Experimental Investigation And Numerical Simulation By A Discrete Vortex Method On VIV Of Marine Risers

As the worldwhile activities of oil and gas exploration and production continue apace to deeper water, many kinds of platforms have been built. A key element of the platform is the marine riser system, which plays an important part in transporting the oil and gas from the seabed to the platform. Marine risers can be thousands of meters long as the prodution depth goes. The vortex-induced vibration (VIV) response of marine risers subjected to the ocean current accounts for the greatest contribution to overall riser fatigue damage. Since their failure may cause great economic damage and environmental problems, marine risers are designed with a great safety factor to guarantee the safe, but which leads to huge cost. Accurate prediction and/or suppression of the VIV response of the marine risers, which can greatly reduce the cost of the platform, has led to an intensification of research activity in ocean engineering in recent years.The present research presented in this dissertation is supported by the "National High-Tech Research and Development Program (863Program)"of China with Grant no.2006AA09A103. Experimental investigation has been conducted to improve the understanding of the VIV of marine riser and develop new VIV suppression method. A CFD model has been built based on a discrete vortex method to simulate the VIV response of marine risers. The main content is as follows:First, experimental tests of VIV of a small vertical riser pipe have been conducted in a wave flume. The length of the riser model is about2m with a diameter of16mm. In order to vary the mass ratio, the riser pipe is filled with air, water and sand, respectively. Fiber brag grating (FBG), a new technology, is used to measure the strain response in the present experiment. By modal analysis method, the vibrating modes of VIV response are analysed at different flow velocities. The dimensionless vibrating frequency and amplitude versus reduced velocity are also presented in this work. Moreover, the distributions of the displacement of VIV response along the length of the riser are analysed at different flow velocities.Second, a passive flow control method to suppress VIV has been tested, which is accomplished by arranging three identical small circular cylinder closed to the riser model with120°angle. The riser model with control rods is placed in a water flume subject to four different current velocities and two extreme attack angles. The experimental results show that the control rods can remarkably reduce the transverse responses of riser model while the vibration frequency of riser model is almost as same as that of a bare riser. The experiments also demonstrate that present flow control measure is not sensitive to the incoming flow direction which is a very important advantage for practical engineering applications.Third, laboratory tests have been conducted on vortex-induced vibration (VIV) of a long flexible riser towed horizontally in a wave basin in order to study high mode VIV responses. The riser model has a total length of28.0m with an aspect ratio of about1750. Reynolds numbers ranged from about3000to10000. Fiber optic grating strain gauges are adopted to measure the dynamic response in both cross flow and in-line directions. The cross-flow vibrations were observed to vibrate at modes up to6and the in-line reached up to12. The fundamental response frequencies can be predicted by a Strouhal number of about0.18. Multi-mode responses and the asymmetry of the bare pipe response in uniform flow were observed and analyzed. The experimental results confirmed that the riser pipe vibrated multi-modally despite it being subject to a uniform current profile and all of the excited modes vibrated at the Strouhal frequency. The asymmetrical distribution of displacement was mainly due to more than one mode participating in the resposne. Higher harmonics of the VIV response such as the third, fourth and fifth harmonics frequencies were found to be steady over the entire duration of the test even if they varied along the length of the riser pipe.Fourth, a2D grid-free discrete vortex model is established in order to predict VIV responses. Fast multipole method is used in the model in order to improve the efficiency of the calculation of the vortex-induced velocity. Not only flow past a fixed cylinder,but also flow past an oscillating cylinder has been simulated in present work. For flow past an oscillating cylinder, both VIV responses of an elastically mounted rigid cylinder and fluid forces of a forced rigid cylinder are concerned. Drag force, lift force and displacement responses are presented. The relationship between the response amplitude and reduced velocity is analysed in the present work.Fifth, coupled with the strip method, a quasi-3D DVM model is established by combining the2D DVM model with the FEM structure model in order to predict the VIV of long flexible risers. Then, numerical simulations of VIV of long flexible risers are conducted. The simulation results are compared with the experimental results, and the reasonable agreements are obtained,which show that the present DVM model can be used to predict the VIV of long flexible risers.