The next step in floating wind

The energy transition is a hot topic within the offshore industry nowadays, and for good reason. Effects of climate change are imminent and it is necessary to invest in the development of large scale offshore renewables. What the exact energy mix will look like in the future is difficult to predict, but it is clear that offshore renewable energy will grow immensely in upcoming years.

Floating wind potential

According expectations, 1800 Gigawatts (GW) of offshore wind energy will be generated by 2050, of which 250 GW from floating wind farms. This was stated in the DNV Report “Floating offshore wind: The next five years”, published in 2022. Suitable regions to generate floating offshore wind can be found in California and South-East Asia, but also in Europe, where the main focus is on the Mediterranean, North Sea, Bay of Biscay, Baltic Sea and the Aegean Sea.  All of these areas have deep water and have suitable wind conditions for generating sustainable energy.

To efficiently transport 250GW of renewable energy from sea to shore, offshore substations are required. These substations collect the wind energy (AC) from the farm, convert the electricity to high-voltage direct current (HVDC) or high-voltage alternative current (HVAC) and transports this through 2 or 4 export cables to shore. By doing so, the power losses during transport from substation to shore are significantly reduced. These substations are already widely used in shallow waters, where the station is supported by a jacket or other bottom-founded solutions. Each substation is typically connected to 100 to 150 wind turbines, generating between 1.0 and 2.0 GW of power.

At water depths beyond 150 meter, the costs of the renowned ‘bottom-founded’ solutions increase exponentially, making floating solutions an interesting alternative. It is expected that in water depths up to  300 meters, the substation might still be bottom-founded, while the turbines already utilize a floating solution. In fields with water depths beyond 300 meters, a floating offshore substation (FOSS) will be required.

With an expected required floating wind energy capacity of 250GW in 2050, Ref. [1], the market is presented with a significant challenge. Assuming that the first full-scale floating substation will be commissioned in 2035. Eight (8) to ten (10) substations of 2 GW are to be built each year over the period of 2035 to 2050, to reach this forecast.

Current market challenges

At the moment, there is a significant amount of floating concepts in development to support the floating wind turbine. Recently, companies have started to investigate the technologies for floating offshore substations as well, but it is fair to say that there are no proven designs in the market.

The main challenge at the moment is that the dynamic conditions, as encountered by a FOSS, are uncharted territories for equipment and cable manufacturers (OEMs). The high-voltage equipment, as existing today, has not been designed for continuous motions and accelerations as will be experienced on a floating platform. 

For a financial perspective, floating wind solutions are expected to be more expensive than bottom-founded solutions. This is of course dependent on a large range of factors, like design life, water depth, size of the wind farm and many other aspects. On the other hand, being able to develop deep water sites would decrease the constraints on hub heights, shipping routes as well as the social impact on offshore wind. And even though these aspects will reduce project expenses, it is difficult to express this in reduced cost for the overall development. And in the end, differences in material cost between bottom-founded and floating substations are more prominent and easier to express.  One could argue that the cheapest option for a floating substation is to refurbish an existing tanker hull and transform it into floating substation, as is being done for floating production, storage and offloading (FPSO) units. However, this solution is not considered feasible, due to the expected allowable accelerations of the HV equipment, which are still being investigated by the OEMs.

Technical challenges

From a technical point of view, conceptual studies are challenging and complex, since there are still a lot of unknowns regarding:

  1. the standards to be used in overall design of the floater,
  2. the allowable motion criteria of the HV equipment,
  3. the fatigue capacity of the inter-array and export cables.

In recent years Nevesbu and Iv have focussed on investigating these three topics in more detail and the current status is discussed in the remainder of this article. It is important to realize that wind turbines are capable of operating in conditions up to Beaufort 8, which means that the floating substation must be operational in waves of 8 to 12 meters high. This makes bullet point 2 and 3 even more challenging.

1. FOSS substation standards 

In 2021, the Floating Offshore Substations (FOSS) Joint Industry Project (JIP), initiated by DNV, was launched. The main goal was to study gaps in the current standards required for floating offshore substations, proposals to bridge those gaps, and explore the current state of technology and design. In addition to developing this standard, a recommended practice is being prepared for the design and analysis of high-voltage export cables. Nevesbu and Iv were closely involved in this JIP, together with numerous companies from the industry. This JIP has brought more insight into the remaining challenges and technology gaps for floating offshore wind substations and the preferred standards for floating solutions. A kickoff meeting was held in June to launch phase II of the project.

2. Concept development 

Nevesbu and Iv have investigated and compared several floating substation concepts in recent years. Different floater types were investigated, like SPARs, buoys, semi-submersibles and tension leg platforms.

Each concept must adhere to the established requirements for offshore wind energy, avoiding excessive steel weight and maintaining simplicity in terms of fabrication. Furthermore, it is essential to ensure safety and reliability, while also guaranteeing very high availability and a platform lifespan of no less than 30 to 40 years. In addition, the social costs of supplying sustainable electricity must remain affordable, and the solution itself should therefore not be too expensive in terms of costs.

Based on these principles and requirements, a selection was made for the most promising floater type. At this moment, the main focus is on the development of a substation concept, which is based on proven Tension Leg Platform (TLP) technology.

The concept is developed for transforming 1.4 to 2.0 Gigawatt of power, with a DC export link of 300 to 525kV. A typical HVDC topside weighs about 13,000 to 20,000 ton. The floating HVDC platform has a deck area of 85 by 85 metres and raises approximately 25 metres above the water’s surface. The overall arrangement has been optimized for application on a floating substructure. When the platform is installed at sea, it will be held in position with the help of so-called ‘tendons’ that are vertically anchored to the seabed, which restricts the vertical motions and accelerations.

3. Cable guide system 

Dynamic inter-array cables are being developed and tested at full scale already in the floating wind turbine pilots around the world and are therefore considered to have matured before deployment of the first FOSS units. The DC export cables, on the other hand, are even more fatigue sensitive, due to the large core and surrounding metallic sheath. A cable guide system that solves the fatigue problem for the DC export cable(CGS) has been developed by Nevesbu .

Floating substation model test at MARIN

MARIN MKB model test

In October 2023, Nevesbu has tested the floating concept with different configurations of the cable guide system (CGS) in model scale. The model test has been performed within the MKB slot provided by MARIN.

Two primary objectives have been established for the model testing campaign. The first series of model tests is conducted to calibrate the numerical model. The second series of tests is centered around the CGS, where the system’s response is examined under different CGS pretension levels in a range of different sea states. To achieve these objectives, the model was equipped with a motion and acceleration sensors and load sensors on each tendon and cable guide.

This information is logged for the expected sea states the substation will be operating in. The maximum wave height tested in the model basin corresponds to the once in 100 year event occurring West of Shetlands, which is Hs > 17.0 meter. A first high-level check has shown that the measured maximum offset, accelerations and CGS loads in irregular waves are comparable to the outcomes from the numerical model. The exact results of these tests are now carefully compared and verified by Nevesbu, using the in-house developed numerical model and the data from MARIN.

Concept status and outlook 

After the successful testing campaign of the floating substation, in combination with the in-house developed cable guide system, the FOSS concept is at Technology Readiness Level 3 and ready for further development. Based on this solid concept, more detailed fatigue life estimations of the floater, tendons, cable guide system and HV equipment will be carried out to further mature our design solution. Close collaborations with OEMs should be established in parallel to better understand equipment limitations.

Curious about the possibilities for your project?

Bart, managing director Nevesbu, would be delighted to discuss this with you! Get in touch via +31 88 943 3400 or send a message. 

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Bart van Rijssen