This paper describes aspects of the foundation design methodology developed for the Borkum West II offshore wind farm in the German North Sea, comprising 40 turbines supported on piled tripods in water depths of approximately 30m. The foundation design evolved during a technical due diligence process, which offered the opportunity to review the site investigation data and cyclic loads, to reconsider the effects of cyclic loads on pile resistance and to modify pile lengths and wall thicknesses to mitigate pile tip integrity risk during driving in very dense sands. The re-evaluation of the design storm concluded that axial pile capacities could decrease by up to 25% because of cyclic loading at some turbine locations, but could be almost unaffected by cycling at others. The technical review involved a collegiate process that contributed to the development of acceptable foundation designs and mitigated risks relating to pile installation and foundation performance.

1. Introduction

The Borkum West II wind farm is currently being developed by Trianel Windkraftwerk Borkum GmbH in the North Sea, approximately 45km offshore northern Germany (Figure 1). The first phase of this project includes the construction of forty 5MW turbines. The hub height is approximately 90m above sea level, and the rotor diameter is 116m. The turbines are supported in water depths of 26m to 33m by tripod structures designed by Offshore Wind Technologie GmbH. Figure 2 illustrates the general arrangement of the steel tripods, which have an outer footprint diameter of 28m. Geotechnical engineering was undertaken for the project by Cathie Associates (CA) SA/NV, Belgium. The ground investigations, interpretation and foundation designs were performed in accordance with the standards for offshore wind farms, published by the Bundesamt für Seeschifffahrt und Hydrographie (BSH, 2007, 2008; also known as the Federal Maritime and Hydrographic Agency).

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