Floating Offshore Wind: Attention Changes Underwater

Being an ugly duckling, floating offshore wind is now Cinderella in the world of offshore renewable energy. Currently, attention is being paid to even the concept of floating and substation substations to help bring this power to the shore.

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Over the past year, expectations for growth in floating offshore wind turbine development have increased significantly. Rapidly increase renewable energy capacity as many companies enter the market, 15 MW turbines are imminent, and GW-scale projects may be imminent by the end of the decade. I think we will have a chance.

Much has been said about the various floating offshore wind concepts. However, once you enter a floating or substation substation, you may want to help the enclosure and then raise the voltage of the power to a level sufficient to export it where you need it.

Justin Jones, a consultant at engineering firm Petrofac, said: water. At depths of 100 m, fixed structures are relatively large and heavy, so it is expected that floating substations will be used to collect, convert, and transmit high-voltage power to the shore. As the wave load increases, a larger footprint is required, and the natural cycle and fatigue of the structure become more important. “

In countries such as Japan and the west coast of the United States, where the depth drops rapidly to 700-900 m, buoyancy is needed if you want to build offshore wind power on a large scale. In fact, Japan has already tried a small 25MVA, 100 ton floating transformer in the Fukushima Forward project. Installed in 2013, it was recently decided to abolish the structure (a kind of spar) for the profitability of operating the structure and the three demonstration turbines from which it collected power.

Some are considering the concept of floating, where research is being conducted on these types of structures for offshore wind power, such as Newfoundland and Labrador. Meanwhile, substation substations, including the current Scot Wind offshore wind farm, have also been proposed. Some of the people working on the concept build on their experience in the oil and gas industry, where transformers, seabeds and tops have been developed and provided for the oil and gas industry.

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Image courtesy of Aker Offshore

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Kenneth Simonsen, SVP of Aker Solutions offshore wind power, has designed its floating substation focused on servicing wind power plants in the range of 250 MW to 1.3 GW and 220 KV, developed in oil and gas. And states that it is based on a proven standardized building block. “The simple hull is based on the design of the Blind Faith (semi-submersible production designed by Aker Kværner) platform and the Njord platform (semi-submersible manufactured by Kværner Stord), which have been in operation for over 10 years. The (installed in 1997) used as a deck reference) has been in operation for 20 years, “using HV electrical equipment from a partner brand supplier whose system has been proven in the operating environment. The design can be quite similar, but the North Sea may require more enclosures compared to warmer locations such as the Mediterranean and the western United States.

In 2018, Electricity and Automation Group ABB announced that it has joined the French floating offshore wind turbine designers Ideol and STX Europe Offshore Energy to develop the design of floating substations. The idea was also involved by the French industrial process company SNEF and was developed under a research and development project called Optiflot, which was based on Ideol’s concept of dumping pools. In June of this year, BW Ideol (who owns BW Offshore as a shareholder since February 2021) said he was working with Hitachi ABB PowerGrids on a “scalable floating substation” using Ideol’s shallow draft floating platform. Said. BW Ideol said the collaboration follows the currently completed Optiflot project. ABB was unable to provide an update.

Substation concept under Optiflot project between Ideol, ABB and STX. Image from Ideol.

Earlier this year, construction engineering company Linxon and its parent companies Hitachi ABB PowerGrids and SNC-Lavalin / Atkins partnered to “realize a floating substation for offshore wind power.” Daewoo Shipbuilding & Marine and Korea Electric Power Corporation also said they are working on products for the floating offshore wind substation market in both deep and shallow waters.

Petrofac is working on the concept of floating substations in detail. As the project scales from a 250 MW pilot-scale project to a GW-scale farm, and from high-voltage AC to DC transformers, Jones says there are different design requirements. HVAC is used in pilot projects. As projects grow to gigawatt scale far from the coast, HVDC substations will be needed. For HVDC projects, the floater will be larger because the DC will take up more space because more equipment and high voltage DC equipment need to be properly separated. Semi-submersibles are a good solution, as they are also sensitive to acceleration, Jones says. “If you need topside mass and footprint, deep spurs can be an issue. There are other options such as the Tension Leg Platform (TLP), but you’ll need a hull size similar to the Hatton TLP. increase.”

For the HDVC solution, Jones said he would consider a 66m x 70m footprint on a four-row semi-submersible with a stressed skin top for strength and stiffness. In fact, this is a type of structure that is laid in recession, especially in the form of drilling rigs or floating production units, and can potentially be transformed, especially subject to many warnings related to fatigue. Yes, Jones says.

HVAC substations have a smaller footprint and are lighter than HVDC with the same capacity, making semi-submarines less attractive. They are really optimal only if the structure has a top footprint that exceeds 50m x 50m, Jones says.

In a GW-scale project, Petrovak devised a design for a deep-draft semi-submersible with a low-movement but shallow draft for transportation and maintenance, and a minimal braided top structure. “This design has the added benefit of having the cable outlet well below the surface of the water, reducing cable fatigue,” he says. The first project using a substation could have a capacity close to 250 MW connecting 20-25 turbines. Pilot-scale substations are the smallest TLP facilities, with highly efficient weight structures and good vertical movement, but with high mooring costs. Early commercial HVAC substation projects require innovative thinking to achieve the most efficient design. “He says.

Dynamic export cables are an important element needed to power the shore from floating structures. These cables above 220 kV and about 300 mm in diameter require a low fatigue environment and there is currently no cable that can do the job. Jones believes there are solutions within the oil and gas industry that may meet the bill. However, he says, the floater still needs to be designed to minimize the fatigue load on the cable. “We recommend paying a higher capital investment in advance to reduce the possibility of cable failure over a 25-year life, and to design the cable with less movement and therefore less cable fatigue.” He says. “You may want to choose a floater with deeper, lower movements and lower the cable outlet under a surface with a lower wave load.”

Work together to set industry standards

DNV hopes to launch a collaborative industry project to coordinate best practices in this area for technological development to take place and scale up more quickly. The organization says there is a standard for offshore substations (DNV-ST-0145), but wanted to develop rules for floating substations in line with the trend for floating offshore wind turbines.

More than 20 companies have attended the first workshop and 29 partners are currently interested and are scheduled to kick off in the first quarter of 2022. “Kristin Nergaard Berg, Senior Principal Consultant and JIP Project Manager, said. “Core Scope is in the design and certification of new technologies and incorporates appropriate design and certification requirements into the standard to mitigate the risk of using these systems. Related to HV power supplies and dynamic cables. We expect to see major gaps, but there may be other areas that need to be addressed. “

Many of the equipment is the same, but there are differences between floating and fixed substations that must be considered. “The main differences are the movement and acceleration of the floater, the high capacity specifications found in power equipment, and the dynamic features required for cables combined with high voltage,” says Berg. “We have experience building from onboard power equipment, but we have not met the specifications required for this equipment. We also have experience with dynamic power cables from oil and gas, but this uses a floating substation. Limited compared to what is needed for, and both AC and DC applications. For small projects, less equipment is required, which reduces the technical gap. Some floats For formula wind power plants, a fixed substation at the bottom may be available if the water is not too deep, but for large commercial wind power plants far from the deep sea or coast, overwater or underwater. You need one of the larger capacity substations. “

Uses Floatgen 3, BWIdeol damping pool technology. Image from Ideol & VJoncheray.Go to the seabed

Others are developing substation solutions that do not require dynamic export cables. ABB announced the undersea power system in 2019. While developed to meet oil and gas requirements, it has also confirmed its applicability to offshore wind. With a capacity of 100 MW and a depth of 3000 m, the system consists of a submarine transformer, switchgear, and a submarine power distribution of up to 600 km. This year, ABB added offshore wind to its agreement with Aker Solutions on submarine solutions, and Aker Solutions announced the concept of a floating offshore substation.

Egil Birkmore, Vice President of Renewable Energy Infrastructure at Aker Solutions, said: Placing a transformer underwater can reduce capital investment by 50-70% compared to placing it in a semi-submersible, but this is not one size that fits all solutions. There are sweet spots depending on the water depth and the amount of electricity required. He says it will be sent. There is also the opportunity to share the infrastructure with oil and gas facilities and power existing facilities. This enables a step-by-step approach to wind power plant development. Birkmore says ABB and Kellas infrastructure partners are working on this idea.

Developer Aker Offshore Wind has announced that it will deploy an undersea substation if it wins Scot Wind’s bid. This is a major licensing round in Scotland and the winners will be announced next year. Leif Holst-SVP Project-Aker Offshore Wind states, “This is an innovation taken from the energy sector and applied to offshore wind.” He says that putting it on the seabed has many advantages, including using the natural cooling provided by seawater, making it completely unmanned and easier to maintain. increase. “This is an area where there is technology that can push LCOE further down and leverage the (Aker) group,” he says.

However, depending on the water depth, these can face challenges. In short, very deep waters with long power cables pose economic challenges, says Ralphtor, program manager for the UK’s Offshore Renewable Energy (ORE) Catapult.

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Njord A – Template for future substation floaters? Image from Equinor by Thomas Sola.