Ultimate Capacity Study For Unbonded Flexible Pipes Under Pressures

As the demand for energy increases,the exploitation of oil and gas resources has gradually expanded from the land and shallow water into the deep water.The risers and submarine pipelines as the important engineering equipment is facing various new challenges.Due to its unique structure and its material characteristics,the unbonded flexible pipe(Note:hereinafter referred to as unbonded pipe)has large tensile bearing capacity,resistance to internal/external pressure and small bending stiffness.Therefore,the unbonded pipe becomes a very good choice since it has better corrosion resistance and bending compliance compared to traditional steel pipe.However,it is very essential to control the failure modes of unbonded pipes in deep and ultra-deep water with complex environmental conditions.Once the failure occurs,it may cause a series of engineering accidents resulting in immeasurable economic losses and environmental pollution.Therefore,the in-depth study for the ultimate capacity of the unbonded pipe is of great needs in order to design unbonded pipe effectively.This paper focuses on the mechanical properties of unbonded pipe section under pressures,especially the collapse failure of unbonded pipe under the external pressure and the burst failure caused by the internal pressure.For the collapse failure,the main study is the wet collapse mode of the inner layer resulted by the external pressure applied on the outer wall of the inner layer when a leakage exists to the annulus of the unbonded pipe.This issue is simplified to the confined buckling of the inner layer of unbonded pipe.For the burst failure,a three-dimensional model considering the original cross section of pressure armor layer is adopted to study the factors that affect the burst pressure.Specifically,the main contents of this paper include:(1)Studies on the instability theory of circular pipe with elasticity confinement under external pressure.The instability theory of the pipe under external pressure is analyzed and summarized.The spring constraints are introduced to simulate the outer layer confinement,and the instability theory formula of the pipe is derived from the principle of resident potential energy.(2)Numerical studies on the confined inner-layer collapse of double-layer elastic unbonded pipe.The general finite element program ABAQUS is adopted to construct two-dimensional plane models.The influence of the ratio of inner and outer layer bending stiffness on the buckling pressures are studied using Riks method.Instability modes of confined pipes and the effect of different diameter to thickness ratios are summarized.Numerical analysis results are fitted to the calculation formula and compared using the analytical method.Meanwhile,a three-dimensional spiral model of inner layer is established in terms of the minimum defect value.The buckling pressures of inner spiral layer under different bending stiffness ratios are calculated using dynamic implicit algorithm.Instability modes of confined pipes and the effect of different diameter to thickness ratios are concluded as well.The design formula is proposed.(3)Studies on the confined inner-layer collapse parameters of elastic unbonded pipes.The influence of interlayer friction,inner layer non-roundness,interlayer gap and uneven thickness of inner layer on buckling pressure is investigated by dynamic implicit algorithm.A middle layer is added to the two-layer plane model in order to simulate the inner sheath between the carcass and the armor layer.The influence of the inner sheath on the confined collapse is studied.(4)Numerical studies on the confined elastic-plastic inner-layer collapse of double-layer unbonded pipe.Considering the ideal elastic-plastic and bilinear hardening material used in the inner layer,the effects of the yield strength and the hardening rate on the collapse capacity of the inner layer are investigated.The elastic-plastic empirical formulae are fitted by combining the confined elastic collapse calculation formulae.(5)Collapse experimental study of unbonded pipes.Intact collapse experiments and confined collapse experiments with the broken outer sheath are carried out using two types of steel strip reinforced unbonded pipes.The results are compared with the finite element analysis and the calculation formulae to verify the correctness of the formulae.(6)Burst pressure study of the unbonded pipe uner internal pressure.The nonlinearity of the material is considered in the theoretical analysis and the related results are compared with the results from the refined finite element model established by ABAQUS.In addition,the influence of the tension on the burst pressure is analyzed as well.