FEA Of Stiffness And Bearing Capacity Of The Carcass Layer Of Unbonded Flexible Pipeline

With the depletion of land oil and gas resources,China began to vigorously develop the exploration and exploitation of marine oil and gas resources.As one of the most important equipment of offshore oil and gas exploitation,marine unbonded flexible pipeline plays the role of oil and gas medium transportation.In the actual working condition,due to factors such as the motion of floating platform,the action of sea wind and wave and the dead weight of pipeline,the unbonded flexible pipeline bears various loads such as axial tension and compression,torque and bending moment;in the deep sea area,the unbonded flexible pipeline also bears higher external pressure.This requires the stiffness and load resistance of unbonded flexible pipes.In order to study the response of unbonded flexible pipe to various loads,many researches have been done in the past.In the finite element method,due to the complexity of pipeline structure and the limitation of computer processing ability,the pipeline model is often equivalent to a simplified model.This simplified model not only can not reflect the stress distribution of the structure,but also has limitations in the analysis of deformation.Taking the innermost carcass layer of pipeline as an example,the friction factor between steel strips cannot be considered in the simplified model,and some even ignore the axial self-locking of the carcass layer,and assume that the axial elastic modulus is zero.In addition,in the wet collapse finite element analysis,which the external pressure is mainly borne by the carcass layer,previous studies only determine the critical collapse value by analyzing change trend of the radial displacement of the carcass layer with the external pressure,but this method could not find the critical point accurately.In order to make up for the above defects,a three-dimensional finite element model of the carcass layer of unbonded flexible pipe is established by solid element.Considering the factors of self-locking and friction between the steel strips,the axial tension and compression stiffness,torsional stiffness and bending stiffness of the pipeline of the carcass layer are calculated,and the effects of laying angle of steel strips in the skeleton layer,friction coefficient between steel strips and thickness diameter ratio of the pipeline on various stiffness are studied.By studying the stress distribution of the FE model under axial force,torque and bending moment,the ultimate loads of the carcass layer are calculated,and the causes of stress concentration in the steel strip under various loads are found,and the structural size of the steel strip cross section is optimized.In the aspect of radial load on the carcass layer,two different finite element models of the pipeline under external pressure are established.A new method for judging the critical collapse value of the carcass layer is proposed,and the results are compared with the theoretical results,which shows the rationality of the new method.In addition,the effects of friction coefficient between steel strips,initial ovality of pipeline,thickness diameter ratio of pipeline,length of pipeline and laying angle of steel strip on radial bearing capacity are analyzed.Suggestions are provided to improve the critical collapse value of the carcass layer.This paper has a certain significance for the study of mechanical properties of the carcass layer of unbonded flexible pipeline.