The UK is the world leader in offshore wind energy; almost 40% of global capacity is installed in UK waters. A new ambitious target of 40GW of wind power by 2030 aims to produce sufficient offshore wind capacity to power every home, helping to achieve net zero carbon emissions by 2050. Offshore wind turbine (OWT) foundations, which are typically steel monopiles, contribute approximately 25% to a windfarm's capital cost. The size of OWTs is increasing rapidly and continued optimisation of foundation design is paramount. Recent research has led to significant advances through theoretical developments combined with high-quality field-testing. Despite recent advances, there remains significant uncertainty in the measurement and interpretation of key soil deformation parameters that underpin new and existing design approaches. The central aim of SOURCE is to use rigorous measurement and interpretation in the field and laboratory to quantify and reduce material parameter uncertainty and minimise the impact on the predictive capability of OWT foundation design methods. Improved site characterisation will contribute to increased security in design, lowering capital costs, subsidies and carbon emissions and meeting the UK's ambitious new energy targets.