Fig. 8 (a) von Mises stress profile in the OWT structure obtained for
the 6 load cases from Table. 2, (b) sub-model result for load case 6 to
highlight the local stress concentration at the weld toe, (c) axial
stress (in y-direction) vs. axial strain variation at the weld toe for
the (cyclic) load case 6.
The peak local stress values in the OWT structure was obtained from FE
analysis and the results are shown in Fig. 8. As seen in Fig. 8(b), the
higher stress values were found near the weld toe with lower values
found further away from the weld region. Also seen in this figure is
that the maximum stress values at the weld toe was below the yield
stress of the material, indicating zero or limited plastic deformation
during the operational life of the OWT monopile. The deformation in the
OWT caused bending stresses, therefore the stress at the weld toe was
compared to a nearby region instead of the nominal stress further away.
Moreover, since the weld geometry was included in the model, the effect
of stress concentration at the weldment has been considered. It should
be mentioned that the weld geometry used in the FE analysis was taken
from literature where S355 welds were performed on 90mm thick plates as
considered in this study [19]. The stress concentration factor for
the chosen weld geometry was found to be 1.18 which falls between the
values reported by Kolios et al. [19]. The maximum stress produced
at the weld toe of the circumferential weld in the monopile was fully
reversed, i.e mean stress was zero. Assuming that the weld is free from
any residual stresses, a null mean stress condition would indicate that
the structure will have better resistance to fatigue crack growth since
only half of the cycle (when stress is tensile in nature) can contribute
towards fatigue crack growth. For all the six load cases (refer Table 2
for values) the stress vs. strain curve showed elastic behaviour similar
to that of shown in Fig. 8(c). This suggests that the stresses in the
monopile are well within the design criterion and the S-N fatigue design
life approach can be employed to estimate the remaining life of the
service exposed monopile. However, this study has not taken the effect
of residual stress and environment interaction which can have
significant effects on the result. Further, the calibrated material
model’s contribution was negligible since the stress values were found
be within the elastic regime of the material. However, the proposed
cyclic plasticity material model would be useful for applications
looking into the effect of hammering loads on the monopile during the
installation process in future work as well as the fatigue life
assessment in the case of storm events (i.e. over loads).
Fig. 9(a) shows the maximum stress and strain values for the respective
load cases obtained from the FE model. The selected weld geometry showed
a stress concentration value of 1.18 but it is worth mentioning that
depending on the weld profile, the stress concentration factor for OWT
weldments are reported to vary between 1.1 to 1.6 [19]. Therefore,
according to the DNVGL-RP-C203 standard [32] (Eq. 7 for as welded
joints in free corrosion environment which is referred to as D curve),
the number of fatigue cycles to failure corresponding to each load case
was determined.