A novel real-time estimate method of wave drift force for wave feed-forward in dynamic positioning system

Abstract

Traditionally, 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 and current speed and direction. The feed-forward control is further extended to wave feed-forward, thanks to the research development on the real-time estimate of the wave drift force. Wave feed-forward allows the dynamic positioning system (DPS) to directly compensate the slowly varying wave drift force in order to avoid sudden drift off caused by large unexpected wave force. A novel real-time estimate method of wave drift force is proposed in this paper. The estimated wave drift force is used in the dynamically positioned control system for wave feed-forward dedicated to enhancing the performance of the DPS. A time domain simulation for a dynamically positioned semi-submersible is conducted. It is demonstrated from the results that the novel proposed wave drift force estimate method is effective and efficient to be applied in wave feed-forward. And the positioning performance of the vessel is improved under the employment of wave feed-forward.