where B is the number of rotor blades in the turbine, athe axial induction factor, aʹ the factor to account for any rotational speed in the wake (non-ideal rotor condition), ϕ the flow angle of incoming wind, CN the coefficient of normal component of force, r the radial distance from centre of rotation of rotor blades, ω the rotational speed of rotor andCT the coefficient of tangential component of force.
In order to calculate the wind load, the wind velocity profile was regrouped into six categories using rainflow counting algorithm [29] and implemented in MATLAB. Each group of wind speed was characterised by the frequency of occurrence through the 2 years of monitoring. For each group of wind speed, the mean wind speed of the group was used to calculate the corresponding wind loads using Eq. 5 and Eq. 6. For any given moment in time, one of the load case (or wind speed and its corresponding frequency) was assumed to be acting on the OWT structure [24].
3.2 Finite element modelling3.2.1 Material model
The elastic properties used in the FE model were taken from a previous study as shown in Fig. 4(a) [20]. Further, the stabilised hysteresis loop obtained for obtained from the cyclic load tests conducted in this study. The tests were performed on the base metal and cross-weld specimens as shown in Fig. 4(b-d). The experimentally measured values of the tensile and cyclic properties of S355 G10+M are given in Table 1. Following this, an elastic-plastic constitutive equation was used to simulate the experimental results. Calibrated parameters for the mixed isotropic-kinematic model is given in Table 1. Further details of the model can be found in the Abaqus manual [31]. In the cyclic load test, it was observed that the stress vs. strain response (hysteresis loops) were stabilised within the first 20 cycles, nevertheless, the experiment was conducted for 200 cycles (or until failure if cycles to failure was less than 200). Therefore, the 20th cycle from test was used to calibrate the model, where the set parameters resulted in a good fit between the stress vs. strain response of the test and FE model in 20 cycles. Fig. 4(b-d) shows the stabilised cyclic stress vs. strain response obtained for the 20th cycle from experiment as well as the FE model.