As offshore wind projects move into deeper waters, tripod and jacket structures are becoming more favourable as support structures for offshore wind turbines. These structures are much lighter than traditional jackets for oil and gas platforms, and the cyclic loading is a larger proportion of the loads. The response of piles in dense to very dense North Sea sands subject to long-term cyclic axial loads is complex, and there are no standard methodologies to predict the effect of cyclic loading on the pile capacity. This paper briefly reviews available methods for assessing degradation of axial pile capacity due to cyclic loading. Foundation response under irregular storm loading is considered, and a new analytical approach for calculating pile capacity degradation is proposed. The method has been successfully validated against the Dunkirk pile load tests (Health and Safety Executive (HSE), 2000a) compared with the results of cyclic (T-Z) analysis, and applied to several large wind farm projects in the German sector.
The response of tripod and jacket piles to long-term cyclic axial loads is complex, and there are no generally accepted methodologies to predict the effect of cyclic loading in sand. Moreover, the majority of recent research on the effects of cyclic loading focused on cyclic lateral loads on monopiles. The analysis of cyclic loads for subsequent capacity degradation analysis is also outlined. Section 3 of this paper contains a brief review of the response of the soil adjacent to the pile during cyclic axial loading. During high-intensity cyclic axial loading, the pile capacity will reduce, and non-recoverable displacement at the pile head will accumulate. The effect of cyclic lateral loading on the axial capacity is also considered. Methods for axial degradation analyses are reviewed in section 4, together with the available pile test data.