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.