Investigation On Heave Motion Induced VIV On A Steel Lazy Wave Riser

Steel lazy-wave riser(SLWR)is one of the most promising riser types applied in deep-water and ultra-deep-water oil and gas exploration.Results from steel catenary riser(SCR)indicates that the relative oscillation current between the riser and the surrounding fluid caused by the platform motion may also induce the vortex-induced vibration(VIV).Considering the complex structural form of SLWRs,their in-plane motion characteristics and out-of-plane VIV characteristics are likely to be different from SCRS,which still need to study through the model test method.Meanwhile,fatigue damage on SLWRS caused by platform motion-induced VIV is one of the most concerned problems in the engineering field.Thus,in this paper,the experiment on platform motion induced-VIV of a SLWR is carried out in the ocean basin.The experimental data are processed by statistical analysis,spectrum analysis and modal analysis,answering the following questions:1)How to obtain more accurate in-plane motion response of SLWRs based on the experimental data?2)What are the characteristics of the in-plane motion response of SLWRs under platform motion and whether it is some similarities and differences with the steel catenary riser?3)Whether the platform motion will induced the VIV response of a SLWR and what is the relationship between the VIV response,in-plane motion and platform motion parameters?4)How is the fatigue behavior of the SLWRs under platform motion-induced VIV,and whether it has to be taken into account in the engineering application?5)How to suppress the VIV response on SLWRs caused by platform motion.Results indicate that the numerical reconstruction method of in-plane motion can successfully reconstruct the real in-plane normal displacement and velocity of the SLWRs.On the basis of the above results,it is proved that the VIV actually could be induced by the platform motion by the comparison of the response frequency and the vortex frequency.This kind of VIV has typical ‘time-sharing' characteristics such as‘amplitude modulation' and ‘mode transition'.The vibration is dominated by the‘travelling wave' which is produced at the hung-off section and transfer to the two ends of SLWR.The presence of traveling wave amplifies the VIV response of the buoyancy section,which is expected to suffer little vibration due to its weak in-plane movement.Meanwhile,the results of fatigue analysis show that the out-of-plane vortex-induced vibration dominates the fatigue damage of the hung-off section or even the whole length of the riser in comparison with the fatigue damage caused by the in-plane global motion.The efficiency of helical strakes on suppressing the VIV response of SLWRs is more than 70% so that the fatigue life is greatly improved.In addition,the general laws of the VIV of the SLWRs caused by platform motion are established.