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.