Effect of installation advancement ratio on two-way cyclic response of a single-helix screw pile for offshore wind

Screw piles have been proposed as a silent and efficient type of foundations and anchors for offshore renewable energy application (e.g. floating wind). For large screw piles, conventional pitch-matched installation (advancement ratio, AR = 1.0) may be impractical due to the prohibitively high installation crowd force. By over-flighting (AR < 1), it is possible to reduce installation crowd force to zero and create pull in forces, as well as improving in-service tensile (both monotonic and cyclic) response at expense of reduced compressive response. For some applications e.g. serving as a foundation of a jacket structure, the pile may be subject to two-way cyclic loading. This study uses centrifuge tests to illustrate symmetric two-way, i.e. being in tension and compression alternatively, cyclic performance of a screw pile installed in sand, with effects of variation of advancement ratio being considered. Compared to previous one-way tensile cyclic tests, change of direction of displacement accumulation creates a more abrupt accumulation for the two-way loading cases depending on the magnitude of the load applied. The optimal AR value for two-way cyclic performance (AR = 0.5 in this study) is higher than that seen as optimal (AR = 0.25 in this study) for purely one-way tensile cyclic performance, as a balance of compressive and tensile response is needed. In all cases though, over-flighted installation it is observed to have a beneficial effect on cyclic performance.