On Internal Pressure Capacity Of Thermoplastic Composite Pipes

With the gradual depletion of onshore crude oil,the environment of oil exploration and production is gradually changing from shallow sea to deep water.Composite materials have become a good substitute for carbon steel due to their inherent advantages such as corrosion resistance,fatigue resistance,light weight and good thermal insulation performance.In order to resist the high pressure and high temperature operating environment,thermoplastic composite pipe(TCP)has become a new development direction of offshore oil and gas transportation pipeline.TCP has multi-layer complex structure and anisotropic material characteristics,and is used in a more complex environment,so its mechanical behavior analysis and failure prediction are much more complex than ordinary steel pipes.This paper investigates the mechanical properties of TCP under internal pressure and varying temperature.This paper mainly analyzes and predicts the mechanical behavior of TCP when it is subjected to the internal pressure during transportation.At the same time,it considers the influence of the temperature change inside and outside the pipe and the non-linearity of the material in the working environment.Based on the three-dimensional anisotropic elastic theory,the mechanical model of TCP is established.Combined with the elastoplastic constitutive equation of the material,the nonlinearity of the material is further considered.A variety of macro failure criteria were selected to predict the internal pressure capacity of TCP with 55°winding angle,and the predicted results were compared with the finite element simulation results and experimental results to verify the reliability of the model and the applicability of the failure criteria.Further explore mechanical behavior under thermal loading,through thermodynamics steady state heat conduction equation and the first and the third type of boundary conditions to determine the temperature distribution through the thickness,and substitute it into the mechanics model to solve the stress and strain of each part for TCPs under different temperature conditions,and consider the influence of material stiffness degradation caused by the temperature on the pressure capacity in the pipes.The results show that the analytical results with the three kinds of theoretical models proposed in this study are in good agreement with which of the finite element analysis.The thermodynamic coupling model considers the non-uniform distribution of temperature through the thickness,and the corresponding analytical solutions are derived,which fill in the gaps in the theoretical models of this kind of problem.Both the winding angle and the number of reinforcement layers have a significant effect on the internal pressure capacity of the TCP,and when the nonlinear behavior of isotropic layers is introduced the predicted burst pressure are more consistent with the experimental results.For a 55° anti-symmetrically laminated TCP,the fiber failure criterion is more accurate in predicting the internal pressure capacity.On the one hand,the increase of the temperature of the medium in the TCP causes the thermal stress to reduce the bearing capacity of the pipe,and on the other hand,causes the stiffness degradation of the material to further weaken the resistance for each direction of the TCP.