Vortex-induced Vibration Of A Steel Catenary Riser Induced By Platform Heave Motion

With the progress of the deep-water oil and gas production,the flexible slender structures used to connect the floating structures and the seafloor are getting longer and therefore becoming more flexible.The hydrodynamic responses of these slender structures,such as risers,are extremely complex owing to the coupling effects of the wind,wave,current and platform motion.Currently,the study on the flexible risers can be divided into two independent categories:1)riser global dynamic response analysis neglecting the VIV effect;2)VIV analysis considering steady ocean current.However,to the compliant risers?take SCR for example?,there would be equivalent oscillating current around the riser under platform motion even in still water.Such equivalent oscillating current would also lead to alternating vortex shedding,and therefore it's possible to induce out-of-plane vibration of the SCR.However,there are no conclusive answers to the question that whether platform motion-induced VIV is possible?This dissertation is fundamentally based on the oscillating current induced VIV and large-scale SCR VIV model tests and try to addressthe question that whether platform motion-induced VIV is possible?The intrinsic relationships among platform motion,global dynamic response of the SCR,the oscillating current around the riser and the VIV characteristics are also uncovered by the integration of theoretic analysis and model test.Combined with the typical frequency-domain VIV analysis,a time-domain VIV algorithm would be developed which is capable of considering the“time-varying”feature?different VIV response may happen at different time window?under oscillating current.Generally speaking,the dissertation includes the following 7 parts:1)Recent research status on riser global dynamic responses,steady current-induced VIV and platform motion-induced VIV are introduced;2)To investigate“platform motion-induced VIV”,SCR model test under platform motion?SCR test?and flexible riser VIV model test under oscillating current?forced oscillation test?are designed and performed;3)Systematic data analysis theory,methodology and case study are introduced including six aspects:data pre-analysis,frequency analysis,time-frequency analysis and fatigue damage estimation;4)Experimental results from forced oscillation test are discussed,the“time-varying”and“hysteresis”features of VIV in oscillating current are revealled.It further studies how VIV is affected by the KC number and maximum reduced velocityU_Rmax.Results indicate that the VIV strain,displacement,dominant frequency and fatigue damage would increase with the increasing U_Rmax?within 4⁸ based on the current experimental results?.Besides,these VIV features seems to be independent on KC number when KC number is sufficiently large?above 50 based on the current experimental results?.However,owing to the“small KC number effect”,the VIV response would be more stable in amplitude and its fatigue damage would be larger considering the absence of“amplitude modulation”.What's more,compared to VIV in steady current,VIV in oscillating current has a larger maximum strain and displacement but a slightly smaller RMS strain and displacement.And there is a distinct“time-varying”feature for VIV response frequency in oscillating current compared to the relatively constant dominant frequency for VIV in steady current.Fatigue damage caused by VIV in oscillating current is slightly smaller than that in steady current,but remains in the same damage level.5)Platform motion-induced VIV from SCR test is performed,platform motion-induced VIV is quantitatively confirmed to be fundamentally the same problem as VIV in oscillating current based on the dominant frequency analysis.It further reveals the“time-varying”features for platform motion-induced VIV including“amplitude modulation”and“time-varying response frequency”,which are also discovered for VIV in oscillating current.Besides,three factors are found to have affected platform motion-induced VIV:1)space and time-varying SCR in-plane equivalent current by platform motion;2)axial tension variation;3)touch down point?TDP?variation.What's more,platform motion-induced VIV is found to have the similar response amplitude but broader response frequency range compared to that in steady current.And fatigue damage caused by platform motion-induced VIV is at the same level as that caused by VIV in steady current and that caused by global riser motion.6)VIV developing process for VIV in oscillating current is proposed including three stages:“building up”,“locking-in”and“dying out”.Maximum equivalent current velocity V_nmax and maximum KC number KC_max are found to be the two major controlling parameter for platform motion-induced VIV:the VIV features are determined only by V_nmax when KC_max is sufficiently large?at least larger than 39 based on the current experimental results?,but VIV features are determined by both V_nmax and KC_max when KC_max is smaller.Most importantly,the response frequency model is proposed for platform motion-induced VIV,which indicates that the dominant frequencyf_{do mi},V_nmax and KC_max satisfy the strouhal relationship with St=0.14.7)Based on the VIV semi-empirical model,two modification methods were proposed to predict platform motion-induced VIV:“maximum current profile method”and“time-varying current profile method”.Results indicate that the“time-varying current profile method”has a better approach to the VIV prediction,it divided the platform motion period into several quasi-static time windows,where instantaneous equivalent current profile,instantaneous axial tension distribution and instantaneous TDP location can be considered to address the“time-varying”feature for platform motion-induced VIV.Result comparison also suggests that the time windows should be reduced for the small KC_max cases.This dissertation is supposed to reveal the VIV features and its mechanisms of the SCR under platform motion.It may also lay a solid foundation for the future VIV research of the slender flexible structure under oscillating current.Finally,this dissertation shall provide a fresh idea to the better understanding of the dynamic characteristics for the SCR and the improvement of the vulnerabilities of the riser systems.