Cost and noise-reducing foundation for Deutsche Bucht sets sail

The mono bucket – a cost-effective concept that combines the benefits of a gravity base foundation, suction bucket and monopile – is being transported to the windfarm on Seajack’s offshore installation vessel Scylla.

Scylla is currently heading to the construction site, which is 100 km from the mainland.

Northland Power’s Deutsche Bucht is the first commercial project to test this new type of offshore foundation, which consists of a single steel cylinder – the suction bucket – upon which a shaft is mounted. A transition piece and a turbine are installed on the shaft.

The foundation is installed using a patented installation process in which the pressure in the cavity between the foundation and the seabed generates water flow, which reduces resistance around the edge of the skirt of the foundation. This reduced resistance allows seabed penetration.

An installation control system controls vertical alignment, ensuring that the foundation is installed within prescribed tolerances. Installation is almost completely noise free. For decommissioning, the foundation can be removed for reuse or recycling by reversing the suction process. This involves applying water pressure to the foundation skirt so that it can be lifted from the seabed.

The mono bucket concept has been in development at Universal Foundation since 2001, based on experience gained from the oil and gas industry, and was successfully used in applications such as the foundations for offshore met masts.

The 269-MW Deutsche Bucht supplied its first power to the grid in July 2019. A total of 33 MHI Vestas 8.4-MW turbines are due to be installed on the windfarm using conventional foundations.

In May 2018, Northland was granted a licence by the German authorities to install two more turbines using mono buckets.

Production from the turbines in the 17-MW mono bucket demo project will earn the same fixed feed-in tariff as the base project for approximately 13 years under the German Renewable Energy Act, equating to approximately €184/MWh (US$205/MWhr) for eight years and €149/MWhr (US$166/MWh) for the remainder.

Northland Power said the mono bucket demonstration could be of significant benefit for future offshore windfarms and enable Northland to construct projects more quickly and with lower costs.

Deutsche Bucht is approximately 95 km to the northwest of the North Sea island of Borkum in Germany’s exclusive economic zone.

In March 2018, Universal Foundation and Hitachi Zosen teamed up to undertake a feasibility study to assess the application of mono bucket foundation technology for a Japanese offshore wind project.

The project will use experience gained from commercial projects in Europe and the US and apply it to conditions and requirements relevant to the Japanese offshore wind market.

Hitachi Zosen Co Ltd is executing the work in partnership with Kyoto University and Toyo Construction Co Ltd.