Beatrice Offshore Wind Project, Wind Turbine Generator Foundation Design

Abstract

The Beatrice Offshore Wind project comprises the development of 84 number 7MW turbines located in the Moray Firth in the North of Scotland. It is one of the most northerly and exposed sites globally and also the deepest site for fixed foundations for offshore wind. The paper describes the design challenges and how they were addressed. The solutions are likely to be of interest to anyone else developing a deep water offshore wind project, especially with variable soil conditions and a significant water depth range. The Beatrice WTG foundations were completed as an EPCI project with close integration between the EPCI contractor, designer, fabrication and installation teams. Across the site water depths ranged from 35-68m, however, this range was reduced when a small number of outliers were discounted. The final range was 38-55m. There was also a significant variation in soil conditions across the site and this created significant challenges when attempting to standardise the design. Global analysis of the selected structures under wind and wave conditions was performed using Sequential Coupled Analysis (SCA) with BHawC and ANSYS ASAS software packages. The analysis was performed by passing information between the turbine supplier and the substructure designer to produce coupled wind and wave loading on the integrated jacket, WTG and tower system. The jackets are the largest ever designed and installed for offshore wind. The solution developed was a 4 legged pre-piled jacket. The design was split up in to 5 clusters to address the water depth range. Associated with this a pile stick up range of 2-6m was adopted. No scour protection was used. The top half of the jacket, transition piece and much of the secondary steelwork was standardised across the site. The base dimension for the jacket and pile diameter was also standardised across the site to allow for re-use of a pre-piling template. The final foundations were installed in 2018. The grouted connection between the jacket and the pre-piles includes the first application offshore of Masterflow 9800 grout. Control of early age cycling was a key consideration in design of the jacket to pile interface. Piles were designed in accordance with the Imperial College Pile ‘ICP’ effective-stress pile design approaches for offshore foundations.