Research On The Boundary Value Problems Of Subsea Free-spanning Pipeline Dynamics Subject To Internal Flow And External Marine Current

The world offshore oil and gas exploitation has now expanded from shallow water to deep and ultra-deep areas.The complicated ocean environment brings great challenges to deepwater pipeline design.Vortex-induced vibration(VIV)is one of the major causes of failures in deepwater pipelines.However,an accurate prediction of the dynamic behaviors of the pipelines often involves great amount of parametric studies,and methods such as numerical and experimental studies will cost tremendous amount of time.Therefore,it is necessary to develop an analytical solution which is accurate and time-saving.First of all,the prediction method based on the generalized integral transform technique(GITT)for the dynamic characteristics of the subsea pipeline free span subject to both internal flow and external marine current is established.A wake oscillator model is adopted to simulate the vortex-shedding effects on the pipeline system.The partial differential governing equation system is solved by the method of GITT.Convergence behaviors are analyzed,and results are verified to validate the correctness of the mathematical model and the proposed solution method.Secondly,the majority of the researches on this subject considered free-spanning pipelines to be lying on the horizontal seabed,ignoring the fact that the seabed supporting subsea pipelines is in most cases uneven.In order to make up the limitations of the mathematical model for the vibration of horizontal free-spanning pipelines,the mathematical model for the vibration of the subsea free-spanning pipeline over a slope subject to both the internal fluid and the external marine current is established.The natural frequency of the free-spanning pipeline over a slope is obtained,and how the angle of slope affects the structural natural frequency is discussed.And also,the effect of the slope angle,together with the internal and external flow velocities on the dynamic behaviors of the pipeline free span and on the lock-in region are summarized.The newly built mathematical model which considers the effect of the slope angle is compared to the existing horizontal pipeline model,and the validity of the new model is justified.Furthermore,due to the intricacy of the vibration behaviors of the subsea pipeline system and the complexity of the marine environment,it is,to a certain extent,difficult to determine the corresponding boundary conditions,which consequently make it even more complicated to solve the eigenvalue problems.The eigenvalue problems for different boundary conditions are solved,and the eigenfunctions as well as the eigenvalues are obtained.Besides,the soil spring is employed to simulate the pipe-soil interaction along the span shoulders.The natural frequency and the critical internal velocity of dynamic destabilization of the structure is obtained for different boundary conditions using GITT.The mode shapes of the pipeline system under different boundary conditions are studied,and the integrated effects of fluid parameters,seabed inclination and soil parameters on the vibration behaviors of the free span are discussed.