Study On Fatigue Failure Mechanism And Life Prediction Of Subsea Dynamic Cable

Dynamic cables include deep-water dynamic umbilical cables and offshore wind power dynamic cables,etc.They are important equipment for deep-sea oil and gas resource development and offshore wind power systems.When they are installing and laying or working in place,due to the joint action of offshore platform movement,wind,wave,and current loads,fatigue failures will occur under repeated alternating loads,resulting in huge economic losses.Due to the complex cross-section of dynamic cables,it is difficult to accurately predict their fatigue life.Accurate fatigue analysis and failure mechanism research of dynamic cables have always been the focus and difficulty of academic research.This paper first introduces the structural characteristics and failure modes of dynamic cables,points out the fatigue failure problems of dynamic cables under the working conditions,and introduces the fatigue failure theory,leads to the fatigue failure mechanism of armored steel wire as the key element of dynamic cables,and clarifies the fatigue failure mechanism of the key element of dynamic cables.It is necessary to study the fatigue problem of the armored steel wire.In this regard,this paper first designs the friction and wear test of the steel wire based on the friction and wear theory of metal materials,analyzes changes of the friction coefficient between the steel wires under different pressures and contact angles,and corrects the friction coefficient in the dynamic cable fatigue calculation.Then based on the Archard wear model and its modified model,a theoretical model of steel wire interlayer wear was established,and the model was verified by experiments.The maximum wear depth is selected to characterize the wear between the steel wires.The increase rate of the maximum wear depth between the steel wires decreases as the number of reciprocating wear increases.The combined tension and bending load is the main load condition that the dynamic cable is subjected to when it is fatigued.Based on the theoretical model,this paper calculates the tension and bending stiffness and critical curvature of the dynamic cable,designs and conducts the combined tension and bending test.Aiming at the error caused by the dynamic cable's own weight and the end problem,the tensile-bending combination test method was improved to verify the accuracy of the theoretical method,and the influence of the tensile force on the bending stiffness of the dynamic cable was further analyzed.This paper clarifies the process of predicting the fatigue life of dynamic cables,and then takes wind power dynamic cables as the research object,and estimates the fatigue life through Ocarflex software,focusing on the fatigue of its metal elements.Based on the classic fatigue life prediction process,the in-position sea state of the dynamic cable is determined first,and then establishs the numerical model of the floating body and the dynamic cable,clarifies the tension and curvature distribution of the dynamic cable,calculates and analyzes the local stress of dangerous part,and predicts the overall life of the dynamic cable.The fatigue life of the armored steel wire under different friction coefficients is considered,and the fatigue life of the copper conductor is calculated.The results show that the friction coefficient has a greater influence on the fatigue life of the armored steel wire.The installed steel wire is more prone to fatigue failure.In the study of the fatigue problem of dynamic cables,attention should be paid to the fatigue failure of copper wires.This paper studies the fatigue failure mechanism of dynamic cables,and is dedicated to improving the accuracy of the fatigue life prediction of dynamic cables.This paper modifies the friction coefficient of dynamic cable,optimizes the dynamic cable tension and bending combination test method,improves the accuracy of dynamic cable bending stiffness parameters,and determines the curvature,tension,and stress calculation and analysis methods in the fatigue life prediction process.It has certain reference value for engineering.