Centrifuge Modeling of the Installation Advancement Ratio Effect on the Cyclic Response of a Single-Helix Screw Pile for Floating Offshore Wind

Upscaled screw piles have been proposed as anchors for offshore floating wind applications, but this upscaling can result in a significant increase in vertical installation forces. Previous studies have shown that the use of over-flighting techniques during installation (installation advancement ratio, AR < 1.0) can reduce or eliminate these forces and improve the capacity and stiffness of screw piles under monotonic tensile loading conditions. However, the impact of over-flighting installation on the cyclic response of screw piles has not received adequate attention. To address this, a series of drained cyclic uplift tests in a geotechnical centrifuge were conducted. The tests involved different AR values during installation and varying one-way cyclic tensile loading amplitudes. The results revealed that reducing the installation AR can significantly decrease displacement accumulation and improve cyclic loading stiffness, resulting in a more stable cyclic response. The cyclic axial loading stiffness tends to stabilize or slightly decrease with cycling for stable cases, whilst unstable and meta-stable cases exhibit an initial reduction of loading stiffness followed by a stabilization or slow recovery. The post-cyclic monotonic uplift tests also show that capacity degradation was predominately due to the displacement accumulation itself rather than any additional cyclic effects. The cyclic stability of the screw pile investigated was found to be comparable with straight-shafted and screw piles from previous studies and beneficial installation effects were maintained under cyclic loading. A predictive framework for displacement accumulation and capacity degradation is also presented and developed within this paper.