Development of a Practical Soil Constitutive Model Suitable for the Design of Offshore Wind Turbine Foundations in Sand Considering Cyclic Loading

Offshore wind turbines are subject to cycles of loading and unloading from their environment i.e. wind and waves. This cyclic loading can significantly weaken the foundations over time leading to a reduced loading capacity when compared with a static load of the same magnitude. Structures can fail due to the build-up of plastic strain in the soil over time through successive loading cycles. In addition, there are failure mechanisms caused by liquefaction (where pore water pressure builds up and soil friction decreases). Through laboratory experiments this project will seek to develop a constitutive model of sandy soil under cyclic loading. This will provide a better understanding of the behaviour of sandy soils with cyclic loads applied. Therefore, it should be possible to improve the design of offshore wind turbine foundations. This is of particular significance to the UK as much of the seabed surrounding Britain has at least partial presence of sandy layers, with some areas comprised almost entirely of sandy soil e.g. southern North Sea. Improvements in the design of offshore wind turbine foundations should lead to material and cost savings, helping to drive down the levelised cost of energy.

The project methodology will be focussed around gathering results from cyclic triaxial tests. Triaxial tests are a standard industry test used to measure the mechanical response of a soil sample to prescribed stress and/or strain boundary conditions. The cyclic triaxial tests will apply computer controlled cyclic loads to a range of samples prepared with different initial densities and stress histories. The samples will be prepared in the lab, as opposed to testing cored samples from the field due to the tendency of sandy soils to break apart when loads are released. The results will be used to develop and calibrate a constitutive soil model, which will seek to improve the currently available models' shortcomings with respect to cyclic loading. Sandy soils are complex and therefore difficult to define fully with a constitutive model. Existing models prioritise modelling either static or cyclic loads at the expense of reducing accuracy with respect to the other load type. This project is novel in seeking to combine both cyclic and static loads into a single constitutive model.

This project is part of the Wind and Marine Energy Systems and Structures (WAMESS) Centre for Doctoral Training and falls within the EPSRC Wind Power, Ground Engineering and Energy research areas. This project will be run in partnership with Fugro, making use of their large commercial soil testing laboratory in Wallingford (near Oxford). The project will work in tandem with another doctoral project of a similar aim, but more focussed on mathematical and computational modelling aspects.

This outward-facing doctoral training centre will create impact through knowledge enhancement and leadership development which will have significant benefit for society, people and the economy.

Societal Impacts:
A very large increase in renewable energy generation, mainly wind, wave and tidal, is expected in the coming years and decades to meet the UK Government and international obligations to reducing greenhouse gas emissions by at least 80 per cent by 2050 when compared to 1990 levels. In particular, the Offshore Wind Industry Council is proposing, under a Sector Deal, to deliver 30GW of offshore wind by 2030 and 50GW by 2050, whilst reducing the average price of electricity by 18%. The longer term societal and economic impacts arise from the difference that the CDT programme and its graduates make to the UK realising this medium-term and longer-term target. The societal impact of meeting these targets, over failing to meet them, can be calculated in avoided CO2, increased sustainability, security and resilience of the energy system in a safe, affordable and environmentally sensitive manner.

People Pipeline and Skills:
There is a widely recognised skills gap in renewable energy both in UK and Europe. Hence, the proposed CDT is timely contributing significantly to meeting the sector's skills demand by the provision of highly trained engineering leaders, expert in a broad range of wind and marine energy technologies and engineering. Most of the CDT graduates will be expected to take up posts in the growing commercial wind and marine energy sectors, and quickly rise to positions of leadership and influence. Some graduates will remain in the higher-education sector and develop academic careers providing much needed increased capacity and capability resulting in a positive impact through an expanded research-base and capability to deal with the inevitable research challenges of the sector as it develops further commercially.

Students will be mentored and encouraged to take a proactive role in creating impact with their research whilst observing Responsible Research and Innovation (RRI). All the Universities participating in this CDT proposal have explicit policies and resources in place to support knowledge exchange and impact and also public engagement. These support the students throughout their studies to engage in broader dialogue and deliberation and to be aware of the potential impacts and implications of their research.

Our CDT students will also engage in outreach activities and impact the wider community through the well-established Professional Engineering Training Scheme (PETS): this scheme is managed and directed by the students and provides opportunities to engage in outreach activities and to work with peers. e.g. PETS runs a schools and colleges programme wherein the students organise visits to schools and colleges to provide information about renewable energy and a basic introduction to the technology involved.

Economic Impact:
The low-carbon and renewable energy sector is estimated to increase five-fold by 2030, potentially bringing two million jobs to the UK. In particular, an ambitious Sector Deal for industry proposed by Government as part of its Clean Growth Strategy could see a total installed capacity of 30 GW of offshore wind by 2030 with the potential to create at least 50,000 jobs across the UK. If achieved, this would be a six fold increase from the current installed capacity and would make offshore wind the largest source of domestic electricity. To ensure resilience, it is also important to retain and develop the leading UK Wave and Tidal position. With the direct and indirect value added to the UK supply and installation chain in terms of job creation, intellectual property exploitation, and sales of wind, wave and tidal technology and services, the proposed CDT will make an important contribution through knowledge enhancement and leadership development.