Development of current-induced scour beneath elevated subsea pipelines

Abstract

When scour occurs beneath a subsea pipeline and develops to a certain extent, the pipeline may experience vortex-induced vibrations, through which there can be a potential accumulation of fatigue damage. However, when a pipeline is laid on an uneven seabed, certain sections may have an elevation with respect to the far-field seabed, eo, at which the development of scour would vary. This work focused on predicting the development of the scour depth beneath subsea pipelines with an elevation under steady flow conditions. A range of pipe elevation-to-diameter ratios (i.e. 0 ≤ eo/D ≤ 0.5) have been considered for laboratory experiments conducted in a sediment flume. The corresponding equilibrium scour depths and scour time scales were obtained; experimental data from published literature have been collected and added to the present study to produce a more complete analysis database. The correlation between existing empirical equations for predicting the time scale and the experimental data was assessed, resulting in a new set of constants. A new manner of converting the scour time scale into a non-dimensional form was found to aid the empirical equations in attaining a better correlation to the experimental data. Subsequently, a new empirical equation has also been proposed in this work, which accounts for the influence of eo/D on the non-dimensional scour time scale. It was found to have the best overall correlation with the experimental data. Finally, full-scale predictions of the seabed gaps and time scales were made for the Tasmanian Gas Pipeline (TGP). Keywords: Scour time scale, Equilibrium scour depth, Subsea pipelines, Pipe elevation, Steady currents