Drilling for natural gas offshore, in some instances hundreds of miles away from the nearest landmass, poses a number of different challenges over drilling onshore. The actual drilling mechanism used to delve into the sea floor is much the same as can be found on an onshore rig. However, with drilling at sea, the sea floor can sometimes be thousands of feet below sea level. Therefore, while with onshore drilling the ground provides a platform from which to drill, at sea an artificial drilling platform must be constructed.
|Source: ChevronTexaco Corporation|
Drilling offshore dates back as early as 1869, when one of the first patents was granted to T.F. Rowland for his offshore drilling rig design. This rig was designed to operate in very shallow water, but the anchored four legged tower bears much resemblance to modern offshore rigs. It wasn’t until after World War II that the first offshore well, completely out of sight from land, was drilled in the Gulf of Mexico in 1947. Since then, offshore production, particularly in the Gulf of Mexico, has resulted in the discovery and delivery of a great number of large natural gas deposits.
The Drilling Template
Since the land that is going to be drilled cannot provide a base for offshore drilling as it does for onshore drilling, an artificial platform must be created. This artificial platform can take many forms, depending on the characteristics of the well to be drilled, including how far underwater the drilling target is. One of the most important pieces of equipment for offshore drilling is the subsea drilling template. Essentially, this piece of equipment connects the underwater well site to the drilling platform on the surface of the water. This device, resembling a cookie cutter, consists of an open steel box with multiple holes in it, dependent on the number of wells to be drilled. This drilling template is placed over the well site, and usually lowered into the exact position required using satellite and GPS technology. A relatively shallow hole is then dug, in which the drilling template is cemented into place. The drilling template, secured to the sea floor and attached to the drilling platform above with cables, allows for accurate drilling to take place, but allows for the movement of the platform, which will inevitably be affected by shifting wind and water currents.
In addition to the drilling template, a blowout preventer is installed on the sea floor. This system, much the same as that used in onshore drilling, prevents any oil or gas from seeping out into the water. Above the blowout preventer, a specialized system known as a ‘marine riser’ extends from the sea floor to the drilling platform above. The marine riser is designed to house the drill bit and drillstring, and yet be flexible enough to deal with the movement of the drilling platform. Strategically placed slip and ball joints in the marine riser allow the subsea well to be unaffected by the pitching and rolling of the drilling platform.
Moveable Offshore Drilling Rigs
There are two basic types of offshore drilling rigs: those that can be moved from place to place, allowing for drilling in multiple locations, and those rigs that are permanently placed. Moveable rigs are often used for exploratory purposes because they are much cheaper to use than permanent platforms. Once large deposits of hydrocarbons have been found, a permanent platform is built to allow their extraction. The sections below describe a number of different types of moveable offshore platforms.
|A Drilling Barge|
|Source: California Department of Transportation|
Drilling barges are used mostly for inland, shallow water drilling. This typically takes place in lakes, swamps, rivers, and canals. Drilling barges are large, floating platforms, which must be towed by tugboat from location to location. Suitable for still, shallow waters, drilling barges are not able to withstand the water movement experienced in large open water situations.
|A Jack-Up Rig|
|Source: National Oceanic and Atmospheric Administration|
Jack-up rigs are similar to drilling barges, with one difference. Once a jack-up rig is towed to the drilling site, three or four ‘legs’ are lowered until they rest on the sea bottom. This allows the working platform to rest above the surface of the water, as opposed to a floating barge. However, jack-up rigs are suitable for shallower waters, as extending these legs down too deeply would be impractical. These rigs are typically safer to operate than drilling barges, as their working platform is elevated above the water level.
Submersible rigs, also suitable for shallow water, are like jack-up rigs in that they come in contact with the ocean or lake floor. These rigs consist of platforms with two hulls positioned on top of one another. The upper hull contains the living quarters for the crew, as well as the actual drilling platform. The lower hull works much like the outer hull in a submarine – when the platform is being moved from one place to another, the lower hull is filled with air – making the entire rig buoyant. When the rig is positioned over the drill site, the air is let out of the lower hull, and the rig submerses to the sea or lake floor. This type of rig has the advantage of mobility in the water, however once again its use is limited to shallow water areas.
|A Semisubmersible Rig|
|Source: Department of the Interior|
Semisubmersible rigs are the most common type of offshore drilling rigs, combining the advantages of submersible rigs with the ability to drill in deep water. A semisubmersible rig works on the same principle as a submersible rig: through the ‘inflating’ and ‘deflating’ of its lower hull. The main difference with a semisubmersible rig, however, is that when the air is let out of the lower hull, the rig does not submerge to the sea floor. Instead, the rig is partially submerged, but still floats above the drill site. When drilling, the lower hull, filled with water, provides stability to the rig. Semisubmersible rigs are held in place by huge anchors, each weighing upwards of 10 tons. These anchors, combined with the submerged portion of the rig, ensure that the platform is stable and safe enough to be used in turbulent offshore waters. Semisubmersible rigs can be used to drill in much deeper water than the rigs mentioned above.
|A Drillship in the Beaufort Sea|
|Source: Mining and Minerals Service|
Drillships are exactly as they sound: ships designed to carry out drilling operations. These boats are specially designed to carry drilling platforms out to deep-sea locations. A typical drillship will have, in addition to all of the equipment normally found on a large ocean ship, a drilling platform and derrick located on the middle of its deck. In addition, drillships contain a hole (or ‘moonpool’), extending right through the ship down through the hull, which allows for the drill string to extend through the boat down into the water. Drillships are often used to drill in very deep water, which can often be turbulent. Drillships use what is known as ‘dynamic positioning’ systems. Drillships are equipped with electric motors on the underside of the ship’s hull, capable of propelling the ship in any direction. These motors are integrated into the ship’s computer system, which uses satellite positioning technology, in conjunction with sensors located on the drilling template, to ensure that the ship is directly above the drill site at all times.
Offshore Drilling and Production Platforms
As mentioned, moveable rigs are commonly used to drill exploratory wells. In some instances, when exploratory wells find commercially viable natural gas or petroleum deposits, it is economical to build a permanent platform from which well completion, extraction, and production can occur. These large, permanent platforms are extremely expensive, however, and generally require large expected hydrocarbon deposits to be economical to construct. Some of the largest offshore platforms are located in the North Sea, where because of almost constant inclement weather, structures able to withstand high winds and large waves are necessary. A typical permanent platform in the North Sea must be able to withstand wind speeds of over 90 knots, and waves over 60 feet high. Correspondingly, these platforms are among the largest structures built by man. There are a number of different types of permanent offshore platforms, each useful for a particular depth range.
|An Offshore Platform|
|Source: Duke Energy Gas Transmission Canada|
This depiction of offshore drilling and completion platforms gives an idea of just how massive these offshore rigs can be. For reference, the fixed platform (the shallowest shown) is usually in no more than 1,700 feet of water – whereas the height of the Hoover Dam, from top to bottom, is less than half that, at just under 730 feet. Because of their size, most permanent offshore rigs are constructed near land, in pieces. As the components of the rig are completed, they are taken out to the drilling location. Sometimes construction or assembly can even take place as the rig is being transported to its intended destination.
In certain instances, in shallower water, it is possible to physically attach a platform to the sea floor. This is what is shown above as a fixed platform rig. The ‘legs’ are constructed with concrete or steel, extending down from the platform, and
|Offshore Drilling Platforms|
fixed to the seafloor with piles. With some concrete structures, the weight of the legs and seafloor platform is so great, that they do not have to be physically attached to the seafloor, but instead simply rest on their own mass. There are many possible designs for these fixed, permanent platforms. The main advantages of these types of platforms are their stability, as they are attached to the sea floor there is limited exposure to movement due to wind and water forces. However, these platforms cannot be used in extremely deep water, it simply is not economical to build legs that long.
Compliant towers are much like fixed platforms. Each consists of a narrow tower, attached to a foundation on the seafloor and extending up to the platform. This tower is flexible, as opposed to the relatively rigid legs of a fixed platform. This flexibility allows it to operate in much deeper water, as it can ‘absorb’ much of the pressure exerted on it by the wind and sea. Despite its flexibility, the compliant tower system is strong enough to withstand hurricane conditions.
Seastar platforms are like miniature tension leg platforms. The platform consists of a floating rig, much like the semisubmersible type discussed above. A lower hull is filled with water when drilling, which increases the stability of the platform against wind and water movement. In addition to this semisubmersible rig, however, Seastar platforms also incorporate the tension leg system employed in larger platforms. Tension legs are long, hollow tendons that extend from the seafloor to the floating platform. These legs are kept under constant tension, and do not allow for any up or down movement of the platform. However, their flexibility does allow for side-to-side motion, which allows the platform to withstand the force of the ocean and wind, without breaking the legs off. Seastar platforms are typically used for smaller deep-water reservoirs, when it is not economical to build a larger platform. They can operate in water depths of up to 3,500 feet.
|A Floating Production System|
|Source: Minerals Management Service|
Floating Production Systems
Floating production systems are essentially semisubmersible drilling rigs, as discussed above, except that they contain petroleum production equipment, as well as drilling equipment. Ships can also be used as floating production systems. The platforms can be kept in place through large, heavy anchors, or through the dynamic positioning system used by drillships. With a floating production system, the wellhead is actually attached to the seafloor once the drilling is completed, rather than being attached up to the platform. The extracted petroleum is transported via risers from this wellhead to the production facilities on the semisubmersible platform. These production systems can operate in water depths of up to 6,000 feet.
|A Tension Leg Platform|
|Source: Minerals Management Service|
Tension Leg Platforms
Tension leg platforms are larger versions of the Seastar platform. The long, flexible legs are attached to the sea floor, and run up to the platform itself. As with the Seastar platform, these legs allow for significant side-to-side movement (up to 20 feet), with little vertical movement. Tension leg platforms can operate at around 7,000 feet.
Subsea production systems are wells located on the sea floor, as opposed to at the surface. Just as in a floating production system, the petroleum is extracted at the seafloor, and then ‘tied-back’ to an already existing production platform. The well is drilled by a moveable rig, and instead of building a production platform for that well, the extracted natural gas and oil are transported by riser or even undersea pipeline to a nearby production platform. This allows one strategically-placed production platform to service many wells over a reasonably large area. Subsea systems are typically in use at depths of 7,000 feet or more, and do not have the ability to drill, only to extract and transport. To see an example of a subsea production system in the Gulf of Mexico, click here.
Spar platforms are among the largest offshore platforms in use. These huge platforms consist of a large cylinder supporting a typical fixed rig platform. The cylinder does not extend all the way to the seafloor, but instead is tethered to the bottom by a series of cables and lines. The large cylinder serves to stabilize the platform in the water, and allows for movement to absorb the force of potential hurricanes. The first Spar platform in the Gulf of Mexico was installed in September of 1996. Its cylinder measured 770 feet long and was 70 feet in diameter, and the platform operated in 1,930 feet of water. To see an example of a Spar platform in the Gulf of Mexico, click here.
To learn more about offshore drilling in the Gulf of Mexico, visit the U.S. Bureau of Ocean Management Regulation and Enforcement here.