Research On Vortex Induced Fatigue Of Unbonded Flexible Pipes On Seabed Suspension Span

Due to its unique mechanical properties,non-bonded flexible pipes are widely used in offshore oil and gas transportation.Unbonded flexible pipes laid on the seabed may have overhangs.Due to the influence of vortex-induced vibrations,the pipes may fail due to fatigue.Once the damage occurs,it will have a serious impact on the economy and the environment.Therefore,it is necessary to study the vortex-induced fatigue problem of submarine suspensionspan unbonded flexible pipes.Based on the research on the fatigue failure of deep-sea non-bonded flexible risers,this paper establishes a vortex-induced fatigue prediction method for submarine suspension-span non-bonded flexible pipes.The main tasks are as follows:The fatigue failure model of non-bonded flexible pipe is established.The theoretical model of the axisymmetric load response of the unbonded flexible pipe is established by using the energy method,and the solution method is given by combining the static balance equation of the flexible pipe and the deformation coordination relationship between the pipe layers;the current mature bending load response model is introduced,And based on the bending load response model combined with the axisymmetric load response model to establish the fatigue failure model of the unbonded flexible pipe.The local mechanical properties of a typical eight-layer non-bonded flexible pipe were studied.The theoretical and numerical simulation methods are used to calculate the axial tensile and pure bending load responses of the non-bonded flexible pipe,and the experimental results are compared to verify the validity of the theory and numerical simulation in this paper;for the flexible pipe under axisymmetric load The bending performance was explored,and the influence of the interaction between the tube layers on the bending performance was analyzed.The vortex-induced vibration of the unbonded flexible pipe overhanging section is simulated.The Fluent-Newmark method was used to simulate the two-dimensional vortexinduced vibration of the pipeline;the maximum amplitude ratio under different reduced speeds was explored,and the influence of different span lengths on the maximum vortex-induced vibration amplitude in the resonance region was studied at the same time.The calculation of fatigue stress provides strain input.The vortex-induced vibration fatigue prediction method of the suspension pipe is established.The alternating fatigue stress induced by vortex-induced vibration is calculated,the fatigue life of the tensile armor layer is calculated based on the rain flow method and the miner linear damage theory,and the sensitivity parameters of the fatigue life are analyzed.Safety design provides theoretical guidance.