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Offshore Wind Journal

Planning ahead – and planning for every eventuality – speeds construction of UK test centre

Mon 18 Jun 2018 by David Foxwell

Planning ahead – and planning for every eventuality – speeds construction of UK test centre
As of mid-June 2018, offshore work for the EOWDC was all but complete

The European Offshore Wind Deployment Centre will test next-generation offshore wind technology. First power is due to be transmitted on time despite adverse weather and a very tight timescale, as the project's director told OWJ

A court challenge, tight timescale, adoption of new technology, a challenging site with varying water depths and seabed conditions – even unexpected contamination found onshore where the substation for the project was to be built – proved no obstacle to the development of the European Offshore Wind Deployment Centre (EOWDC), which is expected to produce first power in mid-July and be fully commissioned shortly afterwards.

Famous for the fact that the current incumbent of the White House, Donald Trump, launched a court case to try to prevent it being built, the EOWDC was fast-approaching completion in mid-June, when OWJ spoke to Vattenfall’s project director, Adam Ezzamel. With the court case now a distant memory, the EOWDC is becoming rightly known for several world-firsts and for some important lessons learned during the construction phase.

As Mr Ezzamel explained, one of the biggest challenges to be overcome building the facility was that, by the time the court case had been resolved, there was very little time between the company’s final investment decision (FID), and a 30 September deadline for any windfarm seeking to be accredited under the Scottish Government’s enhanced renewables obligation certificate scheme for offshore wind test and demonstration projects to be completed.

Two years is no time at all for a complex project using so much new technology, including the world’s most powerful offshore wind turbines, new 66 kV cables and potentially most challenging of all – at least so it seemed prior to the project getting under way – the first large-scale use of a new type of foundation for the turbines at the EOWDC, the suction bucket jacket foundation.

Located in Aberdeen Bay, the main aim of the innovative 93.2 MW, 11-turbine offshore wind project is to test next-generation technology and, once operational, boost the industry's drive to competitive clean power. It is not merely a testing facility however, and once operational will generate more than the equivalent of 70% of Aberdeen’s domestic electricity demand and 23% of the city’s total demand.

Construction of the facility began in October 2016 and at the time OWJ interviewed Mr Ezzamel most of the offshore work had been completed, bar installation of scour protection around the foundations. That the project has nearly been completed in such a short timescale is all the more remarkable given the sometimes-atrocious weather experienced earlier this year.

“It is true that we sometimes experienced quite long delays due to the weather,” Mr Ezzamel told OWJ. “There were long periods of time when easterlies plagued work at the site and made it very difficult. Foundation work and installation of the cabling offshore were affected, but we caught up and the performance of the contractors on the project, Boskalis and Swire Blue Ocean has been terrific.”

Using a totally new type of foundation was always going to be potentially challenging but as the project progressed installation of the suction bucket jacket foundations turned into one of the project’s biggest successes.

The novel foundation type supports a three-legged jacket foundation which in turn supports a turbine. Unlike a conventional foundation, such as a monopile which is driven into the seabed using a pile driver, the suction bucket jacket can be installed without a pile hammer and without the underwater noise that pile driving causes and its potential adverse effects on marine life.

The foundations are lowered into place from an installation vessel – in this case two vessels, Asian Hercules III and Heerema's heavy-lift vessel Aegir. Once in place on the seabed air and water are pumped out of the suction buckets, anchoring them in the seabed. Unlike pile driving, the process is virtually noiseless. It has the added advantage that, when the project is decommissioned, the installation process can be reversed, removing the entire foundation.

“There is no doubt about how well the suction bucket jacket foundations worked,” Mr Ezzamel told OWJ. “It was a massive success for the project. There was undoubtedly a learning curve involved for the installation contractor, but I am in no doubt that they are quicker and easier to install than many other foundations. It also helped a great deal that we carried out test installations using suction bucket jacket foundations in 2016 and 2017 and that we were well-prepared.”

This was borne out by the fact that the final foundation was installed in not much more than two hours, using Aegir, operating in dynamic positioning mode.

A DP vessel isn’t essential to the process – foundations installed earlier in the project using Asian Hercules III required an anchor spread, which took more time. Even with a less sophisticated vessel, if you take the time it took to install the anchor spread out of the equation the actual installation process took 2-4 hours, Mr Ezzamel explained.

The key to the process, he said, is that whatever vessel is used to install the suction bucket jacket foundation, a single lift is all that is required, unlike a monopile installation, which needs the foundation to be lifted from the deck of the vessel, up-ended, positioned – often using specialised equipment fitted to the vessel such as monopile upending tools – and then driven into the seabed, only after which is the transition piece and tower installed.

The jackets, which are between 68 m and 81 m high and weigh in excess of 1,300 tonnes, were constructed by Belgian-owned Smulders UK. As Mr Ezzamel explained, due to the particularly tight timescale for the project, some creative thinking was required to ensure it ran to schedule, so the steel for the foundations was ordered before the design process for them had been completed, to ensure it was available when needed. Smulders also began fabrication work before the detailed design of the suction bucket foundations was completed, to make sure they would be available and ready to be shipped when needed.

Although somewhat larger than early-generation turbines, MHI Vestas Offshore Wind’s V164 8.4 MW turbines proved relatively straightforward to install, said Mr Ezzamel. As he also noted, one of the ironies of the Trump challenges to the project and the delay that resulted from it is that Vattenfall was able to take advantage of newly-developed, more powerful turbines than would have been available had construction proceeded when originally planned. In fact, two of the 11 turbines installed off Aberdeen will be capable of 8.8 MW, this being the first time an 8.8 MW model has been deployed commercially in the offshore wind industry.

However, a lot of careful work and analysis went into ensuring the turbine installation vessel, Swire’s Blue Ocean’s Pacific Orca, could jack-up and then extract its legs from the varying seabed conditions that prevail at the EOWDC site.

Although small compared with a commercial-scale project which might have a hundred turbines compared with the EOWDC’s 11, the water depths at the site vary and the subsoil varies a good deal too. Over what is a relatively small area, there are four different types of subsoil at the locations for the turbines, ranging from stiff clay underlain by different material to a much sandier substrate.

“We knew that the leg penetration into the seabed Pacific Orca would require when it jacked up could, potentially, be problematic, given the differing sub-surface conditions,” Mr Ezzamel told OWJ. The offshore industry is littered with instances of problems encountered when vessels jack-up, such as punch-through, that is rapid penetration through a stronger soil layer overlying a weaker one, instances of problems when the seabed is uneven or when there are footprints from previous projects or when scour occurs. Extracting legs from the seabed can also be problematic if conditions vary.

“At some of the locations where turbines were going to be installed the legs needed to penetrate to a depth of 22 m. That would make removing them from the seabed quite challenging, so we collaborated with Swire Pacific Offshore to develop a solution.” He explained this mainly involved strengthening the hull of the vessel in and around the jack-up legs to take account of potential non-linear forces imparted to it, and a thorough pre-installation analysis of conditions at the site of each turbine.

The contract to install the ground-breaking 66 kV inter-array and export cables for the Vattenfall project was awarded to VBMS, now part of Boskalis. Cable manufacturer JDR supplied more than 20 km of inter-array and export cables for the test centre.

As highlighted above, cable lay operations were affected by the weather earlier in 2018, but otherwise installation of the 66 kV cabling has gone according to plan. From an installation and operational point of view, the 66 kV cable differs little from a conventional 33 kV cable although the termination for the cable is slightly different to its predecessors.

Work onshore is almost completed too. Work started on the onshore substation for the project in October 2016 with preliminary work constructing the substation and associated cabling works at Blackdog in Aberdeenshire, and is on schedule to be completed as planned. This is despite construction company J Murphy & Sons, appointed by Vattenfall to deliver the onshore substation and the associated cabling, encountering asbestos and other potentially dangerous material at the former landfill site.

With the construction phase of the project approaching completion, the EOWDC will soon transition into the operations and maintenance (O&M) phase, for which an O&M facility has been built in a refurbished warehouse in Aberdeen Harbour. Windfarm technicians will be transported to and from the windfarm in a crew transfer vessel contracted from Windcat Workboats.

A programme of scientific work associated with the EOWDC has been underway for some time. “It is important to harness the EOWDC as an opportunity to conduct in-depth research into offshore wind at a full-scale, near-shore facility,” Mr Ezzamel concluded. “Each of the projects being undertaken has the potential to offer new insights into the sector.”

They include a project that aims to provide information on the extent to which offshore windfarms influence salmon and sea trout, another that involves undertaking a comprehensive study of bottlenose dolphin movements throughout the development and operational phase of the EOWDC, a third measuring connectivity between bird special protection area populations and offshore windfarms, and one addressing the socio-economic impact of offshore wind on the human environment that aims to help understanding of how offshore wind developments can maximise benefits to local and regional communities.

 

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