Assessment Of Riser Fatigue Life On Vortex-Induced Vibration In Random Wave And Current

Riser's fatigue caused vortex-induced vibration under combined action of wave and current is a major failure mode.Once the riser fails,serious economic losses and environmental pollution would result,it is great significance to assessment of riser's fatigue life on vortex-induced vibration.To solve this problem,developing vortex-induced vibration fatigue life assessment technique for top tension risers in typical sea areas,when the effects of random wave,shear flow and platform random excitation were considered comprehensively.The main research work and achievements of the paper were as follows.With statistical analysis of distribution stochastic wave and current,the parameters of long-term sea state and the return period shear flow were obtained.A single short-term sea conditions was simulated under the random wave in time domain,based on random vibration theory,harmony superposition method and Wen shengchang spectrum according with china seas.Furthermore,the accuracy of simulation was verified through the autocorrelation coefficient and theoretical in frequency domain,the simulation results were in good agreement with target values.The TLP motions of time history curves were provided a boundary condition for risers.The response of IL and CF vortex-induced vibration of the riser under different sea conditions and platforms were simulated in time domain,based on improved two-dimensional coupled parametric excitation on VIV prediction model foundation of vortex strength wake oscillator theory.The governing equations were dispersed by finite element method that was written by Matlab software.And compared with the simulation results and experimental results from BP and MARINTEK,the results are agreed well.On this basis,the effects of current velocity,top tension factor,and different sea conditions on the two-dimensional vortex-induced vibration response of the riser were studied.Developing a fatigue damage assessment method for riser under combined of waves and currents,based on random vibration theory,rain flow method and Miner fatigue damage theory.The cumulative fatigue damage of risers were analyzed and calculated in different sea conditions and different directions under combined wave and current.According to the Norwegian DNV-RP-C205 standard analyzes the effects of welding quality and different defects on the riser fatigue life.A vortex-induced vibration numerical simulation and fatigue evaluation system for the top tension riser was developed.The fatigue damage evaluation method was verified by a full-scale riser fatigue test with surface defects,based on 20 years of expected service life requirements.In this paper,developing vortex-induced vibration fatigue life assessment technique for top tension risers provides technical support for riser design and in-service safety assessment.