Experimental Investigation On Vortex-Induced-Vibration Of A Long Flexible Riser In A Stepped Current

Currently, world offshore oil & gas industry are developing quickly in deepwater. It is becoming the hot research area for researching VIV of deepwater riser in deepwater oil & gas engineering research field. Experimental method is one of the most credible and widely used research methods in researching deepwater riser VIV currently.There are near-surface flow, near-seabed flow, and even middle jets which are often breaking out suddenly in the whole water depth area. The deepwater risers are influenced by the step current for long time. Currently, a lot of scholars have researched VIV of long flexible risers in uniform and linearly sheared currents. However, there are few scholars who researched VIV of long flexible risers in a stepped current. After inventing a rig used in towing tank for accelerating flow, an experimental investigation of VIV of a long flexible riser in a stepped current has been achieved.Depending on the results from numerical test and towing tank test, we can see that the flow in one space of the flow-accelerating rig can be accelerated much faster than the carriage on water towing tank and the flow-accelerating rig may disturb flow field out of it a little.It is doable to use the flow-accelerating rig to accelerate one section flow around riser and finish the experiment of VIV of a long flexible riser in a stepped current. The main measuring meters are strain gauges which are located along the riser evenly. After the analysis of the test data, we get several main conclusions, as follows:1) The riser will be bending in-line because of the flow drag, and the up in-line bending point will be moved to direction of the accelerated flow section.2) Standard deviation of in-line strain on riser could be increasing by Re number level of riser. However, standard deviation of cross-flow strain on riser could be up or down by Re number level of riser.3) The mode numbers of both in-line and cross-flow riser vibrations increase by Re number level of riser. The mode numbers of in-line riser vibrations would be the same as the numbers of cross-flow riser vibrations.4) The cross-flow vibration of riser is similar to beat vibration in some condition of stepped current.5) In the condition of stepped current, the in-line vibration could distribute widely in frequency field when the Re is a bit low or high, and the in-line vibration could distribute centrally in frequency field when the Re is on middle level.6) The frequency level of in-line vibration which distribute widely in frequency field when the Re is a bit high could be lower than the frequency level of in-line vibration which distribute centrally in frequency field when the Re is on middle level.7) In the condition of stepped current, the cross-flow vibration could distribute widely in frequency field when the Re is a bit high, and the cross-flow vibration could distribute centrally in frequency field when the Re is a bit low or on middle level.8) The frequency level of the cross-flow vibration would increase by Re number level of riser.9) The centrality of the frequency distribution of cross-flow vibration is better than in-line vibration. 10) The frequency level of the cross-flow vibration is close to the frequency level of in-line vibration.