Research On Optimization Method Of Mooring System Of Submerged Mobile Offloading Terminal

There are many marginal oil fields in China,which have small annual output,short development life,and are not surrounded by oilfields with supporting facilities.There is a certain demand for the "bee-style" development model.However,the commonly used CALM system is expensive and is greatly affected by the weather in the sea area.In the sea area with strong typhoons or monsoons,the effective operation time may be relatively reduced,and the maintenance and operation costs may increase.Therefore,the design and research of a new offloading system that can resist the harsh environment and effectively control the cost are of great significance to the development of marginal oil fields.A series of studies on a conceptual design of a new submersible mobile offloading terminal were carried out in this paper.Hydrodynamic calculation of the mobile offloading terminal is the basis for the coupled analysis of the offloading system.In this paper,four hydrodynamic simulation methods of the mobile offloading terminal were studied.By comparing and analyzing the first-order wave force RAO,motion RAO and second-order wave force,it was found that the method based on the mass and all stiffness matrix of the mobile offloading terminal was better to model and could simulate the motion response of the mobile offloading terminal well.Based on the results of the hydrodynamic calculation of the mobile offloading terminal,coupled analyses of the mooring system under the survival condition and the offloading condition in time-domain were carried out.In this work,the coupled dynamic model of the offloading condition was established in time-domain to study the influence of different loading conditions on the dynamic responses of the offloading system.The coupled dynamic models of survival condition were established in time-domain to analyze the influence of environmental parameters and mooring parameters on the motion responses and mooring forces of the system.In the design of mooring system,the change of mooring parameters has an important effect on motion responses and mooring forces of floating platforms.An optimized mooring system can improve the mooring performance of the mooring system and effectively reduce the cost.Therefore,the mooring system optimization frameworks based on NSGA-II and multi-objective PSO were established respectively,and were used to optimize the design of the mooring system in this paper.By comparing the mooring optimization results of NSGA-II and multi-objective PSO,it was found that both of them could solve the multi-objective optimization problem of mooring system,but both had their own advantages and disadvantages.Using PSO to solve the multi-objective optimization problem of mooring system was simpler in principle,faster in operation,and the distribution of the obtained Pareto front was better.The convergence of the obtained Pareto front was better when NSGA-II was used to solve the multi-objective optimization problem of the mooring system,so NSGA-II was more suitable for the optimization of the mooring system of the submersible mobile offloading terminal.