Investigation On Buckle Propagation Mechanisms Of Deep-sea Pipelines And Arresting Performance Of Integral Buckle Arrestors

Subsea pipelines are the most primary transportation means of offshore oil-gas resources,which can be classified as single pipe,pipe-in-pipe(PIP)and sandwich pipe according to the pipe cross-section structures.With the exploration and exploitation of offshore oil-gas resources extending into deep and ultra-deep water areas,the high hydrostatic pressure in deep sea is prone to induce the local buckle instability of the pipes,and the buckle can propagate along the length and collapse the whole line,which will cause significant economic loss.Therefore,it is very necessary to carry out the research on the buckle propagation mechanisms and its arresting control of deep-sea pipelines.However,the complex structures of deep-sea pipelines,especially the PIP and the sandwich pipe,bring great difficulties to precisely characterize the buckle propagation scenarios.Besides,the buckle crossing over an integral buckle arrestor is influenced by many factors,and is closely related to the structures and the material mechanical properties of the pipe and the arrestor.The purpose of this paper is to investigate non-linear buckle propagation mechanisms of deep-sea oil-gas pipelines and arresting performances of integral buckle arrestors under the high hydrostatic pressure.Through small scale model experiments of buckle propagation and buckle crossover for deep-sea composite pipes,the numerical models are developed to simulate the buckle propagation and its crossing over an integral buckle arrestor for deep-sea pipelines under the quasi-static,external pressure.The influencing mechanisms of the structures and the material mechanical properties on the buckle propagation pressure of deep-sea pipelines and the buckle crossover pressure of the arrestors are revealed.The methods of evaluating the arresting performance and the design for integral buckle arrestors are established,which lays a solid theoretical foundation for the design of deep-sea oil-gas pipelines and the arresting control of integral buckle arrestors.The main contents of the research works can be summarized as follows:(1)Buckle propagation and buckle crossover model experiments for deep-sea composite pipes.Using independently designed hyperbaric chamber experiment devices and equipments,the small scale model tests of the buckle propagation and the buckle crossover on the deep-sea composite pipe and integral buckle arrestor specimens with different structures and material mechanical properties are carried out under the high hydrostatic pressure.The local collapse pressure and the buckle propagation pressure of the pipes,and the buckle crossover pressure of the integral buckle arrestors,as well as the change in volume of the specimens are measured,respectively.The influencing of inter-layer adhesion behaviour on the buckle propagation and the buckle crossover of sandwich pipe specimens are investigated.(2)Study on non-linear buckle propagation mechanisms of pipe-in-pipe systems.Based upon the rigid-perfectly plastic ring collapse models,the theoretical formulas for calculating the buckle propagation pressures of the single pipe,the pipe with a solid rod insert and the PIP system are derived,respectively.Using the finite element software ABAQUS,a three-dimensional numerical model is developed to simulate the initiation of the local buckle instability and the consequent steady-state buckle propagation of the PIP system under the external pressure.Through a series of parameter sensitivity analyses,four types of buckle propagation modes are identified.Furthermore,a more accurate empirical formula for the buckle propagation pressure Pps of a pipe with a solid rod insert and a more reasonable empirical formula for the buckle propagation pressure Pp2 of PIP systems are proposed,respectively.The good agreements between experimental and calculated results of the buckle propagation pressure for the PIP systems demonstrate the effectiveness and practicality of the empirical formula.(3)Study on non-linear buckle propagation mechanisms of sandwich pipes.In combination with small scale model test and numerical simulation,the influencing mechanisms of the non-linear buckle propagation of sandwich pipes under the external pressure are systematically investigated,and the influencing mechanisms of geometric dimensions and material mechanical properties of the inner and outer pipes,the thickness and material mechanical property of the core layer,as well as the inter-layer adhesion behaviour on the buckle propagation pressure and deformation configurations of sandwich pipes are revealed.The sensitivity parameters influencing the buckle propagation pressure of sandwich pipes are identified.An empirical formula is proposed to predict the buckle propagation pressure of sandwich pipes accurately.(4)On the arresting performance of integral buckle arrestors for single pipes.Dedicated finite element models are developed to simulate the initiation of the local buckle instability,the steady-state buckle propagation along the length,as well as the buckle crossing over an integral buckle arrestor into the downstream pipe under the external pressure,reproducing two types of crossover modes,i.e.,the flattening crossover and the flipping crossover.Through the numerical simulation method,the effects of geometric dimensions and material mechanical properties of the pipe and the integral buckle arrestor on the buckle crossover pressure are systematically investigated.Based on a large number of numerical simulation results,the more reasonable empirical formulas are proposed to calculate the buckle crossover pressure and the arresting efficiency,respectively.In comparison with the existing test data,it is verified that the empirical formula and the lower envelope line can be used to availably evaluate the arresting performance of the integral buckle arrestors for single pipes.(5)On the arresting performance of integral buckle arrestors for pipe-in-pipe systems.By improving the traditional integral buckle arrestor,a new type of device with higher arresting efficiency,designated as inward convex integral buckle arrestor,is put forward for the PIP systems.According to the scale model test results,dedicated three-dimensional finite element models are developed to simulate the whole process of a propagating buckle crossing over an inward convex integral arrestor under the external pressure.Through extensive parametric dependence analyses,the influences of geometric dimensions and material mechanical properties of the PIP and the arrestor on the buckle crossover pressure are systematically studied,and the potential buckle propagation and crossover modes are identified,and an empirical formula for calculating the buckle crossover pressure of the inward convex integral buckle arrestors for the PIP systems is obtained by the data fittings.The corresponding empirical formula and the lower bound envelope line for calculating the arresting efficiency are established to conveniently estimate the arresting performance of the inward convex integral buckle arrestors in the preliminary design stage.(6)On the arresting performance of integral buckle arrestors for sandwich pipes.Based on the scale model test results,finite element models are developed to simulate the scenarios of the steady-state buckle propagation and the propagating buckle crossing over an integral buckle arrestor in sandwich pipes under the external pressure.Through the numerical simulation technique,the buckle crossover mechanisms and the deformation configurations of the sandwich pipes and the integral buckle arrestors are investigated,and the influencing mechanisms of geometric dimensions and material mechanical properties of the sandwich pipes and the integral buckle arrestors,as well as the inter-layer adhesion behaviour on the buckle crossover pressure are revealed.Furthermore,an empirical formula is proposed to calculate the buckle crossover pressure of the integral buckle arrestors for the sandwich pipes.The empirical formula and the lower bound envelope line are established to estimate the arresting efficiency of the integral buckle arrestors,which provides the theoretical basis for the design of sandwich pipes and integral buckle arrestors.