Key Problems Of Marine Flexible Pipe And Cable's Fatigue Test And Research On Fatigue Failure Mechanism

Marine flexible pipe and cable includes flexible pipes,umbilicals,optical cables and other equipments.It's used to connect floating equipment such as platform with oil and gas production equipment,which occupies an important position in offshore oil and gas production systems.Due to the complexity of the marine environment,flexible cables often undergo various failures during service.Among them,fatigue failure has become a kind of failure mode that is frequent but difficult to predict.Then it has become the focus of current academic research in marine engineering.Due to the complex structure of the flexible pipe/cable,the non-bonding forms are often used between the layers.The behavior of the layers during bending is difficult to predict,resulting in inaccurate calculation of the fatigue stress.At present,it is difficult for academic circles to obtain the accurate fatigue life of flexible pipe/cable through life prediction theory.It is necessary to perform fatigue tests on flexible cable.In order to obtain the accurate fatigue life of the flexible pipe/cable,it is necessary to highly simulate the behavior of the flexible cable under fatigue loading under actual working conditions.Firstly,this paper introduces the structural characteristics of marine flexible pipe/cable,and introduces the basic theory of metal material fatigue.The problem of fatigue failure of flexible pipe/cable under the working condition of complex ocean loading is derived.Based on the failure mechanism,the method of predicting the fatigue life of pipe/cable is discussed.The huge calculation error is analyzed and the necessity of the fatigue experiment simulating actual working conditions.According to the progress of the current fatigue test,the key issue that has not been solved in the test is proposed,the design of the tester's pendulum head.In the test,the pendulum part bears the function of applying cyclic bending load.The length of pendulum has a great influence on the line type of the pipe/cable under the bending load.In this paper,firstly,a simple theoretical model is established,and the problem of pipe/cable's line type is simulated and analyzed through ABAQUS software.Based on this,the related scheme of the pendulum head design in the simulation condition fatigue test was proposed and verified by the linear measurement experiment.This article describes the test system and the test flow of the fatigue test that has been completed.The test method and the test results are briefly analyzed from various aspects.The fatigue test that simulates actual working conditions requires a lot of manpower and material resources.At the same time,the test takes a long time period,and merely based on fatigue failure of the entire pipe/cable body.Therefore,this paper furtherly studies two aspects of the fatigue mechanism of the flexible cable: 1.Due to the reciprocating sliding of the layers under the working conditions,friction and wear between the layers of the armored steel wire are accelerated so as the fatigue process.In this paper,the inter-layer wear behavior was tested and the effect of this behavior on the life of the armor layer was analyzed.2.In the process of reciprocating bending,the flexural response of the flexible pipe due to the friction between the layers leads to a non-linear bending response,but there is a hysteresis effect.In this paper,the hysteresis effect is studied by the tensile-bending combination experiments.This paper is based on the basic goal of completing a highly-reduced fatigue test simulating the actual working condition of flexible pipe/cable structure.Furtherly,the mechanism of fatigue failure is analyzed.It is certain that there is a reference and enlightenment significance for current academic community concerning about the analysis of fatigue problems in flexible pipe/cable structure.