Dynamic Analysis And Optimization On Mooring System Of Deepwater Semi-submersible In South China Sea

The oil and gas resource in South China Sea is rich,and oil and gas exploitation depth increases gradually in recent years,which stimuluses the demand for deepwater equipment such as deepwater semi-submersible platforms and so on.Mooring system is capable of maintaining position for deepwater semi-submersible platforms and guarantee its safe operation.The environment condition in South China Sea is severe,especially typhoon occurs frequently,which tend to cause mooring failure accident.The hydrodynamic performance of deepwater mooring system,which refers to nonlinear dynamic coupling problems,is complex and difficult to analyse.In our country the study for deepwater mooring system starts late,and research results are rarely used in practice application.To improve operation safety level for deepwater mooring system,it's essential to apply more research on mooring system for deepwater semi-submersible platforms.The effect factors of hydrodynamic performance of deepwater mooring system include the mooring type,equipment option,mooring line component,anchor pattern,environment load and so on.The relevant nonlinear problem include material nonlinearity,geometric nonlinearity,load nonlinearity,seabed effect and so on.Based on practice application,this thesis takes a deepwater semi-submersible platform in South China Sea for example,uses the environment data in Liuhua field,and analyses the hydrodynamic performance of its mooring system.The evaluation criterion of platform offset and mooring line tension refers to the standard API.Because of the weakness such as mooring line breaking and anchor dragging,the ultimate bearing capacity of mooring system is evaluated,failure mechanism is analysed and control strategies are proposed.Based on different requirement in different operation condition,related optimization scheme is discussed.The main work of this thesis can be concluded as follows:(1)The hydrodynamic performance analysis of deepwater semi-submersible platform mooring system in operating condition.Based on 3D potential flow theory and morison equation,coupled analysis model of platform-riser-mooring system is established using the software ANSYS-AQWA.Quasi static method,frequency domain method and time domain method are adopted in hydrodynamic performance analysis of mooring system.In detail,load distribution and geometric shape of mooring lines are analysed with catenary method.Transfer function of platform's dynamic response in six degrees of freedom are analysed with frequency domain method using AQWA-LINE module.Platform offset and mooring line tension are calculated with time domain method using AQWA-DRIFT module.Comparing the analysis results between models with riser and that without riser,it's reasonable to ignore the influence of riser because of the minor difference.Based on the analysis results under different operation condition which are evaluated according to standard API,operation window in specific operation condition is divided.(2)The ultimate bearing capacity evaluation of deepwater semi-submersible platform mooring system.Hydrodynamic performances in two survival conditons are analysed,getting the conclusion that mooring lines in the first survival condition don't break while mooring lines in the second survival condition turn to break because of low safety factor.The left bearing capacity of mooring systems which lose one mooring line in the two survival conditons are analysed,getting the conclusion that the mooring system in the first survival condition can meet the safety requirements while mooring lines in the second survival condition break in succession,so the break sequence of mooring lines is discussed.On the basis of dragging theory and multicomponent catenary equation,dragging judgement formula is derived,which combined AQWA calculation and manual calculation and realizes time domain analysis of anchor dragging,and a practice example in specific condition is analysed.(3)The study on hydrodynamic performance optimization of deepwater semi-submersible platform mooring system.The influence effect and sensibility of influence factors upon platform offset and mooring line tension are analysed using orthogonal analysis,and it come to the conclusion that pretension optimization is the best effective and practical way for hydrodynamic performance optimization of mooring system.The pretension in operation condition is optimized by comparing the results of platform offsets and minimum safety factors of mooring lines,and analyzing the operation windows in different operation conditions.The pretension in survival condition is optimized by comparing the minimum safety factors of mooring lines.(4)The study on failure modes and control strategies of deepwater semi-submersible platform mooring system.Several common failure types of mooring system are discussed such as mooring line breaking,anchor dragging,braking system failure and so on,the causes,modes and risk factors of which are summarized,and the prevention,control and response strategies are proposed.The common external influence factors such as environment,personel,equipment and management are analysed,and their influence modes and control strategies are put forward.