Oil rigs play a crucial role in the petroleum industry, enabling the extraction of oil from underground reservoirs to meet global demands.
What is Oil Rig?
Oil rigs are structures used for drilling and extracting oil from underground reservoirs.
An oil rig (also called an oil platform, offshore platform, oil production platform, etc.) is a large structure with facilities to extract and process petroleum and natural gas that lie in rock formations beneath the seabed.
There are several types of oil rigs, including:
- Land Rigs: Used for onshore drilling
- Offshore Rigs: Used for drilling in oceans and seas
- Jackup Rigs: Used for drilling in shallow water
- Semi-Submersible Rigs: Used for drilling in deep water
- Drill ships: Used for drilling in deep water
- Platform Rigs: Used for drilling and production on fixed platforms
- Inland Barge Rigs: Used for drilling in inland waterways
READ: WELL COMPLETION
The major types of offshore oil rigs include the following:
Fixed Platform
Anchored directly into the seabed, fixed-platform rigs consist of 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 about 1,500 feet below the surface, but they’re costly to build, so they usually require a large oil discovery to justify their construction.
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 same principle 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 than some 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.
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 wind and 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 at greater depths than platform rigs, potentially up to 3,000 feet below the surface. 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, floating partly 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. A machine 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.
Tension-Leg Platform
Another rig that can drill beyond a mile is the tension-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 to depths from 600 to 3,500 feet, and are also sometimes used as satellite or early-production platforms for large Deepwater discoveries.
Subsea System
Floating production systems, drill ships and even some pre-existing platform 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 (pictured at right while still on land), 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, the inaccessibility of such deep oil wells also makes it difficult to fix leaks.
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 is hidden 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.