Research On Vertical Thermal Buckling Of High Temperature And High Pressure Submarine Pipelines

China has a vast territory and a coastline of tens of thousands of kilometers.The sea area is vast and contains abundant marine oil and gas resources.Submarine pipelines are widely used in offshore oil and gas field production,long-distance transmission,gathering and transportation,etc.because of their high efficiency,safety,economy,stability and convenience.They are an indispensable part of offshore engineering equipments.However,when the submarine pipelines transport oil and gas of high temperature and high pressure,the large additional thermal stress will be accumulated on the pipe wall,causing the pipelines to excite the vertical thermal buckling,thus posing a potential safety hazard to the normal operation of the pipelines.In this paper,according to the vertical thermal buckling characteristics of submarine pipelines of high temperature and high pressure,the theoretical calculation and numerical simulation were used to verify each other,and the theoretical calculations,numerical simulations and control measures were systematically studied.Based on the energy method that satisfies the theory of large buckling deformation,the relationships between the buckling length L,the buckling height vm and the temperature load ?T of the ideal straight pipelines on the rigid seabed were derived.Based on the practical engineering example,the variations of the buckling displacement and buckling length of the pipelines with different friction coefficients and pipe cross-sectional areas were studied by using MATLAB software.Aiming at the shortages of rigid seabed,the analytical solutions of the vertical thermal buckling of the ideal straight pipelines on the rigid plastic seabed were deduced.The effects of different subsoil strengths and buckling directions on the vertical thermal buckling deformation were studied.Based on the explicit dynamic algorithm in ABAQUS software,the silty clay of Bohai sea and Mohr-Coulomb model were used to model and analyze the vertical thermal buckling of the submarine pipelines laid on the sunken seabed with a single initial imperfection under the working temperature of 90? and the working pressure of 20MPa.The variations of vertical displacements and stress values with the temperatures during the process of vertical thermal buckling of pipelines on the pit seabed,trench seabed,single uplift seabed and bilateral uplift seabed separately.The sensitivity of the parameters affecting the vertical thermal buckling induced by the pipelines were discussed,including the influences of diameter-thick ratio of pipeline,temperature load,pressure load,friction coefficient of pipe-soil and initial imperfection size of seabed.Combined with the control measures of the pipeline buckling commonly used in marine engineering in China,the natural backfilling method after trenching,the engineering backfilling method after trenching and the briquetting protection method were studied,and the effectivenesses of controlling vertical thermal buckling induced by pipelines were analyzed.The numerical simulations of the engineering backfilling method were carried out,and the sensitivity and influence of the backfilling parameters to the vertical thermal buckling of the pipelines were discussed,including soil density,elastic modulus,Poisson's ratio,internal friction angle and cohesion force.It is of great significance for the laying,maintenance,safety evaluation and buckling control of submarine pipelines,and provides references for the application of practical marine engineering.