Investigation of the Positioning Performances for DP Vessels with Thruster Failure Modes by a Novel Synthesized Criterion

摘要

Dynamic positioning system (DPS) is usually adopted to keep offshorevessels at a desired position. For safety critical offshore operations, thedynamically positioned (DP) vessel should maintain reasonable positioningperformance even under the circumstance of thruster failure. In thiscase, thruster failure mode analysis is often taken into account when designinga new DP vessel. In this paper, the positioning performances ofthe vessels with thruster failure modes are investigated by time domainsimulations. A novel synthesized positioning performance criterion isproposed to quantify the positioning performance including the aspectsof positioning accuracy and power consumption. The synthesized criterionconcerns how well the vessel is positioned rather than how largeof the environmental conditions that the vessel can counteract. A semisubmersibleemployed with eight azimuth thrusters is adopted to performthe investigation. It can be concluded from the obtained results that, ifthe thruster system is well designed, thruster failure may not affect thethrust system in supplying sufficient thrust force with two or less than twothrusters failure for the semi-submersible. With 4 thrusters failure, thethruster system try its best to counteract the environmental yaw momentin order to keep its heading, the thrust system cannot supply sufficientforces to assist the vessel going back to the desired position, which is verydangerous in practical operation. Therefore, the failed thruster should betimely maintained to guarantee safe operation and to save power.