Globally, the offshore wind sector is receiving greater attention than ever before. This is driven by its huge potential to add large quantities of green electricity, maturing technology and falling costs. In the case of countries with offshore areas with deep coastal waters and seabed conditions that do not support fixed-bottom offshore structures, the development of floating offshore wind farms (FOWF) has gained significance. Still in the nascent stages of development with only two commercial installations till date (Portugal and Scotland), the technology is gradually maturing with several demonstrations in multiple locations worldwide. The technology is being explored by countries such as UK, Norway, Portugal, Spain, France, Japan, US (west coast) and South Korea where water depths are more conducive for FOWF. In some areas, the potential for the technology is very high. For instance, the technical potential for floating offshore wind power in France is estimated at 155 GW, of which 33 GW would be accessible, considering the limits linked to other uses of the sea. The continuing development of this technology opens up new areas and opportunities for offshore wind farms.
The development of FOWF requires simultaneous advancements in several new supporting technologies, such as floating structures for mounting the wind turbines, floating offshore substations as well as dynamic export cables. Dynamic cables are submarine power cables that float in the sea and dynamically follow the movements of the floating equipment such as offshore floating high voltage substations, as well as of the waves and tides. Technology players such as Nexans, JDR Cables, BPP-Cables, Furukawa Electric Co. (FEC), etc. are investigating and developing high voltage (HV) dynamic cables suitable for FOWF. So far, medium voltage (MV) dynamic power cables (35 kV AC or less) have been applied for inter-array and export cable systems in demonstration projects. Though there are efforts to establish a new 66 kV alternating current (AC) standard for cost reduction in large offshore windfarms, there is a gap in the market for HV dynamic power cables up to 220 kV.
Unlike conventional transmission cables, which are static and designed to operate in conditions that are very different from those experienced by cables for floating offshore structures in marine renewable energy industries, cables for floating platform wind turbines must run through the water column from the platform base at the water surface to the touchdown point on the seabed. This trajectory exposes the cable to dynamic environmental forces. While conventional submarine cables are installed or secured on the seabed, the cables for floating offshore wind power generation facilities have floating components to enable them to move with the floater. These floating cables are subject to greater levels of mechanical and electrical stress due to the platform’s motion and sea conditions. The structure must therefore have excellent resistance to mechanical fatigue caused by repetitive bending. Its design must ensure high durability.
Generally, the inter-array transmission systems of offshore wind farms are based on three-phase AC technology with three conductor cores. The conductors are electrically insulated and water sealed by a sheath (made of polymer). An armour of twisted steel wires provide mechanical protection. A double wire armour is recommended for additional stiffness and protection.
According to a research paper on dynamic cables by Ruben Weeheim of the Delft University of Technology, the lazy wave and steep wave hanging configurations are the most appropriate for dynamic power cables. In case the waters are too deep, a W-shape configuration can be used for inter-array networks. This is, however, a new configuration and needs greater research prior to implementation. The critical points of the lazy-wave configuration are at the hang-off and touchdown points. Solutions to make the cables resistant to fatigue are the use of static and dynamic bend stiffeners and Uraduct (a protection system to protect cables from abrasion, dropped objects and impact). For improved fatigue life and flexibility, metallic corrugated tubular sheathing can replace the metallic sheath. The key operation and maintenance challenges relate to accessibility and costs. Dynamic cables can be deployed only for high voltage alternating current (HVAC) systems and are not qualified for direct current (DC) systems. Floating offshore installations at water depths of up to 400-500 m are likely to be feasible for dynamic power cables. Further investigation of fatigue and hydrostatic pressure resistance of such cables is necessary for applications at locations at greater depths (for instance California, which has depths of 800 m).
Figure 1: Possible dynamic cable configurations
Source: Research paper on assessing mechanical loading regimes and fatigue life of marine power cables in marine energy applications by PR Thies, L Johanning and GH Smith
Figure 2: Example of dynamic cable configuration for floating offshore wind farm
Source: Paper on Dynamic Cable System for Floating Offshore Wind Power Generation by Ryota Taninoki, et al.
It may be noted that dynamic cables are already being investigated and deployed for application in the oil and gas industry. Given the rise in the number of floating production facilities used for offshore production of oil and gas and exploitation of hydrocarbon reserves in deeper waters, the performance demands on power cables have also risen. Presently, Hitachi ABB Power Grids (formerly ABB’s power grid division) is the only company that has developed HVAC dynamic power cables of 115 kV and 123 kV. It has deployed them at two Norwegian offshore platforms—Gjøa (2010) and Goliat (2013)—to supply electricity to these platforms (semi-submersible and FPSO – floating, production, storage and offloading) from the shore. For the two projects, Hitachi ABB Power Grids’ power cable system included 1.5-km corrugated copper sheath HVAC dynamic cables for water depths of 350 m and 380 m. In order to mitigate the technological and monopoly risks, FOWF project developers are looking at developing other options to fill the gap in the market.
Carbon Trust of UK-Floating Wind JIP competition
In an effort to address the lack of availability of HV dynamic export cables for transmission of power from wind farms to the shore, UK’s Carbon Trust in association with 12 offshore wind developers and supported by BPP-Cables, launched a dynamic export cable competition in 2018. In 2019, five competition winners—Aker Solutions (Norway); Furukawa Electric Co. (FEC) (Japan); Hellenic Cables S.A. (Greece); JDR Cable Systems (UK); and Zhongtian Technology Submarine Cable Co. Ltd (ZTT) (China)—were announced. The competition was conducted as part of the Floating Wind Joint Industry Project (Floating Wind JIP), which aims to accelerate and support the development of commercial-scale floating wind farms. The Floating Wind JIP is a collaboration involving EnBW, ENGIE, Eolfi, E.ON, Equinor, innogy, Kyuden Mirai Energy, Ørsted, ScottishPower Renewables, Shell, Vattenfall and Wpd, with support from the Scottish government. The key competition objective is to accelerate the development of this technology to ensure that HV dynamic cables are available for the first commercial floating wind projects within the next five to ten years.
The winners will receive funding to support the design, initial testing and development of dynamic cables ranging from 130 kV to 250 kV to enable the efficient transmission of power from floating wind farms to the shore.
Notably, BPP-Cables, a subsidiary of BPP-TECH and Keppel Offshore and Marine, which supported the competition, has itself been actively investigating dynamic cable technology. In 2013, it developed a prototype 100 MW, 132 kV dynamic AC marine cable for high power transmission through the moving water column from surface vessels or installations, down to depths of over 2,000 m. It was part of the PowerCab JIP comprising four oil and gas majors (BG Group, BP Exploration, DUCO and Shell UK). The cable was fatigue tested to verify electrical and mechanical performance in conditions representative of the West of Shetlands. The design and development programme allowed BPP-Cables to realise key features such as light weight, extended fatigue life and relatively low fabrication costs. It used component technologies such as fluid barriers, optimal materials, configuration design and load management providing improved physical, electrical and mechanical properties combined with economic benefits.
Key project developments
The 30 MW Hywind Scotland Pilot Park project, commissioned in 2017, is the world’s first commercial scale FOWF. Developed by Norway’s Equinor (formerly known as Statoil) 30 km off the coast of Peterhead, Aberdeenshire, Scotland, the project deploys 36-km, 36 kV cross-linked polyethylene (XLPE)-insulated cables supplied by Nexans under a EUR10.2 million contract. The offshore cables systems were tested and manufactured at Nexans’ specialised factory at Halden, Norway.
Previously, Nexans, in collaboration with Equinor, started the 2.3 MW Hywind Demo project, the first full-scale floating wind turbine in 2009, which was put into test operation off the island of Karmoy in Norway. For this project, Nexans Norway supplied the 12-km, 24 kV XLPE-insulated submarine cable, also from its Halden factory. The company’s manufacturing plant in Halden is equipped with special advanced full-scale test rigs for dynamic cables and umbilicals. It has established test programmes that subject test specimens to the fatigue loads to which they will be exposed during their defined lifecycle. With its long experience in dynamic umbilicals used in the oil and gas industry, Nexans is one of the forerunners in designing and manufacturing dynamic power cable systems. It has manufactured and qualified a 145 kV, three-core prototype dynamic power cable, which is suitable for water depths of up to 1,500 m. It is currently developing and qualifying future HV dynamic cables with metallic sheath and laminated foil.
Meanwhile, Equinor is also developing another large scale commercial FOWF—88 MW Hywind Tampen—in Norway, where around 3 GW of floating offshore wind is expected to be installed by 2030. Significantly, in April 2020, Norway’s Ministry of Petroleum and Energy approved the Hywind Tampen project to power the Snorre and Gullfaks oil and gas platforms and help in decarbonising them. Also, the EFTA Surveillance Authority (ESA), which monitors compliance with the agreement on the European Economic Area in Norway allowing it to participate in the internal market of the European Union, approved Norway’s state aid from the NOx fund for the project, covering 43 per cent of the total investment costs. The project is the world’s first FOWF directly connected to oil and gas platforms.
For this project, Eqiunor awarded the contract for inter-array and export cables to JDR Cable Systems, part of TFKable Group, in late 2019. JDR will design and manufacture eleven 66 kV dynamic inter-array cables (2.5 km) and two static export cables (12.9 km and 16 km), each equipped with a JDR-designed breakaway system and a range of cable accessories to be delivered by 2022. The inter-array cables will connect to the 11 turbines in a loop and the static cables will connect the loop to oil and gas platforms. The power cores for the cables will be manufactured by JDR’s parent company TFKable at its Bydgoszcz factory in Poland. All the cables and accessories will be assembled at JDR’s facilities in Hartlepool, UK.
Recently, in July 2020, JDR completed HV termination and testing of the 66 kV export and inter-array cables associated with the 25 MW Windfloat Atlantic FOWF, off the coast of Viana de Castelo, northern Portugal. With this, the project became fully operational and commenced supplying clean energy to Portugal’s electricity grid. Developed by the WindPlus consortium (comprising EDP Renováveis, ENGIE, Repsol and Principle Power Inc.), it is world’s first large-scale commercial FOWF operating at 66 kV. It is also the first offshore floating wind farm in continental Europe.
The first of the three turbines (8.4 MW) for the Atlantic FOWF was connected to the mainland grid via a 20-km cable supplied by JDR at the end of 2019. The European Investment Bank (EIB), European Commission (EC) and the Government of Portugal provided financial support to the project and it is the first floating wind farm that is bank-financed.
The project’s three semi-submersible platforms, anchored with chains to the seabed at a depth of 100 m, are connected through a network of inter-array cables to one export cable. For the project, JDR deployed its innovative dynamic 66 kV technology and breakaway system [BSR Latch and T-connector break away system (T-BAS)], which protects the floating platform in the unlikely event of a mooring line failure. JDR selected Nexans to supply the 66 kV T-connectors to provide the connection for JDR’s phase cores of the dynamic 66 kV floating wind inter-array cables. The robust EPDM (ethylene propylene diene monomer) material of the accessories provides strong resistance to rough offshore conditions.
The WindFloat Atlantic project builds on the success of the 2-MW WindFloat1 prototype, which was in continuous operation for over five years from 2011 to 2016. It operated at high availability producing over 17 GWh at sea states up to 7 m of significant wave weight and surviving waves of 17 m. In the second phase, WindPlus has planned an installation of a 125 MW FOWF, which can further build on the success of the first phase of the commercial project.
Another FOWF project that is currently under construction and scheduled for commissioning by end-2020 is the 50 MW Kincardine FOWF, located 15 km southeast of Aberdeen, Scotland. The project developer Cobra Wind International awarded the export and inter-array cable design and supply contract to Prysmian. It is responsible for supplying two export cables of 17 km route length comprising a static cable design combined with around 0.5 km of dynamic route section to complete the connection to the floating turbine tower. The 33 kV three-core submarine cable will utilise an EPR insulation system throughout. The static section length will be completed with single-wire armouring while the dynamic section will apply double-wire armoured design. The cables will be produced by the company at its plants in Vilanova, Spain, and Drammen, Norway. Offshore cable installation company Global Offshore is responsible for cable installation and burial for the project.
Fukushima Forward project (demonstration)
Among the winners of the Carbon Trust competition, FEC has prior experience with HV dynamic cables from its association with the Fukushima Floating Offshore Wind Farm Demonstration Project (Fukushima Forward). The first stage of the project, funded by Japan’s Ministry of Economy, Trade and Industry (METI), was commissioned in 2013 with the world’s first 66 kV dynamic or riser cables. In the first stage, a 2 MW offshore floating wind turbine generator and a 25 MVA offshore floating substation (about 25 km from the shore) were constructed. In the second stage (which ended in 2017), one 7 MW and one 5 MW floating wind turbine generator was built. FEC along with VISCAS Corporation (a joint venture of FEC and Fujikura Limited) successfully developed and manufactured the special high voltage riser cable system for the project. FEC was responsible for the overall development of the power transmission system for the floating offshore wind farm, and VISCAS Corporation was responsible for the manufacturing and the jointing of the special high voltage riser cable. Meanwhile, Shimizu Corporation was responsible for the towing and the anchoring of the floating offshore wind turbine equipment to the sea area, the laying and burying of the submarine cable connecting Naraha and the anchoring sea area and the connection of the power generation equipment and the substation equipment with the riser cables.
Under the project, the riser cable was suspended from the floating structure (substation and wind turbine generator) into the sea. For designing the dynamic cable system, the undersea behaviour was forecasted by simulations based on oceanographic conditions, the floating structure shaking property and the floating structure anchoring design condition among other things. The joint units were laid by two methods—in catenary shape and by hanging horizontally and hence the design incorporated mechanical properties to withstand mechanical and electrical stress in both methods. It was laid after conducting the requisite water pressure and permeability tests. A special cable laying vessel (KAIYO) equipped with a turntable and operated by dynamic positioning system was used for cable laying. The vessel was capable of loading the riser cable, the submarine cable and accessories such as a modular buoy.
In 2019, the 7 MW turbine (supplied by Mitsubishi Heavy Industries) was decommissioned due to underperformance. Another downside of the pilot highlighted by industry experts is the high cost (though exact figures are not available) compared to other demonstration projects in Europe. Notwithstanding the setbacks, the pilot project has demonstrated the viability of key floating offshore wind technologies.
Figure 3: Structure of 66 kV and 22 kV riser cables installed at the Fukushima Forward project in Japan
Source: FEC
Table 1: Key parameters of dynamic cables at the Fukushima Forward project in Japan
| Parameter | Dimensions (66 kV riser cable) | Dimensions (22 kV riser cable) |
| Nominal voltage | 66 kV | 22 kV |
| Conductor | 3 x 100 mm2 | 3 x 150 mm2 |
| Optical fibre count | SM 8×3 | SM 8 |
| XLPE insulation | 11 mm | 6 mm |
| Metallic sheath | Corrugated stainless steel sheath | Stainless steel foil laminated tape (0.6 mm) |
| Inner PE jacket | 3.5 mm | 3.5 mm |
| Armour | Double galvanised steel wire (6 mm) | Double galvanised steel wire (6 mm) |
| Outer PE jacket | 6 mm | 6 mm |
| Outer diameter | 175 mm | 147 mm |
| Weight | 53 kg/m in air; 29 kg/m in water | 43 kg/m in air; 27 kg/m in water |
| Length | 860 m | 2,340 m |
Source: FEC
Figure 4: Layout of the dynamic cables at the Fukushima Forward project in Japan
Source: FEC
Table 2: Key floating offshore wind projects
| Project | Transmission developer | Capacity (MW) | Export cable length and voltage | Expected completion |
| Hywind Scotland Pilot Park project, Scotland | Equinor | 30 | 36 km; 36 kV AC | 2017 (commissioned) |
| WindFloat Atlantic FOWF (Phase 1), Portugal | Windplus consortium (EDP Renováveis, ENGIE, Repsol and Principle Power Inc.) | 25 | 20 km; 66 kV AC | 2020 (commissioned) |
| Kincardine FOWF, UK | Kincardine Offshore Windfarm Limited (Cobra Wind International) | 50 | 17 km; 33 kV AC | 2020 (under construction) |
| Griox FOWF, France | Réseau de Transport d’Électricité (RTE) | 24 | 29 km; 63 kV AC | 2020-21 |
| Faraman FOWF, France | RTE | 24 | 19 km; 63 kV AC | 2020-21 |
| Grussain FOWF, France | RTE | 24 | NA; 33 kV AC | 2020-21 |
| Leucate FOWF, France | RTE | 24 | 18 km; 63 kV AC | 2022 |
| Hywind Tampen FOWF, Norway | Equinor | 88 | 29 km; 66 kV AC | 2022 |
| WindFloat Atlantic FOWF (WFA Phase 2), Portugal* | Windplus S.A. | 125 | NA | NA |
| Castle Wind, California, US* | Castle Wind LLC (Trident Winds and EnBW North America) | 650-1,000 | NA | NA |
| Equinor and Hitachi Zosen Corporation collaboration, Japan* | Equinor and Hitachi Zosen Corporation collaboration | 400 | NA | NA |
| South Korea FOWF, South Korea* | Macquarie and Gyeongbuk Floating Offshore Wind Power | 1,000 | NA | NA |
Note: *proposed projects
Source: Global Transmission Research
The way forward
The large-scale development, manufacturing and supply of HV dynamic cables is necessary to ensure fast commercial deployment of floating wind technology. In fact, the industry views this as one of the potential bottlenecks for floating offshore wind development. Given that floating platforms are in the nascent stages of development in the offshore renewables sector, ground experience with dynamic cables and other new floating components is still limited, requiring greater investigation. Specifically, there is still considerable room for research and development in dynamic cable technology with respect to its application in greater water depths and at higher voltages. As more floating offshore wind projects are commissioned, developers and technology players will gain greater experience, which will result in further advancements in technology.
