Nonlinear Dynamic Investigation Of Vortex-induced Vibration For Deepwater Risers Under Complicated Marine Environment

Marine riser is one of the key equipment in deepwater exploitation,acting an important role to transport oil & gas resource from seabed well to floating production platform.As a slender flexible structure,the riser's vortex-induced vibration(VIV)occurring when current flowing across and subsequently caused fatigue damage should not be ignored.The existing numerical simulations concerning riser's VIV are generally based on some assumptions and simplifications,which to a certain extent influences their prediction accuracy.In-line VIV research is relatively later than the cross-flow one,and its prediction technique is still not mature enough.The floating platform connected to the riser's top-end will experience multi-degree-of-freedom motion response with wind,wave and current excitations,making the riser's encountered fluid field and axial tension change with time varying.However,the previous relevant research mostly had not taken the effect of platform motion excitation on riser's VIV into account.The relevant experimental investigations indicate that the in-line structural response and the time-varying axial tension as well as unsteady flow induced by platform motions all have a significant impact on the riser's fatigue damage,which should not be ignored.Therefore,improving the traditional numerical prediction models and investigating the riser's VIV in consideration of top-end platform motion excitation will significantly increase the prediction accuracy of slender structure fatigue damage under the actual marine environment.This dissertation is aimed to furtherly investigate the effect of platform motion excitations on the riser's VIV dynamic response characteristics.Based on the international latest progress of the VIV nonlinear dynamic response for deepwater risers,this dissertation investigated the time domain prediction of riser's cross-flow and in-line VIV coupled response,added mass coefficient variation effect,VIV response characteristics with time-varying tension,VIV response characteristics under unsteady flow as well as VIV fatigue damage with platform motion excitations in detail.The main research works in this doctoral thesis are introduced as follows:(1)The background and significance of the research subject are elaborated,and the basic concepts as well as relevant parameters of riser's VIV are introduced.The domestic and foreign experimental and numerical researches are summarized systematically,including riser's VIV research method overview,VIV research with time-varying tension and VIV research under steady flow.Besides,the research frontiers that need to be supplemented and improved in the relevant fields are also generalized.(2)Based on hydrodynamic force-decomposition model and forced vibration experimental data,the time domain numerical analysis procedure of riser's cross-flow and in-line VIV is developed.An available lock-in judgement and amplitude allocation criterion is proposed for the 2St and 3St excitation regions in the in-line direction.Compared with model test results,the proposed time domain method is validated well,and its advantages over the traditional frequency domain prediction tools are discussed.The variable added mass element,which could simulate how the added mass coefficient changes with structural real-time response effectively,is integrated into the numerical model.By comparing the proposed method and the traditional simplified method assuming Ca=1.0,the added mass coefficient variation effect on riser's VIV response is investigated.(3)The time domain prediction method for riser's VIV with time-varying tension is established,the structural stiffness matrix will be updated according to the real-time tension at each time step.Compared with the test results of small-scale riser model,the proposed method is validated well.With another large-scale riser model,the initial phase effect,amplitude effect and frequency effect of the time-varying tension on riser's VIV are investigated systematically.The Mathieu-type VIV resonance under the combined excitation of VIV and time-varying tension is captured.(4)The si-frequency time-varying tension is extended to a more complex excitation form of multi-frequency component superposition.Based on the prediction method established by this paper,the riser's VIV response under 32 multi-frequency cases are calculated in time domain.The structural dynamic response characteristics under four typical kinds of complicated time-varying tension cases(i.e.si-frequency & multi-amplitude case,multi-frequency & si-amplitude case,HF&SA+LF&LA case and HF&LA+LF&SA case)are investigated respectively,and the mechanism of multi-frequency parametric excitation on riser's VIV is analyzed.(5)By improving the calculation formula of excitation force,the time domain prediction method for riser's VIV under unsteady flow is established.Compared with the test results of small-scale riser model under oscillatory flow cases,the proposed numerical model is validated well.From the level of VIV mechanism,the essential reasons of the presented time-sharing response characteristics under oscillatory flow are analyzed.(6)The encountered fluid field of deepwater risers under the actual marine environment is usually very complicated,combined with steady flow and unsteady flow.The VIV response characteristics under three typical kinds of unsteady flows(including uniform combined flow,sheared-oscillatory flow and complicated combined flow)are investigated respectively.The effects of different fluid field parameters on riser's VIV response are discussed,and their mechanisms are explained reasonably with physical essence.(7)Based on three-dimensional potential flow theory and indirect time domain method,the heave and surge motion response of a tension leg platform servicing in 1500 m depth of South China Sea under one-year-return-period sea state is predicted.Platform heave excitation is simulated as multi-frequency time-varying tension,and surge excitation is equivalent to sheared-oscillatory flow field.Considering background steady flow and platform multi-degree-of-freedom motion excitations,the VIV response of the full-scale deepwater riser is simulated in time domain.A fatigue damage calculation procedure is developed with the help of rainflow counting method and S-N curve approach.Steady flow case,heave case,surge cases and combined case are analyzed respectively,and the fatigue characteristics of full-scale riser under complicated marine environment are investigated in detail.