Erosion of cuttings pile sediments: A laboratory flume study

The best practical option for remediation of drill cuttings piles is directly contingent on a firm understanding of the properties and attributes of these sediments. Whilst there is an increasing literature on the geochemical characteristics of cuttings piles, comparatively little is known regarding the transport by tidal currents and waves. This paper reports some laboratory flume experiments conducted to ascertain the threshold entrainment shear stress (tau(crit)) of drill spoil sediments from two locations (near to the drilling axis and outside the rig periphery) on the NW Hutton platform. tau(crit) varies between 0.12-1.52 Nm(-2) and statistical analysis shows a difference in sediment mobility wherein sediments farthest from the drilling axis are comparatively more stable. Specialised low-temperature scanning electron microscopy (LTSEM) and compositional analysis of sediments (particle size, metal and organic content, inhabitant macrobenthos) are used to provide some insight into the differences between sites. Swelling attributable to the greater clay content of sediment close to the drilling axis may, in part, explain the observations. Comparison of the measured entrainment thresholds with both tidal and wave-induced bed stresses (separately) predicted using numerical models indicates that the sediments are unlikely to be moved by the prevailing tidal currents, but may be moved or agitated by extreme winter storms (wave height similar to9-10 m). However, observations made during the flume experiments suggest a winnowing of the fine particles from the sediment matrix under sub-threshold stress, which may be important if these particles are geochemically reactive.