Offshore wind ready to float?

July 14, 2014

The Fukushima Forward 2MW floating offshore wind turbine. Photo from Fukushima Forward.

A growing number of companies are eying options for floating offshore wind. Elaine Maslin learned why at All Energy in Aberdeen.

This year could see something of a sea-change in off- shore wind power development. What has, until now, been a mostly fixed-foundation industry, using mono- piles and jackets, is increasingly looking to floating solutions.

Floating wind designs have been driven by a move into deeper waters, areas where wind energy potential is thought to be greater. There has also been a shift in geographic focus. Europe has been a leader in offshore wind development, but other countries are now looking to offshore wind, using floating designs.

“While Europe has both shallow and deeper waters, other areas such as the US and Japan can only have offshore wind in deeper waters,” says Adrian Fox, program manager, technology and supply chain, at the UK’s Crown Estate, which manages offshore leases for wind parks. This is because the water depths in their offshore areas make jacket or monopile-based foundations hard or impossible.

As well as practicality, cost is a key issue, Fox told a session focusing on floating offshore wind solutions at the All Energy conference in Aberdeen, in May. There has been an increase, not decrease, in costs for shallow water offshore wind projects, says Nico Bolleman, managing director Blue H Engineering, which has a tension leg platform wind turbine design. “This is because turbines have become bigger and in deeper waters,” he told the All Energy session.

By moving to floating structures, developers could help reduce costs, Fox suggests, by building and commissioning floating wind turbines onshore, removing the need for offshore lifting vessels, reducing offshore installation time. In addition, facilities could potentially be brought back to shore for operations and maintenance.

The Fukushima Forward spar-based floating substation. Photo from Fukushima Forward. 

By using buoyancy instead of seafloor-fixed support structures, developers could reduce the amount of material they use, and therefore costs, Bolleman says. In addition, floating solutions will better deal with deep water conditions, avoiding having to pile or anchor into rocky seabeds, and higher winds and wave heights, which cause high dynamic loads, he says. Nor will a floating turbine require a transition piece—a component which has created a number of issues on fixed offshore wind turbines.

Using TLP designs, turbine structures would take up a smaller footprint, limiting incursions into fishing grounds. In addition, he suggests floating wind economics could be boosted through a tie-up with oil and gas operators, by using floating wind units to power enhanced oil recovery projects, such as subsea compression or boosting, says Johan Sandberg, service line leader, offshore renewable energy, DNV GL.

In fact, DNV GL recently launched a joint industry project to research the idea.

There are challenges, including non-linear frequency dynamics, says Patrick Rainey, control engineer, DNV GL, which could in turn make control optimization costly. Consideration will also need to be given to inter-turbine and substation cabling, as well as structures for transformers or substations.

But, says Fox: “The only real limitations offshore are imagination and engineering—capability to innovate. We thought 7-8MW turbines would be pushing it 3-4 years ago. But we do need to go further and I believe we will.”

Japan

Japan has been quietly developing its floating wind capabilities. According to Main(e) International Consulting (MIC), 80% of Japan’s offshore resources are in 100m+ water depth. In June last year, Prime Minister Shinzō Abe said: “Japan intends to become the first country to commercialize floating offshore wind power technology by around 2018.”

Statoil's Hywind floating wind turbine, offshore Norway. 

DNV GL’s Sandberg says Japan has an increased focus on renewables post-Fukushima and that its expertise in steel, experience in lean manufacturing, and spare yard capacity, means it is in a strong position from which to develop floating wind technology.

A number of scale prototypes have already been installed offshore Japan. Last year saw a 66kV spar-based substation and a 2MW downwind turbine on a semisubmersible base installed at Fukushima. In 2014-15, as part of the Fukushima Forward project, two turbines, one on a Mitsubishi-designed, V-shape-semisubmersible base and one on a Japan Marine United designed-advanced spar, are due to be installed, both using Mitsubishi turbines, in 100-200m water depth, and aver- age 7m/s wind speed, 20km offshore Fukushima. Fukushima Forward is funded by Japan’s Ministry of Economy, Trade and Industry and consists of a consortium with Marubeni Corp. acting as project integrator.

Japan’s Hitachi Zosen has also been working with Statoil on floating wind development. A 2012 agreement between the two companies was extended in April this year and HZ is reported to be planning 7.5MW pilot plants by 2016, before building wind farms with a combined capacity of 300MW in 2022.

Scotland/Norway

A site offshore Scotland could be home to the UK’s first floating wind park, led by Norway’s Statoil, using its Hywind design. Kelly Meulepas, senior engineer, Statoil, told All Energy Hywind is a ballasted steel structure, which can be towed to site and moored with three mooring lines.

A demonstrator Hywind unit was installed off Norway in 2009, using a 2.3MW Siemens turbine. It has produced more than 37.6GW hours of electricity, and survived 44m/s wind speed and 19m wave heights, Meulepas says. The latest Hywind design will be used in the proposed Hywind Scotland Pilot Park Project (HSPP), a five-turbine development, 5km offshore Peterhead,in an area with 10.1m/s average wind speed, 95-120m water depth, and in 1.8m mean wave height.

The DeepCwind consortium is investigating three general designs for modeling at the University of Maine Deepwater Offshore Wind. Image from Advanced Structures and Composites Center, Maine. 

The structure below sea level will be 75m-long (25m shorter than the first Hywind demonstrator). It will use dynamic power cables in a lazy S formation, with 33vV transmission voltage to shore. The company carried out geotechnical studies this spring. A final investment decision is due to be made on HSPP in 2015.

“Our goal is a large-scale commercial park, producing 500- 1000MW, which we believe could be cost-competitive with bot- tom fixed turbines from 2020. We need to build a small wind farm to demonstrate the cost reductions and risk,” Meulepas says.

Challenges for the larger units offshore Peterhead will include installation, which will need to be different to the Hywind offshore Norway, Muelepas says, as well as maintenance and marine systems.

Spain

In Spain, the Nautilus Floating Solutions consortium is developing a floating wind turbine. The consortium comprises four companies in northern Spain (the Astilleros de Murueta shipyard, Tamoin industrial services firm, Velatia, an industrial and technology group working in electronics and communications, and mooring system firm Vicinay) and the Tecnalia technology center. The consortium has a collaboration agreement with Spanish utility group Iberdrola.

Gonzalo Fornos, business managing director at Tamoin and a board member on Nautilus, told All Energy the group has developed a floating, stabilized, semisubmersible platform, for 50-250m water depth. As well as accommodating “any type of wind turbine,” it could take a substation, Fornos says. Concept definition was achieved in 2011, conceptual design was achieved in 2013, and tank testing will be carried out this year, at the University of Cork, Ireland, and in Cantabria, Spain. Detailed engineering will start in 2015, and a prototype is due to be deployed in 2016, Fornos says, with a commercial unit ready by 2018. The aim is to pre-assemble the unit in port, and tow it to its installation location.

Floating concepts

The Crown Estate’s Adrian Fox says there are three main types of floating turbine structure: semisubmersible, TLP, and spar.

In October, 2011, US-based Principle Power deployed a full- scale prototype WindFloat using a 2MW Vestas turbine, 5km off the coast of Aguçadoura, Portugal. The semisubmersible is connected by subsea cable to the local grid. The structure was completely assembled and commissioned onshore before being towed some 400km along the Portuguese coast.

US-based PelaStar’s TLP was conceived in 2006, by naval architects at Glosten Associates. It has been designed for water depths greater than 65m. In 2013, the UK’s Energy Technologies Institute (ETI) commissioned Glosten to carry out a FEED study for the deployment of PeleStar TLP technology. The demonstra- tion TLP will use Alstom’s 6MW Haliade 150 offshore wind turbine and the structure will be designed for installation and opera- tion at Wave Hub, off the southwest coast of England. Belfast’s Harland and Wolff is shipyard partner and Dockwise is project partner.

Statoil's Hywind floating wind turbine, offshore Norway. 

Netherlands-based Blue H Engineering has developed a TLP design using a ballasted (semisubmersible anchor blocks), tension-leg mooring system. It would be assembled onshore and able to be installed with one anchor handling vessel, without lifting vessels or divers, Nico Bolleman says. Installation could be in a large weather window, with 3m-high seas for tow-out and 2m-high seas for installation. Yard harbor draught would not need to be above 10m. Blue H installed a large scale (75% full size) prototype with a small wind turbine in 2008, in 113m water depth, 21.3km offshore southern Italy.

US-based Maine Aqua Ventus I has approval for a pilot-scale offshore wind farm, which will comprise two floating wind turbines, with 12MW capacity, in the Gulf of Maine, 2.5mi. off the southern coast of Monhegan Island and 12mi. off the coast of the mainland. The DeepCwind consortium, led by the University of Maine, launched its 1:8 scale floating prototype VolturnUS off Maine last year. Its research is funded by the US Department of Energy, the National Science Foundation, and others.

Winflo is a consortium between French group DCNS and Nass et Wind. Winflo has a semisubmersible design, planned for deployment in 2014, offshore France using a 1MW turbine.

Germany’s GICON is developing the GICON SOF (Schwimmendes Offshore Fundament, or “Floating Offshore Foundation”). The development is in cooperation with partners, including the Technical University and Mining Academy Freiberg, Rostock University and Jaehnig GmbH. Construction of a full scale prototype and deployment in the German Baltic Sea is planned during 2014.

A French, EDF-led consortium, comprised of turbine designer Nénuphar, and Technip, is working on two 2MW prototype, floating Vertiwind vertical-axis turbines, due to be installed 5km off France’s south coast in 2015.

Poseidon Floating Power, from Denmark, has developed the P37, a 1:2.3 scale of its P80 design, a wave and wind harvesting unit. P37 is a 37m-wide model, weighing about 320-tonne. It is in its fourth test phase, since 2008, sited off Onsevig Harbor, at the north coast of Lolland.

US-based Nautica Windpower has an advanced floating turbine (AFT) design, using a buoyant tower and single mooring point. It hopes to deploy a medium-scale AFT in 2016.