offshore drilling basics topic for beginers

offshore drilling basics

1. OFFSHORE DRILLING

2. During 1998 in the United States, the
American Petroleum Institute reported that
the average offshore oil well was drilled to
10,845 ft (3306 m) at an average cost of
$585/ft. Offshore drilling operations and
equipment are similar to those on land.
The major difference is a top drive and
the platform upon which the rig is
mounted.

3. 1.Top Drive
The drilling rig on an offshore rig is similar
to a drilling rig on land. A major difference is
the top drive used on an offshore drilling rig
(fig.1). A top drive is a power swivel located
below the traveling block that drives the
drillstring. It is either a large electrical or
hydraulic motor that generates more than
1000 horsepower.

4. Fig.1 top drive
It is hung from the hook
on the traveling block
and turns a shaft into
which the drillstring is
screwed. The top drive
moves up and down
vertical rails to prevent it
from swaying with any
motion from waves.

5. Drillpipe is added to the drillstring three
joints at a time when using a top drive. The
top drive saves time during making a
connection. Slips are still used in the master
bushing on a stationary rotary table to
prevent the drillstring from falling down the
well.

6. 2.Offshore Crews
There are usually three crews on an
offshore rig (fig.2), two working on the rig
and one off duty ashore. An onshore and
an offshore crew are rotated once every
two weeks. The crews work 12 hours from
11 a.m. to 11p.m. and 11 p.m. to 11 a.m.

7. An offshore crew can
have a driller, assistant
driller, derrickman,
roughnecks, motorman,
diesel engine operator,
pump operator, mud
man, crane operator,
and roustabouts.
Fig.2 Fixed production platform

8. The driller handles the rotary controls
drawworks and pumps, while the assistant
driller handles the pipe and racking system
and the iron roughnecks. The assistant
driller can also relieve the driller when
necessary. The crane operator operates the
crane used to lift equipment and supplies
from supply ships and barges onto the
offshore rig.

9. The crane operator is also usually the
head roustabout charge of the roustabouts
who handle the supplies and equipment. A
tool pusher and company man will also be
aboard. On some offshore rigs, a pit
watcher can be responsible for the drilling
mud and circulating equipment. There are
often rig mechanics and electricians to
maintain the rig.

10. On a large drillship, there can be quarters for
200 people. If the production platform is close
to a port, the crew can be ferried out to the
platform on fast crew boats for a crew change
every two weeks. On many, however, the
crew and other personnel are transported to
and from the production platform by
helicopters that land on the helideck (a flat
platform).

11. 3.Exploratory Drilling
In shallow, protected waters such as
lagoons and canals up to about 25ft (7 5 m)
deep, the rig can be mounted on a drilling
barge. A pasted barge is a drilling barge
designed to be sunk and rest on the bottom
while drilling. The drilling deck is mounted
on posts to keep it above the surface of the
ocean.

12. Offshore, a mobile offshore drilling unit
(MODU) is used. Three types arejackup,
semisubmersible, anddrillship.

13. A jackup rig usually has two barge-like hulls
and at least three vertical legs through the
hulls (Fig. 3). The legs are either 1) open-
truss with tubular steel members that are
cross braced, or2) columnar made of large
diameter steel tubes.

14. Fig.3 Jackup rig(自升式平台 )

15. The cantilevered jackup rigis most common
with the derrick mounted on two large steel
beams that protrude over the edge of the
deck. Occasionally the derrick is mounted
on the deck over a slot or keyway in the
deck.

16. The jackup rigis usually towed into position
although some are self-propelled. While
moving, both hulls are together and float like
a barge with the legs raised high. At the
drillsite, the lower hull (mat) is flooded and
positioned on the seafloor.

17. On each of the legs is a jack housethat
uses a rack-and-pinion arrangement
powered by an electric or hydraulic motor to
raise and lower the upper hull. The upper
hull is jacked up on the legs until usually
about 25 ft (8 m) above the sea surface. The
drilling rig on the upper hull is then secure
above the waves, and the mat acts as a
stable foundation, even with a soft bottom.

18. If the ocean bottom is relatively hard,
smaller cylinders (cams) with a point on the
bottom can be used on the bottom of each
leg instead of a mat. After drilling, the hulls
can be joined again and the rig towed or
steamed to another site.

19. Legs on these rigs are constructed up to
550 ft (170 m) high. Jackups are generally
used in water depths up to 300 ft (91 m)
although some can drill in deeper waters.

20. For deep-water drilling, a floater, a semi-
submersible, or drillshipis used. A semi-
submersible is a floating, rectangular-
shaped drilling platform (Fig. 4).
The most common type is the column-
stabilized semisubmersible.Most of the rig
flotation is in the pontoons, located 30 to 50
ft (9 to 15 m) below sea level when on
station. Square or circular columns connect
the drilling platform to the pontoons .

21. Fig.4 semisubmersible rig(半潜式平台 )

22. If it is relatively shallow water, the semi-
submersible is anchored on station with a
mooring pattern of anchors and chains
radiating out from the rig. In deeper waters,
it uses dynamic positioning. Because most
of the flotation is below sea level in the
pontoons, the rig is very stable even during
high sea and winds.

23. Once drilling is completed, the semi-
submersible can be towed to another
drillsite. To move the semisubmersible, the
pontoons are emptied, and the rig floats
high in the transit modefor easier towing.

24. A ballast-control specialistsupervises the
raising and lowering of the semi and keeps it
stable. For long-distance transport, the semi
can be carried on the deck of a special ship
during a dry tow. Some semis can drill in
water depths up to 10,000 ft.

25. A drillshipis a ship with a drilling rig
mounted in the center (Fig. 5).The ship
steams out to the drillsite and then drills
through a hole in the hull, called the moon
pool. The ship floats over the drillsite.

26. Fig.5 drillship (钻井船)

27. On the drillship, a satellite dish is used to
track navigational satellites.A computer
aboard the drillship constantly recalculates
the drillship's location. If the drillship drifts off
the drillsite, the computer engages the
ship's propellers and puts the drillship back
on location.

28. Drillships have propellers on the side of the
ship (bow and stern thrusters) and can
move both back and forth and sideways.
Keeping on station by computer is called
dynamic positioning.

29. Drillships are very expensive.For efficiency,
some modern drillships have the equipment
and ability to drill two wells at the same time
from one derrick. The derrick contains two
traveling blocks and top drives, and the ship
has two independent drillers and assistant
drillers stations and two setback areas to
rack the pipe. Some drill ships have the
capability of drilling in 35,000 ft water depth.

30. Spudding an Offshore Exploratory Well
On a well drilled by a jackup rig, several
hundred feet of large diameter (26 or 30 in. -
66 or 76 cm), conductor casing is set into
the sea bottom.
On a very soft sea bottom,the conductor
casing is jetted into the bottom by pumping
seawater through the center.

31. On a harder bottom, the conductor casing is
pile-driven into the bottom.
On a very hard bottom, a hole is drilled, and
then the casing is run into the hole and
cemented.

32. Theconductorcasingextendsabovesea
leveltojustbelowthedrillingdeck.Asmaller
diameterholeisthendrilledthroughthecasing
intotheseafloortoseveralhundredfeetbelow
thebottomoftheconductorcasing.Surface
casingisrunintotheholeandcemented.Next,
ablowoutpreventerstackisboltedtothetopof
thesurfacecasing.Therestofthewellis
thendrilledandcasedsimilartoawellonland.

33. On a well drilled by a floater (a semi or
drillship), a temporary guide baseor drilling
template is installed on the sea bottom. The
temporary guide base is a hexagonal-
shaped steel framework with a hole in the
center for the well. It is attached to bottom of
a drillstring and lowered to the sea bottom.
Four steel guidelinesrun from the sides of
the temporary guide base up to the floater.
They are used to lower and position other
equipment in and on the well.

34. The drillstring is then raised back up to the
floater leaving the temporary guide base on
the sea bottom. A guide frame is then
attached to the bottom of the drillstring. It
has two or four arms through which the
guidelines run.

35. The drillstring and guide frame is then
lowered down the guidelines to the
temporary guide base. A large diameter hole
(30 or 36 in -76 r 9cm.) is drilled through the
center of the temporary guide base to about
100ft (30 m) below the seafloor.

36. The drillstring and guide frame are then
raised back to the floater. The guide frame is
then attached to the lowest joint on the
foundation pile, the first casing string run
into the well. A fundation pile housing and
permanent guide struture is attached to the
top foundation pile joint.

37. The Location pile is then run into the hole
and cemented. The permanent guide
structure is attached to the temporary guide
base on the sea bottom. The hole is then
drilled deeper, and a string of conductor
casing is run and cemented into the hole.

38. Asubseablowoutpreventstackthatcan
beactivatedfromthefloateristhen
loweredandlockedontothewellheadwith
ahydraulicwellheadconnector.The
drillingrigisthenconnectedtothe
blowoutpreventersby,flexible,metal
hollowtube(marineriser).Thedrillstring
goesthroughthemarineriserintothewell.

39. A tensioner system of wire rope and pulleys
on the floater supports the upper part of the
marine riser. The marine riser completes a
closed system to circulate drilling mud down
the drillstring and up the annulus between
the drillstring and marine riser.

40. To compensate for the up-down motion
(heave) of the floater on the surface, a
telescoping joint that expands and
contracts is used on the top o the marine
rise. A heave compensator is also located
between the traveling block and hook on
the drilling rig. The compensator has
pistons in cylinders that hold the hook and
drillstring stationary as the floater heaves.

41. During an emergency such as severe
weather, the blowout preventers can be
closed and the marine riser disconnected
from the BOP stack. The floater can then be
moved off station to safety. After the
emergency has passed, the well can be
relocated and reentered.

42. Developmental Drilling and Production
After a commercial, offshore field has been
discovered, it can be developed with a fixed
production, tension leg, or compliant platform. A
fixed production platform has legs and sits on the
bottom. One type is called a gravity-base platform
because it has a large mass of steel-reinforced
concrete on the bottom of the legs, and gravity
holds it in position (Fig. 6).

43. Fig.6 Gravity-base production platform
Themassivebasehashollow
cellsthatcanbeusedforfloata-
tionwhenassemblingand
towingtheplatformintoposi-
tion.Onlocation,thecellscan
beusedforballastorstorageof
crudeanddieseloil.Itiscons-
tructedinasheltered,deepwater
portalongtheshorelineandthen
towedintoposition.Thistypeis
usedinareasofveryroughseas.

44. A more common type is the steel-jacket
platform that has legs, the steel jacket,
that sit on the bottom (Fig.2and Fig.7). It
is constructed on land and either floated
horizontally or carried on a barge out into
position. It is then flooded and rotated
vertically. Piles are driven into the sea
bottom and bolted, welded, or cemented
to the legs to hold it in position.

45. A crane is used to lift the
deck and modules such as
power generation, crew
quarters, and mud storage
off deck barges and
position them on the
platform.
Fig.7 Steel-jacket production platform

46. Offshore platforms often have several decks (flat
surfaces) on top of each other to serve various
functions such as power and drilling (Fig.2).
Wellheads are usually located on the lower cellar
deck. Separators, treaters, and gas compressors
are located on the platform. The treated oil or gas
is then usually sent ashore through a submarine
pipeline. A crane is used to lift supplies and
equipment aboard the platform. Usually one or two
derricks are left on deep-water platforms after the
wells have been drilled to use for workovers.

47. The deepest water depth for a production
platform is 1350 ft. (411 m). In relatively
shallow waters, there can be a separate
quarters platform for the crew next to the
production platform as a safety precaution. A
bridge connects the platforms.

48. A tension-leg platform (TLP) floats above
the offshore field. It is held in position by
heavy weights on the seafloor (Fig. 8). The
weights are connected to the tension-leg
platform by hollow, steel tubes 1 to 2 ft (0.3
to 0.6m) in diameter, called tendons. The
tendons pull the platform down in the water
to prevent it from rising and falling with
waves and tides.

49. Fig.8 tension-leg platform

50. Tension leg platforms can be installed in
water depths up to about 3500ft (1065 m).
A tension-leg well platform, in contrast,
has only wellheads and no production
treating facilities onboard. The produced
fluids are sent by seabed pipeline to a
production platform in shallow water for
treatment.

51. Fig.9 Compliant tower)
A compliant platform (Fig.9)
is a relatively light
production platform that is
designed to sway with wind,
waves, and currents.

52. One type,a guyed-tower, is attached to a
pivot on the ocean bottom.
Another type,a spar, is a floating production
platform in the shape of a closed, vertical
cylinder like a buoy. The spar is designed to
not rise and fall with the waves. Both the
guyed-tower and spar are held in position
with radiating guy wires and ocean bottom
weights.

53. Wells are drilled through a well template on the
ocean bottom that is used to position and separate
the wells. The template is a steel frame with slots for
each well. It can be either
1) on the bottom of a leg and the wells are drilled
through the leg.
or 2) on the sea bottom between the legs or tendons.
It supports the equipment necessary to drill and
produce the wells. Each slot on the template locates
and standardizes the instillation of a well.

54. The offshore field is developed by deviation
drilling from one platform. If the offshore well
flows, it is completed with a Christmas tree.
An offshore oil well that needs artificial lift is
usually completed with gas lift. Offshore wells
are required by law to be equipped with storm
chokes. The choke is installed on the bottom
of the well and is closed either manually or
automatically during an emergency.

55. Subsea Work
Installation and work on a well underwater can be
done by three methods: a diver, a diver in a one-
atmosphere diving suit and a remotely operated
vehicle. A saturation diverbreathing a helium and
oxygen mix can work down to 1000 ft (300 m). The
one-atmosphere diving suit(ADS) is a hard diving
suit with one atmosphere air pressure in it and
human-powered limbs. It can operate down to
2300 ft (700 m).

56. A remotely operated vehicle(ROV)is an
unmanned submersible that can effectively
operate down to 15,000 ft (4600 m). It is
connected to a mother ship on the surface
by a cable (umbilical). A closed-circuit
television camera on the ROV allows
operators on the surface to manipulate the
ROV with thrusters and do work with
manipulator arms. ROVs used to work on
offshore wells are very similar to those used
to discover and explore sunken ships.

57. Subsea Completions and Wells
A subsea completion consists of a wellhead and
production equipment such as a Christmas tree or
gas lift on the bottom of the ocean. The subsea
wellis drilled from a floater instead of a production
platform. The completion can be either dry, with an
atmospheric chamber surrounding the equipment
(Fig. 10 a), or wet, which is exposed to sea water
(Fig. 10 b).

58. Fig.10 Subsea wells (a) dry (b) wet

59. The production from a subsea well can be
taken by flowline to a subsea manifold
where it is commingled with production from
other subsea wells. It is then taken by a
flowline to a production platform in shallow
water, a tension leg platform, a semisub-
mersible facility, a spar tower, or up a pro-
duction riser to a floating production, storage,
and offloading vessel (Fig.11) for processing.

60. Fig.11 Subsea wells tied in to an FPSO vessel

61. The floating production, storage, and offload-
ing (FPSO) vessel is a converted tanker, a
semisubmersible or a specially built ship that
contains separation and treating facilities. It
can be kept on position by an anchoring
system or by dynamic positioning. The treated
oil is then transferred from the FPSO to a
shuttle tanker to be brought ashore. Ultra
deep-water wells are done with subsea
completions.

62. Remote portions of an offshore field and
smaller fields that are not economic by
themselves can be developed by satellite
wells. The satellite well is drilled by a mobile
offshore drilling unit and completed as a
subsea well. The satellite well is then tied to
an existing production platform by flowline.