Types of offshore oil rigs
Offshore drilling has come a long way since the wharf-based rigs of the late 1800s, giving oil companies an arsenal of options for tapping deep-sea deposits.
Wed, May 19, 2010 at 12:30 PM
Illustration: U.S. Minerals Management Service
The earliest offshore drilling was limited to coastal oil deposits that were accessible from piers, but oil companies today can choose from a variety of elaborate methods, letting them drill almost anywhere at almost any depth. From roving, computer-controlled contraptions to giant "spar" platforms held up by 10,000-foot poles, today's deepwater rigs are going far beyond anything their offshore forefathers could have imagined.
Such engineering wonders also come with big risks, however, as demonstrated by the 2010 Deepwater Horizon explosion, which killed 11 people and launched a torrent of oil into the Gulf of Mexico. Anything from human or mechanical error to corrosion, methane bubbles or earthquakes can spiral into a runaway disaster when drilling for oil offshore, and the struggles to control the Deepwater Horizon spill highlighted the difficulty of doing anything 5,000 feet deep in the ocean.
But with potentially vast oil reserves located on North America's Outer Continental Shelf, and the United States still leading the world in oil consumption at 19.5 million barrels a day, oil companies and offshore-drilling advocates argue that extracting oil from the ocean is economically vital and environmentally safe. There are currently about 4,000 offshore drilling rigs and production platforms in the Gulf of Mexico, and under the Obama administration's new offshore energy strategy, more may soon appear off the North Slope of Alaska and even the U.S. East Coast.
Oil-drilling technology is constantly improving, and some rigs combine elements from different models to achieve specific abilities. But in general, 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, drillships 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.
To learn more about offshore rigs, oil production and oil spills, see MNN's Translating Uncle Sam feature on offshore oil drilling and these related links:
- What went wrong at Gulf oil rig? A lot, investigators find
- Is this spill the Three Mile Island for the oil industry?
- How Kevin Costner's oil filtration tech works
- Betty White lends a hand to animals in oil spill
- Stopping the oil leak: Golf balls vs. nukes
Jack-up rig: Library of Congress
Deepwater Horizon rig: ZUMA Press
Subsea production module: U.S. National Energy Technology Laboratory