Study Of Buckling Behaviors In Deepwater Unbonded Flexible Pipe

Unbonded flexible pipe is widely used in offshore engineering projects recently with lots of advantages,such as low installed cost,excellent performance in harsh environment,flexibility to align with subsea structures and low weight.It will be widely applied in the South China Sea also.However,flexible pipe is a complex multi-layer structure including lots of contact and friction interaction between layers,which make numerical simulation of flexible pipe diffcult.During installation and pipe-laying,deepwater unbonded flexible pipe may undertake axial force due to external pressure and bending cycling moment,especially in touch down point or overbend zone.The condition includes several types of loads,and the bend cycling is always changing,which may cause buckling failure of tensile armor layers in flexible pipe.The buckling failure includes two modes: radial buckling mode(bird-caging)and lateral buckling failure,which will result in unacceptable economic damage.Traditional method to solve this problem in industry companies is full-scale experiment,but the cost of experiments is extremely high.And the numerical study about local buckling of tensile armor layer in flexible pipe is not mature and perfect.Therefore,it is necessary to study the buckling behavior systematically to give reference to design of cross-section of flexible pipe.This paper applied curved beam method and loxodromic/geodsic assuptions to study analytical solutions of tensile armor wires' structural responses.A piece of code was written to solve the equations and a FEM(Finite Element Method)model was created to compare with analytical results.The results suggested that the method was acceptable in structural analysis of flexible pipe tensile armor wires.Then stiffness matrix of curved beam element based on loxodromic assumption and relative displacements was built.This paper applied the curved beam element and used equivalent helical wires to represent the whole tensile armor layers.Hybrid Mixed formulations and Coulomb friction model were used to model contact and friction interation between layers.The flexible pipes under axial symmetrical load were analyzed.Meanwhile,an Euler equivalent beam FEM model was created and the dummy surface was applied to simulate the contact and friction between layers.Sensitivity analyses,initial imperfection analyses and buckling modes were presented in the paper.The results of curved beam element model were compared with ABAQUS Euler beam FEM model.Unbonded flexible pipe need undertake not only axial compressive force and external pressure but also non-symmetrical load as bending moment in harsh environment.And the former models including equivalent wires cannot be applied in non-symmetrical load at all,which will increase the difficulties and time consumption greatly.In order to analyze flexible pipe buckling behaviors with bend,a new curved beam FEM model was created.Sensitivity analyses and buckling critical loads were studied.The results are compared with analytical results based on other researchers' reference to analyze bend moment's effect.This paper built a curved beam element based on the curved beam FEM to simulate flexible pipe under long term bending cycling and axial compressive load.The buckling behaviors including structural responses and twist degrees during buckling had been studied.Moreover,an alternative model using Euler beam,equivalent shell layer and ABAQUS software was presented as well.The two models were compared with experimental results in Denmark to study bend cycling's effect on buckling of flexible pipe.This paper summarized all results of these models to study the main factors of buckling behaviors.The results indicate the computation effciency of curve beam method is higher than the one of Euler beam FE model.A fast and accurate curved beam element model can be created to study buckling behaviors of unbonded flexible pipe.It will give references to design of flexible pipe cross-section and make contributions to manufactor and design of flexible pipes in China.