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Sofia HVDC converter platform demands innovation in technology and engineering Iv-Offshore & Energy Engineering HVDC Platform Offshore wind

One of the greatest technical offshore challenges

One of the largest wind farms in Europe, Sofia Offshore Wind Farm, will be realised 195 kilometres off the coast of North East England, in the area known as Dogger Bank. This wind farm is the largest project in RWE Renewables’ current development portfolio. With 100 wind turbines, totalling a capacity of 1.4 gigawatts, and an innovative High Voltage Direct Current (HVDC) converter platform, this wind farm will supply approximately 1.2 million British homes with sustainable energy in the future. The Sofia HVDC converter platform, to be installed at the heart of the wind farm, will be one of the largest and most powerful offshore HVDC converter platforms currently in existence or under construction. The fact that the platform will be installed far from the coast and will be operated remotely, poses an interesting transportation challenge in relation to the 25 metre telecom mast.

Grand in dimensions and project scope

The Sofia HVDC converter platform is one of the largest offshore wind platforms Iv-Offshore & Energy has ever worked on. In recent years, Iv-Offshore & Energy has been involved in the Borwin Alpha (400 MW), Dolwin Alpha (800 MW) and Helwin Béta (690 MW) platforms and various engineering and FEED (Front-End Engineering Design) studies, such as the FEED study for the 2 GW offshore grid connections for TenneT.

In July 2020, Iv-Offshore & Energy signed the early works contract with Sembcorp Marine Offshore Platforms Pte Ltd for the detailed design of the topside and jacket of the Sofia HVDC converter platform. Iv-Offshore & Energy will support Sembcorp Marine for the purchase of all auxiliary equipment, and GE Renewable Energy’s Grid Solutions business (GE) will supply the HVDC equipment. As an engineering partner of Sembcorp Marine, Iv-Offshore & Energy is fully responsible for the engineering and platform system integration, and we support Sembcorp Marine with engineering-related issues in consultations with RWE Renewables (end customer) and GE (partner), among others. Sembcorp Marine will eventually construct the platform at one of their fabrication yards in Batam, Indonesia.

From wind to green energy

The Sofia Offshore Wind Farm covers a total area of 593 km2. A network of approximately 370 kilometres of
inter-array cables will transport the electricity generated by the wind turbines to the Sofia HVDC converter platform. The platform will then convert this electricity from 66 kilovolts (kV) alternating current (AC) to 320 kV direct current (DC). Two export cables of approximately 220 kilometres in length, which together form a single high voltage direct current circuit, will transport the power ashore from the wind farm to the onshore substation in Lackenby.

The 10,000-tonne topside of the Sofia HVDC converter platform will measure approximately 80 metres in length, 40 metres in width and 40 metres in height. The topside will be placed on a jacket foundation with eight legs positioned on the seabed just under 30 metres below sea-level (BSL). The jacket foundation is then secured with monopiles driven deep into the seabed. The 18 strings of inter-array cables, two export cables and the fibre optic cable are integrated into the jacket and fed via J-tubes to the platform where these are assembled into the topside.

A 25-metre-high telecom mast will be placed on the top deck of the topside. This telecom mast will provide communications coverage of the array area. However, the challenge here is that when the platform is ready, it will be transported by ship through the Suez Canal, and the topside together with the telecom mast is too large to pass freely under the bridge that spans the Suez Canal. Iv-Offshore & Energy is responsible for devising a suitable solution for this issue.

Giving equipment a place remains a puzzle

The dimensions of the platform are enormous and yet giving all the necessary equipment a place remains a puzzle. The vast capacity of the wind farm, of course, requires the presence of larger or more equipment on the platform. The layout of the platform must be such that everything remains safe, easily accessible, and maintainable; all this in combination with the high availability of the platform. Integrating all the equipment forces us to think outside the box and to use the available space as smartly as possible.

When converting alternating current to direct current heat is produced, which must be dissipated elsewhere. But the system that is needed for cooling the HVA/C equipment (an air conditioning system) demands a lot of space. Iv-Offshore & Energy has therefore applied a new cooling concept for the Sofia HVDC converter platform. Conventionally, the cooling system is subcontracted to an HVA/C contractor who designs the system and supplies the components. But for this project, Iv-Offshore & Energy is responsible for everything, including creating the design, the purchasing of the necessary components and the integration with the process systems.

For the Sofia HVDC converter platform, it has been chosen to use an HVA/C system, whereby the condenser is cooled with cold water instead of with air. Cooling with cold water has the advantage that the highest design temperature of the seawater used is lower than the temperature of the outside air. Furthermore, the heat transfer of water is better than that of air, which allows the system to be designed more compactly. With the water-cooling system, lower temperatures can be achieved in the air conditioning system, and smaller ducts can be used, which in turn also require less space.

Further development of sustainable energy

Iv-Offshore & Energy has been devising the design of the platform since July 2020. In every project, we strive to utilise our technical knowledge to achieve the utmost and to make a valuable contribution to the further development of sustainable energy.

The cutting of the first steel for the fabrication of the platform is scheduled for August 2021. The offshore installation is expected to commence in 2024.

Would you like to know more about this project? Fedor will be pleased to tell you more. Contact Wim via 088 943 2802 or send him an email.

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