The calculation of the required power to maintain a vessel keeping a fixed position with the aid of a Dynamic Positioning (DP) system is not easy. Wind, waves and current forces should be analyzed together with the propulsion configuration of the vessel. But some rough estimation could be done easily with the HSSC number as explained here below.
A Dynamic Positioning (DP) system is a computer controlled system to automatically maintain a vessel’s position and heading by using its own propellers and thrusters.
Any vessel has 6 freedoms of movement, namely: yaw, surge, sway, heave, pitch and roll. The heave, pitch and roll of the vessel cannot be controlled by a DP system: these movements belong to the sea-keeping behaviour of the ship.
A seagoing vessel is subjected to forces from wind, waves and current as well as from forces generated by the propulsion system. The function of the DP system is to control automatically the yaw, surge and sway, by using propellers and thrusters of the vessel, and therefore maintain the vessel in the desired location or maintain the required heading control.
The basic elements of a Dynamic Positioning system are:
- Sensor System.
- Controller.
- Thruster System.
- Power System.
The focus of this post is on the thruster system. The determination of the minimum thrust to successfully dynamically position a vessel is complex. But Ref [1] provides a very simple estimation. It is proposed to use as a design criteria the performance of a DP vessel with a high wind on the beam with no sea or current. They define a dimensionless number HSSC, defined simply as the ratio of 80% of available thrust to the load from a 61 knot beam wind. For a well designed system, the HSSC number should be greater than 1.
As a very rough estimate, we could conclude that 80% of the total DP power installed in the vessel should be enough to maintain the position of the vessel with a wind of 61 knots acting on the beam. Forces of waves and current are ignored. That means that if we are in an area where wind speed is not so high, the used DP power will be much lower than the installed power.
References:
- Steddum, Riddle; Herrmann, Robert P. “Thrust Required and Holding Capacity”. Houston, USA: Dynamic Positioning Conference, 21 – 22 October, 1997.
- Kongsberg Maritime A/S.
