7.5. Dynamic positioning
Dynamic positioning (DP) is required by ships which are required to stay in position without the use of anchors or other means fixing them to seabed, e.g. crane vessels, ships for cable laying, pipe laying, pipe trenching, stone dumping, diving support, dredgers and bunker boats. [1]
DP consists of hardware (propellers, thrusters) which ensure keeping the vessel in position and software, that processes input information (wind, position, heading) and controls the output devices to ensure the ship either stays in position or proceeds according to a defined course along a defined track with waypoints and over a defined distance.
The environmental sensors include:
Gyrocompass determines the heading of the ship.
Vertical reference units determine the roll and pitch of the ship.
DGPS (dynamic global positioning) systems determine the position of the vessel.
Taut wires - a self-tensioning winch keeping a constantly tensioned wire connected with weight on the seabed; computers will receive data from transmitters collecting data about directional angle of a wire).
Radar based position systems measure the distance and heading from one or more transmitters located at a fixed location.
Combined sensors, e.g. Octan combines gyrocompass and motion sensor.
Laser based systems, e.g. FANBEAM, measures distance and heading from a reflector fixed at the location.
Underwater position systems – their transponder replies to the sound signals transmitted from the ship and measures distance and heading.
The ship computers ascertain the heading of the ship, its exact position and give corrective signals to propellers and thrusters to keep the ship in position or follow the set track.