Investigation On The Fatigue Damage Mechanism Of A Cyclic Loaded Wellhead Conductor

Subsea wellhead system is a key component of the entire underwater oil drilling and production equipment.The response of the riser to the platform movement,current and wave will generate cyclic loads on the wellhead system,causing the bearing capacity degradation and fatigue damage of the wellhead system.This paper is divided into two parts.In this paper,analyses the degradation mechanism of a laterally loaded conductor and fatigue damage of wellhead.Firstly,analyses the degradation mechanism of a laterally loaded conductor.Based on the strength degradation mechanism of cohesive soil under disturbance,a p-y model degrading with accumulated displacement has been proposed and user element for cyclic p-y curve has been developed in ANSYS.And by comparing with the change of soil strength with cumulative displacement in the T-bar test,the correctness of the cyclic p-y curve user unit is verified.With the p-y curve user element,the degradation mechanism of a laterally loaded conductor has been modeled in ANSYS.Results indicate that,the resistance p of the developed cyclic p-y element degrades with the accumulated displacement to a residual value,and the larger the cyclic displacement amplitude,the faster the degradation of lateral soil resistance.Under the cyclic displacement load,the maximum conductor bending moment and the lateral bearing capacity decrease to the stable value with the number of cycles,and the larger the cyclic displacement amplitude,the more significant the degradation.Under the cyclic force,the conductor displacement,maximum conductor bending moment and the BOP angle increase with the number of cycles,and the larger the amplitude of cyclic force,the more significant the increase.Under the cyclic displacement load,the smaller the residual soil resistance,the more significant the degradation of the maximum conductor bending moment and the lateral bearing capacity.In addition,after storm sea condition,the conductor lateral bearing capacity lower than the value of normal sea condition,because the storm sea condition will increase the degradation rate of soil strength.Then,carry out the global coupling model response analysis of wellhead system.The global coupling model of riser-wellhead-conduit-soil for fatigue analysis is established in ANSYS,and apply the platform motion calculated by AQWA under storm and normal sea conditions to the top of the riser,and analyses the global coupling response of the wellhead system.The results show that under the two sea conditions,the soil resistance decreases continuously,and the soil resistance decreases more significant then storm sea conditions.The maximum bending moment of the wellhead system appears at 1m below the mud line,so use this as a fatigue hotspot for the next fatigue analysis of the wellhead system.Finally,analyzes the fatigue damage of the wellhead,and the effect of the storm sea conditions and residual soil resistance has been investigated.Results indicate that,the storm sea condition will increase the fatigue damage of the wellhead.The smaller the residual soil resistance,the greater the fatigue damage of the wellhead.