On compensation for wave-induced roll in dynamic positioning control

Abstract

Dynamic positioning (DP) for station-keeping control keeps the position and heading of the vessel constant regardless of environmental disturbances from wind, waves and currents. If the point of interest on the vessel has a significant vertical offset, it will be influenced also by the roll, pitch and heave motions. Although such motions are usually neglected in DP control, there is an opportunity to also influence these since thrusters will not only influence the horizontal plane motion of a vessel, but also generate roll and pitch moments. In this paper, we formulate the problem of DP control when explicitly considering the first-order wave-induced roll motion and thruster roll moment. It is shown that the control performance can be significantly improved when the DP system’s point of interest, and the location of the thrusters, have a significant vertical offset compared to the vertical position of the center of rotation. The proposed control algorithm is demonstrated in a realistic simulation of a relatively small ship during launch-and-recovery of a remotely operated vehicle (ROV) where the DP’s point of interest is at the lower end of the launch-and-recovery-system (LARS) that extends below the keel of the ship. It is shown that the positioning performance can be improved with 20–45% reduced error, depending on the sea state.