Research On Tensile Fatigue Performance Of Fiber Reinforced Thermoplastic Pipe(FRTP)

With the development of offshore oil and gas resources from shallow sea to deep sea,there are new requirements for riser equipment in the field of offshore oil and gas development.Compared with the traditional steel risers,the flexible risers made of fiber reinforced thermoplastic composites have the advantages of smaller bending stiffness,lighter self weight and good corrosion resistance,which have broad application prospects in the field of marine engineering.Under cyclic loading,the marine riser structure is prone to fatigue failure,resulting in marine engineering accidents.In view of the above problems,this paper studies the Tensile fatigue performance of fiber reinforced thermoplastic composite pipe(FRTP).This paper takes FRTP as the research object,carries out the pull-pull uniaxial fatigue test of FRTP,obtains the S-N curve,and determines the stiffness attenuation law based on the residual stiffness theory.The three-dimensional finite element model of FRTP was established by using ABAQUS finite element software.The secondary development of ABAQUS was carried out by using UMAT subroutine written in FORTRAN language.The fatigue process of FRTP was simulated.The results of finite element model and experimental model were compared.The fatigue damage analysis and fatigue life prediction of FRTP were carried out.The nonlinear finite element software orcaflex is used to establish the FRTP marine flexible riser model.The time domain analysis of the riser under the combined action of wave,current and upper floating platform motion is carried out.The influence of wave,current action direction,fluid density in the riser and seabed soil type on the riser is analyzed.The fatigue performance of the FRTP flexible riser is studied in the fatigue analysis module.The results of FRTP fatigue test show that the stiffness of the pipeline decreases gradually with the increase of cycle cycle of fatigue load.In the early fatigue life,the higher the fatigue load stress level,the faster the attenuation of the pipe stiffness.The simulation results of FRTP based on ABAQUS are in good agreement with the experimental results.The stiffness attenuation of FRTP in the early stage of fatigue life is mainly caused by the damage of the matrix of FRTP reinforcement layer.In the middle of fatigue life,for a long period of time,the fiber in the reinforcement layer is not damaged,the FRTP stiffness attenuation is not obvious,and it is in a flat state.At the later stage of fatigue life,the fiber in reinforcement layer is damaged,the stiffness of FRTP decreases sharply,and the outer protective layer and inner liner of FRTP are damaged,which eventually leads to the fatigue failure of FRTP.Orcaflex is used to analyze the FRTP flexible riser.It is found that the maximum effective stress and tension of the riser occur at the suspension point,and the maximum bending moment and curvature of the riser occur at the touchdown point.Under the action of 180° wave and current,the maximum effective tension and maximum stress of the riser are the largest,and the maximum bending moment and maximum curvature are the smallest.The fluid density in the riser has little effect on the bending moment of the riser,but the greater the fluid density in the riser,the greater the maximum effective tension,penetration depth and soil resistance of the riser.The greater the undrained shear strength of seabed soil is,the smaller the penetration depth of riser in soil is,and the greater the soil resistance is.The fatigue damage of riser at suspension point and touchdown point is serious,and its fatigue life value is small,but it meets the engineering design requirements.