Peter Jones, Engineering Manager Grid Systems ABB UK, explains the background to the development of ABB’s new self-installing gravity-base structure (GBS) platform concept for offshore HVDC projects
ABB is currently working on a contract worth around $1 billion for TenneT, the Dutch-German transmission grid operator, to create the 900 MW DolWin2 power link that will connect offshore wind farms in the North Sea to the German mainland grid.
DolWin2 is ABB’s third offshore wind connection order for TenneT, following the 800 MW Dolwin1 link, which is now drawing to completion, and previously the BorWin1 project. Both the BorWin1 and Dolwin1 projects feature conventional fixed platforms to house the offshore converter stations. However, the DolWin beta platform is based on a new GBS design concept, building on experience gained from semi-submersible floating platforms for the oil and gas sector.
Wind farms in the DolW in cluster – including the 400 MW Gode Wind II project and other wind farms – will be connected by 155 kV AC cables to the HVDC Light® converter station platform situated in the North Sea. This will then transmit the electricity at +320 kV DC via 45 km of subsea cable and 90 km of land cable to the onshore HVDC station at D√∂rpen-West, where it will be converted to 380 kV AC and connected to the German main grid.
The platform challenge
The first HVDC link to connect an offshore wind farm with an AC grid was the 400 MW BorWin1 project. Based on HVDC Light® technology, this 200 km link connects the Bard Offshore 1 wind farm located off Germany’s North Sea coast to the HVAC grid on the German mainland.
The drawback of the conventional fixed platform (jacket and topside) featured on BorWin1 is that installation/lifting is only possible in the better sea conditions found during May to September. Furthermore, with a 1,000 MW HVDC station weighing in at around 10,000 tons, it requires the world’s largest crane vessel, which has implications for both costs and availability, and multiple offshore lifts. Most designs are also not suitable for shallow water.
One alternative is the jack-up self-installing (floating) platform. Its installation has no need for a large crane vessel, instead using a complex design of jack-up system and platform to handle the offshore jack-up operation. Many yards have the capability to fabricate platforms with no design risk, although current experience is limited to large platforms of over 10,000 tonnes.
ABB has worked with Aibel, the Norwegian offshore engineering company, to develop a new self-installing gravity-base structure (GBS) platform design, based on proven technology from the oil and gas sector. It provides a global design for 700 to 1,100 MW projects and detailed designs for 800 and 900 MW applications with a design life of 30 years.
Intended for use with wind farms in sea depths of between 15 and 45 metres, the GBS platform is constructed onshore. All the platform systems are fully commissioned in dock, minimising offshore hook up works.
The platform is then towed into position by tugs and secured on the seabed by its own weight and ballasting. This approach significantly reduces the weather dependence of the installation operation and offshore commissioning is limited to energisation and trial runs following installation of the HV cables.
Reduced environmental impact
The GBS platform is designed to reduce environmental impact. It has the potential to simplify the requirements for seabed preparation, while the elimination of noisy piling operations ensures there is no impact on wildlife. The platform is also easy to remove and decommission at the end of its service life.
Optimised operation and maintenance
Optimisation of operation and maintenance were key factors in the design of the GBS platform. It is intended for remote operation, but is also prepared for helicopter and boat access. The provision of living quarters and equipment storage facilitates the use of the platform as a service point. Indoor walkways are provided and the living and working sections are localised to ensure working safety. Systems for large equipment replacement projects are incorporated.
GBS moves from concept to reality
The first ever GBS platform, for Dolwin2, which is believed to be the world’s largest offshore wind platform, is currently under construction on behalf of ABB and Aibel at the Dubai shipyard, Drydocks World.
The Dolwin beta platform will have a capacity of 900 MW and include accommodation facilities and a helicopter deck. Following fabrication, it will be delivered to Aibel’s yard in Haugesund, Norway, for fitting out. DolWin2 is scheduled to come on line in 2015.