Coupled Dynamic Analysis And Fatigue Reliability Assessment Of Deepsea Riser

Deepsea riser connects floating body on water surface and subsea equipment as a key structure,which plays an important role in offshore oil and gas exploitation and output,and it is a key part for the development of national energy strategy towards the deepsea.The complexity of the deepsea environment makes the riser become a weak link in the development of offshore oil and gas system.Therefore,it's urgent to carry out the dynamic behavior of deepsea riser and its reliability under complex environment in order to predict accurately the motion situation of the riser and evaluate its fatigue reliability,which can provide reference data for the design and safe operation of the deepsea riser.This paper aims to study the dynamic response and fatigue reliability of the deepsea riser by combining theoretical calculation with finite element simulation.The applicability and reliability of the methods are verified by the examples,and obtains some conclusions that are of great consultable value:(1)The nonlinear vibration equation of marine riser is established to analyze the influences of vibration model difference,external excitation loading mode,internal flow viscosity and velocity on the dynamic response of the riser.It is observed that the calculation results of the nonlinear beam model of the riser ars greater than thoes of the linear beam model.And the riser vibration modes and vibration peak under the coupling excitation of wave,current,platform motion and the single excitation of platform motion are found to be very different,so the direct effect of wave and current on the marine riser can't be ignored.The influences of kinematic viscosity and flow velocity of the inner flow on vibration of the riser are limited,and increasing top pre-tension of the riser can counteract this effect.(2)Time domain coupled analysis model of the deepsea riser is established in the Sesam/DeepC module,and the influences of the different environmental load direction,pre-tension and the number of the tendon and riser on dynamic response of the riser are analyzed.It is observed that the platform motion,the top pre-tension and the number of tendon have significant effects on the dynamic response of the riser.(3)Based on the fatigue analysis program in time domain,the fatigue lives of the riser nodes under different working conditions are obtained.The fatigue reliability indexs of typical nodes are calculated,and the influences of the environmental parameters on the fatigue reliability of the riser are also analyzed.The results show that the fatigue lives of the riser nodes are greater than the designed life,and decreasing the top pretension significantly will obviously increase the fatigue failure probability of the riser.The fatigue reliability of the riser decreases with the increase of wave height and current flow velocity,and increases with the increase of the wave period.(4)The improved response surface method is proposed to solve convergence failure in the iterative calculation of the fatigue reliability index of the riser,and the calculation results have enough accuracy.This method is used to analyze the influence of the variation coefficient of the random variables on the fatigue reliability.The results show that the fatigue reliability of the riser decreases with the increase of random variable coefficient of variation,and the variation coefficient of the random variable B has the greater influence than variable A and ?.