Analysis On Mechanical Performance For Deepwater Pipe-laying

Submarine pipeline is an important assembly part of oil and gas exploitation projects in the deep offshore waters. Pipes inevitably encounter unpredictable risk due to the severe ocean environment, the high ambient pressure makes such structures collapse critical, they are particularly vulnerable during the installation process when in addition to the external pressure, they must sustain bending and tension loads. The work presented in this paper is motivated by the design needs of structures such as pipelines and risers, therefore it is very necessary to address the study on mechanical performance of pipes in the pipe-laying process. In general there are three kinds of pipe-laying techniques can be used in the deepwater projects, which are respectively S-Lay, J-Lay and reel-lay methods. The current state-of-arts and characteristics of the three methods are detailedly introduced, the special studies are focused on the deepwater S-lay method, as the pipeline is laid into the sea off a special lay vessel, the configuration of suspended section induces bending which is usually highest in the sagbend region close to the seafloor, where the hydrostatic pressure also is very large.Several analytical procedures are developed to solve the buckling and stress state of pipes. According to numerical analysis results, some design advices and installation techniques for deepwater pipe-laying are presented, which not only provide the theoretical basis and reference standards, but also increase the security of the exploitation projects. The major contents of the article are summarized as follows:(1) A software mainly used in reel lay method is developed to calculate the mechanical behavior of the pipes under various loads such as pure bending moment, combined tension and bending moment, combined press and bending moment, and radial pressure through the analytically nonlinear solution. It is based on the theory of material mechanics, and contains two constitutive assumptions, which are respectively elastic-perfectly plastic and linear hardening. The output of calculating results is in the forms of intuitional graphics and documents.(2) A suitable formulation of the problem based on the assumption of uniform deformation along the tube length and symmetrical buckle is established to predict the nonlinear response and elliptical collapse of steel tubes for different diameter to thickness (D/t) ratios under pure bending or combined bending and external pressure. In this formulation, the displacement-strain response is deduced from the nonliner ring theory, the constitutive relationship is represented by the Ramberg-Osgood representation, J₂ flow theory of plastic is used to model the inelastic material behavior, and the principle of virtual work is used to satisfy equilibrium. The analysis results show that the response of long tubes in pure bending is characterized by a limit load type of instability, the limit moment is directly related to the ovalization experienced by the tube cross section, and the presence of initial curvature leads to lower limit external pressure.(3) A series of FEM pipe models are established to simulate the pure bending experiment. Through numerical solution, the stress and buckling rules of different D/t are obtained. And the analysis results of FEM are taken to compare with the ones from the other two calculating procedures mentioned above.(4) The use of buckle arrestor is an effective and economical measure to restrict the buckle propagating along the pipelines. Four types of buckle arrestors are described in detail, in which the integral arrestor is widely believed to be most efficient to prevent buckle propagating and crossover during pipe-laying operation. Based on existing experiment results, the performance and technical specification of integral arrestor is examined, the corresponding parameters effecting on the efficiency of integral arrestor are analyzed, and an empirical equation is put forth to design the integral buckle arrestors for deepwater offshore pipeline in this context.This study is partly sponsored by the special subject of the National High Technology Research and Development Program of China ("863" program) with its title as "Technique of deepwater pipe-laying". And it is an important part of the subject titled as "Investigation on technological design and computational analysis for deepwater pipe-laying" (Grant No. 2006AA09A105-1).