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.