sometimes required. This generic turbine is largely open to all technologies
(in terms of offshore, onshore application, direct drive, gearbox, rotor shaft bearing,
etc.) and serves as a kind of common thread for exemplary design cases, which may
be supplemented by the teams of authors of the following chapters, according to
their requirements.
A team of experienced wind turbine modelers wrote this chapter under the lead
of Dipl.-Ing. Philipp Thomas, who studied mechanical engineering at the Otto-von-
Guericke University Magdeburg. Since 2012, he researches in the area of holistic
modelling and load analysis of wind turbines and is one of the main programmers
of the load analysis tool MoWiT. Today he heads the group global turbine dynamics

23. xviii Wind turbine system design
at the Fraunhofer-Institute for Wind Energy Systems. Philipps had competent sup-
port from his co-authors, who were; M.Sc. Matthias L. Huhn studied mechanical
engineering at the Hamburg University of Technology, RWTH Aachen University,
and EPFL in Lausanne. Since 2017, he is a research associate at Fraunhofer-IWES
in the group for global turbine dynamics and works on modelling, verification, and
validation of aeroelastic models of wind turbines. The latter is also the subject of his
PhD he is working on. He is a member of the international committee Joint Working
Forum on Model Validation of the IECRE;
Dr. Anna Wegner studied Physics at the University of Heidelberg. She received
her PhD at the University of Bremen. She worked as a postdoctoral research fellow
both in Germany and in the U.S. Today she heads the Application Center for Wind
Energy Field Measurements at the Fraunhofer-IWES; and
Dr.Eng. Mareike Leimeister is an offshore wind engineer with special interest
in floating offshore renewable energy systems. She holds a double master’s degree
from TU Delft and NTNU and received her EngD from the University of Strathclyde
in 2020. Since 2017, she is also a research associate at the Fraunhofer Institute for
Wind Energy Systems, where she both coordinates and works on joint research pro-
jects. Her expertise lies in global turbine dynamics, numerical modelling, simula-
tion, and optimization as well as floating wind turbines.
The authors of Chapter 2 dig much deeper into the topic of models and simulation
with a specific focus on rotor and drivetrain modelling. The rotas have to be dis-
cussed in detail, despite it is not part of nacelle or drivetrain, but the main source of
drivetrain loading and with strong interaction in-between, of course not exclusively.
M.Sc. Paul Robert Feja, who studied mechanical engineering at RWTH Aachen with
a focus on renewable energy, managed the team. In 2015, he joined Fraunhofer-
IWES, where he worked on global wind turbine model development and simulation.
He was responsible for the implementation of a real-time virtual rotor model for
hardware-in-the-loop tests at the nacelle test bench DyNaLab. Since 2020, he is the
group manager for test and method development, focusing on simulation of wind
turbine drivetrains and test benches. Paul was supported from his co-author team,
whose members are;
M.Sc. Muhammad Omer Siddiqui, a mechanical engineer with a focus on
modelling and simulation of mechanical systems. He has a master’s degree in
Simulation Sciences from RWTH Aachen University. In 2018, he joined Fraunhofer
IWES as a research associate where he is primarily involved in developing high fidel-
ity simulation models of the test bench and nacelle drivetrains; and again Dr.Eng.
Mareike Leimeister, who was already introduced in the remarks to Chapter 1.
In Chapter 3 M.Sc. Karsten Behnke and his team from wind industry and IWES
co-authors give an insight into the concepts and design of pitch systems which is
still considered as one of the most failure prone subsystems within the entire turbine.
Karsten Behnke studied mechanical engineering at the Otto von Guericke University
Magdeburg and he wrote his master’s thesis on the topic of multibody simulation of
rolling bearings. In 2017 he joined Fraunhofer-IWES as a research associate. Since

24. Preface xix
2018 he is part of the group slewing bearings, where he researches in the field of
pitch bearing and gearbox bearing damages. The team of authors for chapter 3 is
complemented by;
M.Sc. Arne Bartschat, who is the manager of the group slewing bearings in the
department validation and reliability at Fraunhofer IWES. He has professional expe-
rience with blade bearing and reliability dedicated research and project management
since 2014. His research interests include finite element modeling at various levels
from single components to complex assemblies, design, execution and evaluation
of blade bearing tests, SCADA analysis of 1000+ turbines, model development and
load simulation of wind turbines;
M.Sc. Matthis Graßmann studied mechanical engineering at the University
Rostock with focus on simulation. In his master thesis, he created a fully para-
metrized FE bearing model and run simulations considering realistic surrounding
structures of an experimental test rig. He joined the Fraunhofer-Institute of Wind
Energy Systems IWES as a research associate in 2019. There, he is part of the group
Slewing Bearings and researches in the field of large slewing bearings in wind tur-
bines. He is responsible for FE calculations of bearings and their surrounding struc-
tures like test rigs and rotor hubs.
M.Sc. Florian Schleich studied mechanical engineering at the Hamburg
University of Technology. In 2017 he joined Fraunhofer-Institute for Wind Energy
Systems to write his master thesis in the field of finite element modelling of blade
bearings. Since 2018 he is working as a research associate in the department vali-
dation and reliability. He has been working on several projects related with blade
bearing simulation and testing. His focus is on the development of FE wind tur-
bine rotor models and the simulation of blade bearings internal load distributions;
furthermore,
Dipl.-Ing. Eike Blechschmidt, who studied mechanical engineering with
mechatronics as a major field of study. He wrote his master thesis on condition mon-
itoring of slowly rotating bearings. Eike worked eight years for REpower/Senvion
in different functions in the fields of condition monitoring, test & validation and
pitch & yaw systems. Since 2021 he is working for Fraunhofer-IWES as a research
associate with a focus on data analysis and artificial intelligence as well as on grease
comparison tests.; and
M.Sc. Oliver Menck studied mechanical engineering and mechatronics at the
Hamburg University of Technology. He joined Fraunhofer-IWES as a research asso-
ciate. Here he is involved in the planning and execution of tests, data analysis, and
lifetime calculation of bearings in wind turbines. His interests include anything related
to mechanical engineering, mechatronics and what else wind energy; and finally, from
wind industry Dipl.-Ing. Heiko Jungermann, who has studied electrical engineer -
ing at the “Fachhochschule Osnabrück” from 1996-2000. He started as an application
engineer at Nidec SSB Windsystems from 2000 with the main tasks, design and layout
of customized pitch systems for customers world-wide. In 2013 Heiko Jungermann
became the head of the systems engineering at Nidec SSB Windsystems. Since 2020
he has the responsibility for the systems engineering and the electronic development
team at Nidec SSB Windsystems in the role of Director E&D.

25. xx Wind turbine system design
Chapter 4 deals with another wind turbine subsystem, which applies large bearing
devices, the yaw-system, usually only dealt with very briefly in the current literature
but explained here in detail by professionals from wind industry. The yaw system
author-team consists of;
Dipl.-Ing. Christian Bulligk, who graduated in mechanical engineering at
Dresden University of Technology in 2009. He has been working in the wind indus-
try since 2010. For the wind turbine manufacturer REpower/Senvion, he developed
mechanical components for pitch and yaw systems for turbines up to 6 MW. Since
2020, he has been lead engineer for pitch and yaw systems at bewind GmbH, an
engineering office with extensive knowledge and experience in design, transport,
installation, and operation of wind turbines; and his Co-author, Dipl.-Ing. Daniel
von dem Berge, who studied mechanical engineering at the University of Applied
Sciences Gelsenkirchen, department Bocholt. He started working in the wind indus-
try in 2009 at the wind turbine manufacturer Kenersys, where he was responsible for
pitch and yaw system, drivetrains and various auxiliary systems for 2 up to 2.5MW
turbines. Since 2015, he has been working as engineer for mechanical systems and
since 2019 as project manager for the maxcap project at engineering office windwise
GmbH, an service provider that specializes in the wind industry - from the develop-
ment and construction of multi-megawatt wind turbine generators to support with
purchasing, quality assurance, project management and the technical management
for wind-energy projects.
Just explaining and discussing the astonishing range of drivetrain concepts of tur-
bine manufactures and over time since the 1980s is the main content of Chapter 5.
Development lines divided into on- and offshore application are discussed in detail
as well as general aspects from literature, scaling laws and performance indicators.
The author and editor of this book put in his experience from discussion with many
wind energy experts to provide a broad overview of drivetrains in wind turbines.
Prof.  Dr.-Ing. Jan Wenske studied mechanical engineering at the Technical
University of Clausthal with focus on high performance drives and power electron-
ics. He received his PhD in 1999 at the Institute of Electrical Engineering at the TU
Clausthal on the field of power electronic application for grid stabilization under
high share of wind energy feed in. He worked another year as senior scientist and
leader of research group distributed renewable energy systems. In 2000 he changed
to industry, as project manager within the pre-development division for forklift truck
drivetrains at the STILL GmbH. Subsequently he was in charge of the Department
for Power Electronic Development at Jenoptik Defense & Civil Systems from 2005
to 2010 with focus on high performance hybrid drivetrains, high-voltage vehicle
power supplies and more electric aircraft projects. Since 2011 he has been deputy
director of Fraunhofer IWES. 2013 he become Professor at the University of Bremen
for Wind Energy Systems and is Chief Technology Officer (CTO) at the IWES.
A true pioneer in the field of gearbox design for wind turbines describes in Chapter 6
very personally his experiences, old and new innovations and the example of a hands-
on design process for the design of a gearbox for the 7.5MW IWT. He also presents

26. Preface xxi
an outlook on a possible future with very high ratio, multi power split gears for
future double digit rated power turbines. Dipl.-Ing. Urs Giger is a Senior Mechanical
Engineer, holds a HTL Diploma in Mechanical Engineering from the FHNW School
of Engineering and run his own company GGS in Andermatt. His most recent devel-
opment work has focused to the design of Multi Rotor (MR) wind turbines. He
holds three patents and has evolved the flexible pin for PTGs into a low-cost and
effective element. His long-term collaboration with Ray Hicks † (Wales) and Kiril
Arnaudov † (Sofia) has resulted in innovative drivetrains for the wind industry. He
is an active member of the JWG 1 ISO TC 60 IEC TC 88 JWG GEARBOXES
FOR WIND TURBINES, and active member in IEC 61400-8. He is representa-
tive of Switzerland in the International Electrotechnical Commission TC88: WIND
TURBINES and lives in Mühlau, Switzerland.
An expert team from HAWE has compiled all relevant information regarding the
hydraulic assistance systems inside the nacelle. Safe operation of the WT is not pos-
sible without these systems. They control and supply centralized or decentralized
hydraulic actuators for controlling the brakes, the rotor lock, the on-board crane, the
nacelle-roof opener and quite often the entire pitch system of the turbine. System
properties such as leakage free, reliability and safety are of essential importance.
The authors are;
Dipl.-Ing. Andreas Nocker who is with HAWE Hydraulik since 2000. More
than ten years he worked as Product Manager. Since 2011 he is in charge as Key
Market Manager for the application field Energy worldwide and especially for the use
of hydraulics in wind turbines. After finishing his studies in 1991, he began his career
with Bosch Rexroth, Lohr am Main/Germany working in the R&D department for
mobile hydraulics. At Oil Control Deutschland, Augsburg/Germany he was assistant
of the head of technology from 1993 to 1999. He studied at the Technical University
Munich/Germany and holds a degree (Dipl.-Ing. TU) as mechanical engineer.
After studying mechanical engineering at the University of Applied Sciences
in Kiel, Arved Hildebrandt directly started his professional career in the sector of
wind turbines in 2009. In more than 10 years he designed various components and
systems for different wind turbine manufacturers and engineering offices in Germany.
In 2021 he has started in the technical sales team of HAWE SE and is responsible
for wind turbine related products. Besides his engineering and sales activities, Arved
Hildebrandt has a passion for innovation management and is part of several patent
applications. Also in this chapter Daniel von dem Berge and Christian Bulligk
provide detailed information and experiences, here about the central lubrications
system for bearings in wind turbine, an important auxiliary system and essential for
the overall reliability of the entire turbine.
The importance of well-designed cooling systems within wind turbines is often
underestimated. Cooling circuits which are at least temporarily to hot or cold cause
significant trouble (power derating or insufficient coolant flow respectively). The
cooling system is one key enabler for the performance and also efficiency of the
overall drivetrain system. Gearbox, Generator and Converter always need sufficient

27. xxii Wind turbine system design
cooling. Within the gearbox the oil additionally serves the lubrication and therefore
reliability and mitigation wear-out. Combined systems are efficient but not easy to
design. Ernst-W. Langhoff gives a deeper look in the secrets of the design of such
systems. In closely cooperation with Urs Giger (Chapter 6) he designs and explains a
suitable cooling and lubrications concepts for the 7.5MW geared drivetrain concept
with power split and dry lube system, introduced by Urs Giger. Ernst-W. Langhoff
is employed by the Hydac Group since May 1985, the first years as a sales engineer,
later as a key account manager for wind energy Industry solutions for gearboxes and
wind turbines. In technical cooperation together with the employer’s development
department he developed the two-stage filter element for gearboxes, not only for
WTB`s but also for industrial application. Beside the lubrication systems, he also
designed water glycol systems for combined gear cooling circuits with converter as
well as generator and in general further more special wind industry solutions. Now
68 years old and retired, he continues work with the Hydac Group, with passion.
The Chapter 9 presents the recommended verification and validation process
according to the V-model for complex product development, exemplary for the
wind turbine, with the focus on rotor and drivetrain. The lead at the experienced
team of authors had Dipl.-Ing. Hans Kyling, who studied aeronautical engineer -
ing at the RWTH Aachen. For more than a decade he has worked in different roles
in both numerical and experimental investigations of complete drivetrains as well
as individual subsystems and components of wind turbines. Today he heads the
department System Validation of Mechanical Drivetrains at the Fraunhofer Institute
for Wind Energy Systems. Specific parts as Co-authors and specialists for test and
validation took over M.Sc. Karsten Behnke and Dr. Anna Wegner, both already
introduced above as well as M.Sc. Malo Rosemeier a mechanical engineer. Since
2013, he works as Research Associate at the Fraunhofer-IWES. In the Department
of Rotor Blades, he is responsible for the applied research on rotor blade structures.
His focus areas are among others the development of validation tests and structural
analysis methods.; and
Dr. Alexandros Antoniou, who is a PhD Mechanical Engineer with 22 years’
experience in design, manufacturing, and testing of composite materials and
sub-structures for wind turbine rotor blades. Currently, he is heading the Group
Modelling of Polymers and Composite Materials at Fraunhofer-Institute for Wind
Energy Systems.
Finally, as a real expert in bearing systems for wind turbine main suspension sys-
tem Dipl.-Ing. Marc Reichert and his Co-authors M.Sc. Tobias Baumgratz and
M.Sc. Clemens Brachmann both from Eolotec GmbH give a comprehensive
insight in the corresponding design process, the requirements and challenges in
Chapter 10. After graduating in mechanical engineering in 2005, Marc Reichhart
started working in the application-engineering department of a bearing manufac-
turer and later became the head of application engineering. In 2010, he moved to
a wind turbine development company where he was able to expand his detailed
knowledge of rolling bearings to the overall drivetrain system and the corresponding

28. Preface xxiii
interactions. Finally, in 2012, he joined the newly founded company Eolotec GmbH.
Since then, his tasks have included the new and further development of bearing and
system calculation methods as well as the development of new measurement sys-
tems for large size bearing arrangements in wind turbine drivetrains. In the recent
years, it has become more and more his vision to bring together the extensive field
experience regarding bearing damages, results from measurement campaigns and
system calculation results, in order to gain a better understanding of the influence of
load dynamics. This should help to avoid bearing failures in the future and thus to
increase the reliability of large size bearing systems.
The Co-author Tobias Baumgratz holds a master’s degree in mechanical engi-
neering with focus on product development, he joined Eolotec GmbH as a working
student in 2019. Through this occupation, he was able to build up initial knowledge
in the field of rolling bearings for wind turbines and to extend this knowledge while
writing his master thesis on the development of rolling bearing calculation models
in FEM. After the master’s degree in mechanical engineering, he started to work
as development engineer at eolotec GmbH, where Tobias Baumgratz have now
acquired further expertise in the field of calculation methods for rolling bearings
and structural components as well as in the design of drivetrain concepts.
The second Co-author Clemens Brachmann gathered his first theoretical and
practical experiences about laser additive manufacturing and about polymer pow-
der deposition for laser sintering during his studies and interning in Taiwan and
Germany. After his Master Thesis about the computational implementation of a heat
conduction model, he joined eolotec in 2022 as a project engineer, now coordinating
engineering services around roller bearings for wind turbines.
Volume 2: Electrical Systems, Grid Integration, Control & Monitoring
The content of Vol.2 shall just explained briefly here. The chapters in more detail
and the authors are described in the equivalent preface of Volume 2 of this book.
In contrast to Vol.1, Volume 2 focuses on the content of the electrical drivetrain
(generator, converter systems) of the wind turbine. In addition, the topics turbine
control, bus systems and monitoring are discussed in detail. Another extensive focus
are wind turbine HiL test systems, not exclusively but specifically for measuring
and certifying their electrical properties, grid integration testing and model valida-
tion. The Volume 2 concludes with chapters related to the topics advanced control
for smarter turbines and wind farms as well as system integration in an anticipated,
highly decentralized electric energy supply systems of the future (principles, mod-
eling and grid-forming control).
The Editor and the whole team of authors, which work all with great commitment
and general passion for wind energy and wrote this book for whom interested, hope
all readers enjoy reading and a successful future work in the fascinating world of
wind energy systems.
With best regards
Jan Wenske (Ed.)

29. xxiv Wind turbine system design
Acknowledgements
On behalf of all authors of this book, the editor would like to thank the following
companies for their kind support in the publication of this book. The information
and images provided are of great value for understanding and explaining the com-
plex areas of knowledge.
bewind GmbH
Bonfiglioli Deutschland GmbH
DNV Denmark A/S
Eolotec GmbH
Federal-Mogul DEVA GmbH (a Tenneco Group Company)
Flender GmbH
Fraunhofer-IWES
GGS
Groeneveld-BEKA GmbH
HAWE Hydraulik SE
Hydac Group
Kendrion INTORQ GmbH
Liebherr-Components Biberach GmbH
NIDEC SSB WIND SYSTEMS
Svendborg Brakes A/S
Trebu Technology B.V.
windwise GmbH
Please note that all images and tables marked accordingly are subject to copyright
of the respective companies or institutions and have been reproduced exclusively for
use in this book by individually permission.

30. Abbreviations and Terminologies
1D One-dimensional
3D Three-dimensional
AC Alternating current
ASME American Society of Mechanical Engineers
B2B Back-to-back
BEM Blade element momentum
BTC Bend-twist coupling
CAB Controlled atmosphere brazing
CARB Toroidal roller bearing
CCV Cold climate version
CFD Computational fluid dynamics
CMS Component mode synthesis
COG Compact orbital gear
CRB Cylindrical roller bearing
CTOD Crack tip opening displacement
CWD Center for Wind Power Drives
DC Direct current
DD Direct drive
DEL Damage equivalent load
DFIG Doubly-fed induction generator
DFMEA Design failure mode and effect analysis
DGD Distributed generation drivetrain
DIN Deutsches Institut für Normung
DLC Design load case
DNV Det Norske Veritas
DOFs Degree of freedom
DRTRB Double-row tapered roller bearing
DT Drivetrain
DUT Device under test
DyNaLab Dynamic Nacelle Testing Laboratory
EC European Commission
ECM Extreme current model
EESG Electrically excited synchronous generator
EFC Emergency feather command
EP Extreme pressure
ESS Extreme sea state
ETM Extreme turbulence model

31. xxvi Wind turbine system design
EU European Union
EWH Extreme wave height
EWM Extreme wind speed model
F2F Face-to-face
FDC Force-distributed constraints
FE Finite element
FEA Finite element analysis
FEM Finite element method
FFST Fatigue full-scale blade testing
FMEA Failure mode and effect analysis
FMECA Failure mode, effects, and criticality analysis
FST Full-scale blade testing
FTA Fault tree analysis
GBTC Geometric bend-twist coupling
GD Geared drivetrain
GDW Generalised dynamic wake
GEBT Geometrically exact beam theory
GFRP Glass Fiber Reinforced Plastic
GL Germanischer Lloyd
GPS Global positioning system
GRC Gearbox reliability collaborative
GRP Glass reinforced polyester
HAPT Highly Accelerated Pitch Bearing Test
HCV Hot climate version
HIL Hardware-in-the-loop
HSS High-speed shaft
HTS High-temperature superconductor
IEC International Electrotechnical Commission
IG Induction generator
IPC Individual pitch control
IR Inner ring
ISO International Organization for Standardization
ITGS Integrated tubular gear system
IWES Institute for Wind Energy Systems
JONSWAP Joint North Sea Wave Project
LCC Life cycle cost
LCoE Levelized cost of energy
LDD Load duration distribution
LEFM Linear-elastic fracture mechanics
LEP Leading edge protection
LES Large eddy simulation
LiDAR Light detection and ranging
LRD Load revolution distribution
LSS Low-speed shaft
LVRT Low voltage ride through
MAN Maschinenfabrik Augsburg-Nürnberg

32. Abbreviations and terminologies xxvii
MBS Multibody simulation
MLC Measurement load case
NCM Normal current model
NCV Normal climate version
NLGI National Lubricating Grease Institute
NREL National Renewable Energy Laboratory
NSS Normal sea state
NTM Normal turbulence model
NVH Noise, vibration and harshness
NWH Normal wave height
OEM Original equipment manufacturer
OVRT Over voltage ride through
PA Polyamide tube
PAO Poly-alfa olefin
PC Point contact
PGT Planetary gear train
PL Performance level
PMBOK Project Management Body of Knowledge
PMSG Permanent magnet synchronous generator
PSF Partial safety factor
RANS Reynolds-averaged Navier–Stokes
RCF Rolling contact fatigue
SBTC Structural bend-twist coupling
SFST Static full-scale blade tests
SG Synchronous generator
SODAR Sound detection and ranging
SPMT Self-propelled modular transporter
SRB Spherical roller bearing
SRP/CS Safety-related parts of control systems
SSS Severe sea state
SWH Severe wave height
TANDEM Towards an Advanced Design of Large Monopiles
TCO Total cost of ownership
TI Turbulence intensity
TR Technical reports
TRB Tapered roller bearing
TRL Technology readiness level
TS Technical specifications
UC Ultra-caps
UMP Unbalanced magnetic pull
VDI Verein Deutscher Ingenieure
V&V Verification and Validation
WBS Work breakdown structure
WP Work package
WT Wind turbine
YFM Yielding fracture mechanics

33. This page intentionally left blank

34. 1
Fraunhofer Institute for Wind Energy System, Großer Westring, Bremerhaven, Germany
Chapter 1
Load calculation and load validation
Philipp Thomas
 1
, Mareike Leimeister
 1
, Anna Wegner
 1
, and
Matthias L. Huhn
 1
The design process of wind turbine (WT) generators is an iterative process. In the
beginning, there are requirements regarding the electrical power or the specific power
(i.e., power per swept area) for certain locations as well as the topology of the WTs.
These requirements form the basis for an initial design of the rotor, which then pro-
vides loads for the design/selection of the load-­carrying components and drivetrain.
This results in a design of the overall system, whose interaction is examined with numerical simulation tools, and requirements for the next iteration of the WT com-
ponents are provided. The load assumptions for the individual components and the dynamics of the overall system are constantly being refined until the requirements for the system design are deemed to have been met. To ensure that the load assump-
tions always contain the same operating states that are relevant for the lifetime of the WTs, regardless of the manufacturer, they are determined based on the specifica-
tions of international standards. After completion of the numerical design process, the design loads and the system dynamics are verified by independent certification bodies before a prototype of the WT can be built. The numerical design loads must be validated on the prototype in the field. This proves that the real loads and dynam- ics are within the limits of the numerical design. At the same time, the quality of the numerical simulation tools used in the design process can be quantified.
Typically, the design process of the entire system takes place at the manufac-
turer of the WT. Specific components are purchased from specialized companies, such as bearings or gears. The supplier companies receive the necessary load pro-
files for the component design from the manufacturers and return numerical model parameters to the manufacturer, which integrates these into the simulation of the overall system and checks the requirements for the system dynamics. If the compo-
nent manufacturer also wants to verify the requirements for its design in the over-
all system, he/she usually has to fall back on freely available simulation models
from WTs. These so-­called generic WTs exist for various power classes from a
few hundred kilowatts to 15 megawatts and beyond. This means that component

35. 2 Wind turbine system design
manufacturers are able to understand the load simulations in accordance with the
requirements of international standards, independently of the WT manufacturer, and
to generate relevant load information for the design process themselves. Although
the parameters of generic WTs are never 100% consistent with the parameters of
the manufacturers and generic loads are always subject to uncertainty compared to
manufacturer loads, generic WTs offer indispensable added value for suppliers and
research. After all, they offer the only way to determine and research the loads and
dynamics of WTs independently of the manufacturer.
Fraunhofer Institute for Wind Energy Systems IWES develops generic WTs
itself and has already published a 7.5 MW design, the onshore WT IWES Wind
Turbine IWT-­7.5-­164 [1]. The 7.5 MW WT has a rotor diameter of approximately
164 m, a hub height of 119.3 m (hybrid tower) and a total mass of 2004 t. The design combines a large onshore WT with relatively low specific power and a focus on detailed blade design with a load mitigation control strategy. Therefore, the WT was developed for coastal locations with high average wind speeds and turbulence inten-
sity (IEC IA). The key figures of the WT can be found in Table 1.1.
An excerpt of previous use cases shows the potential of generic WTs. The IWT-­
7.5-­164 was designed in the Smart Blades project [2] and used to investigate various
concepts for passive and active load reduction techniques. The loads generated were later used for the IWES Highly Accelerated Pitch Bearing Test (HAPT) test bench [3]. In the Towards an Advanced Design of Large Monopiles (TANDEM) research project [4], it was used to calculate loads for the design of an XL monopile, in the InterWiLa research project [5] for load simulations with new types of wind fields, and for the optimised [6] and automatic [7] upscaling of the IEA Wind Task 30 OC4
Table 1.1  Summary of IWT-­7.5-­164 main properties
Description Value Unit
Rated electrical power 7542 kW
Rotor diameter 163.96 m
Hub radius 2 m
Arc length of the blade 79.98 m
Hub height 119.3 m
Cut-­in wind speed 3 m/s
Rated wind speed 11.7 m/s
Cut-­out wind speed 25 m/s
Rated tip speed 85.53 m/s
Cut-­in rotor speed 5 rpm
Rated rotor speed 10 rpm
Rated tip speed ratio 7.31 –
Blade cone angle 2 degrees
Shaft tilt angle 5 degrees
Mass of rotor-­nacelle assembly 536.78 t
Single blade mass 30.93 t
Specific power per rotor swept area360 W/m²

36. Load calculation and load validation 3
semi-­submersible [8] and OC3 spar floater [9], respectively, from a 5 MW to the 7.5
MW WT.
This chapter deals with the requirements from the following standards for deter-
mining the loads of WTs:
••IEC 61400-­1 design requirements
••IEC 61400-­3 design requirements for fixed offshore WTs
••IEC 61400-­13 measurement of mechanical loads
Section 1.1 first introduces the topic of load calculation and then goes into load
determination requirements for onshore and offshore WTs. Topics such as taking
environmental conditions into account when determining the loads by simulation
and the procedure for determining the design loads are considered. At the end of
the section, two use cases for load calculation are presented. Section 1.2 deals with
the validation of the load calculation models. After the general procedure for model
validation is explained, the acquisition of measurement data is presented. Finally,
the specific procedure for validating the load calculation models is outlined.
1.1 Design loads of wind turbines
The design loads are determined in accordance with the requirements of interna-
tional standards. IEC 61400-­1 [10] for onshore WTs and 61400-­3 [11] for offshore
WTs are mentioned here as examples. The standards define requirements regarding the consideration of environmental conditions and how these are to be considered when designing specific components of the WT. The aim is to consider all operating and extreme load conditions that are likely to occur during the service life of the WT and their worst combinations when determining the design loads.
The standards contain requirements for the simulative determination of the
design loads. These requirements are sometimes very specific and sometimes leave a lot of room for interpretation. Detailed simulation scenarios are prescribed, the
so-­called design load cases (DLCs), but the requirements for the numerical simula-
tion tools are only outlined schematically. With the so-­called fully coupled simula-
tion tools, the DLCs specified in the standards can be implemented numerically and converted into time series. The time series contain the dynamic reaction loads and deformations for a period of typically 10 minutes. Load spectra are extracted from
the time series and converted into component-­specific operating loads for a service
life of 20 or more years as well as extreme loads. Furthermore, the simulation pro- vides the natural frequencies of the individual components and the coupled natural frequencies of the overall WT system as well as the specific characteristic curves, such as power and thrust depending on the wind speed.
The results of the load simulation are checked by measurements on the WT
in the field to show their validity. The load calculation and load validation are two essential steps in the type certification process, which is essential for the commercial sale of the WT.

37. 4 Wind turbine system design
1.1.1 Standard load calculation
International standards define different requirements for determining the design
loads for the design and certification of onshore and offshore WTs. The require-
ments for onshore locations are taken here as an example from IEC 61400-­1 and
reproduced to the extent necessary for understanding. The standard defines essential design requirements to ensure the structural integrity of WTs. Its purpose is to pro- vide adequate protection against damage from all hazards during the designed life-
time. Requirements for the numerical simulation models, the consideration of dif-
ferent environmental conditions, the type and implementation of simulations and the processing and evaluation of the simulation results are specified. In addition, IEC
61400-­1 describes the use of an aeroelastic dynamics model to predict design loads.
Such a model is intended to be used to determine loads over a range of wind speeds using turbulence and other wind conditions. All relevant combinations of external conditions and design situations shall be analysed. The standard also defines a set
of simulations to be performed. These DLCs (cf. Table 1.2) can be seen as the mini-
mum requirement for the scope of the simulations to be carried out and the operating situations to be considered.
For onshore WTs, the DLC simulations differ mainly in terms of the operat-
ing and wind conditions and the way they are evaluated. The operating conditions include all states that are expected during the lifetime of the WT. This includes
normal operation, start-­up and shut-­down, standstill as well as various error states.
Each operating state is intended to simulate either normal or extreme operating con-
ditions, or sometimes a combination of these. This is also reflected in the type of evaluation, where a distinction is made between fatigue loads (F) and ultimate loads (U). Furthermore, suitable partial safety factors (PSFs) are assigned to normal (N) and abnormal (A) operating conditions and the resulting favourable or unfavourable loads in the evaluation. Deterministic or stochastic models are used to represent the wind conditions.
While onshore the most relevant environmental condition is the wind, there are
also waves and currents for offshore locations.
1.1.1.1 W
Since the locations of WTs and the wind conditions prevailing there always dif-
fer, the wind conditions have been categorised and WT classes introduced (cf.
Table 1.3). This means that a WT can be assigned to an annual average wind speed
‍V
ave‍, a reference wind speed ‍V
ref‍ (usually ‍5V
ave‍, except for areas subject to tropical
cyclones (‍V
ref,T‍)) averaged over 10 minutes, and a reference wind turbulence ‍I
ref‍, for
which the WT is designed.
The standard differentiates between the four wind speed classes I, II, III and S. Class
I is for high wind speed locations typically found offshore. A class III WT would be pre-
destined for a weak wind site. In addition, a distinction is made between four turbulence classes A+, A, B and C. Turbulence is the temporally and spatially varying fluctuation in wind speed, which is caused by meteorological interactions and the interaction of the wind with obstacles. These include the influence of the wind by buildings, such as other WTs,

38. Table 1.2
  
Onshore DLC according to IEC 61400-
­1 and MLC according to IEC 61400-
­13
Design situationDLCMLCWind conditionOther condition/objectives/remarksType of
analysis
PSF
1) Power production1.1NTM v
in
< v
hub
< v
out
For extrapolation of extreme eventsUN
1.21.1NTM v
in
< v
hub
< v
out
WT is running and connected to the gridF*
1.3ETM v
in
< v
hub
< v
out
UN
1.4ECD v
hub
= v
r
+/- 2
 
m/s and v
r
UN
1.5EWS v
in
< v
hub
< v
out
UN
3.1v
in
< v
hub
< v
out
Normal operation below rated wind speed and
above rated wind speed with relatively steady rotational speed;
Objectives: Blade, tower and drivetrain frequencies
2) Power production
plus occurrence of fault
2.1NTM v
in
< v
hub
< v
out
Control system fault or loss of electrical network UN
2.2NTM v
in
< v
hub
< v
out
Protection system or preceding internal electrical
fault
UA
2.3EOG v
hub
= v
r
± 2
 
m/s and v
out
External or internal electrical fault, including loss
of electrical network
UA
2.42.4NTM v
in
< v
hub
< v
out
Control, protection, or electrical system faults,
including loss of electrical network
F*
3) Start up3.12.1NWP v
in
< v
hub
< v
out
v
in
and > v
r
+ 2
 
m/s
F*
3.2EOG v
hub
= v
in
, v
r
± 2
 
m/s and v
out
UN
3.3EDC v
hub
= v
in
, v
r
± 2
 
m/s and v
out
UN
4) Normal shutdown4.12.2NWP v
in
< v
hub
< v
out
v
in
, v
r and > v
r
+ 2
 
m/s
F*
4.2EOG v
hub
= v
r
± 2
 
m/s and v
out
UN
5) Emergency
shutdown/stop
5.12.3NTM v
hub
= v
r
± 2
 
m/s and v
out
Rated powerUN
3.3v
hub
> v
r
Emergency stop from rated power; Objectives: Blade, tower and drivetrain frequencies
(Continues)

39. Design situationDLCMLCWind conditionOther condition/objectives/remarksType of
analysis
PSF
6) Parked (standing
still or idling)
6.1
EWM 50-
­year recurrence period
UN
6.2
EWM 50-
­year recurrence period
Loss of electrical network connectionUA
6.3
EWM 1-
­year recurrence period
Extreme yaw misalignmentUN
6.41.2NTM v
hub
<0.7
 
v
ref
Rotor either at standstill or idlingF*
3.2High wind speed (high enough to get
sufficient excitation, this will be WT specific)
WT is parked (standstill or idling); Objectives: Blade and tower frequencies
7) Parked and fault
conditions
7.1
EWM 1-
­year recurrence period
UA
8) Transport, assembly,
maintenance and repair
8.1
8.2
NTM v
maint
to be stated by the manufacturer
EWM 1-
­year recurrence period
U U
A A
9) Yaw start/stop3.4Low wind speedWith an instrumented blade in a horizontal
position, the blade gets excited by starting and
stopping the nacelle yaw rotation. Test shall
be conducted with blades in normal operating
position (targeting the flatwise frequencies)
and with blades feathered (targeting the
edgewise frequencies);
Objectives: Blade frequencies
Table 1.2
 
Continued

40. Load calculation and load validation 7
and by the terrain, such as hills and forests. A typical low turbulence class C site would
be offshore, and an A+ site with very high turbulence would more likely be onshore in
complex terrain. The combination of wind and turbulence class results in the WT class,
such as IC or IIIA+. Theoretically, all WTs could be designed for an IA+ location, which
would also make them suitable for all other locations. However, this would mean that the
majority of the WTs (if not all of them) would be oversized, which is uneconomical for the
manufacturers and would unnecessarily drive up the costs of the WTs. The future location
of a WT is therefore already of great relevance in the design phase. The class S WTs are
intended for very specific locations for which the manufacturer must determine the wind
and turbulence parameters himself.
Further parameters are derived from the parameters
‍V
ave‍, ‍V
ref‍ and ‍I
ref‍, which are
required to describe the specific wind conditions for the simulation of each DLC. A distinction is made between stochastic and deterministic wind conditions. While the stochastic wind conditions attempt to examine the inflow occurring during everyday operation and its influence on the loads and dynamics of the WTs as realistically as possible, the deterministic wind conditions are intended to depict special operating conditions.
For the stochastic wind conditions, turbulence models are used to generate the
random fluctuations in wind speed in the time domain. A distinction is made between normal (normal turbulence model, NTM) and extreme (extreme turbulence model, ETM) turbulence intensity (TI). For the simulation of the DLCs with stochastic wind conditions, wind files are generated during the preparation of the simulation. The turbulence models provide the necessary parameters, namely the mean wind speed and TI at hub height over a period of 10 minutes. For NTM, for example, the stan-
dard deviation is determined by the following equation:
‍
=I
ref

0.75V
hub+ 5.6 m/s

‍ (1.1)
The standard deviation ‍‍ is the ‍TI‍ times the mean wind speed ‍V
hub‍. Depending on the
average wind speed and reference TI, the associated TI for each DLC simulation can
be determined (cf. Figure 1.1).
To avoid having to simulate any number of wind speeds in the operating range
of the WTs, wind speed ranges are defined, and suitable wind files are generated for each range. A common subdivision would be to start a new range every 2 m/s. Each
Table 1.3  Basic parameters for WT classes according to IEC 61400-­1
WT class UnitI II III S
‍V
ave‍
m/s 10 8.5 7.5
Designer specific values
‍V
ref‍
m/s 50 42.537.5
‍V
ref,T‍
m/s 57 57 57
A+
‍I
ref‍
– 0.18
A – 0.16
B – 0.14
C – 0.12

41. 8 Wind turbine system design
range represents an average wind speed and TI. Common software tools for calcu-
lating stochastic wind fields use pseudo-­random numbers to determine the temporal
fluctuations in wind speed. The sequence of these random numbers is determined by
a starting value (seed) and is thus made reproducible. So that not all stochastic DLCs
use the same stochastic fluctuation, each seed is only used once in a load simulation.
To ensure the statistical significance of the stochastic DLC simulations, each wind
speed range is simulated with several seeds. At least six seeds must be used in DLC
1.2, which corresponds to a total of 60 minutes of turbulent simulation per average
wind speed. IEC 61400-­1 specifies 12 seeds for DLCs 2.1, 2.2 and 5.1, and even 15
seeds for DLC 1.1. For the implementation of DLC 1.2, which is intended to map normal power production, there are at least 72 simulations each with a length of 600 s (with an operating range of 3–25 m/s with a range width of 2 m/s and 6 seeds each) and even more if yaw errors are considered. The stochastic DLCs represent both normal and extreme operating situations of WTs and are evaluated according to fatigue or extreme loads, depending on the DLC.
The deterministic wind conditions consist of laminar inflow, which is partially
overlaid with specific wind speed profiles. Since there are no stochastic fluctua- tions and therefore no turbulence, these DLCs are only suitable for investigating specific wind and operating conditions and their impact on the load and the dynamic behaviour of the WT. (Extreme) gusts, changes in wind direction, wind shear and combinations thereof are simulated. In addition, the effect of extreme wind speeds (extreme wind speed model, EWM) is examined, which can be implemented sto-
chastically or deterministically. This should take into account particularly strong storms, which only occur every year or once in 50 years.
Figure 1.1  TI for the normal turbulence model

42. Load calculation and load validation 9
Each DLC uses horizontal and vertical oblique flow. Oblique flow is the devia-
tion of the main wind direction from the rotor plane. Horizontal oblique flow is
taken into account differently for operating load cases and standstill load cases.
While the maximum dead range of the yaw control, in which the yaw control is not
yet active (e.g., ±10 degrees), is to be examined in the case of operating load cases,
special attention is paid to the failure of the yaw control in the standstill load cases,
with angles of up to ±180 degrees. Here, DLC 6.2 in particular leads to numerical
instabilities with many load simulation tools at a 90-­degree oblique flow. Vertical
oblique flow takes into account the effect of terrain on the wind direction. It is rec- ommended to turn the main wind direction downwards by 8 degrees against the horizon for onshore load cases. Furthermore, the wind speed, which changes with
altitude, must be taken into account. In IEC 61400-­1, an exponential shear model is
specified with an exponent of 0.2 or 0.11 for onshore load cases, depending on the wind model.
In addition to specifications on wind conditions, the standard contains other
requirements that must be taken into account. The natural frequencies for the tower, drivetrain and rotor must be determined. If coupled system frequencies are in the range of up to the sixth harmonic of the speed (6 P), further analysis must be carried out to check for any resonance points and, if necessary, countermeasures must be taken. If there is a risk of earthquakes at the potential installation site of the WT, the load simulations must also be adapted accordingly. The same applies to regions where ice formation on the rotor blades is to be expected.
1.1.1.2 Waves and current conditions
In the case of offshore WTs, also sea conditions need to be taken into account in
addition to wind conditions. The underlying standard IEC 61400-­3 [11], hence, is
mainly based on IEC 61400-­1 and extends it by offshore-­specific design require-
ments. Thus, in addition to the wind and operating conditions detailed in 0, different waves, the directionality between wind and waves, the presence and type of sea cur-
rents as well as the water level need to be considered and are included in the offshore DLC specification.
The waves occurring offshore are irregular and stochastic. They can therefore be
represented by a wave spectrum, which is based on a significant wave height, peak spectral period and the mean direction of the waves. Similar to the wind conditions, it is differentiated between normal (normal sea state, NSS), severe (severe sea state, SSS) and extreme (extreme sea state, ESS) conditions with the corresponding nor- mal (NWH), severe (SWH) and extreme (EWH) wave heights, and associated peak spectral periods. While the SSS is only used in combination with NTM to represent
the severe conditions at a wave-­dominated site, the ESS in conjunction with EWM
reflects the extreme environmental condition that has a recurrence period of one year or 50 years.
Sea currents are only taken into account in DLCs for ultimate strength analyses
and not for fatigue analyses. Depending on the type of environmental and site condi-
tion, it is differentiated between normal (normal current model, NCM) and extreme

43. 10 Wind turbine system design
(extreme current model, ECM) currents. Both types contain currents that are gen-
erated by the wind and, hence, only reach down to a limited depth (mostly 20 m)
below the water surface. Depending on the location of the offshore WT, there might
also be surf currents to be considered due to breaking waves occurring close to the
coast. Only in the ECM, subsurface currents need to be additionally considered,
which may source from storm surge or tides prevailing at the offshore site.
With the consideration of both wind, waves and currents in the DLCs, the direc-
tionality of the environmental factors may have a significant impact on the resulting
loads on the offshore system. For ultimate and, hence, worst-­case scenarios, wind
and waves may be considered as codirectional and unidirectional. In some fatigue DLCs, also multidirectionality is considered for aligned wind and waves. The mis-
alignment of wind and waves along with their multidirectionality mainly needs to be taken into account in parked DLCs. A separate third combination of the directionali-
ties of wind and waves with the current direction is not required. For the currents, the
direction of the main source of the specific subtype is applied, i.e., the near-­surface
wind-­generated currents follow the wind direction and the subsurface currents are
codirectional with the waves, while the direction of the breaking wave induced surf currents is, due to the nature of this current type, parallel to the coastline.
1.1.1.3 Fatigue and extreme loads
The fatigue and extreme loads are required when designing the individual compo- nents of the WT. The simulations to determine the loads use aeroelastic models. This allows the complex, nonlinear interaction of the WT to be examined in the time domain, and the transient loads and deformations that occur in the process to be determined. The models account for gravitational, inertial, aerodynamic, actua-
tion and other relevant loads as required by the IEC 61400-­1. More specifically, it
requires the following to be taken into account:
••The influence on the wind field by the WTs
••The influence of the three-­dimensional (3D) flow on the aerodynamic properties
of the rotor blades
••Transient aerodynamic effects
••Structural dynamics and the coupling of vibrational modes
••Aeroelastic effects
••The interaction of the control system with the WT.
In addition to the requirements for the capabilities of the load model, the
standard also requires subsequent validation of the aeroelastic simulation model through measurements that should be carried out in accordance with the require-
ments of IEC 61400-­13. This is to ensure that the simulated loads and dynamics
reflect reality.
The extreme loads are determined from the results of all extreme load DLCs at
different positions of each component of the WT. The evaluation is carried out for all degrees of freedom and determines the largest load in terms of absolute value. PSFs

44. Load calculation and load validation 11
are used to take into account the uncertainties in loads, analysis methods and the
importance of components with regard to the consequences of failure. The design
load consists of the PSF and the characteristic load. The characteristic load is deter-
mined from the simulated loads and, if necessary, applied with a specific safety fac-
tor per component, which should take into account the effect of a failure, depending
on the requirements of the respective DLC. The standard defines minimum PSF
values, the use of which also requires a validated load model. The extreme loads
are required for the design of the respective component and indicate the load level
to be endured. Therefore, extreme loads are also referred to as design driving loads.
The fatigue loads are a measure of the damage caused by the cyclic load changes
during operation of the corresponding DLC. The limit state is reached when the
component reaches damage 1 at the combination of a load level, a number of cycles
and an oscillation frequency. The number of cycles depends on the fatigue strength
range of the Wöhler curve of the respective material of the component, and the oscil-
lation frequency on the desired service life of the WT, e.g. 20 years. In the standard,
the use of Miner’s rule is recommended, which uses a rainflow counting method.
The Weibull distribution of the wind speeds measured at the site is used to take into
account the occurrences of the various mean wind speeds during the lifetime of the
WT. This allows the loads simulated in the DLC to be extrapolated for the desired
lifetime of the WT.
In addition to determining the loads, the load model is also used to investigate
the deformation behaviour. For this purpose, the DLCs are evaluated for critical
deformations, such as a sufficiently large distance between the rotor blade and the
tower at each operating point.
1.1.2 Use cases and exemplary loads
The generic WT IWT-­7.5-­164 has already been used many times to determine design
loads. These two use cases are presented below as examples:
••Rotor blade design
••Monopile design
The right-­hand Cartesian coordinate systems of the following two examples
are shown in Figure 1.2 and are defined as follows. The tower coordinate system
origin is located at the point where the tower bottom horizontal plane crosses the vertical centreline of the tower. The
‍xt‍-­axis points downwind with respect to the
main wind direction, the ‍yt‍-­axis points to the side, and the ‍zt‍-­axis points vertically
upwards.
The blade root coordinate system is located at the centre of the blade root and
rotates with the rotor. It is tilted with the main shaft tilt angle, coned with the rotor cone angle and pitched with the blade pitch angle. This means that, in the case of zero pitch angle, the
‍xb‍-­axis points downwind, the ‍yb‍-­axis is parallel to the rotor
plane and points against the rotational direction, and the pitch ‍zb‍-­axis points radially
outwards.

45. 12 Wind turbine system design
1.1.2.1 Rotor blade design
During the design process of the IWT-­7.5-­164 rotor blades, a selection of the five
DLCs 1.1, 1.2, 2.3, 6.1 and 6.2 were simulated in order to determine the loads rel-
evant to the design of the rotor blades. This selection of DLCs was then used to eval-
uate different blade design approaches. For this purpose, the DLCs were repeated
several times and the results were compared.
The loads along the complete rotor blade are required for the rotor blade design.
For the sake of clarity, only the results at the blade root are shown here. Fatigue
results in Table 1.4 are presented in terms of damage equivalent loads (DELs) cal-
culated for all force and moment components: ‍F
x‍,
‍
F
y‍
, ‍F
z‍, ‍M
x‍,
‍
M
y‍
and ‍M
z‍, with the
following settings:
••20 years WT lifetime
••10
7
load cycles
••Wöhler slope exponent m of 4, 8, 10 and 14 for blade outputs
Table 1.4  Fatigue loads at the blade root of the IWT-­7.5-­164
m F
x
[kN]
F
y
[kN]
F
z
[kN]
M
x
[kNm]
M
y
[kNm]
M
z
[kNm]
4 261.5 549.9 557.3 11879.3 10469.2 304.9
8 252.6 420.6 464.5 9229.3 10623.8 409.0
10 261.0 399.4 461.1 8844.9 11193.0 458.9
14 281.2 377.7 471.1 8540.6 12456.6 535.4
Figure 1.2  Coordinate systems at tower and blade root

46. Load calculation and load validation 13
Ultimate loads are presented in terms of the minimum and maximum load com-
ponents located on the main diagonal of an ultimate load table with the correspond-
ing (same time step) load components listed in the rows. Each ultimate load table
also lists DLC names (with wsp: wind speed in m/s, yaw: yaw misalignment in
degrees, seed: wind seed), at which the ultimate loads occurred, with the corre-
sponding PSFs. For reasons of space, only the moments in the three spatial direc-
tions
‍x‍, ‍y‍ and ‍z‍ are shown in Table 1.5, and the forces are left out.
With the help of the load calculation, variables influencing the loads can also
be examined. The IWT-­7.5-­164 reference blade design was modified in two ways to
account for bend-­twist coupling (BTC). BTC couples the blade bending to the blade
torsion and automatically twists the rotor blade out of the wind at high bending. This reduces the aerodynamic torque with large blade deflection and thus the loads. Two BTC approaches were investigated: structural (SBTC) and geometric (GBTC)
bend-­twist coupling.
Table 1.6 compares the relative change in fatigue loads at the blade root with
the reference design, as an example for S -­N slope ‍m= 8‍. The three different blade
design methods affect the loads differently. With these exemplary numbers, GBTC seems the most promising for load reduction. Only the torsional loads
‍M
z‍ increase
with BTC, which is to be expected.
In addition to evaluating different design approaches for rotor blades, the blade
root loads of the IWT-­7.5-­164 offer other practical uses. For example, they can be
used for the design of WT manufacturer-­independent test stands for which no other
loads are available. The IWT-­7.5-­164 blade root loads were used to design the IWES
Table 1.5  Extreme loads at the blade root of the IWT-­7.5-­164
DLC M
x
[kNm]
M
y
[kNm]
M
z
[kNm]
PSF
[-]
M
x
maxDLC11_wsp25yaw0seed6 16656.7−9789.1−7.9 1.35
M
x
min
DLC11_wsp25yaw-­8seed5−14939.6−6242.1−199.91.35
M
y
maxDLC62_wsp50yaw300seed2 −1968.830363.4728.5 1.10
M
y
min
DLC11_wsp11yaw-­8seed68983.3 −40819.8−52.5 1.35
M
z
max
DLC11_wsp13yaw-­8seed2−21486.5−756.4 1231.71.35
M
z
minDLC62_wsp50yaw180seed4 −15477.1−1359.5−1194.01.10
Table 1.6  Comparison of fatigue loads for different blade design approaches at
the blade root of the IWT-­7.5-­164
F
x
F
y
F
z
M
x
M
y
M
z
Reference100.00%100.00%100.00%100.00%100.00%100.00%
GBTC 95.59% 99.95% 99.21% 100.04%90.45% 104.69%
SBTC 99.58% 101.35%101.34%101.89%97.78% 112.78%

47. 14 Wind turbine system design
HAPT test stand [3], where endurance tests for large bearings are carried out.
The fatigue and extreme loads served as the basis for defining the capacities of
the test bench, with additional safety factors being taken into account so that the
test bench is also suitable for testing future bearing generations. In addition, the
IWT-­7.5-­164 loads were used for the planning and execution of the pitch bearing
test program.
1.1.2.2 Monopile design
The design process of bottom-­fixed offshore foundation structures usually consists
of the following three sequential steps:
1. Provision of representative cutting loads
2. Design of the monopile
3. Optimization of the overall system
In the example described below, the DLCs 1.1, 1.2, 3.1, 4.1, 6.1, 6.2 and 6.3
were simulated to determine the representative cutting loads at the transition piece of the tower. The environmental conditions were chosen for a representative North Sea location. The fatigue and extreme loads at the transition piece of the tower for six degrees of freedom of the three spatial directions
‍x‍, ‍y‍ and ‍z‍ as well as the
resultant loads in the x − y – horizontal plane are shown in Table 1.7. The fatigue
loads were calculated for ‍10
7
‍ cycles in 20 years lifetime and for the Wöhler slope
exponents ‍m‍ of 4, 5 and 6.
The extreme loads at the transition piece are shown in Table 1.8 for DLC 6.2.
For reasons of space, only the moments in the three spatial directions ‍x‍, ‍y‍ and ‍z‍ and
the resulting moment ‍xy‍ are shown, and the forces are left out.
The values of the extreme loads can be found on the main diagonal (shaded
grey in Table 1.8). The absolute largest and smallest values are given as well as
the subcase of the DLC in which the values occurred (with u: wind speed in m/s, y:
yaw misalignment in degrees, ww: wind-­wave misalignment in degrees, s: seed).
In addition to the extreme values, the other loads that occurred in the simulation at the time of the extreme load are also entered. In addition to the loads, the PSF is also listed.
Table 1.7  Fatigue loads at the transition piece of the IWT-­7.5-­164 in the tower
coordinate system
mF
x
[kN]
F
y
[kN]
F
xy
[kN]
F
z
[kN]
M
x
[kNm]
M
y
[kNm]
M
xy
[kNm]
M
z
[kNm]
4408.5873.7583.01819.623583.418758.720532.920237.1
5468.6975.6600.72113.729210.823378.822087.322992.4
6523.51086.4643.32353.937778.328556.125083.425550.5

48. Table 1.8
  
Extreme loads at the transition piece of the IWT-
­7.5-
­164 in the tower coordinate system
Load caseM
x
[Nm]
M
y
[Nm]
M
xy
[Nm]
M
z
[Nm]
PSF
[-]
M
x
maxDLC62_u37y30ww30s41.10E+081.29E+071.11E+083.42E+061.10
M
x
minDLC62_u37y240ww30s6−1.10E+08−2.97E+071.14E+08−2.53E+061.10
M
y
maxDLC62_u37y0ww0s12.00E+077.52E+077.78E+07−5.86E+051.10
M
y
minDLC62_u37y180ww0s21.02E+07−9.53E+079.59E+071.15E+061.10
M
xy
maxDLC62_u37y120ww30s38.30E+07−8.25E+071.17E+082.78E+061.10
M
xy
min
DLC62_u37y0ww-
­30s1
−2.90E+041.79E+043.41E+04−1.91E+051.10
M
z
max
DLC62_u37y30ww-
­30s4
4.24E+07−1.00E+074.36E+075.80E+061.10
M
z
min
DLC62_u37y330ww-
­30s1
−7.03E+074.96E+067.05E+07−6.79E+061.10

49. 16 Wind turbine system design
1.2 Design load validation
Within the numerical design process, the design loads and system dynamics are
calculated and form the basis for the manufacturing process of the turbine. Before
being built into a serial product, the results from the numerical design process must
be validated on the prototype in the field. This provides proof that the real loads and
dynamics are within the limits of the numerical design.
Based on the numerical design, a prototype of the WT is built, on which the
type certification measurements are carried out. The type certification measurements
for load validation are specified in the standard IEC 61400-­13 ‘Measurement of
mechanical loads’ [12], as part of the IEC 61400 series, that comprises all IEC stan-
dards relevant for WTs. In Chapter 10, details of further IEC standards relevant to the certification process of WTs can be found.
The standard of the American Society of Mechanical Engineers (ASME) for
Verification and Validation in Computational Solid Mechanics ASME V&V 10 defines validation as the ‘process of determining the degree to which the model is an accurate representation of corresponding physical experiments from the perspective of the indented uses of the model’ [13, p. 3]. Accordingly, the case of intended use for which the model is to be validated must be defined. However, the area for which the model can be considered valid after successful validation is not defined by the intended use domain but by the validation experiments that span the validation space
(see Figure 2.3-­2 in ASME V&V 10 [13, p. 5]).
A general process for verification and validation is presented in the standard
ASME V&V 10 [13] and illustrated in Figure 1.3
*
.
This process should be carried out in parallel with the model development.
However, the different steps of verification of aeroelastic models, such as code veri-
fication and calculation verification, are not part of this book. This chapter focuses on validation. First, the load measurements to receive the experimental outputs from the physical experimentation branch are described. Then, the simulation results of the modelling and simulation branch are described.
1.2.1 Standard load measurements
The measurements of mechanical loads in the design validation follow the standard
IEC 61400-­13. The quantities to be measured can be divided into three different
groups:
••Load quantities
••Meteorological quantities
••Operational quantities
* 
Figure 3.3-­1 reprinted from ASME V&V 10-­2019, by permission of The American Society of Me-
chanical Engineers. All rights reserved.

50. Load calculation and load validation 17
Meteorological quantities, specified in the standard IEC 61400-­12-­1 [14], com-
prise wind speed, wind direction, TI, air density and wind shear for the lower rotor
half as mandatory parameters. For the load quantities, the minimum instrumentation
according to the standard is given in Table 1.9.
Operational parameters comprise electrical power, rotor or generator speed,
yaw misalignment, rotor azimuth angle, pitch position and speed, brake status and WT status. The status values are usually taken from the SCADA system of the
Figure 1.3  Verification and validation process [13, p. 9]

51. 18 Wind turbine system design
WT. In the following, the implementation of the corresponding measuring points
is described.
1.2.1.1 Blade bending moment
For the measurement of the bending moments, classical electrical strain gauges are
usually used. Recently, fibre-­optic sensors have also been applied more and more
frequently. Especially when measuring the bending moments of the rotor blades, the
advantage of fibre-­optic sensors is relevant, as no additional lightning protection has
to be considered with this type of sensor.
Table 1.9  Minimum instrumentation for mechanical load measurements
according to IEC 61400-­13
Load quantity WT <1500 kW or rotor
diameter <75 m
WT >1500 kW and rotor
diameter >75 m
Bending moment blade
root flatwise
1 blade mandatory,
2 blades recommended
2 blades mandatory, 3 blades
recommended
Bending moment blade
root edgewise
1 blade mandatory,
2 blades recommended
2 blades mandatory, 3 blades
recommended
Rotor tilt moment Mandatory Mandatory
Rotor yaw moment Mandatory Mandatory
Rotor torque Mandatory Mandatory
Bending moment tower
base (normal)
Mandatory Mandatory
Bending moment tower
base (lateral)
Mandatory Mandatory
Bending moment tower
mid (normal)
Recommended
Bending moment tower
mid (lateral)
Recommended
Bending moment tower
top (normal)
Mandatory
Bending moment tower
top (lateral)
Mandatory
Bending moment
distribution blade
flatwise
2 blades mandatory, 3 blades
recommended
Bending moment
distribution blade
edgewise
2 blades mandatory, 3 blades
recommended
Blade torsional frequency
and damping
Recommended
Pitch actuation loads 1 blade mandatory
Tower top acceleration
(normal)
Mandatory, when used for
controller feedback
Tower top acceleration
(lateral)
Mandatory, when used for
controller feedback
Tower torque Mandatory

52. Load calculation and load validation 19
The strain gauges in the blade root are installed in the cylindrical part of the
rotor blade. Here, two pairs of strain gauges are installed perpendicular to each other
so that each sensor of a pair is in 180° opposite position (Figure 1.4). The best posi-
tion to place the strain sensors can be determined gravitationally. To do this, the
blade is brought into the feather position, and the lowest point in the rotor blade
circumference is determined with the help of a cylindrical body. The other three
positions can be determined by measuring or using the blade bolts as a reference.
Following the standard, the measurement requirements are different for turbines
with a rotor diameter of more than 75 m and a rated power of more than 1500 W
compared to smaller turbines. Almost all modern WTs fall into the category of larger
turbines. Therewith, the measurement of the bending moment distribution in at least
two rotor blades is required. Especially here, the advantages of fibre-­optic strain
sensors come into play to reduce the risk of lightning strikes. To realize the bending moment distribution, typically one additional set of strain sensors is installed further inside the blade. At this position, another challenge is posed in the installation of the sensors. Further into the blade, it is not as straightforward to identify the most suitable position for the strain sensor. Whereas for the leading edge the position is clear, the opposite position at the trailing edge does not allow the installation of the sensor due to the tapered shape of the blade. Here, the position on the pressure or suction side of the blade has to be chosen. For the flatwise bending moment, the best position is in between the webs (the middle position displayed on the right
in Figure 1.4). However, depending on the construction of the blade, this position
might be difficult to access. Then, the strain gauges are applied on one side of the web on the pressure side and on the other side of the web on the suction side. This alignment is also used for blades with only one web. Typically, signals from the sensors are transmitted to a control cabinet located in the hub of the turbine. When installing the cables, the additional cableways due to pitching have to be considered. This is typically addressed by applying an additional cable loop that expands during pitching.
As the measured quantity for both fibre-­optic and conventional strain gauges
is strain, both systems have to be calibrated to convert strain into the correspond-
ing bending moment. The most accurate way to calibrate the strain gauges is the
Figure 1.4  Installation of strain sensors in the rotor blade in the blade root
(left) and at a larger rotor radius (right)

53. 20 Wind turbine system design
application of a defined force on the rotor blade. This type of calibration can be
carried out well on a test stand, but on a real WT in the field, it involves a large
technical and logistical effort and is therefore usually not applied. Here, the mass
and the centre of gravity of the rotor blade are used for calibration. By turning the
blade into a horizontal position on either side of the turbine, the relation between the
applied moment of gravity and the output of the strain gauge reveals the calibration
factors for the bending moment at the blade root. To reduce external forces other
than the gravitational force during the calibration process, aerodynamic loads on the
blade must be minimized. Therefore, during the calibration, the mean wind speed
must be below 5 m/s. Alternatively, a data set from the operating turbine, which
fulfils the conditions of the calibration, can be extracted and used for the calibration.
An additional calibration at the end of the measurement campaign allows for drift
correction.
1.2.1.2 T
The measurement of tower moments typically is realized using electrical strain gauges, as most towers are made of armoured concrete and lightning protection is therefore not relevant. Both for larger and smaller turbines, the measurement of the bending moments at the tower bottom is mandatory for normal and lateral direc-
tions. For larger turbines, an additional set of sensors is required at the tower top. A third set of sensors is recommended in the middle of the tower. As in the blade root, a set of four strain gauges are installed in pairs in lateral or normal direction. The sensors should be installed not too close to the turbine door and to tower flanges in order to avoid any effects from these elements.
For the tower moments, an analytical calibration can be applied. To this end,
the mass, the overhang moment and centre of gravity of the nacelle as well as the Young’s modulus are needed as input from the turbine manufacturer. For the cal-
culation, additionally, the geometry of the tower is measured. The wall thickness can be quantified using an ultrasonic gauge. To find the offset of the calibration, the nacelle has to be yawed several times over 360°. To minimize aerodynamic effects, the wind speed should be below 5 m/s.
1.2.1.3 M
The measurement of the moments on the main shaft poses an additional challenge to the measurement system. The sensors installed on the shaft are in the rotating system
of the turbine, whereas the data acquisition system is typically installed in the non-­
rotating system of the turbine.
The rotational forces pose a higher demand for the stability of the sensor. This is
particularly important for the fast-­rotating high-­speed shaft, which is not part of the
certification process. However, also for the low-­speed shaft, the rotation of the shaft
has to be considered. Besides the rotational forces, data and energy transfer from
the rotating to the non-­rotating part of the turbine poses additional challenges to the
measurement of the main shaft, if a slip ring cannot be used. Here, battery packs mounted on the shaft can serve as a solution.

54. Load calculation and load validation 21
1.2.2 Data evaluation process
During the measurement campaign, regular plausibility checks are applied to the
data. The end of the measurement campaign is reached with the completion of the
capture matrix. After a final plausibility check, the data evaluation process starts.
The final documentation is implemented by the report, which end is defined by the
IEC 61400-­13. Additionally, special measurement load cases (MLCs) have to be ful-
filled. Some of these MLCs correspond to specific DLCs. An overview of all MLCs
and DLCs is given in Table 1.2.
Before evaluation, certain data are rejected from the data set. This includes, e.g.,
wind directions outside the valid sector, defined by the site evaluation (for details, see Chapter 10) or spikes in the data sets.
1.2.3 Standard load validation
Aeroelastic models, like all models, are subject to assumptions about the state and physics they represent. In order to have confidence in the results of the models,
they must be validated. The standard IEC 61400-­1 specifies that load calculations
must be based on validated methods and approved codes. The aeroelastic simulation model that is used for the specific design calculations must be subsequently vali-
dated by measurements on a dynamically and structurally similar turbine. However, they may differ in detail, e.g., in alternative tower designs [10, p. 40].
1.2.3.1 G
To validate models, output variables are generally compared between the model and the real system to match the response of the model and the real system and to assess whether the model validly describes the real system. This requires the model input to represent the system and its excitation as accurately as possible. However, all model input data are subject to uncertainties. The model input can be divided into system
data and surroundings data (cf. Figure 1.5). System data include geometry, initial
conditions and physical modelling parameters. Surrounding data include boundary
Figure 1.5  Source of uncertainty, adapted from Reference [15]

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56. that "It was a peety! She was a pratty shot, and a fery tight
shentlemans on a hill."
It was work the General required, not amusement; so he journeyed
sadly back, to await in London the command he hoped would ere
long recall him to a profession he had always loved, that seemed
now to offer the sympathy and solace of a home.
Sometimes, but this only in moments of which he was ashamed, he
would speculate on the possibility of meeting Miss Douglas by
accident in the great city, and it soothed him to fancy the
explanations that would ensue. He never dreamed of their resuming
their old footing; for the General's forbearance hitherto had sprung
from the strength, not the weakness of his character, and the same
stubborn gallantry that held his position was available to cover his
defeat; but it would be a keen pleasure, he thought, though a sad
one, to look in her face just once more. After that he might turn
contentedly Eastward, go back into harness, and never come to
England again.
In the meantime, the days that dragged so wearily with St. Josephs,
danced like waves in the sunshine through many of those other lives
with which he had been associated in his late history. Amongst all
gregarious animals, it is the custom for a sick or wounded beast to
withdraw from the herd, who in no way concern themselves about
its fate, but continue their browsings, baskings, croppings,
waterings, and friskings, with a well-bred resignation to another's
plight worthy of the human race. If the General's friends and
acquaintance asked each other what had become of him, and waited
for an answer, they were satisfied with the conventional surmise—
"Gone to Scotland, I fancy. They tell me it's a wonderful year for
grouse!"
Mrs. Lushington, yachting at Cowes, and remaining a good deal at
anchor, because it was "blowing fresh outside," thought of him
perhaps more than anybody else. Not that she felt the least
remorseful for the break-up she believed to have originated solely in

57. her own manœuvres. She was persuaded that her information
conveyed through the anonymous letter had aroused suspicions
which, becoming certainties on inquiry, detached him from Satanella,
and, completely mistaking his character, considered it impossible,
but that their dissolution of partnership originated with the
gentleman. How the lady fared interested her but little, and in
conversation with other dearest friends, she usually summed up the
fate of this one by explaining—
"It was impossible to keep poor Blanche straight. Always excitable,
and unlike other people, you know. Latterly, I am afraid, more than
flighty, my dear, and more than odd."
Besides, Mrs. Lushington, as usual, had a great deal of business on
hand. For herself and her set Cowes was nothing in the world but
London gone down to the sea. Shorter petticoats, and hats instead
of bonnets, made the whole difference. There were the same
attractions, the same interests, the same intrigues. Even the same
bores went to and fro, and bored, as they breathed, more freely in
the soft, Channel air. Altogether, it was fresher and quieter, but, if
possible, stupider than Pall Mall.
Nevertheless, Mrs. Lushington, being in her natural element,
exercised her natural functions. She was hard at work, trying to
mate Bessie Gordon, nothing loth, with a crafty widower, who
seemed as shy of the bait as an old gudgeon under Kew Bridge. She
had undertaken, in conspiracy with other frisky matrons, to spoil
poor Rosie Barton's game with young Wideacres, the catch of the
season; and they liked each other so well that this job alone kept
her in constant employment. She had picnics to organise, yachting
parties to arrange, and Frank to keep in good humour; the latter no
easy task, for Cowes bored him extremely, and, to use his own
words, "he wished the whole place at the devil!" She felt also vexed
and disappointed that the General had withdrawn himself so entirely
from the sphere of her attractions, reflecting that she saw a great
deal more of him before he was free. Added to her other troubles
was the unpardonable defection of Soldier Bill. That volatile light

58. dragoon had never been near her since Daisy's marriage—a
ceremony in which he took the most lively interest, comporting
himself as "best man" with an unparalleled audacity, and a joyous
flow of spirits, that possessed, for a gathering composed of
Hibernians, the greatest attractions. People said, indeed, that Bill
had shown himself not entirely unaffected by the charms of a lovely
bridesmaid, the eldest of Lady Mary's daughters; and it was
impossible to over-estimate the danger of his position under such
suggestive circumstances as must arise from a wedding in the
house.
Then a grey hair or two had lately shown themselves in her
abundant brown locks; while of the people she chose to flirt with,
some neglected her society for a cruise, others afforded her more of
the excitement produced by rivalry than she relished, none paid her
the devoted attention she had learned to consider her due.
Altogether, Mrs. Lushington began to find life less couleur de rose
than she could wish, and to suspect the career she had adopted was
not conducive to happiness, or even comfort. Many people make the
same discovery when it is too late to abandon the groove in which
they have elected to run.
Daisy, in the meantime, true to his expressed intention of turning
over a new leaf, found no reason to be dissatisfied with his lot. You
might search Ireland through, and it is saying a good deal, without
finding a more joyous couple than Captain and Mrs. Walters. The
looked-for promotion arrived at last, and the bridegroom had the
satisfaction of seeing himself gazetted to a troop on the very
morning that provided him with a wife. Old Macormac was pleased,
Lady Mary was pleased, everybody was pleased. The Castle blazed
with light and revelry, the tenants drank, danced, and shouted. The
"boys" burnt the mountain with a score of bonfires, consuming
whisky, and breaking each other's heads to their own unbounded
satisfaction. In short, to use the words of Peter Corrigan, the oldest
solvent tenant on the estate, "The masther's wedding was a fool
to't! May I never see glory av' it wasn't betther divarsion than a
wake!"

59. But Norah's gentle heart, even in her own new-found happiness, had
a thought for the beautiful and stately Englishwoman, whom, if she
somewhat feared her as a rival, she yet loved dearly as a friend.
"What's gone with her, Daisy?" she asked her young husband, before
they had been married a fortnight. "Sure she would never take up
with the nice old gentleman, a general he was, that marked the
race-cards for us at Punchestown. Oh, Daisy! how I cried that night,
because you didn't win!"
They were walking by the river-side, where they landed the big fish
at an early period of their acquaintance, and Norah brought the gaff
to bear in more ways than she suspected; where they parted so
hopelessly, when, because of his very desolation, the true and
generous girl had consented to plight him her troth; and where they
had hardly dared to hope they would meet again in such a glow of
happiness as shone round them to-day. It was bright spring weather
when they wished each other that sorrowful good-bye. Now, the
dead leaves were falling thick and fast in the grey autumn gloom.
Nevertheless, this was the real vernal season of joy and promise for
both those loving hearts.
"What a goose you were to back me!" observed Daisy, with a
pressure of the arm that clung so tight round his own. "It served you
right, and I hope cured you of betting once for all!"
"That's no answer to my question," persisted Mrs. Walters. "I'm
asking you to tell me about my beautiful Blanche Douglas, and why
wouldn't the old General marry her if she'd have him."
"That's it, dear!" replied her husband. "She wouldn't have him! She
—she accepted him, I know, and then she threw him over."
"What a shame!" exclaimed Norah. "Though, to be sure, he might
have been her father." Then a shadow passed over her fair young
brow, and she added wistfully, "Ah, Daisy! I'm thinking I know who
she wanted all the time."
"Meaning me?" said Daisy, with a frank, saucy smile, that brought
the mirth back to her face, and the sunshine to her heart.

60. "Meaning you, sir!" she repeated playfully. "But it's very conceited of
you to think it, and very wrong to let it out. It's not so wonderful,
after all," she added, looking proudly in his handsome young face. "I
suppose I'm not the only girl that's liked you, dear, by a many. I
oughtn't to expect it!"
"The only one that's landed the fish," laughed Daisy, stopping in the
most effectual manner a little sigh with which she was about to
conclude her peroration. "You're mistaken about Miss Douglas,
though," he added, "I give you my word. She hadn't your good
taste, my dear, and didn't see it! Look, Norah, there's the very place
I left Sullivan's fishing-rod. He'll never get it again, so it's lucky I
bought his little brown horse. I wonder who found it? What a day
that was! Norah, do you remember?"
"Remember!"
So the conversation turned on that most interesting of topics—
themselves, and did not revert to Satanella nor her doings. If Norah
was satisfied, Daisy felt no wish to pursue the subject. However
indiscreet concerning his successes, I think when a man has been
refused by another lady, he says nothing about it to his wife.
CHAPTER XXIX
UNDIVIDED
The late autumn was merging into early winter, that pleasantest of
all seasons for those sportsmen who exult in the stride of a good
horse, and the stirring music of the hound. Even in Pall Mall true
lovers of the chase felt stealing over them the annual epidemic,
which winter after winter rages with unabated virulence, incurable
by any known remedy. A sufferer—it would be a misnomer to call
him a patient—from this November malady was gaping at a print-
shop window, near the bottom of St. James's Street, wholly

61. engrossed in the performances of a very bright bay horse, with a
high-coloured rider, flying an impossible fence, surrounded by happy
hunting-grounds, where perspective seemed unknown.
"D'ye think he'll get over, Bill?" said a familiar voice, that could only
belong to Daisy Walters, who had stolen unperceived behind his
friend.
"Not if the fool on his back can pull him into it," answered the other
indignantly. And these comrades, linking arms, turned eastward, in
the direction of their club.
"How's the Missis?" said Bill, whose boast it was that he never forgot
his manners.
"Fit as a fiddle," replied the happy husband. "Had a long letter from
Molly this morning. Sent her best love—no, scratched that out, and
desired to be kindly remembered to you."
Molly, called after Lady Mary, was the eldest and, in Bill's opinion,
the handsomest daughter, so he changed the subject with rather a
red face.
"About to-morrow now," said Bill. "I've got Martingale to do my
orderly. Are you game for a day with the stag?"
"Will a duck swim!" was the answer. "Norah is coming too. I shall
mount her on Boneen; he's own brother to the little horse that beat
our mare at Punchestown."
"Couldn't do better in that country," asserted his friend. "He'll carry
her like a bird, if she'll wake him up a bit, and it's simply impossible
to get him down. By Jove, Daisy, there's St. Josephs going into the
Club. How seedy he looks, and how old! Hang me, if I won't offer
him a mount to-morrow. I wonder if he'll come?"
So this kind-hearted young sportsman, in whose opinion a day's
hunting was the panacea for all ills, mental or bodily, followed his
senior into the morning-room, and proffered his best horse, with the
winning frankness of manner that his friends found it impossible to
resist.

62. "He's good enough to carry the Commander-in-Chief," said Bill. "I've
more than I can ride till I get my long leave. I should be so proud if
you'd have a day on him; and if he makes a mistake, I'll give him to
you. There!"
St. Josephs was now on the eve of departure for the employment he
had solicited. While his outfit was preparing, the time hung heavy on
his hands, and he had done so many kindnesses by this young
subaltern that he felt it would be only graceful and friendly to accept
a favour in return, so he assented willingly, and Bill's face glowed
with pleasure.
"Don't be late," said he. "Nine o'clock train from Euston. Mind you
get into the drop-carriage, or they'll take you on to the Shires. I'll
join you at Willesden. And if we don't have a real clinker, I'll make a
vow never to go hunting again."
Then he departed on certain errands of his own connected with the
pugilistic art, and the General reflected sadly how it was a quarter of
a century since he used to feel as keen as that reckless light-hearted
boy.
He waited on high authorities at the War-office, dined with the field-
marshal, and, through a restless night, dreamed of Satanella, for the
first time since her disappearance.
A foggy November morning, and a lame horse in the cab that took
him to Euston Station did not serve to raise his spirits. But for Bill's
anticipations of "a clinker," and the disappointment he knew it would
cause that enthusiast, the General might have turned back to spend
one more day in vain brooding and regret. Arrived on the platform,
however he got into a large saloon-carriage, according to directions,
and found himself at once in the midst of so cheerful a party that he
felt it impossible to resist the fun and merriment of the hour.
St. Josephs was too well known in general society not to find
acquaintances even here, though he was hardly prepared to meet
representatives of so many pursuits and professions, booted and

63. spurred for the chase, and judging by the ceaseless banter they
interchanged,
"All determined to ride, each resolved to be first."
Soldiers, sailors, diplomatists, bankers, lawyers, artists, authors, men
of pleasure, and men of business, holding daily papers they never
looked at, were all talking across each other, and laughing
incessantly, while enthroned at one end of the carriage sat the best
sportsman and most popular member of the assemblage, whose
opinions, like his horses, carried great weight, and were of as
unflinching a nature as his riding, so that he was esteemed a sort of
president in jack-boots. Opposite him was placed pretty Irish Norah,
now Mrs. Walters, intensely excited by her first appearance at what
she called "an English hunt," while she imparted to Daisy, in a
mellower brogue than usual, very original ideas on things in general,
and especially on the country through which they were now flying at
the rate of forty miles an hour.
"It's like a garden where it's in tillage, and a croquet-lawn where it's
in pasture," said Norah, after a gracious recognition of the General,
and cordial greeting to Bill, who was bundled in at Willesden,
panting, with his spurs in his hand. "Ah! now, Daisy, it's little of the
whip poor Boneen will be wanting for easy leaps like them."
"Wait till you get into the vale," said Daisy; "and whatever you do let
his head alone. Follow me close, and if I'm down, ride over me. It's
the custom of the country."
The General smiled.
"I haven't been there for twenty years," said he; "but I can
remember in my time we were not very particular. I shall follow my
old friend," he added, nodding to the president, whose nether
garments were of the strongest and most workmanlike materials;
"when a man has no regular hunting things, he wants a leader to
turn the thorns, and from all I hear, if I can only stick to mine, I shall
be in a very good place."

64. Everybody agreed to this, scanning the speaker with approving
glances, the while, St. Josephs, though wearing trousers and a
common morning coat, had something in his appearance that
denoted the practised horseman; and when he talked of "twenty
years ago," his listeners gave him credit for those successes which in
all times, are attributed to the men of the past.
"Mrs. Walters must be a little careful at the doubles," hazarded a
quiet good-looking man who had not yet spoken, but whose nature
it was to be exceedingly courteous, where ladies were concerned. "A
wise horse that knows its rider is everything in the vale."
Norah looked into the speaker's dark eyes with a quaint smile.
"Ah, then! if the horse wasn't wiser than the rider," said she, "it's not
many leaps any of us would take without a fall!" and in the laughter
provoked by this incontestable assertion, a slight jerk announced
that their carriage was detached from the train, and they had
arrived.
Though it requires a long time to settle a lady in the saddle for
hunting, even when in the regular swing of twice or thrice a week,
and though Norah was about to enjoy her first gallop of the season
in a new habit, on a new horse, she and Daisy had ample leisure for
a sober ride to the place of meeting, arriving cool and calm, pleased
with the weather, the scenery, the company, and, above all,
delighted with Boneen.
They were accompanied by the General on a first-class hunter
belonging to Bill, and soon overtaken by its owner, who, having
lingered behind to jump a four-year-old over a tempting stile for
educational purposes, had crushed a new hat, besides daubing his
coat in the process.
"Down already?" said St. Josephs. "What happened to him? What
did he do?"
"Rapped very hard," answered Bill; "found his friend at home, and
went in without waiting to be announced;" but he patted the young
pupil on its neck, and promised to teach it the trade before

65. Christmas, nevertheless. Certainly, if practice makes perfect, no man
should have possessed a stud of cleverer fencers than Soldier Bill.
And now, as she reached the summit of a grassy ascent, there broke
on Norah's vision so extensive and beautiful a landscape as elicited
an exclamation of amazement and delight.
Mile after mile, to the dim grey horizon, stretched a sweep of
smooth wide pastures, intersected by massive hedges, not yet bare
of their summer luxuriance, dotted by lofty standard trees, rich in
the gaudy hues of autumn, lit up by flashes of a winding stream that
gleamed here and there under the willows with which its banks were
fringed. Enclosures varying from fifty to a hundred acres, gave
promise of as much galloping as the heart of man, or even woman,
could desire. And scanning those fences the Irish lady admitted to
herself, though not to her companions, that from a distance they
looked as formidable obstacles as any she had confronted in Kildare.
"It's beautiful," said Norah. "It's made on purpose for a hunt. Look,
Daisy, there's the hounds! Oh, the darlings! And little Boneen, he
sees them, too!"
Gathered round their huntsman, a wiry, sporting-looking man on a
thorough-bred bay horse, they were moving into sight from behind a
hay-stack that stood in a corner of the neighbouring field. Rich in
colour, beautiful in shape, and with a family likeness pervading the
lot as if they were all one litter, a fox-hunter would have grudged
them for the game they were about to pursue—a noble red deer, in
so far tame, that he was fed in the paddock, and brought to a
condition that could tax the speed and endurance even of this
famous pack. The animal had already arrived in a large van on
wheels, drawn by a pair of horses, and surrounded by a levee of
gaping rustics, whose eagerness and love for the sport reminded
Norah of her countrymen on the other side of the Channel.
"Will they let him out here, Daisy?" said she, in accents of trembling
excitement. "I wish they'd begin. What are we waiting for?"

66. "Your patience will not be tried much longer," said the General,
lighting a cigar. "Here comes the master, at a pace as if the mare
that landed him the Thousand Guineas, the Oaks, and the St. Leger,
had been made a cover-hack for the occasion!"
"With the Derby-winner of the same year for second horse!" added
her husband. "If you want a pilot, Norah, you couldn't do better than
stick to him, heavy as he is!"
"I mean to follow you, sir," was the rejoinder. "If you don't mind,
Daisy, maybe I'll be before ye."
Even while she spoke a stir throughout the whole cavalcade, and a
smothered shout from the foot-people, announced that the deer had
been enlarged.
With a wild leap in the air, as though rejoicing in its recovered
liberty, the animal started off at speed, but in the least favourable
direction it could have taken, heading towards the ascent on the side
of which the horsemen and a few carriages were drawn up. Then
slackened its pace to a jerking, springing trot—paused—changed its
mind—lowered its head—dashed wildly down the hill to disappear
through a high bull-finch, and after a few seconds came again into
view, travelling swift and straight across the vale.
The General smoked quietly, but his eye brightened, and he seemed
ten years younger for the sight.
"It's all right now," said he; "the sooner they lay them on the better."
Soldier Bill, drawing his girths, looked up with a beaming smile.
"They say there's a lady, a mysterious unknown, in a thick veil, who
beats everybody with these hounds," he observed. "I wonder why
she's not out to-day."
"I think she is," replied Daisy, shooting a mischievous glance at his
wife. "I fancied I caught the flutter of a habit just now behind the
hay-stack. I suppose she's determined to get a good start and cut
Norah down!"

67. Ere the latter could reply, the hounds dashed across the line of the
deer. Throwing the tongues in full musical notes, they spread like a
fan, with noses in the air; then, stooping to the scent, converged, in
one melodious crash and chorus, ere they took to running with a
grim, silent determination that denoted the extremity of pace. Every
man set his horse going at speed. Nearly a dozen selected their
places in the first fence—a formidable bull-finch. The rest, turning
rather away from the hounds, thundered wildly down to an open
gate.
Amongst those who meant riding straight, it is needless to say, were
Mrs. Walters and her three cavaliers. These landed in the second
field almost together. Daisy, closely pursued by his wife, stealing
through a weak place under a tree, the General sailing fairly over all,
and Bill, unable to resist the temptation of a gap, made up with four
strong rails, getting to the right side with a scramble, that wanted
very little of a nasty fall.
The hounds were already a quarter of a mile ahead with nobody
near them but a lady on a black hunter, who was well alongside,
going, to all appearance, perfectly at her ease; while her groom, on
a chestnut horse, left hopelessly behind, rode in the wake of the
General, and wished he was at home.
Daisy, whose steeple-chasing experience had taught him never to
lose his head, was the only one of our party who did not feel a little
bewildered by the pace. Taking in everything at a glance, he
observed the black hunter in front sail easily over a fence that few
horses would have looked at. There was no mistaking the style and
form of the animal. "Of course it is!" he muttered. "Satanella, by all
that's inexplicable! We shall not catch them at this pace, however!"
Then, pulling his horse to let his wife come up, he shouted in her
ear, "Norah, that's Miss Douglas!"
Whether she heard him or not, the only answer Mrs. Walters
vouchsafed was to lean back in her saddle and give Boneen a
refresher with the whip.

68. Unlike a fox, whose reasons are logical and well-considered, a deer
will sometimes turn at right angles for no conceivable cause,
pursuing the new line with as much speed and decision as the old.
In the present instance the animal, after leaping a high thorn fence
with two ditches, broke short off in a lateral direction, under the very
shadow of the hedge it had just cleared, and, at the pace they were
going, the hounds, as a natural consequence, over-ran the scent.
Miss Douglas pulled up her horse, and did not interfere. There being,
fortunately, no one to assist them, they flung themselves beautifully,
swinging back to the line and taking it up again with scarcely the
loss of a minute. The President, two fields off, struggling hard to get
nearer, was perhaps the only man out who sufficiently appreciated
their steadiness. Like Coleridge's Ancient Mariner, "he blessed them
unawares." Bill, I fear, did the other thing, for the fence was so high
he never saw them turn, and jumped well into their midst, happily
without doing any damage.
This slight delay, however, had the effect of bringing Daisy, his wife,
Soldier Bill, and the General into the same field with Miss Douglas.
She heard the footfall of their horses, looked round, and set the
black mare going faster than before. If, as indeed seemed probable,
she was resolved not to be overtaken, the pack, streaming away at
speed once more, served her purpose admirable. No horse alive
could catch them; and Satanella herself seemed doing her best to
keep on tolerable terms at that terrific pace. The majority of the field
had already been hopelessly distanced. The General found even the
superior animal he rode fail somewhat in the deep-holding
meadows. Bill was in difficulties, although he had religiously adhered
to the shortest way. Even Daisy began to wish for a pull, and only
little Boneen, quite thorough-bred and as good as he was sluggish,
seemed to keep galloping on, strong and full of running as at the
start. For more than a mile our friends proceeded with but a slight
alteration in their relative positions—Satanella, perhaps, gradually
leaving her followers, and the hounds drawing away from all five. In
this order two or three flying fences were negotiated, and a fair

69. brook cleared. Daisy, looking back in some anxiety, could not but
admire the form in which Norah roused and handled Boneen. That
good little horse, bred and trained in Ireland, seemed to combine
the activity of a cat with the sagacious instincts of a dog. Like all of
his blood, he only left off being lazy when his companions began to
feel tired; and Mrs. Walters, coming up with her husband, as they
rose the hill from the waterside, declared, though he did not hear
her, "I could lead the hunt now, Daisy, if you'd let me. Little Boneen's
as pleased as Punch! He'd like to pull hard, only he's such a good
boy he doesn't know how!"
"Taking fast hold of his horse's head, he got over with a scramble."
Satanella. Page 301
Bill's horse dropped its hind legs in the brook, and fell, but was soon
up again with its rider. The General got over successfully;
nevertheless, his weight was beginning to tell, and the ground being
now on the ascent, he found himself the last of the five people with
the hounds.

70. At the crest of the hill frowned a black, forbidding-looking bull-finch:
on this side a strong rail; on the other, if a horse ever got there, the
uncertainty, which might or might not, culminate in a rattling fall.
Daisy glanced anxiously to right and left, on his wife's behalf, but
there was no forgiveness. They must have it, or go home! Then he
watched how the famous black mare would acquit herself a hundred
yards ahead of him, and felt little reassured to detect such a struggle
in the air while she topped the fence, as by no means inferred a
pleasant landing where she disappeared on its far side.
He wavered, he hesitated, and pulled his horse off for a stride; but
Norah's impatient—"Ah, Daisy! go on now!" urged him to the
attempt, and he chanced it, with his heart in his mouth, for her sake,
not his own.
Taking fast hold of his horse's head, he got over with a scramble,
turning afterwards in the saddle to watch how it fared with his wife
and little Boneen. Her subsequent account described the
performance better than could any words of mine.
"When I loosed him off at it," said she, "I just touched him on the
shoulder with the whip, to let him know he wasn't in Kildare. He
understood well enough, the little darling! for he pricked his ears,
and came back to a slow canter; but I'd like ye to have felt the
bound he made when he rose to it! Such a place beyond! 'Twas as
thick as a cabbage-garden—dog-roses, honeysuckles, I'm not sure
there wasn't cauliflowers, and all twisted up together to conceal a
deep, wide, black-looking hole, like a boreen.
[6]
Well, I just felt him
give a sort of a little kick, while he left the entire perplexity ten feet
behind him, and when he landed, as light as a fairy, Daisy, I'm sure I
heard him laugh!"
Mrs. Walters, like most of her nation, abounded in enthusiasm. She
could not forbear a little cry of delight at the panorama that opened
before her, when she had effected the above-mentioned-feat. To the
very horizon lay stretched a magnificent vale of pasture, brightened
by the slanting rays of a November sun. Far ahead, fleeting across
the level below, sped a dark object, she recognised for the deer; a

71. field nearer were the hounds, running their hardest, in a string that
showed they too had caught sight of their game. Half-way down the
hill she was herself descending, the other lady was urging the black
mare to head-long speed, very dangerous on such a steep incline.
Fifty yards behind Satanella, came Daisy, and close on his heels,
Norah, wild with delight, feeling a strong inclination to give Boneen
his head, and go by them all. The little horse, however, watched his
stable-companion narrowly, while his rider's eyes were riveted on the
hounds. Suddenly she felt him shorten his stride and stop, with a
jerk, that nearly shot her out of the saddle. Glancing at Daisy, for an
explanation, she screamed aloud, and covered her face with her
hands.
When she looked again, she was aware of her husband's horse
staring wildly about with the bridle over its head; of Daisy himself on
foot, and, a few yards off, the good black mare prostrate,
motionless, rolled up in a confused and hideous mass with her
hapless rider.
Down hill, at racing pace, Satanella had put her fore-feet through a
covered drain, with the inevitable result—the surface gave way,
letting her in to the shoulders, and a few yards farther on, she lay
across her mistress, with her neck broken, never to stir those strong,
fleet limbs again.
"Oh! Daisy, they're both killed!" whispered Norah, with a drawn,
white face, while her husband, busying himself to undo the girths,
and thus extricate that limp helpless figure from beneath the weight
that crushed it so sorely, shouted for assistance to Soldier Bill and
the General, who at that moment entered the field together.
"I trust in heaven, not!" he replied aloud; and, below his breath,
even while his heart smote him for the thought, "It might have been
worse. My darling, it might have been you!"
FOOTNOTES:

72. [6] "Boreen," Irish for a deep, stone-paved lane.
CHAPTER XXX
THE BITTER END
It was indeed a sad sight for those joyous riders, exulting but a
moment before, in all the triumph and excitement of their gallop.
Saddest and most pitiable for the General, thus to find and recognise
the woman he had loved and lost. While they took her gently out
from under the dead mare's carcase, she groaned feebly, and they
said, "Thank God!" for at least there seemed left a faint spark of life.
Assistance, too, was near at hand. As Norah observed, "'Twasn't like
Kildare, where ye wouldn't have seen a shealing or may be so much
as a potato-garden for miles! But every farm here was kept like a
domain, and they'd built a dwelling-house almost in every field!"
Within a short distance stood the comfortable mansion, surrounded
by its well-stocked fold-yards, of a substantial yeoman; and Bill, with
two falls, was there in two minutes! A few of the second flight also,
persevering resolutely on the line the hounds had gone, straggled up
and did good service. What became of the Field, and where the deer
was taken, none of these had opportunity to ascertain. All their
energies, all their sympathies, were engrossed by that helpless,
motionless form, that beautiful rigid face, so wan and white, beneath
its folds of glossy raven hair.
Carrying her softly and carefully on a gate to her place of shelter, it
looked as if they formed a funeral procession, of which the General
seemed chief-mourner.
His bearing was stern and composed, his step never faltered, nor did
his hand shake; but he who wrestled with the angel of old, and
prevailed against him, could scarcely have out-done this loving,

73. longing heart in earnestness of purpose and passionate pleading of
prayer.
"But once more!" was his petition. "Only that she may know me, and
look on me once more!" and it was granted.
For two days Blanche Douglas never spoke nor stirred. Mrs. Walters
constituted herself head-nurse, and never left her pillow. The
General remained the whole time at the threshold of her chamber.
The surgeon, a country practitioner of high repute, who saw her
within an hour of her accident, committed himself to no opinion by
word or sign, but shook his head despondingly the moment he found
himself alone. The famous London doctor, telegraphed for at once,
preserved an ominous silence. He, too, getting into the fly that took
him back to the station, looked grave and shook his head. The
hospitable yeoman, who placed his house and all he had freely at
the sufferer's disposal, packing off the very children to their aunt's,
at the next farm, felt, as he described it, "Down-hearted—
uncommon." His kindly wife went about softly and in tears. Daisy
and Bill hurried to and fro, in every direction, as required, by night
and day; while Norah, watching in the darkened room, tried to hope
against hope, and pray for that which she dared not even think it
possible could be granted.
The General looked the quietest and most composed of all. Calm and
still, he seemed less to watch than to wait. Perhaps some subtler
instinct than theirs taught him the disastrous certainty, revealed to
him the inevitable truth.
Towards evening of the second day Norah came into the passage
and laid her hand on his shoulder, as he sat gazing vacantly from the
window, over the fields and orchards about the farm. They loomed
hazy and indistinct in the early winter twilight, but the scene on
which he looked was clear enough—a bright sunny slope, a golden
gleam in the sky above, and on earth a dark heap, with a trailing
habit, and a slender riding-whip clenched in a small gloved hand.

74. "She has just asked for you," whispered Norah. "Go to her—quick!
God bless you, General! Try and bear it like a man!"
The room was very dark. He stole softly to her bedside, and felt his
fingers clasped in the familiar clinging touch once more.
"My darling!" he murmured, and the strong man's tears welled up,
thick and hot, like a child's.
Her voice came, very weak and low. "The poor mare!" she said; "is
she much hurt? It was no fault of hers."
He must have answered, and told her the truth without knowing it;
for she proceeded more feebly than before.
"Both of us! Then it's no use. I was going to give her to you, dear,
and ask you to take care of her for my sake. Have you—have you
forgiven?"
"Forgiven!" His failing accents were even less steady than her own.
"I vexed you dreadfully," she continued. "I was not good enough for
you. I see it all; and, if it could come again, I would never leave you
—never! But I did it for the best. I took great pains to hide myself
away down here; but I'm glad. Yes, I'm very glad you found me out
at last. How dark it is! Don't let go my hand. Kiss me, my own! I
know now that I did love you dearly—far better than I thought."
The feeble grasp tightened, stronger, stronger, yet. The shadows fell,
the night came down, and a pale moon threw its ghostly light into
the chamber. But the face he loved was fixed and grey now, the
hand he still clasped was stiff and cold in death.
The General carried to India a less sore heart, perhaps, than he had
expected. There was no room left for the gnawing anxiety, the bitter

75. sense of humiliation, the persistent struggle against self, that
distressed and troubled him in his previous relations with her he had
loved so dearly, and lost so cruelly even in the hour she became his
own. He was grave and silent, no doubt, in feelings and appearance,
many years beyond his real age; but every fresh grey hair, every
additional symptom of decay, seemed only a milestone nearer home.
Without speculating much on its locality, he cherished an ardent
hope that soon he might follow to the place where she had gone
before. None should come between them there, he thought, and
they need never part again.
Soldier Bill and Daisy saw the last of him when he left England; the
former rather envied every one who was bound for a sphere in
which there seemed a possibility of seeing real service, the latter
comparing his senior's lonely life and blighted hopes with his own
happy lot, felt a humbler, a wiser, and a better man for the contrast.
Mrs. Walters, though losing none of her good nature and genial Irish
humour, became more staid in manner, altogether more matronly;
and though she went out hunting on occasion, certainly rode less
boldly than before the catastrophe. Her sister Mary, however, who
came over to stay with her about this time, kept up the family credit
for daring, and would have taken Bill's heart by storm if she had not
won it already with the fearlessness she displayed in following him
over the most formidable obstacles. After a famous day on Boneen,
when she bustled that lazy little gentleman along in a manner that
perfectly electrified him, Bill could hold out no longer, but placed
himself, his fortunes, Catamount, and Benjamin, at her disposal. All
these she was good enough to accept but the badger; and that
odorous animal was compelled to evacuate his quarters in the
wardrobe for a more suitable residence out of barracks, at a livery-
stable. So they were married in London, and inaugurated the first
day of their honeymoon by a quick thing with the Windsor drag-
hounds.
Of Mrs. Lushington there is little more to be said. The sad fate of her
former friend she accepted with the resignation usually displayed by

76. those of her particular set in the face of such afflictions as do not
immediately effect themselves and their pleasures. She vowed it was
very sad, talked of wearing black—but didn't! and went out to dinner
much as usual. Even Bessie Gordon showed more feeling, for she did
cry when she heard the news, and appeared that night at a ball with
swollen eyelids and a red place under her nose. Many people asked
what had become of Miss Douglas? The answer was usually
something to this effect—
"Don't you remember? Painful business; shocking accident. Killed out
hunting. Odd story; odd girl. Yes, handsome, but peculiar style!"
They buried the good black mare where she fell. Long before the
grass was green over her grave, rider and horse had been very
generally forgotten. Yet in their own circle both had created no small
sensation in their time. But life is so far like the chase, that it admits
of but little leisure for hesitation; none whatever for regret. How
should we ever get to the finish if we must needs stop to pick up the
fallen, or to mourn for the dead?
In certain kind and faithful hearts, however, it is but justice to say
the memory of that hapless pair remains fresh and vivid as on the
day of their fatal downfall.
There is a stern, grey-headed soldier in the East who sees Blanche
Douglas nightly in his dreams; and Daisy Walters, in his highest state
of exultation, when he has been well-carried, as often happens,
through a run, heaves a sigh, and feels something aching at his
heart, that recalls the black mare and her lovely wayward rider, while
it reminds him in a ghostly whisper that "there never was one yet
like Satanella!"
UNWIN BROTHERS, THE GRESHAM PRESS, WOKING AND LONDON.

77. ADVERTISEMENTS
New Comélete Library Edition of
G.J. Whyte=Melville's Novels.
Comélete in about 25 Volumes .
Large Crown 8vo, Cloth Gilt, 3s. 6d. each.
The publishers have pleasure in announcing a monthly issue of
novels by the late G.J. Whyte-Melville, who, uniting, as he did, the
qualities of poet, novelist, sportsman, and leader of society, has long
been acknowledged to stand above rivalry when dealing with sport
and the romance of old. Each volume will be illustrated by front-rank
artists, well printed from type specially cast, on best antique paper,
and handsomely bound.
1 KATERFELTO.      With four illustrations by Lucy E. Kemé-Welch.
2 CERISE.          Wi th four illustrations by G.P. Jacomb-Hood.
3 SARCHEDON.            Wi th four illustrations by S.E. Waller.
4 SONGS AND VERSES, and THE TRUE CROSS.
With five illustrations by S.E.
Waller.
5 MARKET HARBOROUGH, and INSIDE THE BAR.
With four illustrations by John
Charlton.
6 BLACK BUT COMELY.      With four illustrations by S.E. Waller.
7 ROY'S WIFE.      Wi th four illustrations by G.P. Jacomb-Hood.
8 ROSINE, and SISTER LOUISE.
With four illustrations by G.P. Jacomb-
Hood.

78. 9 KATE COVENTRY.
With four illustrations by Lucy E. Kemé-
Welch.
10 THE GLADIATORS.
With four illustrations by J. Ambrose
Walton.
11 RIDING RECOLLECTIONS.
With four illustrations by John
Charlton.
12 THE BROOKES OF BRIDLEMERE.
With four illustrations by S.E.
Waller.
13 SATANELLA.      Wi th four illustrations by Lucy E. Kemé-Welch.
14 HOLMBY HOUSE.  Wi th four illustrations by Lucy E. Kemé-Welch.
Novels by Guy Boothby.
SPECIAL AND ORIGINAL DESIGNS.
Each volume attractively illustrated by Stanley L. Wood and others.
Crown 8vo, Cloth Gilt, Trimmed Edges, 5s.
Mr. RUDYARD KIPLING says:
"Mr. Guy Boothby has come to great honours now. His name is
large upon hoardings, his books sell like hot cakes, and he

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