Dynamic positioning (DP) is a computer controlled system to automatically maintain a ship’s position and heading by using her own propellers and thrusters. Position reference sensors, combined with wind sensors, current 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. 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.
Dynamic positioning ship maintains a correct position and path with the help of its rudder and propeller unit which is controlled by an auto pilot unit. To achieve this, different inputs are feed to the control unit from motion sensor, position sensor, gyrocompass, wind sensor etc.
The most affected systems on DP vessels are rudder and propeller systems, together known as azimuth type system, are subjected to continuous loading and working conditions. Different forces and stresses acting on this unit may damage the system in the long run.
What’s the need of such system?
As the Autopilot or AP of the ship takes input from different sensors, they are all subjected to inertia which results in over shooting or high rudder angle correction.
If such operation continues, unnecessary high forces will act on rudder and steering operation will increase, thus reducing efficiency due to hysteresis. This also increases power intake and hence fuel consumption which results in higher emission from ships.
If a measuring system is provided to measure forces on the rudder, the hysteresis can be eliminated or reduced, which will increase the efficiency of the system and life of the system.
What does the system do?
The sensors of the computer control unit senses resistance, rudder lift, rudder drag and forces on the rudder and feeds this data back to the unit.
The computer control unit Auto pilot, after calculation of received data, gives a better manoeuvrability and reduces forces on rudder.
Advantage of the system
- Conventional rudder propeller can be used.
- No limitation for installed power.
- Usage of the rudder reduces.
- Quieter and efficient rudder operation.
- Response time of rudder reduces.
- Better efficiency of the system.
- Less maintenance cost.
- The most important of all, it reduces fuel consumption and hence emission from ship.
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