thumb|Offshore support vessel Toisa Perseus with, in the background, the fifth-generation deepwater [[drillship Discoverer Enterprise, over the Thunder Horse Oil Field. Both are equipped with DP systems.]]

Dynamic positioning (DP) is a computer-controlled system to automatically maintain a vessel's position and heading by using its own propellers and thrusters. Position reference sensors, combined with wind sensors, motion sensors, and gyrocompasses, provide the computer with information about the vessel's position and the magnitude and direction of environmental forces affecting it. Examples of vessel types that employ DP include ships and semi-submersible mobile offshore drilling units (MODU), oceanographic research vessels, cable layer ships, and cruise ships.

The computer program contains a mathematical model of the vessel, including information on its wind and current drag, as well as the thrusters' locations. This knowledge, combined with sensor data, allows the computer to calculate the required steering angle and the output for each thruster. This allows operations at sea where mooring or anchoring is not feasible due to deep water, seabed congestion (pipelines, templates), or other constraints.

Dynamic positioning may be either absolute, in that the position is locked to a fixed point on the bottom, or relative to a moving object, such as another ship or an underwater vehicle. One may also position the ship at a favorable angle to the wind, waves, and current, a practice called weathervaning.

Dynamic positioning is used by much of the offshore oil industry, for example, in the North Sea, Persian Gulf, Gulf of Mexico, West Africa, and off the coast of Brazil. There are currently more than 1800 DP ships. The applications and availability depends on the type of work and water depth. The most common position reference systems (PRS) and position measuring systems (PME) are:

thumb|[[GPS satellite in orbit]]

  • DGPS, Differential GPS. The GPS position is not accurate enough for DP use. The position is improved by using a fixed ground-based reference station (differential station) that compares the GPS position to the station's known position. The correction is sent to the DGPS receiver by long-wave radio frequency. For use in DP, an even higher accuracy and reliability are needed. Companies such as Veripos, Fugro, or C-Nav supply differential signals via satellite, enabling the combination of several differential stations. The advantage of DGPS is that it is almost always available. Disadvantages include signal degradation due to ionospheric or atmospheric disturbances, satellite blockage from cranes or other structures, and signal degradation at high altitudes. keeping the engine load between 60% and 80%.

The setup depends on the ship's DP class. A Class 1 can be relatively simple, whereas the system of a Class 3 ship is quite complex. On Class 2 and 3 ships, all computers and reference systems should be powered through an UPS.

International Maritime Organization class requirements

Based on IMO (International Maritime Organization) publication 64,5

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Sources

  • List of all offshore vessels
  • IMO, International Maritime Organization
  • Introduction to Dynamic Positioning by the International Marine Contractors Association (IMCA)
  • NMD, Norwegian Maritime Directorate
  • OPL Oilfield Seamanship Series - Volume 9: Dynamic Positioning - 2nd Edition by David Bray
  • NI, The Nautical Institute
  • The Dynamic Positioning Committee of The Marine Technology Society
  • The International Dynamic Positioning Operators Association (IDPOA)