Research On Key Mechanical Problems Of The Pipe During The Reel-lay Process In Deep Water

With the depletion of oil and gas resources in shallow sea day by day, exploitation of marine oil and gas begins to go forward deep sea from shallow sea gradually. However, the transportation of marine oil and gas mainly depends on subsea pipelines, so it is a challenging task laying the pipeline to seabed from land safely. Currently, as one of the main pipelaying methods, Reel-lay method is widely used all over the world for its high efficiency and low cost. But, the Reel-lay process will induce several bending deformation of the pipe which creats large plastic deformation in pipe at the same time. After unloading, large residual ovality and residual stress remain in the pipe, and thus reduce the critical buckling pressure and fatigue life of the pipe seriously. So, it is important to study the plastic deformation process and the reduction of pipe's critical buckling pressure as well as fatigue life.Firstly, FE models according with actual working conditions of the Reel-lay process are built using finite element analysis software ABAQUS, and the dynamic simulation analysis of the pipe is done during reel-lay process. Then, the axial stress and strain history and stress-strain relationship as well as history of the pipe's bending-curvature and ovality are discussed, and the accuracy of FE models is verified.Secondly, numerical simulations for many different working conditions are carried out and then parameter sensitivity analysis is done. And the effects of different outer diameter, diameter to thickness ratio and steel grade on the ovality damage of the pipe are researched.Thirdly, the pipe with ovality damage induced by pipelaying is regarded as the initial model and nonlinear buckling analysis is carried out to the pipe under external pressure. Then, the critical buckling pressure of the pipe with residual ovality is calculated and compared with that of the perfect pipe.Finally, the pipe with residual stress induced by pipelaying is regarded as the free spanning submarine pipeline and stress history of the pipe is calculated during the period of vortex-induced vibration. Then the pipe's stress amplitude and mean stress are obtained. So the fatigue time of the free spanning submarine pipeline can be calculated based on linear cumulative damage theory and the modified S-N curve. Last, research on reduction of the pipe's fatigue time due to residual stress is discussed.