Large-diameter monopiles are an established foundation type for offshore wind energy converters. A crucial aspect in the design of such piles is the bearing behavior under cyclically acting wind and wave loads, namely the accumulation of deflections and rotations under these loads. Regarding the prediction of the behavior under cyclic one-way load, some empirical approaches and also numerical simulation methods exist. In contrast, the behavior under cyclic two-way loading is widely unclear and must be investigated by pile tests. This paper presents the results of 1 g model tests with large-diameter piles in sand under arbitrary cyclic loading types. It was found that for almost rigid large-diameter piles, the effect of the loading type on the rate of displacement accumulation could be accounted for by a function which is almost independent on the system parameters, e.g., pile stiffness or relative density of the sand, but dependent on load eccentricity. The maximum accumulation rate occurs for asymmetric two-way loading. Using the functions found in the tests, a prediction of displacement accumulation for load spectra consisting of packages of arbitrary loads becomes possible. For flexible piles, a different effect of the loading type on the rate of displacement accumulation was observed. Here, asymmetric two-way loading was obviously not more unfavorable than one-way load.

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Abstract. During their service life, monopiles supporting offshore wind turbines are subjected to a large number 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 the 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 test 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 and the pile embedment length.

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