Vortex-induced Vibration And Fatigue Analysis Of Deepwater Risers Considering Parametric Excitations

In recent years, owing to the large demand of crude oil, offshore oil and gasexplorations have been moved to deeper water regions. Risers are one of the basicelements of offshore installations, which are used for drilling and production. Theriser is installed between wellhead at the sea bed and floating platform. Actually, riseris exposed to the strong influences of environmental conditions in unshielded deepwaters, which has complicated behavior under the influence of floating platform’smotions, wind, wave and sea current. In order to assess safety and reliability ofdeep-water risers, investigation of the dynamic behavior of riser under complicatedocean environment is important.The heave of floating platform induces a fluctuation in time of the axial tensionof the riser. A possible and undesirable phenomenon is the parametric excitation of atransverse vibration caused by this fluctuation. The aim of this study is to predictvortex-induced vibration (VIV) response and fatigue damage of deepwatertop-tensioned riser (TTR) under the parametric excitations due to the heave platform.The main contents and conclusions are as follows:(1) The governing equation of VIV of TTR under parametric excitations isestablished considering the composition, structure and external loads characteristics ofTTR. The natural vibration of TTR is detailed investigated and discussed by using theclassical theory considering the effects of bending stiffness, gravity and platformheave. Analytical methods and simplified methods are presented, so as to meet theneeds of the engineering application and further study.(2) Mathieu equation, the classic differential equation which governs theresponse of many physical systems having one degree of freedom, is used to study theparametric instability of TTR with constant tension. The equation of TTR is reducedto Mathieu equation by using the Galerkin method, and the parametric instabilitychart of TTR is obtained by using Mathieu equation. In addition, the results arecompared with those obtained by numerical simulation based on the Floquet theory.The results show that natural frequencies of the riser are very dense due to the highslenderness ratio, which leads to overlapping of different instability zones. The firstinstability zones are more important than higher instability zones which are very narrow in the instability charts.(3) The parametric instability of TTR is investigated by applying the Floquettheory considering the linearly varying tension along the length due to the effect ofgravity. The results show that the natural frequencies of TTR with variable tension areevidently reduced, the parametric instability zones are significantly increased and themaximum allowable amplitude of platform heave is much smaller at the samedamping; The nodes and antinodes of mode shapes are no longer uniformlydistributed along the axial direction and the amplitude also changes with depth, whichleads to coupling between the modes. The combination resonance phenomenon occursas a result of mode coupling, which creates more serious damages.(4) The VIV dynamic response of TTR in shear flow considering parametricexcitations is calculated numerically by the finite difference method based on Van derPol wake-oscillator model. Effects of current speed, top tension and parametricexcitations on VIV of TTR is discussed. The results show that the VIV response ofTTR, especially in the flow with low speed, is much larger due to the parametricexcitation. The vibration displacement and bending stress at the bottom of TTR issignificantly increased under parametric excitations. It also can be found that thevibration frequency of TTR in low speed flow is0.5times larger than the platformheave frequency, while the platform heave amplitude is large and the frequency ishigh. This phenomenon is called parametric resonance, which leads to significantincrease in vibration response.(5) The methods and techniques of VIV experiment under parametric excitationsare presented, which is more important for investigating VIV of TTR. Effects ofcurrent speed, top tension and the parametric excitations on VIV of TTR is discussed(6) A theoretical method of fatigue analysis of TTR under vortex-inducedvibration and parametric excitations is presented, which can be used to predict thefatigue damage of TTR in the design or service. The results show that the fatiguedamage of TTR increases as depth increases, the maximal fatigue damage is at thebottom of the riser. The parametric excitation induces great influence in riser bottom.