| VIV is an important factor leading to the fatigue damage of deepwater riser, the VIVmechanism research on cylindrical structure is gradually tending to be perfect. A large numberof VIV prediction models proposed at the present stage mainly focus on thesingle-degree-of-freedom CF vibration of cylinder, while the IL vibration of the structure isoften restricted or ignored. Recent studies have indicated that, for the low mass ratio structurelike marine riser, the IL fatigue damage caused by VIV is in the same order of magnitude asthe CF’s. Therefore, correctly understand and describe the bidirectional VIV characteristics ofIL and CF, while forecast and assess the comprehensive fatigue life of riser structure in detail,which is of great significance for ensuring the economic benefit and safe operation duringservice of offshore structures.This paper has carried out a series of in-depth research on the structure bidirectional VIVproblems, proposing a semi-empirical model with practical significance of engineering toforecast the IL and CF VIV response of rigid cylinder and slender flexible riser. The maincontents and results include:(1) Systematically induce and review the research status and achievements of thecylinder bidirectional VIV, explain related certain physical concepts and basic physicalparameters of VIV, and summarize the existing semi-empirical models of VIV.(2) Deduce the IL and CF fluctuating hydrodynamic acting on rigid cylinder by fluidbased on two-dimensional discrete point vortex method, redefine the meaning of the controlvariables of traditional wake oscillator, and adopt van der Pol equation describe the nonlinearcharacteristics of the near-wall point vortex, thereby establishing a vortex strength wakeoscillator model which can forecast the VIV response of the two degrees of freedom, andproviding the method to determine the experience parameters in the model. Employ the modelto predict the bidirectional VIV response of rigid cylinder, the variation of obtained CFlock-in, IL initial response peak, phase angle of movement and "8"shape trajectory etc.coinciding with the experimental results, while discuss the effect of mass ratio and dampingratio on the IL and CF amplitude response of the structure.(3) Carry out bidirectional VIV model experiment with different IL and CF frequencyratio for the horizontally arranged outer diameter of5cm, length of120cm rigid cylinder. It is found in the experiment that the frequency ratio may have a significant impact on the VIVcharacteristics of the structure. when the frequency ratio is near1.0, there is no significantdifference between the conclusions of cylinder VIV characteristics and the two degrees offreedom cylinder with the equal frequency ratio, but with the increase in frequency ratio, theIL and CF amplitude together with the phase difference between them change significantly.Apply the vortex strength wake oscillator model to forecast the VIV response of the cylinderwith different frequency ratio, which indicates that under the specific frequency ratio, whileanalyze the effect of frequency ratio and mass ratio on the response amplitude and trajectory.(4) Adopt section method establish the semi-empirical model of three-dimensional time-domainwhich can forecast the bidirectional VIV of slender elastomer. Apply the model to the study onthe VIV response of TTR under the action of step flow, which indicates that the IL amplitudepeak is less than the CF’s, but the order of IL activation mode is about2times the CF. Thevibration in two directions both exists the standing wave and traveling wave, when the IL is inrelatively high vibration mode, the upper half of the riser in the still water presents a standingwave vibration mode and the section trajectory along the pipe is rather changeable, while thelower half of the riser in the uniform flow is mainly on traveling wave vibration and thesection trajectory tends to be more uniform, the vibration characteristics forecast by modelcoinciding with the experimental results.(5) Employ three-dimensional vortex strength wake oscillator model to forecast thebidirectional VIV response of deepwater SCR. Under the action of free shear flow, thetrajectory of each riser node may not form a steady "8" shape, while the response amplitudeand phase angle change with time. The IL vibration mode is significantly higher than the CF’s,and in addition to the riser section near the touchdown point which is in standing wavevibration, the traveling wave has significant effect on the most part of the intermediate sectionof the riser. Due to the high-frequency and high-modal vibration characteristics of IL, thefatigue damage in IL direction is in the same order of magnitude as the CF’s, and has differentdistribution along the pipe, so taking the effect of IL into consideration benefits thecomprehensive assessment of the VIV fatigue damage in riser section. |
PDF Full Text Request