In order to satisfy the demand for clean energy, offshore windfarms are increasingly venturing into deeper, more challenging waters. As a result, leading companies are bringing to life ever-more adaptive, flexible and reliable foundation systems. By Will Daynes
Ever since mankind put sails to the wind, we have been harnessing the power of wind. In the centuries that followed we, as a species, have developed and perfected wind-powered machines of various shapes and sizes for a whole host of applications, from grounding grain and pumping water, to powering steam engines and harnessing electricity.
It is in the latter category that the greatest strides have been made in the last three decades, most notably since the landmark moment in 1991 when the world’s first offshore wind farm was constructed in southern Denmark. At the time, this farm included 11 wind turbines, each of which possessed a capacity of 450kW. Today, the largest offshore wind farm is the 175-turbine London Array, located 20 kilometres off the coast of Kent and with an operating capacity of 630MW.
Those initial windfarms tended to be built close to shore, meaning that virtually all of the key shallow water sites are now either occupied or being developed on. The result of this is that, in order to meet the increased demand for clean energy, consent is now being sought for sites further offshore in deeper waters. At the same time, a similar demand for increased generating capacity has resulted in ever-larger turbines being constructed, meaning that their foundations have become a potentially costly part of the design and a greater logistical challenge. This means that companies have had to look at ever-more innovative solutions to address issues that standard monopiles and jackets are less able to handle.
Float and submerge
A perfect example of such a challenging undertaking is that which has occurred 6.5 kilometres off the coast of Blyth. Wholly owned by EDF Energies Nouvelles, the Blyth Offshore Demonstrator project comprises five MHI Vestas V164 turbines, which together will have a generating capacity of 41.5MW and will ultimately produce enough low carbon electricity to power around 34,000 homes. When complete, the turbines will have the largest power rating used to date for an offshore windfarm. The project is also unique in that its location, with its topography, has resulted in the placement of the first gravity base foundations that were installed using a new ‘float and submerge’ method.
The company responsible for this pioneering approach to offshore wind turbine installation is BAMJV. Part of the Royal BAM Group, the JV comprises BAM Nuttall and BAM Infra who together have forged an enviable reputation for delivering significant civil engineering schemes in the rail, marine, tunnelling, highways and water sectors. “The idea of developing a floating gravity foundation, transporting it to site using conventional marine vessels, and then submerging it to the required depth is something we have been working on for some time now,” explains BAM’s Operations Manager for BAM Offshore Wind, Gavin Gerrard. “Our extensive background working with concrete and steel makes projects like Blyth familiar territory for us, while our past work with immersed tube tunnels and other marine projects makes us experts when it comes to floating concrete structures.
“The Blyth project presented a solid opportunity for us to demonstrate our solution to the problem of providing a strong foundation that not only floated, but could also act as a gravity foundation once submerged onto the sea bed, was viable. The solution, which was designed by in-house consultant BAM Infraconsult, is to basically create a concrete casing surrounding a monopile which has a point of fixture approximately 25 metres above the seabed. The foundation itself is submerged, before being further ballasted to provide the support structures for the turbines themselves.”
Made up of more than 1800 cubic metres of concrete, each gravity based foundation weighs more than 15,000 tonnes when fully installed on the seabed. Once complete, the structures have a total height of around 60 metres from the base to the access platform. The nature of the installation process means that the foundation’s interface with the seabed is relatively limited. It can also be installed without the need for heavy lift vessels, while the fact that no pile driving is required also helps to reduce marine noise, which is a common constraint when it comes to gaining planning permission for new windfarms.
As one can imagine, the challenge of designing a gravity base foundation of this type for the first time was not insignificant. “Despite our aforementioned experience when it comes to developing similar structures, this project still represented new territory for us,” Gavin states. “We worked closely with DNV GL as the verification body during design development leading to certification of the solution. In parallel we undertook the manufacture of the units at the Neptune Yard on the River Tyne and the preparation of the seabed before initialising the installation process, which were understood technologies, just not for this application. We found that this approach worked well and that our confidence in our capabilities was well founded.”
What followed was the process of moving the soft seabed material, preparing a pocket on the seabed and the placing of stone into this pocket to the tolerances required, all of which went extremely well. Equally as smooth was the transportation and submerging of the units themselves. That is not to say that there were not areas that could be improved further; however, as a learning experience, Gavin, his team, the company and indeed the industry as a whole have learnt some potentially game-changing lessons.
Having handed over the foundations, BAM’s involvement with the Blyth Offshore Demonstrator project is now effectively complete. Moving on, it is now Gavin’s task toenter into a period of product development and refinement. “At present, I am leading a team based in the Netherlands working on design, manufacture and installation efficiencies, while also working with a dedicated business developmentteam to facilitate conversations with parties that are interested in the technology we have developed for the Blyth project,” he continues. “We are in the process of refining our thinking regarding the mass production of these units and how we can go about supplying them to a full-scale windfarm. This is work we had already invested in prior to Blyth, however what we have experienced here provides us with a fascinating insight into possible cost savings, greater efficiencies, and improved fabricating and installation processes for installation en masse. This is a process that is ongoing and we are very keen to engage the market regarding new possibilities for the full-scale production of these gravity base foundation units.”
BAM Offshore Wind
Gavin Gerrard – Operations Manager for BAM Offshore Wind
For further information please visit: www.bamnuttall.co.uk