OFFSHORE DRILLING

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.

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.

Fig.1 top drive

It is hung from the hook

on the traveling block an

d turns a shaft into which

the drillstring is screwed.

The top drive

moves up and down verti

cal rails to prevent it from

swaying with any motion

from waves.

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.

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.

An offshore crew can ha

ve a driller, assistant drill

er, derrickman, roughne

cks, motorman, diesel en

gine operator, pump ope

rator, mud man, crane o

perator, and roustabouts.

Fig.2 Fixed production platform

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.

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.

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).

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.

Offshore, a mobile offshore drilling unit (MO

DU) is used. Three types are jackup,

semisubmersible, and drillship.

A jackup rig usually has two barge-like hulls

and at least three vertical legs through the h

ulls (Fig. 3). The legs are either 1) open-trus

s with tubular steel members that are cross

braced, or 2) columnar made of large diamet

er steel tubes.

Fig.3 Jackup rig(自升式平台 )

The cantilevered jackup rig is 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.

The jackup rig is 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.

On each of the legs is a jack house that 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.

If the ocean bottom is relatively hard, small

er cylinders (cams) with a point on the botto

m can be used on the bottom of each leg ins

tead of a mat. After drilling, the hulls can be j

oined again and the rig towed or steamed to

another site.

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.

For deep-water drilling, a floater, a semi-submersible, or drillship is 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 .

Fig.4 semisubmersible rig(半潜式平台 )

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.

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 mode for easier towing.

A ballast-control specialist supervises 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.

A drillship is a ship with a drilling rig mounte

d in the center (Fig. 5).The ship steams out t

o the drillsite and then drills through a hole i

n the hull, called the moon pool. The ship flo

ats over the drillsite.

Fig.5 drillship (钻井船 )

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.

Drillships have propellers on the side of the

ship (bow and stern thrusters) and can mov

e both back and forth and sideways. Keepin

g on station by computer is called dynamic p

ositioning.

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.

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.

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.

The conductor casing extends above sea lev

el to just below the drilling deck. A smaller diam

eter hole is then drilled through the casing into t

he seafloor to several hundred feet below the b

ottom of the conductor casing. Surface casing is

run into the hole and cemented. Next, a blowout

preventer stack is bolted to the top of the surfac

e casing. The rest of the well is thendrilled and

cased similar to a well on land.

On a well drilled by a floater (a semi or drillship), a temporary guide base or 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 guidelines run 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.

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.

The drillstring and guide frame is then lower

ed down the guidelines to the temporary gui

de 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.

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.

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.

A subsea blowout prevent stack that can be activated from the floater is then lowered and locked onto the wellhead with a hydraulic wellhead connector. The drilling rig is then connected to the blowout preventers by, flexible, metal hollow tube (marine riser). The drillstring goes through the marine riser into the well.

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.

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.

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.

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).

Fig.6 Gravity-base production platform

The massive base has hollow cells that can be used for floata-tion when assembling and towing the platform into posi-tion. On location, the cells can be used for ballast or storage of crude and diesel oil. It is cons-tructed in a sheltered, deepwater port along the shoreline and then towed into position. This type is used in areas of very rough seas.

A more common type is the steel-jacket platform that has legs, the steel jacket, that sit on the bottom (Fig.2 and 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.

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

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.

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.

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.

Fig.8 tension-leg platform

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.

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.

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.

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.

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.

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 diver breathing 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).

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.

Subsea Completions and Wells

A subsea completion consists of a wellhead and pr

oduction equipment such as a Christmas tree or g

as lift on the bottom of the ocean. The subsea well

is drilled from a floater instead of a production platf

orm. The completion can be either dry, with an atm

ospheric chamber surrounding the equipment (Fig.

10 a), or wet, which is exposed to sea water (Fig.

10 b).

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

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.

Fig.11 Subsea wells tied in to an FPSO vessel

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.

Remote portions of an offshore field and sm

aller fields that are not economic by themsel

ves can be developed by satellite wells. The

satellite well is drilled by a mobile offshore d

rilling unit and completed as a subsea well.

The satellite well is then tied to an existing p

roduction platform by flowline.