Bending Response Analysis Of Flexible Riser With Built-in Defects

Compared with the traditional rigid vertical risers,the unbonded flexible riser is more and more widely used in offshore oil and gas engineering because it can maintain large axial loading and highly deformable in bending at the same time.The flexible riser is composed of multiple layers which are used to bearing different loads.Tensile layer is an important part of riser which is used to bear axial load.Because of the harsh environment of the flexible riser,the tensile tendons inside the risers are easy to breakage.Such risers with broken tensile tendons were called "built-in defective risers".In the actual engineering environment,if the armor layer in the defective riser is only slightly damaged,there is no need to replace the defective riser if the armor layer is seriously damaged It is necessary to replace the riser as soon as possible.Therefore,it is necessary to regularly evaluate the integrity of the tensile layer,and judge whether to replace the riser according to the evaluation result.The damage of tensile tendons will be reflected in the bending response behavior of the riser.Therefore,the bending response behavior of the riser with built-in defective can be studied and the integrity of tensile layer in the riser can be analyzed.Therefore,it is of great engineering significance to study the bending mechanism of the built-in defective riser.In this paper,the bending response mechanism of built-in defective risers in two different layouts is discussed and the bending response model in different layouts is proposed for the integrity analysis of tensile armor of built-in defective risers.The main contents and contributions of this thesis can be summarized as follows:Firstly,based on the theory of differential geometry,the paper studied the local bending deformation and the axial tension deformation of the tensile layers,and establishes the bending response model of the intact riser.The bending stiffness formulae of the intact flexible riser at different stages are derived by energy method.Secondly,for the two common types of riser layout,the built-in defect factors are imported,and the bending response model of the built-in defect riser under different layouts is established.By using differential geometry and energy method,the bending response model and bending stiffness formula of the built-in defective riser in different layouts are presented.Finally,a parametric analysis of the built-in defect riser was conducted to study its bending response behavior.And a finite element model in the finite element analysis software ABAQUS will be made to compare the finite element analysis result with the theoretical model analysis result,verify the correctness and reliability of the built-in defect riser response model.Through comparison,it is found that the tensile force on the riser will affect the critical slip curvature of the tensile layer,so that the bending of the riser exhibits obvious nonlinear characteristics,and the nonlinear characteristics of the built-in defects risers are more obvious than the intact riser,the bending response curve shows a "three-stage" trend.The existence of broken tendons in the float section riser will affect the bending stiffness of the riser.The magnitude of the influence is related to the angle of the fractured strip at the end of the buoyancy module,and the influence of the defect factor on the float section bending rigidity will vary with the buoyancy module interval.Increase and decrease.