OffshoreAn offshore platform, often referred to as an oil platform or an oil rig, is a large structure used in offshore drilling to house workers and machinery needed to drill wells in the ocean bed, extract oil or natural gas, or both, process the produced fluids, and ship or pipe them to shore. Depending on the circumstances, the platform may be fixed to the ocean floor, may consist of an artificial island, or may float. Remote subsea wells may also be connected to a platform by flow lines and by umbilical connections; these subsea solutions may consist of single wells or of a manifold centre for multiple wells.
Larger lake and sea-based offshore platforms and drilling rigs are some of the largest moveable man-made structures in the world.
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.
Certification of an offshore platform is based on the initial design and construction appraisal, the actual construction phase, and then includes periodical follow-up surveys. During their lifecycle, offshore structures are typically recertified after a major survey.
Drillships are equipped with electric motors on the underside of the ships hull, capable of propelling the ship in any direction. These motors are integrated into the ships 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.
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 allow 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 quite turbulent.
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Floating production systems are essentially semisubmersible drilling rigs, 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, once the drilling has been completed, the wellhead is actually attached to the seafloor, instead of up on 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.
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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. Semisubmersible rigs work on the same principle as submersible rigs; through the ‘inflating’ and ‘deflating’ of the 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.
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These large, permanent platforms are extremely expensive, and generally dedicated to large hydrocarbon deposits in order to be worth the investment. 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. There are a number of different types of permanent offshore platforms, each useful for a particular depth range. In certain instances, in shallower water, it is possible to physically attach a platform to the sea floor.
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A Platform Supply Vessel is a ship specially designed to supply offshore oil platforms. These ships range from 65 to 350 feet in length and accomplish a variety of tasks.
The primary function for most of these vessels is transportation of goods and personnel to and from offshore oil platforms and other offshore structures. In the recent years a new generation of Platform Supply Vessel entered the market, usually equipped with Class 1 or Class 2 Dynamic Positioning System Dynamic positioning.
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