Screw piles for wind energy foundation systems

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Screw (or helical) piles are foundations which are screwed into the ground. They are widely used onshore for supporting motorway signs and gantries as they possess good tensile and compressive resistance. This project aims to make screw piles a more attractive foundation (or anchoring) option offshore for wind farms, which are being deployed in deeper water and subject to increasing performance demands. The UK has challenging targets for expansion of energy from renewables with the potential for over 5000 offshore wind turbines by 2020. The necessary move to deeper water will increase cost and put greater demands on subsea structures and foundations. The current foundation solutions being considered for these applications are driven piles, large monopiles or concrete gravity based structures (GBS). Driving of piles in large numbers offshore causes concerns over plant availability and impact on marine mammals. There are also concerns over the limit of practical monopile development and the high material demands of GBS. Screw piles have the potential to overcome these issues and are scalable for future development from current onshore systems which have relatively low noise installation and are efficient in terms of both tensile and compressive capacity. To meet offshore demands, screw piles will require geometry enhancement but it is envisaged that these will initially be modest to allow de-risked transfer of onshore technology offshore. This will lead to the deployment of several smaller piles or pile groups rather than moving straight to very large single screw piles that may prove difficult to install and require significant investment. To allow screw piles to be considered as a foundation solution for offshore wind this project will develop piles with optimised geometries that minimise resistance to installation but are capable of carrying high lateral and moment loads. In order to install screw piles torque devices are used to effectively screw the anchors into the ground. With increased pile size requirements and potential changes in geometry this project will develop improved, less empirical techniques to predict the torque required in a variety of soil conditions. This will allow confidence in pile installation and investment in appropriately sized installation plant. As new pile geometries are being developed these will need to be tested (through model, numerical and field testing in this project) to verify that they can meet the performance demands of the offshore environment. The project will also develop bespoke analysis techniques to allow consulting geotechnical engineers the tools they require to design the foundations and contractors the tools to inform the installation processes. As piles can be deployed as large single units or smaller units in groups the efficiency of group deployment and multiple foundation geometries will be explored, as using several smaller geometry foundations could reduce the risks during offshore installation and actually be more economic due to lower fabrication costs and demands on installation plant. The areas of investigation above will be combined to produce a design and decision making toolkit for use by geotechnical designers to allow deployment of screw piles as offshore foundations in an efficient and cost effective manner.The research has the potential to make it easier to deploy screw pile foundations for offshore renewables. This project will develop foundations able to deal with current water depths and will provide understanding of the behaviour of piles as water depths and the demands on the foundations increase. By harnessing the installation and performance benefits of screw pile/anchor technology, the results of the project will contribute to an overall cost reduction in electricity generated by renewable means and increase the public's confidence in the future viability of this energy source.