Abstract: The rapid development of renewable energy has become a global consensus, with offshore wind power as a key focus. The upper structures of offshore wind turbines are subjected to complex environmental loads, including wind, waves, and currents, ultimately supported by the anchoring foundation. Therefore, the bearing performance of the anchoring foundation plays a decisive role in ensuring the service stability of the entire system. The piggy-backed anchor is a novel hybrid anchor consisting of two or more traditional anchor plates, such as drag anchors or vertical load anchors. It offers advantages such as low installation cost, high anchoring efficiency, reusability, and high bearing capacity. The paper proposes an upper-bound solution for the plastic limit analysis of piggy-backed anchors to quickly predict their bearing capacity. This study found that the undrained shear strength gradient k does not affect the bearing capacity coefficient. Under the influence of coupled loads, the normal resistance of the piggy-backed anchor predominantly determines the bearing capacity, while the mechanisms of tangential loads and rotational torque are similar to those of a single anchor plate. During service, the piggy-backed anchor requires a larger displacement to mobilize its ultimate bearing capacity compared to a single anchor plate.

Key words: piggy-backed anchors, clays, interference, plasticity, limit analysis, numerical simulation