Research On The Optimal Control Theory For Thruster Assisted Mooring Positioning System

Under the new world economic situation with strained circumstance,the advantages of the deep water energy and sources are becoming outstanding,while the positioning technology in the ocean exploration has been one of the key research fields in maritime engineering.Considering synthetically between the tracking effects and the construction expenditure,the positioning method will change as the operating depth is different.So far there are two methods in position-keeping,which is mooring system in shallow water ares and dynamic positioning(DP)system in deep water areas.Where the former has the performances of simple structure,low costs and so on.The later has the characteristics of flexibility as well as the interoperability.As the matter of fact,the thruster assisted position mooring system is becoming a hot subject in the scholarly debate,which is absorbing those advantages between mooring system and DP system simultaneously,but beyond that it also makes compensation of their disadvantages.In the same disturbances of ocean environment,the mooring system has some worse results,compared to the thruster assisted mooring system which have convenient operations and a better performance in station keeping accuracy.Compared with the dynamic positioning system,the combined system would have lower power consumption and be more stable.At the same time,the thruster assisted mooring system would have a better property in facilities redundancy.If there are some failures just like one mooring line is broken down or one thruster is invalid,the combined system will have less influences on the positioning performance.The application of thruster assisted position mooring system in offshore platform is mainly introduced in this paper,and it also states some model predictive control methods.A four points distributed mooring system is designed for a floating production storage and offloading unit.The whole paper is divided into three modules for the analysis convenience.The first part mainly discusses the static characteristics of catenary mooring line,and the dynamic line response with the environment disturbances.The second part elaborate draws the core control system into the dynamic system,and roughly introduces the calculation about the thrust optimum allocation.The third part especially derives the generalized predictive control(GPC)algorithm,which is applied to the thruster assisted position mooring system,and gives a stair-like control strategy to compensate the disadvantages of GPC.In the design of mooring system,a program is made to compute the static characteristic variables.While an extended Kalman filter and an conventional controller is made during thedesign of dynamic positioning system.The most important thing is that the GPC algorithm is brought to the combined positioning system,and through the simulation results which is indicated the properties of station keeping are quite satisfactory.Since the multi-dimension matrix could be singular in the traditional GPC,an improved algorithm better than it named Stair-like Generalized Predictive control is proposed to solve the above described shortcomings.In this paper,the contents from all parts has made the numerical simulation in time domain.The conclusions can make a good suggestion in analysis and research on the thruster assisted mooring system in deep water offshore manipulating platforms,and it is meaningful to the rapidly expanding of the technology in designing the thruster assisted mooring system in other drilling platforms.