Risk Assessment Of FPSO Soft Yoke Mooring System

Risk assessment is more and more important in engineering and has become one of the indispensable work in the process of design,construction,operation and maintenance,etc.FPSO single point mooring system bears the responsibility for the transmission of the oil,gas,water,electricity and other media in the oilfields,so it is very important to ensure its safe operation.In this paper,risk assessment is conducted for the soft yoke mooring system(SYMS)which is widely used in Bohai sea area.The composition of SYMS is studied,and the failure mode and effect analysis is carried out combined with history failure cases.On the basis of risk identification,the fault tree model of the SYMS connecting parts is set up and qualitative and quantitative analysis is accomplished by combining the method of expert scoring and fuzzy theory.As for the swivel system,to describe its time sequence of failure mode,dynamic logical gate is applied to establish the dynamic fault tree(DFT)which can be solved by Markov and Monte Carlo method.Based on the mixed programming of Matlab and MFC,the fault tree analysis software is developed.At last,a numerical simulation based on hydrodynamics is conducted to get the load effects of FPSO and its mooring system and statistical method is used to complete the quantitative risk analysis.The results show that the SYMS has many first-order minimum cut sets which can directly cause the top event because the connecting components links each other in series.Failure of the single-component can easily lead to a more serious chain reaction of the consequences,thus it is necessary to strengthen the preventive inspection of key components of SYMS.Dynamic fault tree analysis method is more suitable for swivel system with backup.The failure probability of the swivel system is obtained by using Markov and Monte Carlo method respectively,considering spare parts and maintenance.The possibility of mooring leg failure caused by tension is very little and it is much more serious when the mooring leg fails to swing in the longitudinal direction than in the cross direction.In the backward condition,the space between bow and yoke is significantly smaller than in the forward condition,hence a larger space in design helps to reduce the collision risk effectively.