Study On Non-linear Analysis Method On VIV Of Deepwater Risers With Large Deflection

In recent years, owing to the large demand of crude oil and gas in the world,explorations have been moved to deeper water regions. Ocean structures such aslong flexible risers exposing to current flows at deep water are plagued with vortex-induced vibration problems. If failure occurs, it will not only cause economiclosses,but also cause environment pollution. Correctly predicting dynamiccharacters of deepwater risers become more and more important. This paperproposes a non-linear time-domain analysis model for VIV response of deepwaterrisers by model test, theoretic deduction and numerical analysis, and studies thedynamic responses and stress strength of deepwater TTRFirstly, on the base of the results of model test, the paper finds that thevortex-excited force on oscillating riser is not a harmonic force, but a random forcewith narrow bandwidth character. So the VIV responses of risers is random. Andthe paper holds that when the risers oscillate in fluid, the oscillation of riser acts onthe the fluid which is around structure, the fluid conversely acts on structure. Thuscoupling effect exists between structure and fluid which is around risers.Considering the coupling effect, the paper modifies Morrison vortex-excited force,and applies the modified vortex-excited force to analysis on VIV response ofdeepwater risers. A non-linear time-domain analysis model for VIV response ofdeepwater TTR is proposed, and corresponding program TTRTD1.0is designedwith MATLAB. The paper also studies on VIV response and stress strength ofdeepwater TTR operated in a water depth of1500m. The calculated results indicatethat the program TTRTD1.0can effectively predict the VIV responses characters ofdeepwater TTR on base of the proposed analysis model, including multi-modeexcited, high order mode excited, asymmetrical distribution of displacement andlarge displacement. Secondly, the paper modifies the non-linear time-domain analysis model forVIV response of deepwater risers by theoretic deduction considering the largedeformation character of deepwater risers when vortex-induced vibration occurs. Anew analysis model is proposed considering large deformations, Accordingly theprogram TTRLD1.0is designed on the base of the new model. And the dynamiccharacteristic of the new model is analysis ed theoretically. The theoretical analysisresults indicate that large deformation decreases the natural frequency of structure,especially low-order natural frequencies. But large deformation had no effect onvibration mode of simply supported risers. At the same time, The paper also studiesthe large deformation effect on the response and stress strength of deepwater risers.The calculated results compared with the results of no considering large deformationindicate that large deformation increase the amplitude of risers with the increase ofvelocity of current, but it is non-linear relationship between them. And t hecalculated results of stress strength indicate that large deformation increase the stressstrength amplitude of weak location of risers. All the results confirm that largedeformation go against fatigue life, so large deformation effect can’t be neglected.Lastly, deepwater risers are installed between wellhead at the sea bed andfloating platform.The heave of a floating platform induces a fluctuation in time ofthe axial tension of the riser, especially semi-submersible platform. A possible andundesirable phenomenon is the excitation of a transverse riser vibration caused bythis fluctuation. According to the above, the paper further improve the VIVpredicting model considering the heave of a floating platform. Accordingly theprogram TTRPD1.0is designed. The paper studies VIV response and stress strengthof deepwater TTR operated in a water depth of1500m. The calculated resultsindicate that the heave of a floating platform affect the amplitude of VIV responseand stress strength. And the effect of the heave of a floating platform ion VIVresponse of deepwater risers is the function of environmental load, riser parameter,the amplitude of heave and frequency of heave.The new ideas and innovative points lie in the following:1. This paperimproves the Morison vortex-induced force, and proposes a non-linear time-domain analysis model for VIV response of deepwater risers.2. This paper further improvesthe VIV predicting model considering large deformation, and studies the effect onVIV response of deepwater risers.3.This paper further improves the VIV predictingmodel considering the heave of a floating platform, and studies large its effect onVIV response and stress strength of deepwater risers.4. A more corresponded toengineering practice model for predicting VIV response of deepwater risers isproposed finally in this paper.