Numerical investigation of local scour beneath a sagging subsea pipeline in steady currents

Two dimensional (2-D) numerical simulations are conducted to predict scour below the midsection of the suspended span of a pipeline that sags naturally into a scour pit. The deflection of the pipeline is calculated based on the suspension length (or span length) of the pipeline, which is predicted by an empirical formula for the length of the scour pit. The general properties of the scour process are investigated for various pipeline properties, pipeline embedment depths and Shields parameters. The effect of each of these parameters on pipeline sagging are combined into one non-dimensional sagging parameter. A touchdown time is defined as the time when the pipeline touches the bottom of the sand pit. Empirical formulae are proposed for predicting the final scour depth at touchdown and the time scale required for touchdown. It is found that the final sagging depth is a function of the sagging parameter and the Shields parameter, whilst the touchdown time is only a function of the sagging parameter. A formula is also developed for predicting the time history of scour depth prior to touchdown. Collectively, the empirical formulae and numerical simulations are found to be in reasonable agreement with an existing theoretical model available in the literature.