Nonlinear Hysteresis Characteristic Analysis Of Unbonded Flexible Pipes

Flexible pipes play a vital role during the development of marine oil and gas production because of their excellent mechanics performance.Based on the wide application of flexible pipes,this paper focused on the special mechanical properties of flexible pipes such as less bending stiffness,small friction between the layers and hysteresis response.The nonlinear hysteresis characteristics of flexible pipes have been studied,considering in the structure characteristics of unbonded flexible pipes and complex interlayer contact.Excessive linear assumptions were introduced in the analysis of the theoretical model of unbounded flexible pipes,and the nonlinear characteristics such as complex contact,friction and slip between layers were rarely considered in numerical simulation analysis.A reasonable and efficient finite element model of unbounded flexible pipes was established firstly by the finite element analysis software ABAQUS for the analysis of structure response.Quasi-static analysis method was applied for the analysis of structural response in stretch,torsion and bending.Based on the nonlinear analysis of load-displacement curve of three kinds of conditions,compared with other models and experimental data,the proposed numerical model was proved to be effective and accurate.Flexible pipes are in combination conditions in practical engineering application,so the change of the mechanical properties of the structure because of combined load cases was analyzed.This paper also analyzed the coupling effect due to combined load cases and the structural characteristics of hysteresis curve at the same time.In addition,on the basis of the work above,the influence of different types of construction of the spiral layer on tensile and torsional stiffness were analyzed.This paper also made sensitivity analysis of nonlinear hysteresis curve of flexible pipes from the point of view of energy dissipation.Stiffness degradation of hysteresis curve under cycle loading was analyzed.Results show that the model established with anisotropic materials equivalent layer and on the basis of considering interlayer sliding and friction is reasonable,effective and accurate,and can be used as a reasonable model of flexible pipes for structure response analysis too.The friction coefficient and the pressure between the layers have a great impact on the hysteresis curve of structure response under load sand cannot be ignored,and the stiffness in hysteresis curve obviously changed in the partial slip phase under cyclic loading.