Dynamic Positioning (DP) is a vessel capability provided via an integration of a variety of individual systems and functions*. A computer control system automatically maintains 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 favorable angle towards wind, waves and current, called weathervaning.
Based on IMO (International Maritime Organization) publication 645** the Classification Societies have issued rules for Dynamic Positioned Ships described as Class 1, Class 2 and Class 3.
Equipment Class 1 has no redundancy.
Equipment Class 2 has redundancy so that no single fault in an active system will cause the system to fail.
Equipment Class 3 which also has to withstand fire or flood in any one compartment without the system failing.
Classification societies have their own notations DP Class Chart
* Capt. D. Bray FNI – DP Operator’s Handbook. The Nautical Institute, 2010. P.1
** IMO MSC/Circ.645, Guidelines for vessels with dynamic positioning systems
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