Structural Characteristics And Fatigue Failure Analysis Of Dynamic Flexible Pipes Armor Layer

Flexible pipes,as important equipment for deep-sea oil and gas resources transportation,are widely used in offshore oil and gas field development.Flexible pipes are in the dynamic marine environment with complex wave and current,and the armor layer bears the important effects of tensile,compressive,bending and torsional loads,and is prone to structural fatigue.Therefore,it is very important to carry out the structural characteristics and fatigue analysis of the dynamic flexible pipes armor layer.In this paper,the research progress on the structural characteristics and fatigue failure of the dynamic flexible pipes armor layer at home and abroad is firstly investigated,and the structural characteristics and fatigue failure of the tensile armor layer and the pressure armor layer at the end fitting are theoretically analyzed.Studies have shown that the residual stress of the tensile armor layer at the end fitting during processing and manufacturing is easy to cause fatigue failure,and the fretting wear between the local contact areas between the adjacent steel bands of the pressure armor layer is also the main reason for fatigue failure.The finite element model of the tensile armor layer structure at the end fitting is established.The influence of factory acceptance test(FAT)load,epoxy resin material characteristics and friction coefficient on the stress distribution of steel wire in the bending area is analyzed,and the parameter sensitivity analysis is carried out.Studies have shown that the material properties of epoxy resin and friction coefficient have little effect on the stress state of the armor layer,while the FAT load has relatively large effect on the stress state of the armor layer,so it needs to be considered seriously.The finite element model of the pressure armor layer structure is established.The influence of initial displacement,friction coefficient and internal pressure on the compressive stress,normal stress and shear stress of the local contact area of the pressure armor layer structure is analyzed by using the theoretical analysis of cylindrical-plane contact.The research shows that the larger the initial displacement in the coarse sliding stage is,the greater the influence of friction coefficient and internal pressure on the contact compressive stress,normal stress and shear stress is,especially on the shear stress.When the internal pressure is low and the initial displacement is large,the shear stress is large,and the fatigue failure is easy to occur at this time.According to the finite element analysis results of tensile and pressure armor layer,the fatigue failure analysis of tensile armor layer at the end fitting and pressure armor layer is carried out,and the prediction method of fatigue life of dynamic flexible pipes is given.Studies have shown that the traditional tensile armor layer at the end fitting is prone to fatigue failure at the bending point.It is suggested that bending should be avoided in structural optimization design to improve its fatigue resistance.