Permeable Plate Anchors: Accelerating Capacity Gain in Soft Clay

Plate anchors have become an attractive technology for anchoring offshore floating facilities such as floating renewable energy devices because they provide high holding capacity relative to their dry weight. This allows for the use of smaller anchors (relative to a driven or suction-installed pile), which provide cost savings on production, transport, and installation. Loads delivered to the anchor via mooring lines may increase pore water pressure in fine-grained soils. This excess pore pressure will dissipate with time, resulting in a local increase in the undrained shear strength of the soil surrounding the anchor, increasing the capacity. There may be opportunities to consider these capacity increases if the consolidation process occurs over time periods that are short relative to the lifetime of the facility. This paper considers the use of drainage channels in a plate to make the anchor permeable and quicken consolidation times. Experimental data generated from model-scale experiments conducted in a geotechnical centrifuge show (for the anchor design tested) that excess pore pressure just above the anchor dissipated almost an order of magnitude faster for a permeable anchor, and that after full consolidation, the permeable anchor capacity was higher. The latter finding was not anticipated and is believed to be due to changes in load distribution resulting from the rapid reduction in negative excess pore pressure underneath the permeable anchor.