Investigation of the positioning performances for DP vessels considering thruster failure modes by a novel synthesized criterion

Abstract

For safety critical offshore operations, the dynamically positioned (DP) vessel should maintain reasonable positioning performance even under the circumstance of thruster failure. In this case, thruster failure mode analysis is often taken into account when designing a new DP vessel. In this paper, the positioning performances of the vessels with thruster failure modes are investigated by time domain simulations. A novel synthesized positioning performance criterion is proposed to quantify the positioning performance including the aspects of positioning accuracy and power consumption. The synthesized criterion concerns how well the vessel is positioned rather than how large the environmental conditions that the vessel can counteract. A semi-submersible employed with eight azimuth thrusters is adopted to perform the simulation. It can be concluded from the obtained results that, if the thruster system is well designed, thruster failure may not affect the thrust system in supplying sufficient thrust force with two or less than two thrusters failure for the semi-submersible. With four thrusters failure, the thrust system tries its best to counteract the environmental yaw moment in order to keep its heading, however, it cannot supply sufficient forces to assist the semi-submersible going back to its desired position, which is very dangerous in practical operation. Therefore, the failed thrusters should be timely maintained to guarantee safe operation and to save power. The performance of the thrust re-allocation logic dealing with thruster failure during the simulation is also approved. Finally, several design guideline regarding improvement of the positioning performance of the vessel after thruster failure have been summarized.