Dynamic positioning (DP) is a computer controlled system to automatically maintain a vessel's position and heading by using her own propellers and thrusters. Position reference sensors, combined with wind sensors, motion sensors and gyro compasses, provide information to the computer pertaining to the vessel's position and the magnitude and direction of environmental forces affecting its position. Examples of vessel types that employ DP include but are not limited to ships and semi-submersible Mobile Offshore Drilling Units (MODU)
The computer program contains a mathematical model of the vessel that includes information pertaining to the wind and current drag of the vessel and the location of the thrusters. This knowledge, combined with the sensor information, allows the computer to calculate the required steering angle and thruster output for each thruster. This allows operations at sea where mooring or anchoring is not feasible due to deep water, congestion on the sea bottom (pipelines, templates) or other problems.
Dynamic positioning may either be absolute in that the position is locked to a fixed point over the bottom, or relative to a moving object like another ship or an underwater vehicle. One may also position the ship at a favourable angle towards wind, waves and current, called weathervaning.
Dynamic positioning is much used in the offshore oil industry, for example in the North Sea, Persian Gulf, Gulf of Mexico, West Africa and off Brazil. Nowadays there are more than 1000 DP ships.
Dynamic positioning started in the 1960’s for offshore drilling. With drilling moving into ever deeper waters, Jack-up barges could not be used any more and anchoring became less economical.
In 1961 the drillship Cuss 1 was fitted with four steerable propellers, in an attempt to drill the first Moho well. It was possible to keep the ship in position above the well off La Jolla, California, at a depth of 948 meters.
After this, off the coast of Guadalupe, Mexico, five holes were drilled, the deepest at 183 m (601 ft) below the sea floor in 3,500 m (11,700 ft) of water, while maintaining a position within a radius of 180 meters. The ship's position was determined by radar ranging to buoys and sonar ranging from subsea beacons.
Whereas the Cuss 1 was kept in position manually, later in the same year Shell launched the drilling ship Eureka that had an analogue control system interfaced with a taut wire, making it the first true DP ship.
While the first DP ships had analogue controllers and lacked redundancy, since then vast improvements have been made. Besides that, DP nowadays is not only used in the oil industry, but also on various other types of ships. In addition, DP is not limited to maintaining a fixed position any more. One of the possibilities is sailing an exact track, useful for cablelay, pipelay, survey and other tasks.
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