Structural behavior and multi-objective optimization of monopile grouted connections in offshore wind turbines under early-age thermal-mechanical coupling

Abstract

As offshore wind turbines scale up in harsher waters, the structural integrity of monopile foundations has become a central concern. The grouted connection between monopile and transition piece is a critical load-transfer interface, yet its behavior under early-age thermal conditions during grout curing has received limited attention. This study examines monopile grouted connections under coupled early-age thermal and ultimate-limit-state (ULS) loading and develops a multi-objective optimization to mitigate the degradation. A three-dimensional sequentially coupled thermo-mechanical finite element model was built in COMSOL Multiphysics for the NREL 5 MW reference turbine at a North Sea site (Horns Rev), with loads from ECMWF ERA5 data and DLC 6.1. It couples Arrhenius hydration kinetics, a maturity-based elastic modulus, and a concrete damaged plasticity law, validated against the Lotsberg and Azenha benchmarks. Hydration-induced thermal gradients reach 29 °C, raising Tresca shear stress at the shear keys by 16% and producing a tensile damage variable of 0.12 at the endmost key. A Kriging–NSGA-II–TOPSIS framework then reduces the thermal gradient by 19% and improves moment-bearing capacity by 7%. The localized early-age damage (d t = 0.12) at the endmost shear key may serve as a pre-existing defect that accelerates fatigue crack propagation under the 10 8 –10 9 load cycles expected over the turbine's service life, thereby reducing long-term structural margins. Early-age thermal-mechanical coupling therefore warrants explicit consideration, with implications for guidelines such as DNVGL-ST-0126.