Following this, the damage D for each load case was calculated using Palmgren-Miner method as shown in Eq. (9). Depending on the load case probability given by the rainflow algorithm in Table 2, the number of fatigue cycles to which the monopile structure is subjected to over a period of 20 years (ni ) was calculated. Since the wind and wave characteristics were measured for a period of 2 years, it was assumed that similar characteristics are applicable for a period of 20 years, which is the design life of the OWT structure. Subsequently, the number of fatigue cycles to failure (Ni ) was calculated from the stress-life equation given in Eq. (7). Fig. 9(b) shows the number of fatigue cycles sustained by the OWT in the 20 years of operation (ni ) and the number of cycles that the structural material (S355) can withstand before fatigue failure occurs (Ni ). Fig. 9(c) shows that at the end of 20 years, the total fatigue damage in the monopile structure caused due to all the load cases is well within the design limits and the cumulative damage value was found to be 7.13%. This shows a sufficiently large safety margin against failure. It is worth noting that the present study has been conducted on a monopile weldment geometry with a relatively low stress concentration factor as the weld toe and in the absence of welding residual stresses. Therefore, further studies will be conducted in future work to account for the variation in stress concentration factors as well as the residual stress profiles to provide a more accurate estimation of the OWT monopile fatigue life under realistic operational loading conditions.
This study found that the presently followed design criterion is leading to over designing of the OWT structure, thereby adding to the CAPEX cost and impacting the commercial aspect of the wind energy production. The maximum stress and strain in the structure were found to be within the elastic regime of S355 G10+M. Plastic deformation was not observed for any of the load cases, therefore indicating that the monotonic properties of the material are sufficient for determining the design limits. The cyclic deformation behaviour of S355 G10+M was studied experimentally as it is widely used in a number of structural applications, and can be used to investigate the effect of over loads on the structure. Further, the proposed parameters for the cyclic deformation analysis using FE method will be useful for over load analysis.