Cyclic performance of screw piles for offshore floating wind

Abstract

Offshore renewable energy e.g. wind and wave has been regarded as an important solution to achievement of net-zero targets. Screw piles have been proposed as a type of reliable, silent, and cost-effective technology for foundations and anchors for offshore renewable energy application. Although the monotonic loading response of screw piles for this application has been investigated, the cyclic loading response has not received adequate attention. Mechanisms underlying the apparent cyclic response remain unclear. In addition, installation approach has an influence on post-installation in-service performance. Along with a significant reduction of installation vertical force, overflighting (advancement ratio, AR < 1.0) has been shown to enhance monotonic tensile response. It is unclear if this enhancement can be maintained during cyclic loading.

In this thesis, a series of centrifuge experiments was conducted on screw pile installed in medium-dense sand at various AR values. It shows that decreased AR values result in improved cyclic performance i.e. reduced cyclic displacement accumulation with increased loading stiffness. It also suggests that ultimate tensile capacity loss due to cyclic loading is only related to displacement accumulated during cyclic loading and ultimate tensile capacity prior to cyclic loading, regardless of magnitude of cyclic loading. Prediction frameworks for displacement accumulation and capacity loss were developed considering the effect of AR values adopted for installation.

DEM simulations highlight contribution of helix and pile shaft separately, with insights into mechanisms controlling cyclic response. Large loading displacement developed in each single cycle may instigate soil flow around the helices, resulting in significant permanent displacement accumulation. Due to its higher loading stiffness, mobilisation of pile shaft resistance can significantly reduce loading displacement in each cycle, as well as displacement accumulation. Effects of magnitude of cyclic loading, average loading and installation AR values are discussed based upon understanding of the controlling mechanisms.

DEM modelling was also used to investigate effect of helix number and spacing. Increase of helix number, for the medium dense sand studied, increases installation disturbance on soil around piles, leading to reduced pile shaft resistance and cyclic performance. Although it may further reduce installation torque if over-flighted approaches are used incorporated with low helix spacing (e.g. < 2 helix diameters) and may enhance II monotonic tensile capacity. The low helix spacing design may also reduce potential for soil flow around the helices during cyclic loading. Such a design may tend to reduce cyclic displacement accumulation, if the additional installation disturbance induced by additional helices is insignificant.

Date of Award	2025
Original language	English
Awarding Institution	University of Dundee
Sponsors	China Scholarship Council
Supervisor	Michael Brown (Supervisor) & Matteo Ciantia (Supervisor)