Effect of drag rates on drag embedment anchor penetration for cable burial risk assessment

Cable burial risk assessment (CBRA) defines minimum seabed burial depth of cables to avoid interaction with towed objects that penetrate the seabed, for example, drag embedment anchors (DEA). Recent physical modelling studies have shown that model anchors dragged in dry sand may penetrate to much greater depths in loose sand than current CBRA approaches predict. As other seabed penetrating objects (e.g. pipeline ploughs) have shown velocity or rate of pull dependent behaviour in saturated soils, it is necessary to check for similar rate-dependent penetration behaviour for DEAs. This is required to verify the effect of anchor drag rates in CBRA methods and the validity of previous modelling studies that differ in their findings from current CBRA. This study used model centrifuge tests of DEAs pulled at varying rates in saturated conditions. The result show that anchor penetration depths are reduced with increasing drag rates. The results also confirm that CBRA assumptions of anchor orientation are not consistent with observed behaviour and soil penetration factors require updating. The results suggest that CBRA approaches should also consider the drag speed of the penetration event as this may vary during emergency deployment or may be much lower where a vessel drifts at anchor.