Vortex-induced vibration (VIV) is one of the main causes of fatigue failure of submarine pipelines under the action of ocean currents. A reasonable evaluation for the allowable (maximum) length of a pipeline span is vital for the avoidance of VIVs. The length of a pipeline span can alter its natural frequency and further influence the corresponding critical reduced velocity for the onset of VIVs. Nevertheless, the experimental observations indicated that the VIV hysteresis could be significant under the natural flows with increasing/decreasing velocity cycles, especially under the wall-proximity conditions. A case study is then performed to examine VIV hysteresis effects on the allowable span length of submarine pipelines. A dimensionless parameter is proposed and derived for characterizing such hysteresis effect on the allowable span length. It is indicated that lower-critical span lengths are much smaller (up to more than 20%) than those estimated with the commonly used upper-critical flow velocity. As such, the VIV hysteresis effect should be taken into account for determining the allowable span length of submarine pipelines.
Submarine pipelines have been widely employed for transporting offshore oil and gas from subsea wells to platforms or onshore processing facilities, thereby being regarded as the lifelines of offshore oil and gas industries. An uneven seabed is always encountered for long-distance laid pipelines, especially while oil and gas exploitation is currently moving toward deep waters. Meanwhile, a partially embedded pipeline could be suspended as a result of tunnel erosion under the action of ocean currents (Sumer et al., 1988; Gao and Luo, 2010; Gao, 2017). As such, free-spanning pipelines—that is, some sections of a long pipeline untouched with the underlying seabed—are frequently occurring and even inevitable in the subsea environments (see Det Norske Veritas and Germanischer Lloyd, 2017).