Nonlinear Numerical Study Of Deepwater Riser's Static/dynamic Response And Pipelaying Safety Assessment

As the important energy form in the world now,ocean oil and gas resources are playing a more and more important role in the social and economic development.And as a vital component in the marine oil and gas development system,marine riser is proved as the most economic and effective method to transport the oil and gas.With the marine oil and gas engineering constantly extending to the deepwater area,deepwater risers are faced with more and more challenges:complicated marine environment conditions,remarkable floating structure's dynamic response and soil-pipe coupling interaction and so on.In this thesis,first of all,based on the analytical analysis and numerical analysis,the basic theories of two-dimensional static analyses for the deepwater lazy wave riser and steep wave riser are proposed respectively.Reasonable numerical calculation models for the lazy wave riser and steep wave riser are developed to systematically investigate the static performance of deepwater riser and its influencing factors suffering from the hydrodynamic loading caused by the current.Then by improving the rod theory model,a three-dimensional static/dynamic response numerical calculation model,which is available for different riser layout configurations,is put forward to well study the effects of flexible pipe's bending hysteresis characteristic and bend stiffener constraint on the riser motion response.Finally,the sea area,where Zhoushan offshore floating platform "Huajiachi" is located in,is taken as the research background to collect the real marine environment data of this sea area in the field and study the motion response characteristics of the flexible pipe during the offshore pipelaying operation,and a theoretical research system for the riser laying installation safety assessment under the complicated ocean environment is systematically set up.The main research work can be summarized as the following respects:(1)Based on the different riser segment's mechanical characteristic of the deepwater lazy wave riser to conduct the segmented mechanical analysis,a two-dimensional static numerical calculation model is developed by combining the catenary method and linear beam theory,which comprehensively takes the effects of the hydrodynamic loading caused by the current and linear seabed resistance into account.Loop iteration is adopted to numerically solve the problem,which greatly improves the computing speed of the proposed model.The lazy wave riser motion's static characteristics,such as configuration,internal force and pipe's embedment into the seabed and so on,are well investigated and effectively verified by comparing with OrcaFlex finite element simulation results.(2)Based on the large deformation beam theory,a two-dimensional static mechanics analysis model and numerical method for the deepwater steep wave riser are put forward,which fully take account of the riser bending stiffness and hydrodynamic loading.And the finite difference method is employed for the numerical solution.Then according to the numerical result by the proposed model,local mechanical models of the two riser ends with the bend stiffener constraint are developed.Given this two point boundary value problem,they are numerically solved by the bvp4c built-in function in the MATLAB program.And the two proposed models are well verified by comparing with the OrcaFlex finite element simulation result.Finally,the configuration and internal force distribution of the steep wave riser are further studied and the inherent relation between the bend stiffener and riser's bending deformation is revealed.(3)A theoretical analysis model for the riser global static/dynamic response under three-dimensional complicated ocean environment is developed,which is suitable for different riser layout configurations.The theoretical model adopts Winkler foundation model and modified coulomb friction model to simulate the vertical interaction and lateral interaction between the riser and seabed respectively,and employs the plug flow model to simulate the pipe flow effect.Then two case studies about the steel catenary riser and lazy wave riser under the top harmonic heave excitation are conducted to investigate the effect law of the top harmonic heave excitation on the motion responses of the steel catenary riser and lazy wave riser.(4)By taking account of the effects of bend stiffener constraint and bending hysteretic behavior,a theoretical analysis model for the flexible riser global static/dynamic response under three-dimensional complicated ocean environment is specially built,which can more accurately predict the static/dynamic response of a flexible riser,especially for the top-end region and touchdown zone.Combined with the case study of a flexible riser under the top harmonic heave excitation,the influence rules of the bend stiffener constraint and bending hysteretic behavior on the flexible riser's global motion response are revealed and the interaction mechanism between the flexible riser and bend stiffener is also well explored.(5)The real marine environment data are collected in the field from the sea area where Zhoushan offshore floating platform "Huajiachi" is located in.By fully taking the hydrodynamic loadings caused by the wave and current into consideration and conducting the test to acquire the flexible pipe's nonlinear tension-strain and moment-curvature relationships,the calculation analyses of the flexible pipe's motion response under the offshore pipelaying operation are carried out.Finally,a perfect evaluation system for the flexible pipe offshore laying operation is established,which can provide references and suggestions for the flexible pipe laying operation.