Investigation On Asymmetric Buckle And Collapse Of Offshore Pipeline Under Complicated Load Combinations

In recent years, the worldwide exploitation and production of offshore oil-gas resources has expanded into deepwater and ultra-deepwater regions to meet the increasing demands of human for hydrocarbons. The severe marine environment in deep waters will result in the unforeseeable risks in the pipe-laying process. During deepwater installation, offshore pipelines have to withstand combined axial tension, bending and ambient external pressure. Under complicated load combinations, local buckle is prone to occur in offshore pipelines due to excessive bending deformation above the stinger or nearby the touch-down point in the process of deepwater installation. Therefore, in association with practical pipe-laying process in deep water, the current researches focusing on the asymmetric buckle and collapse mechanisms of the pipe under complicated load combinations have an important practical significance.Based upon extensive studies into mechanical characteristics of offshore pipeline during deepwater installation, the asymmetric buckle response of the pipes with arbitrary initial geometric imperfection under complicated load combinations was carried out systemically.The research work presented in this paper can be summarized into three aspects listed as follows.(1) Taking into account the mechanical characteristics of offshore pipeline during deepwater installation, a theoretical model on the asymmetric buckle of the pipe was proposed. This model was based on the non-linear ring theory, the elastic-plastic material constitutive relations, and the principle of virtual work as well as Newton-Raphson method, which can successfully account for the influencing factors such as axial strain, circumferential deformation, initial geometric imperfection, complicated load combinations and paths, etc on the buckle and collapse of offshore pipelines.(2) A three-dimensional numerical model of the pipeline under axial tension and bending as well as combined axial tension, bending and hydrostatic pressure was established within the framework of ABAQUS, the mechanical behavior and the corresponding buckling responses of the pipes under aforementioned load combinations were respectively simulated. Excellent agreements between theoretical and numerical results well validate the feasibility and the accuracy of this theoretical model.(3) Base upon the buckle theoretical model, the parametric analysis on some paramount important factors such as different loading paths, load combinations, diameter-to-thickness ratio, initial geometric imperfections and material properties, were carried out, the influencing mechanisms of the buckle and collapse of the pipes were discussed in detail. Based upon these parametric studies, a few significant conclusions are drawn, which aim to provide a solid theoretical basis for design and installation of offshore pipelines...