3.2.2 Loads and boundary conditions
A three dimensional finite element (FE) model was used to determine the peak stresses in the monopile foundation. The separation between two consecutive circumferential welds in the monopile was assumed to be 6.5m and the first weld close to the mudline was assumed to be 4m above the sea-bed. Since the bending stresses on the monopile decrease rapidly with increasing distance from the fixed end, only the first weld (assumed at 4m from mudline) was modelled in the FE geometry. Fig. 5 shows the FE model dimensions and weld geometry details. Additionally, the regions of load application are also included in the figure and the corresponding values are summarised in Table 2. As discussed in the previous sections, the loads corresponding to each of the six load cases were applied to determine the maximum stress in the structure. The hydrodynamic and aerodynamic forces for the mean wave height and mean wind speed of each group was calculated from section 3.1.1 and section 3.1.2 respectively. Three regions were selected to apply the corresponding loads, i.e. (1) the centre of buoyancy of the submerged section of the monopile was used to apply the wave load, (2) centroid of the tower geometry was used to apply the horizontal wind force, (3) the top edge of the tower was used to apply the wind load acting on the rotor blades and (4) the structural weight consisting of the rotor and nacelle was applied on the tower vertically.