Time-dependent Fatigue Reliability Assessment On The Mooring System Of FPSO

FPSO has become a mainstream tool for offshore oil and gas field development because of short construction period,strong wind and wave resistance and good reusability.However,FPSO is positioned by the mooring system and have a long period when it is subjected to the combined effects of wind,waves and currents.The components of its mooring system bear alternating load,hence fatigue failure is the most common failure mode and is essential to be evaluated.In the marine engineering field,time-dependent reliability evaluation method based on residual strength degradation mechanism is hardly used in fatigue assessment of mooring systems.The thesis of this aritcle is establishing a relatively complete fatigue time-varying reliability assessment process and crucial mooring chains of a FPSO mooring system in the South China Sea is taken as an example.Firstly,the SESAM(hydrodynamic numerical software)is used to establish the FPSO hull finite element model,mooring system model and sea state model.The longterm FPSO sea state and loading status were discretized into 240 short-term work conditions,and the tension time history of mooring system in each short-term work condition is calculated.Secondly,characteristics of marine engineering load are integrated into the component fatigue time-varying reliability model considering the random and timevarying nature of the load and initial strength of the component,thus a marine engineering component time-varying fatigue reliability model is established.The model is based on the strength attenuation model,load-strength interference model,first passage failure criterion and rainflow counting method which is used for counting the tension time history.In addition,a suitable calculation method is proposed for this model in order to correctly solve the reliability and save computing resources,and the process is programmed and calculated with MatlabThen,the Abaqus(finite element numerical simulation software)is used to establish a finite element model of the gearless mooring chain and the influence of tensile residual stress and uniform corrosion on the structural stress is considered to help improve the model.Empirical formula for the influence of component strength and fatigue accumulation rate is established based on the model calculation results.Finally,the above studies are integrated to form a component fatigue time-varying reliability evaluation process considering corrosion effects.The process takes structural design parameters,sea state parameters,component parameters as input and obtains the reliability index of key components of the mooring system during its life,which provides a basis for the design of the components and helps formulate targeted maintenance.