Abstract
During their service life, monopiles supporting offshore wind turbines are subjected to large numbers of lateral cyclic loads resulting from complex environmental conditions such as wind and waves varying in amplitude, direction, load eccentricity and frequency. The consequential accumulation of displacements and rotations of the foundation structure with cyclic loading is one key concern in the design of monopiles. Nevertheless, the relevant offshore guidelines do not provide suitable procedures for predicting such deformations. Although there are several methods for this purpose in literature, some of them produce very different or even contradictory results, which prevents a consistent approach to dimensioning. This paper briefly summarizes the current standardization regarding design of monopiles for cyclic lateral loading and provides some examples of possible prediction models from the literature. To highlight the need for further research, the predictions according to different approaches are compared and evaluated by a calculation example and a parameter study. Further, the results of a small-scale 1 g model test campaign on the load-displacement behaviour of monopile foundations subjected to lateral cyclic loading and the influencing parameters are presented, evaluated and compared with the findings of other research groups. In this way the tests results can help to support or improve model development and provide insight into key issues relevant to monopile design. The parameters that have been assessed include the cyclic load magnitude, cyclic load ratio, load eccentricity, soil relative density, the grain size distribution of the non-cohesive bedding material as well as the pile embedment length.
Highlights
Offshore wind energy is one of the promising solutions for sustainable energy, but for the wind industry to be competitive, it is vital that costs are significantly reduced for future projects
If only the values for system 1 or system 3 are considered, for which the load eccentricity was varied, it becomes clear that this behaviour is more likely to be due to experimental scattering
With respect to the accumulation parameter (α) from Eq 1, the results indicate that it appears to be largely independent of the cyclic load magnitude, the soil relative density (Dr), the load eccentricity (h) and the embedment length of the pile (L) for one-way cyclic loading, as long as the pile-soil system is characterised by an almost rigid load-displacement behaviour
Summary
Offshore wind energy is one of the promising solutions for sustainable energy, but for the wind industry to be competitive, it is vital that costs are significantly reduced for future projects. A governing factor in the design of monopile foundations is compliance with serviceability limit state and associated strict tilting tolerances This means that an accurate prediction of pile displacement and rotation accumulation resulting from cyclic occurring horizontal loads plays a key role for the final dimensions of the foundation structure and its costs. 40 current offshore guidelines do not provide appropriate procedures for the prediction of pile displacement accumulations, which is especially true for monopiles, which due to their large dimensions and low L/D-ratios have a significantly different loaddisplacement behaviour than long and slender piles
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