Development and installation optimisation of screw piles for offshore wind energy foundation systems

Offshore wind energy has rapidly become a mainstream and cost-competitive power source, crucial for meeting rising energy demands sustainably and improving energy security. The anticipated surge in offshore wind capacity will likely require many jacket installations in intermediate water depths, leading to a substantial increase in foundation deployments. Traditional impact or driven piling for these foundations generates prolonged and considerable noise, disturbing marine life and requiring costly and potentially environmentally impactful mitigation measures. Screw piles, comprising helical plates on a central shaft, offer the potential for reduced installation time and noise, and enhanced axial capacity compared to driven piles. Typically, screw piles are used onshore for relatively lightweight structures and the established screw pile industry has empirically derived design methods for installation and capacity prediction. The direct application of these methods to the much larger screw piles needed for offshore wind turbine jackets is unproven, requiring research into their efficacy and optimisation. To investigate the performance of large plugged or solid screw piles, a centrifuge testing programme using scaled models in sand was conducted. Initial tests of piles created using existing design methods and installed at the industry standard advancement ratio (AR), revealed the need for very high installation torque and vertical (crowd) force. For these conditions, CPT-based prediction methods for installation torque and crowd were developed in addition to analytical methods for axial tensile and compressive capacity. Further testing, including with a novel instrumented double-helix pile, explored the impact of varying the advancement ratio on installation requirements and tensile loading performance. Insights gained from these tests, coupled with data from published numerical simulations of a single-helix screw pile using the discrete element method, were used to create new design methods for predicting the installation torque and crowd of a closed-ended screw pile using in situ CPT data. Importantly, these methods include the effects of the advancement ratio and are applicable in loose to dense sand. Similarly, a CPT based tensile capacity prediction method was also developed which accounts for the increase in capacity from low AR installations. These novel design methods are expected to significantly contribute to the development, optimisation, and broader adoption of screw piles as a more sustainable and effective foundation solution for the expanding offshore wind energy sector.