Research On Curvature Featured Dynamics Of Subsea Risers Subject To Internal Flow And External Current

With the increasing depth of deepwater oil and gas resources development,the subsea riser system meets more and more challenges for the complex operation phases,including the curved centerline,internal and external flow,moving boundary,its complex operation dynamics have been highly concerned by the reaserchers and engineers.The subsea riser can be classified by the shapes in the marine current with different centerline variable curvatures,the riser possesses highly nonlinear charateristics in operation phase.The subsea riser is influenced by the ship motion,marine current,seabed soil,internal flow and other factors in operation phase,so the theoretical study and numerial analysis of riser are complicated and difficult.Therefore,the suitable and efficient nonlinear mechanics models for riser with curvature at operation phase are of great sighnidicance.On the background of riser at operation phase,this thesis forcuses on several aspects,such as,riser dynamic model with curvature,dynamic response,influence factors on configuration,internal and external flow,effective numerical methods and solutions.Some achievements are acquired by current research from theory and practice,and conclusions are benefical and helpful to the riser dynamic analysis and engeering applications.The following contents and achievements are included:1.A riser dynamic model about curvature based on the Euler-Bernoulli beam theory considering the effect of marine current amd internal flow with initial conditions and boundary conditions is proposed.Generalized Internal Transform Technique(GITT)is employed to solve the partial differential equations,and highly applicable ofr soving nonlinear problems with its advantages of excellent convergence,effective accuracy control and high computational efficiency.2.On the base of two-phase flow model and two-phase flow damping model,a vertical riser dynamic eaquation with combined damping model using GITT solution is presented.Parametric studies are performed to investigate the effects of damping ratios on the dynamic behavior of the pipe.Besides,the dimensionless critical frequency of the pipe at which instability sets in has been predicted both in the real and imaginary part.Finally,a dimensionless structural parameter?₀ that can govern the normalized stability envelope of the pipe is obtained.3.A 3-D dynamic model of riser with veriable curvature induced by internal flow and marine current is obtained,the riser with veriable curvature can be simplified as a Steel Catenary Riser(SCR)with a curved centerline configuration.The GITT sulotion is employed to solve this kind of nonlinear partical differentical equations.9 typical SCR examples are analyzed to obtain the configuration and mechanical parameters along the entire riser.Comprehensive parametric analysis is carried out to investigate the influences of different curvature models,internal flow velocities and marine current effect on the dynamic behaviors of subsea riser.