Maritime engineering firm Nevesbu, together with Iv, has developed a patented concept for a floating offshore substation (FOSS). The concept has already been successfully tested in MARIN’s basin. This lays an important foundation for the development of floating wind energy.
The energy transition is a hot topic within the offshore industry and the need to invest in large-scale offshore renewables is greater than ever. According to the DNV report “Floating offshore wind: The next five years”, 1800 GW of offshore wind energy is expected to be generated by 2050, of which 250 GW will come from floating wind farms.
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.
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Substations essential part of offshore wind farms
To efficiently transport this huge amount of energy from sea to land, offshore substations are essential. 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 transport this through two or four 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.
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Floating substations needed
At water depths beyond 150 metres, 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 metres, the substation might still be bottom-founded, while the turbines already utilise a floating solution. In fields with water depths beyond 300 metres, a floating offshore substation (FOSS) will be required.
With an expected required floating wind energy capacity of 250 GW in 2050, the market is presented with a significant challenge. Assuming that the first full-scale floating substation will be commissioned in 2035, eight to ten substations of 2 GW are to be built each year over the period of 2035 to 2050, to reach this forecast.
To date, however, there are no proven designs for floating offshore substations on the market. The dynamic conditions faced by a FOSS are uncharted territory for equipment and cable manufacturers.
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Concept based on tension leg platform technology
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 thirty to forty 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 designed to convert 1.4 to 2.0 GW of power, with a DC export link of 300 to 525 kV.
A typical HVDC topside weighs about 13,000 to 20,000 tonnes. 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 optimised 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.
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 has been developed by Nevesbu.
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Tests at MARIN
Nevesbu tested the floating concept on scale model with different configurations of the cable guiding system (CGS). The model test was conducted within the SME 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 centred 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.
The maximum wave height tested in the model basin corresponds to the once-in-100-years event west of Shetland, corresponding to Hs > 17.0 metres. An initial high-level verification showed that the measured maximum offset, accelerations and CGS loads in irregular waves are similar to the results of Nevesbu’s 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 the design solution. Close collaborations with OEMs should be established in parallel to better understand equipment limitations.
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