| Vortex induced vibration (VIV) is one of the important reasons of the structural instability and fatigue damage of deepwater riser. Practically, it is necessary to use VIV suppression measures to reduce or eliminate the influence of VIV and increase the service life of deepwater riser. There is still a big gap in the research work of VIV suppression measures between China and developed countries, especially in the research of large slenderness ratio flexible riser, which is still very limited. The rapid development of the deep-sea oil and gas development projects urgently needs to carry out innovative research of VIV suppression measures of long flexible riser.In this paper, relying on the national863plan project, a new type of VIV suppression measures of multiple control rods is proposed. With long flexible riser as the main research object, the suppression effect, the influencing factors of suppression performance and the suppression mechanism are systematically studied by combining the methods of large scale tank experiment and numerical simulation. In addition, this paper also carried out some basic theoretical exploration on aspect of the multiple cylinders VIV.This paper first briefly introduces the basic theory of VIV, which mainly includes the basic concepts of cylinder VIV, main dimensionless parameters involved in the research and research methods of VIV.In the third chapter, a large scale tank experiment is conducted which using towing method to generate uniform flow. The suppression effect of multiple control rods on VIV of long flexible riser is systematically studied, by using UPVC and steel riser model with slenderness ratio of1750respectively, and with comprehensive consideration of the influence of flow direction, spacing and coverage rate on suppression performance and tension. The experimental results show that the multiple control rods have a significant suppression effect on VIV of long flexible riser, which can simultaneously reduce the displacement response of riser up to90%in both CF and IL direction, and decrease the fatigue damage more than2orders of magnitude, and meanwhile the increase of tension is not obvious. It has robust suppression performance irrespective of flow direction, and the smaller the spacing, the higher the coverage rate produce better suppression effect. The optimized spacing and coverage rate are suggested in this paper.In the fourth chapter, the2D numerical simulation method is used to study the lateral forced vibration of multiple cylinders. Firstly, the numerical model is verified by calculation of the flow around single fixed cylinder and the lateral forced vibration of single cylinder. Then the flow around multiple fixed cylinders and the lateral forced vibration of multiple cylinders are simulated. The numerical results show that fluid-structure energy transfer relationship, phase relationship between structural displacement and lift force, and vortex shedding pattern will significantly change when structural vibration frequency is close to the vortex shedding frequency of stationary body, which cause abrupt change of the amplitude and oscillating mode of lift force. The suppression mechanism of multiple control rods on riser VIV is revealed by analyzing vortex shedding pattern and fluid pressure distribution of single cylinder and multiple cylinders in different attack angle.In the fifth chapter, the quasi3D fluid-structure coupling numerical model is established based on strip theory. By introducing parallel computing technology, the efficiency of the numerical calculation is significantly improved, which enable us to solve the problem of long flexible riser VIV. By selecting the experimental flexible riser model with slenderness ratio of1750and the practical deepwater riser model with slenderness ratio of3691as research objects, this numerical model is used to simulate the lateral VIV of single riser with multiple control rods and tandem double risers. The numerical results show that the lateral displacement amplitude of the riser with multiple control rods is significantly reduced, compared to the bare riser, and the oscillating characteristic of displacement and fluid force and the vortex shedding mode are significantly changed. The VIV response characteristics of the upstream one of tandem double risers is similar to single riser, whose vibration is under periodic and stable lock-in state. Because of the wake interference effect, the VIV response characteristic of the downstream riser is quite different from the upstream one. The maximal lateral displacement of the downstream riser is significantly larger than the upstream one, while the maximal standard deviation of displacement of the downstream riser is smaller than the upstream one. The VIV response of the whole downstream riser shows strong randomness, under nonlocking unstable state, and its vortex shedding pattern is highly irregular. |
PDF Full Text Request