Marine Drilling Rigs

7/28/2019 Marine Drilling Rigs

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JACK UP RIG

For smaller, shallower offshore oil deposits that don't warrant a

permanent platform, or for drilling exploratory wells, oil companies may

use what's called a "jack-up rig." The rig's floating platform is towed into

position by barges, then lowers its support legs down to the sea floor,

raising the rig above the water's surface. The platform can then be

adjusted to varying heights along its tall legs, essentially using the sameprinciple employed by a tire jack (hence the name). Jack-up rigs were

traditionally used in shallow water because it wasn't practical to lower

their legs to great depths, but newer models such as the Tarzan-class

rigs are now stretching those limits. They're also considered safer thansome other types of moveable rigs, such as drilling barges, since their

surface facilities are elevated from the water and less susceptible to

waves and weather.


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Types of Jack up Drilling Rigs

These platforms can be of two types:

Open-Truss:

This type of platform is advantageous because it offers a lot of stability

to the jack-up system. Open-Truss platforms are designed in a pattern

that is criss-crossed which ensures the stability aspect of the oil rig

mentioned in the previous point.

Columnar Platforms:

Unlike the previous variety of jack up platform, the columnar platformsare constructed in a columnar or rectangular form. This reduces the

effective weight-bracing facility of these platforms making them a

problem in case strong wind conditions affect the water in which the oil

rig stands.

A jack-up system is highly unique facility. This rarity along with the

supreme utility is what has enabled the jack up drilling rigs to be a part

of the oil drilling operation for over six decades. It is also this

uniqueness that will enable the jack up rig to be a major part of the oil

drilling industry in the future, for years and years to come.


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Compliant tower

Compliant-tower rigs are similar to fixed platforms, since both

are anchored to the seabed and hold most of their equipment

above the surface. But compliant towers are taller and

narrower, and unlike fixed platforms, they sway with the windand water almost as if they were floating. This is possible

because their jackets are broken into two or more sections,

with the lower part serving as the base for the upper jacket

and surface facilities. This lets compliant towers operate atgreater depths than platform rigs, potentially up to 3,000 feet

below the surface.


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Fixed platform

Anchored directly into the seabed, fixed-platform rigs consistof a tall, steel structure known as a "jacket" that rises up from

the ocean to support a surface deck. The jacket provides the

rig's sturdy base and holds everything else out of the water,

while the drilling modules and crew quarters are located on

the surface deck. Fixed platforms offer stability but no

mobility, and today they're primarily used to tap moderately

shallow, long-term oil deposits. They can drill to depths ofabout 1,500 feet below the surface, but are expensive to

build, so they usually require a large oil discovery to justify

their construction.


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Floating production system

As oil companies expand into ever-deeper waters, they've had to

embrace less traditional methods of getting oil up to the surface. This

often means deepwater rigs are buoyant and semisubmersible, floatingpartly above the surface while pumping up oil from deep wells. Some

use wire and rope to connect with a stabilizing anchor, while others

including the now-sunken Deepwater Horizon, pictured at right in June

2009 are "dynamically positioned," using computer-coordinated

thrusters to keep them in place. These floating production systems are

used in water depths from 600 to 6,000 feet, and are among the most

common types of offshore rigs found in the Gulf of Mexico. Since their

wellheads are located on the sea floor rather than a surface platform, as

on fixed-platform rigs, extra care must be taken to avoid leaks. Amachine on deepwater wellheads known as a "blowout preventer" is

supposed to prevent oil from escaping, but the Deepwater Horizon's

blowout preventer failed after the rig sank, letting crude rush out of the

well.


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Tension-leg platform

Another rig capable of drilling deeper than a mile is thetension-leg platform, which consists of a floating surface

structure held in place by taut, vertical tendons connected to

the sea floor. And for drilling smaller deposits in narrower

areas, an oil company may instead use a miniature version

known as a "Seastar," which allows for relatively low-cost

production of small deepwater oil reserves that would

otherwise be uneconomical to drill. Seastar rigs can drill todepths ranging from 600 to 3,500 feet, and are also

sometimes used as satellite or early-production platforms for

large deepwater discoveries.


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Subsea system

Floating production systems, drillships and even some pre-existingplatform rigs use subsea wellheads to extract oil directly at the seabed,

siphoning the crude up through risers or pipes to the surface. A subsea

drilling system includes a deepwater production module that rests on

the sea floor ,as well as any transportation lines that channel the oil to

surface facilities. Those facilities may be aboard a nearby platform rig, a

ship floating overhead, a centralized production hub or even a faraway

onshore site, which makes subsea oil rigs versatile as well as nimble,

offering oil companies several options for tapping otherwise hard-to-

reach deposits. But as the Deepwater Horizon spill has shown, theinaccessibility of such deep oil wells also makes it difficult to fix leaks.


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Spar platform

Named after the tall, vertical "spar" (aka mast) of a sailing ship, spar-

platform rigs use a single, wide-diameter cylinder to support a surface

deck from the sea floor. A typical spar platform in the Gulf of Mexico has

a 130-foot-wide cylinder, and about 90 percent of its overall structure ishidden underwater. Spar cylinders are available at depths up to 3,000

feet, but existing technology can extend this to about 10,000 feet,

making them one of the deepest-drilling types of offshore rigs in use.


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S i b ibl i


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Semisubmersible rig

If drilling from a bottom supported unit is not possible, the semisubmersible is the

most popular floating unit. The semisubmersible rig is a more costly option than

jackup rigs. The semisubmersible is a column stabilized vessel, because of the deep

draft of its columns, wave energy is dampen, minimizing roll, pitch, yaw, surge, sway

and heave.

It is common to subdivide semisubmersibles rigs into generations. Generations 4 and 5

are the deeper water rigs, with generation 4 being able to drill in 3500 to 4,000 ft and

generation 5 being able to drill in 5000 ft plus depths. Rig that drill in these deepwaters, must be considerably more sophisticated. The variable deck load (VDL) must

be higher. Deep water semisubmersible use of large motors to position the rig over the

well (dynamic positioning) can drill in deeper waters. Generation 1, 2 and 3 use

mooring systems and operate in waters less than 3500 ft.

In evaluating any floating vessel for drilling, computer programs are available tocalculate vessel motions by entering in the wave heights and periods. The result will be

a motion history of that particular rig for a specific drilling period.


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D ill hi


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Drillships

The drillship were popular in the late 1950's to the late 1960's. They

could be mobilized quickly. However, the semisubmersible was far more

a stable drilling platform. The long narrow hull of the drillship results in

motion in all six degrees of freedom (can you name them?), particularly

pitch, roll and heave. Mooring systems and dynamic positioning systems

have greatly improved the drillship's capability of drilling in harsh

environments.


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Ri l ti b d D th C it i


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Rigs selection based on Depth Criteria

Jackup: From approximately 20 to 30 ft minimum depth to 350 to 550 ft maximum depth

The maximum depth is a function of other environmental variables, such as wind, waves, andcurrent conditions at the site. Severe conditions tend to lower the jackup rig's maximum water

depth capacity.

Semisubmersible: Approximately 150 to 8000 ft using dynamically mooring system. Until 1978,

the maximum depth was 2,200 ft using conventional mooring system.

Drillship: Approximately 100 to at least 8000 ft with today's technology. Maximum water depth

limits occur because of riser system limitations.

Marine Drilling Rigs

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