Semi submersible

Usefull for introduction to semi submersible Platform.

1. Definition A  semi-submersible   ( semi submerged ship ) is a specialized marine vessel with legs/pontoons for buoyancy to float, and weight to keep the structure upright, which  used in a number of specific offshore roles as , offshore drilling rigs, heavy lift cranes, oil production platforms , safety vessels. They are designed with good stability and  sea keeping characteristics .

2. Semi-Submersible Exploratory and production purposes; floating structure; towed to the site, ballasted and moored; large vertical columns connected to large pontoons/legs, columns support deck structure and equipment.

3. Semi-Submersible Water depth 200m to 3000m, Offers large deck space, Exceptional stability and sea keeping, Withstand rough waters, Highly flexible and efficient, Can be moved from place to place.

4. Applications Mobile offshore drilling units ( MODU) or Semi–Submersible Drilling Units (SSDU) Semi-Submersible crane vessels (SSCV) Offshore support vessels (OSV) Offshore production platforms

5. Semi-Submersible Drilling Units (SSDU) Based on the way the rig is submerged in the water, there are two main types  Bottle-type semi-subs and Column-stabilized semi-subs

6. Bottle-type semi-subs Hull below the drilling deck is Bottle-shaped. Hull can be submerged by ballasting. Originally  bottles below the rig were completely submerged, resting on ocean floor. As time progressed bottles were only partially submerged. Offers exceptional stability for drilling operations.

7. Column-Stabilized semi-subs More popular design Two horizontal hulls are connected via cylindrical or rectangular columns to the drilling deck above the water Smaller diagonal columns are used to support the structure Mooring lines and dynamic positioning help to keep the semi-sub on location

8. Mobile offshore drilling units (MODU)

9. Semi–Submersible Drilling Units SSDU classified by IMO

10. Semi-Submersible crane vessels (SSCV) De-ballasted during transit; only part of the lower hull is submerged . The vessel Ballasted during lifting operations. High stability allows them to lift extreme high loads

11. Semi-Submersible crane vessels (SSCV)

12. Offshore support vessels (OSV) Semi-submersibles are used as number of offshore support vessels because of their good stability, large deck areas , and variable deck load (VDL ). Construction support vessel, Diving Support Vessel (DSV), Offshore Safety support vessel, Offshore accommodation platform, Offshore Multiservice Vessel (MSV), E.g.- Ocean Odyssey  – Converted semi-submersible drilling rig used as a rocket launch pad.

13. Offshore production platforms When oil fields were first developed in offshore locations, drilling semi-submersibles were converted for use as combined drilling and production platforms. As progressed to deeper water and harsh environments, purpose-built production semi-submersible platforms were designed. Offers very stable and cost effective platforms . Mostly converted semi-submersible vessels.

14. Mooring patterns and Dynamic positioning Spread Mooring patterns and Dynamic positioning used to keep the floating rig in place Mooring is similar to multiple anchors Chosen depending on shape of unit and sea conditions Dynamic positioning uses different motors or propulsion units to counteract motions of water Dynamic positioning system is guided by telemetry signals from beacons, satellite information and the angular movements of a cable.

15. Semi–Submersible Drilling Units – SSDU Advantages Disadvantages Semi-submersibles can achieve good (small) motion response and, therefore, can be more easily positioned over a well template for drilling. Most stable of any floating rig. Semi-submersibles allow for a large number of flexible risers. Provides large deck area. Transportation from location to location is easy. Can be reused and converted to other support vessels. High initial and operating costs. Pipeline infrastructure or other means is required to export produced oil. Building schedules for semi-submersibles are usually longer than jack-up rigs. Limited deck load (low reserve buoyancy). Structural fatigue. Expensive to move large distances. Limited dry-docking facilities available. Difficult to handle mooring systems and riser in rough seas.

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