Floating HVDC platform & Floating Substations: the next challenge on the path to commercial scale floating windfarms

 Floating HVDC platform (SP4) will develop concepts rated at 1.5 GW/320 kV and 3 GW/525 kV for connecting large floating wind farms. Such floating HVDC converter stations are non-existing today but will through this subproject be developed by Aibel together with Hitachi ABB Power Grids, using their strong expertise from the oil and gas sector and as supplier of bottom-fixed HVDC platforms for the offshore wind market.

WHO IS RESPONSIBLE

Aibel leads this subproject, and is responsible for the concept and product development, as well as being product owner. Hitachi Energy will develop HVDC Floaters. DNV will look at rules and regulations. Nexans is responsible for dynamic cables and acceptance criteria. Agder Energi will provide the end-user point of view. Equinor will carry out discussions on the technical and strategic levels. SINTEF will develop a simulation tool.

Expected results

• Development of rules and regulations, philosophies/design principles for unmanned floating HVDC
• Development of HVDC layout for floater motions
• Development of floater hulls, cable/mooring configurations and integrated topsides
• Development of efficient engineering tool for floater simulation including code checking of mooring and cables as an addition to the simulation tool SIMA

Floating Substations: the next challenge on the path to commercial scale floating windfarms

Although essential to the development of commercial scale floating offshore wind farms, floating substations have not received the same degree of attention or full-scale prototypes as their turbine counterparts

Now that the first commercial floating windfarms are about to enter the development phase, however, the industry needs to pay particular attention to this critical element

Figure 1: Elements of a floating Windfarm

Floating wind is attracting increasing investment and public policy support because it can access the 80% of offshore generation potential that is in water depths exceeding 60 meters. These deeper waters tend to be further offshore, where wind is typically more consistent, but where bottom-fixed offshore wind support structures are less feasible technically, logistically, and economically.

In recent years, development of floating offshore wind technologies has been swift, with many concepts emerging. Following the successful deployment of prototypes and demonstration projects, the industry is now transitioning rapidly to commercial projects. However, while demonstration projects generate limited amounts of power that can be exported directly to shore, a commercial scale project will require an offshore substation (OSS). The OSS will host a step-up transformer and the equipment necessary to export power in high voltage (HV). The only floating OSS in the world was installed in 2013 in Fukushima, Japan, and is connected to 3 turbines. The Fukushima OSS handles a total of 16MW and exports power at 66kV, which is not comparable to a commercial scale windfarm.

Figure 2: Semi-submersible (on the left) and Barge (on the right) floating OSS concepts. 

A floating windfarm would typically be installed in depths exceeding approximately 60m, where a bottom fixed monopile or jacket would not be economically relevant. For an OSS, the critical depth for a bottom-fixed foundation could be economically competitive as deep as approximately 100m, a depth not unusual for Oil & Gas fixed platforms. For the first floating wind farms, where water depths allow, bottom fixed substations (using tall jacket foundations) could limit the risks and costs inherent to new technologies such as high voltage dynamic cables. In places like California, however, with depths greater than 500m within the planned offshore wind call areas, bottom fixed substations are not an option.  

Different concepts

The different concepts foreseen for floating OSS foundations are similar to designs utilized for wind turbines: semi-submersibles, tension leg platforms (TLP), barge, or even spars. The barge, semi-submersible, and spar buoy are moored to the seabed with chains, steel cables, or fibre ropes connected to anchors. A TLP is vertically moored with tethers or tendons, these being the “tension legs”. Very strong cables, pipes, or rods link the TLP’s legs to seabed anchoring. Across all types of floating foundations, different anchor types can be used depending on the type of mooring system, soil condition, and expected environmental loads. 

Conclusion

Like floating wind turbines, floating OSS require a number of innovations, each bringing additional risks and costs, especially for pioneering projects. There are ways to mitigate the risks inherent in a floating OSS, thereby providing sufficient confidence to developers and investors. Testing and certification of the critical components will ensure both that the designs are based on best industry practice and that the components meet their design criteria, thus reducing risk of failure. Given the expected schedule for the development of the first commercial floating wind farms, the construction of a full-scale OSS prototype might not be feasible in a relevant timeframe and could be difficult to finance. However, each critical element can be tested individually with appropriate testing technics such as model tests in basins or dynamic test benches. 

Another means to mitigate risk is to implement a high degree of remote monitoring of the OSS to guarantee equipment availability and prevent catastrophic failures. This condition monitoring can be applied to the mooring lines, the cables, the HV equipment, or any other critical elements. In addition, to prevent catastrophic failure, this monitoring will allow for predictive maintenance and thereby reduce O&M costs.  

Combining expertise in bottom-fixed offshore wind, Oil & Gas and power transmission, DNV has taken a leading role in developing floating wind technologies. Proven guidelines on structure, stability, substation design, and position mooring are already available, and cutting-edge monitoring technologies have been developed, such as Smart Mooring, a machine learning system used to detect mooring line failures. DNV is also initiating a Joint Industry Project (JIP) on the floating substations to develop new solutions, standards and recommended practices.

FOR MORE INFORMATION REFER TO LINK

Floating Substations: the next challenge on the path to commercial scale floating windfarms (dnv.com)