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.