Design Of Thrust System For DP Semi-Submersible Rig

Dynamic Positioning System (DPS) is a closed-loop control system, which maintains desired position and heading of the vessels and rigs, by counteracting the environmental forces such as wind, wave, and current, with the forces generated by thrusters. There are four parts in the DPS: sensor system, control system, power system and thrust system. This paper is to design a reasonable and reliable thrust system for the DP semi-submersible rig.There are two basic principles for the thrust system to meet requirement of DPS. One is that there must be enough forces to counteract the environmental forces and moment, and the other is there must be rapid dynamic response for thrusters to make a response to the environment forces. Eight azimuthing thrusters, with fixed pitch propeller and highly efficient nozzle are chosen, which are completely identical. The nozzle can improve the bollard pull. To make more thrusters operated at lighter load can reduce fuel consumption.For dynamic positioning the most important factors determining the positioning capability are behavior of thrusters in the water. All thrust generators work on the principle of accelerating water. So there are some kinds of flows around thrusters. These flows, together with current flow and waves may cause interaction effects leading to degradation of thruster performance. Title angle of the nozzle can reduce the friction loss between thrusters and the pontoon, and forbidden sector can avoid thrust degradation.Simulating the capability plot for the rig under various environment condition, considering one and two thrusters out of work, to validate the reliability of the thrust system.Based on the sequential quadratic programming method, this paper presents a rational and effective method of solving thrust allocation. The objective of the thrust optimum allocation is to minimize the energy consumption, including fuel consumption, tear and wear of thrusters and thrust error etc. Other factors such as thrust limitation, maximum rate that thrust changes, maximum slew rate of thrusters, forbidden sector for the thrust direction angle and singular configuration avoidance are also taken into consideration. Finding a reasonable one dealing with thrust degradation among three methods, and finding a most optimum thruster configuration based on the comparison their fuel consumption.