With offshore wind deployment accelerating, thick-section steel structures must be reliable, weldable, and economically manufactured. Thermo-mechanically controlled processed (TMCP) steels, such as S355ML, are widely adopted for offshore wind turbine (OWT) support structures (e.g., monopiles and transition pieces) because they combine high strength, good low-temperature toughness, lean alloying, and excellent weldability. However, through-thickness variations introduced during a TMCP process and subsequent welding thermal cycles can localise strain, modify residual stresses, and degrade fatigue performance. This thesis investigates the through-thickness microstructural and mechanical variability of a 20 mm thick TMCP S355ML plate (manufactured by Dillinger Group), and the thermo-metallurgical response to butt welding. A computational-experimental workflow is developed, coupling Thermo-Calc (educational version) for phase equilibria, JMatPro for temperature-dependent thermophysical properties, the Materials Algorithms Project (MAP) code for transformation kinetics, and ABAQUS for transient thermal finite-element simulations of single- and multi-pass welds. The experimental program comprises tensile testing, metallography, electron backscatter diffraction (EBSD), and scanning electron microscopy (SEM) fractography on specimens extracted at multiple depths across the plate thickness. Preparation for Gleeble thermal-mechanical simulations, nanoindentation, and welding trials is also described. Tensile tests show ~5% higher ultimate tensile strength at mid-thickness relative to the near-surface of a 20 mm thick TMCP S355ML steel plate, consistent with metallography revealing thicker pearlite bands and slightly finer ferrite grains at the centre of the plate. Welding process simulations predict peak temperatures exceeding the austenite start temperature ~2 mm from the groove edge and indicate that heat accumulation in multi-pass sequences elevates transformation risk above ~0.7t (measured from the bottom surface). SEM fractography confirms ductile failure by micro-void coalescence in both regions; the centre exhibits coarser dimples, consistent with higher strength and locally reduced ductility. Overall, the study emphasises the importance of incorporating through-thickness property gradients and transformation kinetics into the design and qualification of welded TMCP S355ML components for OWT structures, particularly as the sector adopts high-heat-input, high-productivity processes. Recommendations are provided for targeted welding trials and Gleeble-based calibration of phase transformation and transformation plasticity sub-models for TMCP S355ML steel plates.