Horizontal Capacity of Suction Anchor with Retrievable Cap in Clay for Floating Wind Turbine

Abstract

Abstract Recently, the offshore wind industry is transitioning from the fixed-bottom in shallow waters to the floating in deep waters. This shift requires station-keeping anchors to stabilize turbines. Maximizing the cost-effectiveness and installation efficiency of anchor foundations is critical for project success. Suction anchors offer an efficient installation method by creating an underpressure to penetrate the seabed without noise from hammering like driven pile anchors. This foundation concept, well-established in the oil and gas industry, holds promise for floating offshore wind applications. Traditionally, the top cap is fixed on the suction anchor after installation to provide a sealed condition inside for better vertical holding performance. However, in the case of a catenary mooring system in soft clay, where horizontal loads dominate, the conventional fixed top cap may not be necessary. Instead, a retrievable top cap can be designed, offering cost savings and ensuring full embedment of the anchor in the seabed, enhancing its capacity, and mobilizing more soil in deep areas. This study presents a series of numerical modelling to investigate the horizontal capability of the suction anchors in clay with retrievable top caps. Comprehensive numerical simulations were performed, which were validated first against the existing studies in literatures for traditional suction anchor with fixed top cap. A parametric study was then carried out to assess the effects of the retrievable top cap length, anchor length to diameter ratio (L/D), padeye location, and loading angle on the anchor horizontal capacity. Consistent with the existing studies, the failure mechanism of horizontally loaded suction anchor change with the padeye location, which transforms from a rotation failure to horizontal translation failure as the padeye location increase from seabed to around 0.6L (anchor length) and provides the highest horizontal capacity, although it should be noted that the padeye location is also affected by the loading angle. For the anchor with retrievable top cap, almost the same horizontal capacity was observed when the L/D ratio is larger than 1. In addition, when the retrievable top part extends to upper part of anchor, a higher capacity factor can be obtained due to the increased embedment and the mobilization of deep soil. Compared with the anchor with fixed top cap, the optimum padeye location of anchor with retrievable top part will also change to a shallower depth at around 0.5L.