Chapter 6 - Drilling and Completion: Offshore

Question 1

Define movable rigs, why they use them, and types.

Answer 1

Moveable rigs are most often used for exploratory offshore drilling because they are much cheaper to use than permanent platforms. Moveable rigs include drilling barges, drill ships, Jack-up rigs, and semisubmersible rigs.

Question 2

Offshore, what is done once large deposits of hydrocarbons have been found?

Answer 2

Once large deposits of hydrocarbons have been found, permanent platforms of various designs are typically built for development drilling and productions.

Question 3

What is one important difference between offshore and onshore drilling rigs?

Answer 3

The manner in which rotary power is transmitted to the drill string. Offshore rigs use a device called a top drive, instead of the drawworks and associated cables used on land rigs.

Question 4

Define top drive.

Answer 4

The top drive is a large electric or hydraulic motor, typically 1000 horsepower or more, that hangs near the top of the drilling derrick from the hook at the bottom of the traveling block. The top drive turns a shaft into which drill string sections can be screwed, commonly three sections at a time.

Question 5

Define Drilling Barge.

Answer 5

For exporatory drilling in relatively shallow water (as deep as about 25 feet), a drilling barge can be used. One common design is a posted barge, designed to float to the site of interest and then be sunk, resting on the bottom for stability. The actually drilling platform is raised on posts extending upward from the deck of the barge, so that the drilling deck is above the water’s surface.

Question 6

What is a Mobile Offshore Drilling Unit (MODU) and what are the three types?

Answer 6

Further offshore, in deeper water, a mobile offshore drilling unit (MODU) is used. The three types of MODU are the jack-up rig, the semisubmersible rig, and the drill ship.

Question 7

Define Jack-up rig.

Answer 7

The jack-up is the most common, generally used in water as deep as 400 feet. It consist of two bargelike hulls (one above the other) plus three or four verticle legs that pass through the hulls. The drilling platform and derrick are mounted on the upper hull.

Question 8

How are Jack-up rigs put into place?

Answer 8

The whole unit is usually towed into position by tugboats, with the two hulls fastened to each other and the legs jutting into the air (as high as 600 feet above the water).

Question 9

What happens once the Jack-up rig is in position?

Answer 9

Once in position, the lower hull, or mat, is slowly flooded while rack-and-pinion mechanisms simultaneously jack each leg downward. When the mat is securely resting on the seabed, the rack-and-pinion units then jack the platform upward, well above the surface to eliminate the impact of waves and tides.

Question 10

What is the position of the derrick on a jack-up rig typically?

Answer 10

On most jack-up rigs, the drilling derrick is cantilevered-mounted on two large steel beams that extend over the edge of the deck. In other cases, the derrick is mounted closer to the center of the platform and drills down through and opening in the deck.

Question 11

What happens to a jack-up rig once drilling is completed?

Answer 11

Once drilling is completed, the two hulls are brought back together and the legs again raised into the air. The entire rig is then towed to another site.

Question 12

Define Semisubmersible rig.

Answer 12

As the name implies, a semisubmersible rig, or semisub, is partially submerged to increase stability. The unit typically towed to the drilling site. The drilling platform and derrick are connected by columns to pontoons that extend 30-50 feet below the water’s surface.

Question 13

How do semisubmersible rigs stay in position and stable.

Answer 13

Ballast water is pumped into and out of the pontoons to achieve the desired stability. In relatively shallow water, a system of chains and anchors hold the floating rig in place. In deeper water, the dynamic positioning system (using GPS satellites, computers, and shipboard thrusters) keeps the vessel in the desired spot. Some semisubs can drill in water as deep as 10,000 feet.

Question 14

Define Ballast water.

Answer 14

Definition of Ballast water: Fresh or salt water, sometimes containing sediments, held in tanks and cargo holds of ships to increase stability and maneuverability during transit.

Question 15

Define Drill ship.

Answer 15

The third type of MODU is the drill ship. A large vessel specially modified for drilling. The derrick is mounted in the center of the ship, and the operators drill down through a hole in the hull. As with the semisub, the drill ship uses a dynamic positioning system to keep it on station. Some drill ships have two drilling rigs.

Question 16

Define the world record holder for ultra-deepwater drilling.

Answer 16

Transocean reported in April 2011 that its ultra-deepwater drill ship Dhirubhai Deepwater KG2 claimed a world record for operation in the greatest water depth by an offshore drilling rig-10, 194 feet-while working for Reliance Industries offshore India. The vessel is equipped to work in water as deep as 12,000 feet and is outfitted to construct wells as deep as 35,000 feet.

Question 17

Define the completion for a well drilled by a jack-up rig. Phase I.

Answer 17

For a well drilled by a jack-up rig (in relatively shallow water), several hundred feet of large-diameter (26- or 30-inch) conductor casing is set into the seafloor.This can be accomplished by using water jets or a pile driver (in relatively soft soil) or by drilling a hole (in hard rock), running the casing into the hole, and then cementing it in place.This conductor casing extends up out of the water to a level just below the drilling deck.

Question 18

Define the completion for a well drilled by a jack-up rig. Phase II.

Answer 18

A smaller-diameter hole is then drilled down the centerline of the casing to several hundred feet below the bottom of the conductor casing. The next section of surface casing (slightly smaller in diameter) is then run into that hole and cemented. A BOP system is then bolted to the top of the surface casing, and the rest of the well is sequentially drilled and cased in a manner similar to that used for onshore wells.

Question 19

Define the completion for a well drilled by a semisubmersible rig or drill ship.
Phase I.

Answer 19

Completion of wells drilled by a semisubmersible rig or a drill ship (in deeper water) is somewhat more complex. The drill string itself is used first to lower a temporary guide base—a hexagonal steel framework—and then a guide frame to the seabed.Those two units are used to position the drill bit, and a hole 30 or 36 inches in diameter is drilled to about 100 feet.

Question 20

Define the completion for a well drilled by a semisubmersible rig or drill ship.
Phase II.

Answer 20

The drill string and guide frame are then raised to the surface, and attention turns to the foundation pile—the first string of casing that will be placed in the hole. The guide frame is attached to the bottom of the foundation pile, and a permanent guide structure is attached to the top.

Question 21

Define the completion for a well drilled by a semisubmersible rig or drill ship.
Phase III.

Answer 21

The whole assembly is lowered to the seafloor (using steel cables that run from the ship to the temporary guide base), and the foundation pile is run into the drilled hole and cemented.The permanent guide base (now flush with the seafloor) is mated with the temporary guide base.

Question 22

Define the completion for a well drilled by a semisubmersible rig or drill ship.
Phase IV.

Answer 22

The drill string is lowered again, the hole is drilled deeper, and a string of conductor casing is then cemented in place.A subsea BOP is lowered and locked onto the wellhead.

Question 23

Define the completion for a well drilled by a semisubmersible rig or drill ship.
Phase V.

Answer 23

Then, a flexible, hollow metal tube—a marine riser—is attached to the BOP stack and to the drilling rig. Once the drill string is inserted into the marine riser, drilling mud can be directed down through the drill string, returning upward through the annulus between the drill string and the wall of the marine riser.

Question 24

Define the completion for a well drilled by a semisubmersible rig or drill ship.
What happens in the event of severe weather or other emergencies?

Answer 24

A variety of special equipment is used to compensate for the motion of the semisubmersible or drill ship due to wind and waves. In the event of severe weather or other emergency, the BOP can be closed and the stack disconnected from the marine riser, allowing the ship to move to safety. Later, the connection can be reestablished.

Question 25

For offshore drilling, what is the difference between dry and wet wellhead?

Answer 25

The wellhead equipment on a completed subsea well is similar to that on onshore wells. In some cases, the subsea wellhead is dry (enclosed in chamber maintained at atmospheric pressure). In other cases, the equipment is wet (exposed to seawater).

Question 26

Define the purpose of the manifold for offshore production?

Answer 26

Production from a subsea well can be carried by flowline to a manifold where it is combined with production from other wells. From the manifold, oil typically flows to a production platform or to a floating production, storage, and off-loading vessel (FPSO).

Question 27

Define FPSO.

Answer 27

The FPSO is a converted or custom-designed ship with onboard facilities for oil separation and treating. Oil from the FPSO is often then transferred to a shuttle tanker that takes it to an onshore facility for further treatment or storage.

Question 28

Define the role of pipelines in offshore.

Answer 28

In some cases, undersea pipelines are constructed to carry production from a platform to shore.

Question 29

What are the four main types of fixed platforms?

Answer 29

Steel-jacket
Gravity Based
Tension-leg
Compliant-tower designs

Question 30

For fixed development and production platforms, define Steel-Jacket platform.

Answer 30

The steel-jacket platform has four to eight legs of welded steel pipe, which comprise the steel jacket, stabilized with multiple crisscross truss members (fig. 6–5). Typically weighing about 20,000 pounds, the platform is built on land, carried by barge or floated (in a horizontal position) to the site, and then rotated vertically and sunk into position. The bottom of each leg is welded, bolted, or cemented to piles driven into the seafloor.

Question 31

What depth of water is the Steel-Jacket platform used?

Answer 31

500 feet deep.

Question 32

Define Gravity-based platform.

Answer 32

One or more reinforced concrete columns, or caissons, support the work platform that is mounted on top of them (fig. 6–6). Hollow chambers in the columns, plus ballast tanks mounted at the bottom of the columns, can be filled with fluids or solid material to achieve the desired degree of buoyancy.

Question 33

What depth of water is the Gravity-based platform used?

Answer 33

Typically used in water as deep as about 500 feet.

Question 34

Give an example of a gravity-based platform.

Answer 34

the Hibernia platform of Mobil Oil— about 200 miles off the coast of Newfoundland, in 260 feet of water— includes a concrete gravity-based structure weighing about 450,000 tons.

Question 35

For fixed development and production platforms, define Tension-leg platform platform.

Answer 35

In the tension-leg design (fig. 6–7), long, hollow steel tubes, typically two feet in diameter, are connected to the bottom of the platform deck. These tubes, called tendons, extend down to the seabed, where large weights hold them in place, or they are bolted to pilings sunk into the seabed.The tendons and weights prevent the platform from moving up and down because of to wave or tidal forces.

Question 36

For fixed development and production platforms, define Compliant-Tower Platform.

Answer 36

The deck on which the drilling platform rests is attached to a strut- and-truss structure (similar to that of a steel jacket platform) that is in turn connected to a set of special-design pilings driven into the seafloor.

Question 37

What depth is a Compliant-Tower Platform used in and why?

Answer 37

With its smaller “footprint,” the compliant tower is suitable for installation in water 1,000–3,000 feet deep without any need for guy wires or heavy anchors.

Question 38

For floating development and production platforms, what are the three main types?

Answer 38

Semisubmersible Vessel
Monohull Vessel
Spar Platform

Question 39

Define how Semisubmersible Vessels can be used for both exploration and production.

Answer 39

As described earlier, semisubmersible vessels can be used for exploratory drilling. However, much larger and more-complex semisub units also have been built for use as platforms for development drilling and production operations in deep water. These vessels offer greater stability than so-called monohull vessels of conventional shiplike design.

Question 40

Define Semisubmersible Vessels.

Answer 40

On a deepwater semisub, structural columns that support the work platform well above the water’s surface are also connected to a submerged hull (fig. 6–9). This hull consists of pontoons or similar structures to which ballast can be added or removed. Stability is maximized when the hull is fully ballasted and at its lowest depth.

Question 41

How are Semisubmersible Vessels held in place in open water?

Answer 41

Dynamic Positioning Systems

Question 42

Give an example of a Semisubmersible Vessel.

Answer 42

Thunder Horse PDQ—the largest semisub in the world, displacing 129,000 tons—is moored in 6,050 feet of water in the Gulf of Mexico, 150 miles southeast of New Orleans. Operated jointly by BP and ExxonMobil, it began oil production in 2008. Four columns 118 feet tall connect the production platform to the submerged hull.

Question 43

Define Monohull Platform (FPSO).

Answer 43

An FPSO is a widely used type of monohull platform (fig. 6–10). From a distance, it can be hard to distinguish from an oil tanker. It is designed to receive hydrocarbons produced from one or more subsea wells or from nearby platforms.

Question 44

Where/Why are FPSOs popular?

Answer 44

FPSOs are attractive for use in frontier offshore regions because they can be brought to a site easily and do not require the presence of a local oil pipeline infrastructure.

Question 45

If oil is not processed onboard an FPSO, what is it then called?

Answer 45

FSO

Question 46

What’s the worlds first FDPSO?

Answer 46

The world’s first FDPSO—floating drilling, production, storage, and off-loading vessel—was developed in 2009 for Murphy Oil, for operation off the coast of the Republic of Congo.

Question 47

Define Spar Platform.

Answer 47

The last major type of offshore production platform is called a spar. In this design, the work platform is built atop a tall cylinder that is designed to be submerged in the vertical position below the platform.

Question 48

Spars are built in three configurations:

Answer 48

conventional spar, with a one-piece cylindrical hull

truss spar, with a midsection composed of truss elements that connect an upper buoyant hull with a lower tank containing permanent ballast

cell spar, built up from several vertical cylinders 


Question 49

For Spars, what are Strakes?

Answer 49

Strakes (resembling fins) installed on the outer surface of a cylindrical spar can help lessen the effects of wave action and water movement on the platform. 


Question 50

Give an example of a Spar platform.

Answer 50

In March 2010, oil production began from the world’s deepest production platform, the Perdido spar, operated by Royal Dutch Shell.