Dynamic positioning
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Dynamic positioning (DP) is a method of maintaining a ship in a fixed position by using her own propellers and thrusters. This allows operations at sea where mooring or anchoring is not feasible due to deep water, strong winds 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. One may also position the ship at a determined angle towards wind and waves.
Dynamic positioning is much used in the offshore oil industry, for example in the North Sea, Persian Gulf, Gulf of Mexico and off Brazil. Important applications include shuttle tankers, mine sweepers, drilling ships and supply boats. Diving vessels also fix their positions while having divers or remotely operated vehicles in the water.
Other applications include pipe-laying, cable-laying and maritime research.
Scope of dynamic positioning
A ship can be considered to have six degrees of freedom in its motion, i.e. it can move in any of six axes. Three of these involve translation:
- surge (forward/astern)
- sway (starboard/port)
- heave (up/down)
and the other three rotation:
- roll (rotation about surge axis)
- pitch (rotation about sway axis)
- yaw (rotation about heave axis)
Dynamic positioning is concerned primarily with control of the vessel in the horizontal plane, i.e. the three axes surge, sway and yaw.
Requirements for dynamic positioning
A vessel which is to be used for DP requires:
- measurement system(s) to measure its position and heading to a suitable accuracy;
- a control computer to calculate the required control actions to maintain position and correct for position errors;
- thrust elements to apply forces to the vessel as demanded by the control system.
For most applications, the measurement systems and thrust elements must be carefully considered when designing a DP vessel. In particular, for good control of position in adverse weather, the thrust capability of the vessel in three axes must be adequate.