Research On Corrosion Fatigue Life Of Submarine Pipeline

Due to the coupling of corrosive environment and fatigue load,engineering components often suffer from corrosion fatigue and fail(eg,submarine pipeline).It is important to determine the period of safe service for components subject to corrosion fatigue.The corrosion fatigue life is mainly composed of two parts(crack initiation life and crack propagation life)or three stages(pitting stage,pit transition to crack stage and crack propagation stage).This paper mainly studies from the following four parts to predict the safe service life of submarine pipelines.The first part is the initiating of corrosion fatigue crack.It was proposed to apply two parameters,the ratio of depth to diameter(?=d/a)and the position of center(?=b/d),to describe the pit morphology,and to research the sensitivity of parameters of the pit depth(d),the ratio of depth to diameter(?=d/a)and the position of center(?=b/d).Based on the three-dimensional two-parameter pit model,the emanating of corrosion fatigue crack model was established to research the critical conditions of corrosion fatigue crack emanating from the bottom of the pit and the shoulder of the pit,and to predict the initiation life of corrosion fatigue crack of structures in a specific corrosive environment.The second part is the pit-crack.It was proposed to establish a two-parameter pit model,according to the linear elastic fracture mechanics,to analysis the stress intensity factor of the pit with axial I-shape crack on the surface of the pipeline under the action of the hoop stress by the extended finite element method of ABAQUS,and to analysis the trend of the parameters of pit influencing on the stress intensity factor of pit-crack.The third part is the propagating of corrosion fatigue crack.Based on the dynamic law of crack propagation and considering the mechanism of hydrogen embrittlement,a new hydrogen embrittlement-passivation-fracture model of CFCP was proposed.Considering the effect of critical fracture strength factor on crack propagating,a full-stage of corrosion fatigue crack propagating was established,which covers the threshold zone,subcritical propagating and rapid propagating of corrosion fatigue crack.In order to study the corrosion fatigue crack propagating characteristics of materials under the seawater environment,the corrosion fatigue crack growth test of the steel X65 for submarine pipelines was carried out in the air and 3.5% NaCl solution by standard compact CT specimens.The Paris formula was used to analysis the data obtained from the test.The similarities and differences of the crack propagation in the corrosion environment and the air were analyzed,and the crack propagation thresholds under the corresponding system were obtained.The fourth part is the predicting of corrosion fatigue life.Based on the equivalent initial defect size(EIFS)and the linear cumulative damage(P-M)methods,the corrosion fatigue life prediction expression for submarine pipelines is established;the effects of pitting nucleation,pit growth,and corrosion fatigue small crack propagation on the prediction of corrosion fatigue life are eliminated;the drawbacks of the expression of predicting corrosion fatigue life based on a single pit is avoided;the prediction process of corrosion fatigue life under random load is rationally simplified.