Time scale of local scour around pipelines in current, waves, and combined waves and current

This paper presents results of an experimental investigation into the time scale of local scour around subsea pipelines in current-only, wave-only, and combined wave and current flow conditions. To cover a range of geometric scales, the experiments were conducted in a small and a large recirculating (O-tube) flume with model pipelines ranging from 50 to 196 mm in diameter. Based on the experimental results and existing data available in the literature, it is shown that the time scale of the scour process is mainly dependent on the maximum instantaneous Shields parameter in all three flow conditions. Using this result, a new predictive formula for the time scale is presented, which is applicable universally to current-only, wave-only, and combined wave and current conditions. In addition to the time scale analysis, equilibrium scour depth measurements are presented for all three flow conditions. A new method of predicting the equilibrium scour depth in combined wave and current conditions is proposed based on the current component of equilibrium scour depth and the wave component of equilibrium scour depth. Collectively, the empirical formulas developed for time scale and equilibrium scour depth may be used to improve predictive engineering models of scour beneath subsea pipelines.