Existing dynamic positioning systems for ships depend mainly on a feed-back control using the position error as input, supplemented by a feed-forward control signal-based on the instantaneous wind speed and direction. This paper explores the possibility of creating a feed-forward control signal which can reduce the horizontal motions of the vessel which are caused by the low frequency second order wave drifting forces in irregular waves. Results of model tests are presented which indicate that the relative wave height measured directly alongside and from the vessel can be used to create a wave feed-forward control signal which reduces significantly the low frequency wave induced surge, sway and yaw motions of a vessel moored in irregular waves.
The last decade has seen a steady increase in the number of vessels which are stationed by means of dynamic positioning or stationing systems. Up to now, most dynamic positioning systems were used for positioning drill ships in deep water. Nowadays, dynamic positioning is also being used for diving support vessels, supply vessels, fire fighting vessels and maintenance and survey vessels. This increasing interest for dynamic positioning systems stems from the need for a means of maintaining the vessel's position in the horizontal plane which is quick in implementation, does not interfere with systems already lying on the sea-floor and by means of which a reasonably accurate position may be maintained for longer periods of time.
In order to maintain a given average position in the horizontal plane by means of dynamic positioning, the mean and low frequency parts of the environmental forces due to waves, wind and current are counteracted by propulsion units. These propulsion units are either fixed tunnel mounted c.p. propeller, azimuthing right angle drive units with propellers externally fixed to the vessel, or in some cases, vertical axis propeller. The magnitude and direction of the thrust of such units are governed by a control system which has as input the position error of the vessel relative to the required position, (feed-back part) and, in most cases the instantaneous wind speed is used as a measure for the instantaneous value of the wind force to be counteracted by the propulsion units (wind-feed-forward part).
As has been shown in (1), including a wind feed forward signal in the control system can enhance the position keeping performance considerably, especially in gusty weather conditions.
This paper is concerned with the capability to generate what might be termed a wave-feed-forward signal which serves as a measure for the instantaneous second order low frequency horizontal wave drifting force. This force has been shown to be capable of generating large amplitude, low frequency horizontal motions of moored vessels .in irregular waves (2), (3) and (4), and is therefore also of importance for vessels with D.P. systems.
The method dealt with here, is a result of theoretical studies and model tests carried out during the last few years at the N.S.M.B. into the low frequency second order wave forces acting on stationary vessels in irregular waves.