Study On Cross-fault Buckling Behavior Of Subsea Buried Pipeline

Subsea pipelines are key infrastructures for exploration and exploitation of offshore oil and gas and deep-sea mining,but the domestic laying started late so its design,construction,operation,and maintenance urgently need technological breakthroughs.Aiming at the load characteristics of subsea pipelines and the insufficient research on their cross-fault buckling behaviors,the shell buckling and post-buckling behavior of subsea pipelines was intensively studied using the vector form intrinsic finite element(VFIFE)as well as pressure chamber tests.Then,systematical studies were conducted for subsea buried pipelines crossing seismic faults,including the buckling failure mechanism,postearthquake disposal measures,and aseismic and buckling prevention designs.The results provide valuable theoretical and practical guidance for strength assessment,seismic disaster prevention and mitigation of subsea pipelines.The research work and results are as follows:(1)Based on the VFIFE shell element,a buckling failure analysis model of a subsea pipeline with an integral buckle arrestor was established to simulate the whole process of local collapse,buckling propagation,and buckling crossover of subsea pipelines.Fullsize pressure chamber tests,synchronous numerical simulations,and DNV specification checks show that the VFIFE shell model has high accuracy in the simulation of subsea pipelines with a diameter-to-thickness ratio greater than 20.For subsea pipelines with a diameter-to-thickness ratio of less than 20,a VFIFE solid element model was established and its verification was completed through reduced-scale pressure chamber tests and synchronize numerical simulations.Based on the calculation results,a high-precision formula was obtained for calculating the collapse pressure of thick-walled pipelines.(2)Further,the circumferential soil forces on the shell model and the self-contact collision treatment algorithm of subsea pipelines are developed.The cross-fault buckling failure analysis model of the subsea buried pipeline was established and used to simulate the cross-fault buckling behavior.New buckling patterns were found in the simulations,including local collapse with “8” and “U” cross-section shapes,multi-position local collapse,unidirectional buckling propagation,orthogonal buckling propagation,buckling prevention by elbows,crossover around the elbow region and so on.The effects of key parameters such as fault parameters,soil properties,pressure levels,and diameter-tothickness ratios on the critical fault displacement and failure modes were systematically analyzed.(3)Simulations of subsea buried pipeline under another two loading paths,“seismic fault → pressure” and “pressure → seismic fault → pressure”,were further carried out.The effects of loading paths and load histories on the nonlinear deformation responses of subsea buried pipelines were clarified.The residual bearing capacity,weak positions and possible failure behaviors of the pipeline after seismic faults were comprehensively analyzed.Moreover,the feasibility of the post-earthquake production resumption operation for unpressured pipelines and the safety of the post-earthquake maintenance for pressurized pipelines were quantitatively evaluated.(4)Design methods of single integral buckle arrestor and double-integral buckle arrestor were proposed for the cross-fault orthogonal buckling behavior of subsea buried pipelines.The buckling arrest performance of buried pipelines with integral buckling arrestors under seismic fault was studied.Different patterns of the local collapse,buckling propagation,buckling crossover,and buckling prevention under different diameter-tothickness ratios,lengths,and arrangement spacings of the buckle arrestors were analyzed.Through the comparison of the buckling prevention effect,material consumption and difficulty of fabrication,the advantages and disadvantages of the two buckle arrestor schemes and their applicable situations were clarified.