Abstract
A commonly used material in offshore structures is S355 structural steel. For example, during the monopile fabrication process, the material is pre-strained to different levels at different depths through the thickness. Therefore, the influence of pre-straining on fatigue life and crack growth behaviour of the material needs to be examined and considered for design and life assessment procedures. In the present study, uniaxial fatigue and fatigue crack growth tests have been conducted on materials with different pre-strain levels and the results are compared with the un-strained material state. From the test data, it has been seen that the S-N fatigue life will reduce with increasing pre-straining level, while the fatigue crack propagation rate remains largely unchanged in pre-strained material. The results from this study are compared with the recommended S-N fatigue and fatigue crack growth trends available in standards and are discussed in terms of the applicability and level of conservatism in the recommended curves to account for the material pre-straining effects on the fatigue life assessment of offshore structures.
Highlights
There is a push for offshore wind which has resulted in the offshore wind industry developing newer technologies and install more wind farms with turbines of higher capacities in deeper waters [1]
In order to better understand the influence of prestrains introduced during fabrication of offshore structures, the present study extensively investigates the fatigue life and crack growth behaviour in the presence of different levels of tensile pre-strains in S355 structural steel, which is widely used in fabrication of offshore wind turbine foundations, and the results are compared with the as-received material
In order to assess the importance of material pre-straining in fatigue design and life assessment of offshore structures, the obtained uniaxial fatigue data from this study have been compared with the S-N fatigue design curves recommended for the base metal in various design codes and the results are shown in Fig. 11(a) [26,27,28,29,30,31,32,33,34]
Summary
There is a push for offshore wind which has resulted in the offshore wind industry developing newer technologies and install more wind farms with turbines of higher capacities in deeper waters [1]. Monopiles are fabricated via the three roll bending process, a continuous manufacturing process involving cold rolling of previously hot rolled structural steel plates into cylindrical cans, prior to being longitudinally welded. The results from this study showed that tensile pre-strain had no significant effect on fatigue crack initiation and growth behaviour of the materials tested [18]. In order to better understand the influence of prestrains introduced during fabrication of offshore structures, the present study extensively investigates the fatigue life and crack growth behaviour in the presence of different levels of tensile pre-strains in S355 structural steel, which is widely used in fabrication of offshore wind turbine foundations, and the results are compared with the as-received material. The results are discussed in terms of the significance of prestraining effects on design and life assessment of offshore wind turbine monopile foundations, and provide new insights to enhance the life prediction of the offshore structures
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