where Cm and Cd are the
coefficients of inertia force (referred to as mass coefficient) and
viscous drag force (referred to as drag coefficient), respectively. In
the present study, the values of Cm andCd were assumed to be 2 and 0.7 [28]. The
remaining terms are: density of sea-water ρ , diameter of monopile
structure d , wave speed u which is a function of height
from sea-floor z and time t . The length of submerged
section of the monopile structure is L .
Following this, the wave speed variation recorded over the 2 years of
online monitoring was sorted into six groups using the rainflow counting
algorithm [29] implemented in MATLAB. Also, the frequency of
occurrence of these wave speeds over the 2 year period was used to
estimate the weight that should be given to each group during fatigue
life analysis. For any given moment in time, one of the load cases (or
wave force) and its corresponding frequency was assumed to be acting on
the OWT structure [24].
3.1.2 Aerodynamic forces due to wind
The classical Blade Element Momentum (BEM) approach [30] was used to
compute the loads acting on the rotor due to coursing wind. The wind
behaviour was modelled with a one dimensional wave theory. Eq. (3) and
Eq. (4) give the lift and drag forces respectively experienced by an
airfoil.