Vortex-Induced Vibrations of Subsea Free Spanning Pipelines

Results show that the cross-flow response of the near wall case follow the same trend for the case of an isolated cylinder. The cylinder response amplitude in both cross-flow and in-line directions are greatly magnified in the lock-in range, which in the present study happens at around Ur = 5. Outside the lock-in range either reducing or increasing Ur causes the in-line response to decrease. Comparison between the results obtained for the near-wall case and that of the isolated case shows that, the nearby wall decreases the cross-flow response, while at the same time increases the in-line response. In addition, the existence of a nearby wall also shifts the lock-in range to lower Ur, as observed at Ur=4 the responses are larger than the isolated case. Subsea freespan pipelines Subsea pipelines are major and expensive components of offshore deepwater systems. The offshore industry is increasingly considering designs of deepwater pipeline system traversing across escarpment, trough or depressions with the large seabed slopes. The abrupt change in the slope results in a very long freespan on a pipeline which is not continuously supported on the seabed. The span length can easily attain values as large as 100 times the pipeline diameter, with a clearance from the sea bottom ranging from practically nil to greater than 2-3 times the pipeline diameter. Freespanning would cause the stress and vortex-induced vibration fatigue limits and thus greatly reduces the service life of pipelines. There have been several serious incidents when pipelines floated to the surface due to loss of protective concrete coating as a result of flow-induced vibrations