Study On The Thruster Assisted Mooring System Based On PID Control

As the exploration of ocean is extending to the deep water, thepositioning technology of floating structures has been one of the keyresearch fields in ocean engineering. Dynamic positioning (DP) systemand mooring system are the most commonly used tools to positionfloating structures. Depending on DP system and mooring system,thruster assisted mooring system (TAMS) is developed. It’s acombination of the two traditional positioning systems. Compared to themooring system designed under the same environmental criteria, thethruster assisted mooring system would have a better performance inpositioning accuracy; Compared to the dynamic positioning system, theTAMS would have lower power consumption and be more stable. At thesame time, the TAMS would have higher degree of redundancy. Singlemooring lines or thruster failure would have a much less compact on theperformance of the system.Latest research and several main control logic of the thruster assistedmooring system are introduced in this paper. A thruster assisted mooringsystem is designed for a semi-submersible platform, the optimumarrangement of mooring lines; control mode and optimum setpoint arestudied in this paper. Main work includes:(1)Research on the arrangement of the mooring lines of a thrusterassisted mooring systemThe thruster assisted mooring system is required to hold an oceanstructure against wind current and waves with thrusters and mooringlines. The mooring line tensions are due to its weight and elasticproperties. It is obvious that the arrangement of the mooring lines(position of each anchor or the mooring line angle relative to platform)will directly affect the thruster assisted mooring system holdingcapacity because of the influence of the directions of the mooringforces. In order to improve the performance of TAMS, to minimize theresponses of the floating units, find out an optimum arrangement ofthe mooring lines is essential. With the layout of the thruster fixed,different arrangement of mooring lines are analyzed by time domainsimulations. By comparing the results, an optimum arrangement canbe obtained. The holding capacity of the thruster assisted mooring system can be improved.(2)Research on the heading control mode of the thruster assisted mooringsystemThere are three typical control mode of thruster assisted mooringsystem:1) Mooring system working onlyThe low frequency motion of floating structures are compensated bymooring line tensions only, it’s used in the mild environmentalcondition.2) Heading control modeThrusters are used to assisted in heading control3) Full control modeWorking together with mooring lines, thrusters control horizontalmotion (surge, sway, and yaw) of floating structuresTime domain simulations are carried out to study the heading controlmode and the performance are compared with those under full controlmode.(3)Research on the optimum setpoint of the control systemThe restoring forces are provided by thrusters and mooring linetensions. The restoring forces provided by thrusters are determined bythe control logic and setpoint position. Equilibrium position isproposed to be chosen as setpoint, and time domain simulations arecarried out to study the performance of the positioning system.Based on time domain simulation, mooring lines arrangement andcontrol system are studied. The conclusions can serve as a reference indesigning and operation of the thruster assisted mooring system in deepwater semi-submersible drill platforms and this is significant to thedevelopment of the technology in designing the thruster assisted mooringsystem in deep water semi-submersible drilling platforms.