Fig. 3 Variation of speed of waves with increasing depth from free
surface of sea.
Modelling Methodology
3.1 Determination of service loads on OWT monopile structure3.1.1 Hydrodynamic forces due to sea-waves
As Fig. 2(b) denotes, the waves can vary over a wide spectrum of heights
and periods which are non-linear and stochastic. For modelling purpose,
a linear wave theory was adopted to represent the sea-waves. The
Morison’s semi-empirical equations [28] give the hydrodynamic forces
due to an unsteady, viscous flow acting on a fixed body along the flow
(i.e. wave) direction. The total hydrodynamic force is composed of two
components in this study as shown in Eq. (1). The first term represents
the inertia force due to acceleration of the surrounding fluid while the
second term gives the contribution of the viscous drag force against the
monopile surface. Therefore, Morison’s approach was used in this study
with the following critical assumptions:
1. The wave flow is irrotational and incompressible
2. The sea-bed is horizontal, impermeable and provides a fixed support
to the monopile structure
3. Pressure at the sea-surface is constant
4. Force acting on the structure due to undisturbed waves is negligible
5. Wave diffraction and wave splash on the monopile surface is
negligible
6. The mass coefficient and drag coefficient remains constant throughout
the service life
From Fig. 2(b) and Fig. 3 it is evident that wave speed coming across on
a monopile section vary with time and depth. Eq. (1) shows the force
acting on an infinitesimal section of the monopile due to the waves. The
total force on a monopile structure due to sea-waves was therefore
evaluated using an integration shown in Equation (2).