Sofia OFTO

On 25 February 2025 Ofgem launched Offshore Transmission Owner Tender Round 12 with Sofia as the initial project, opening the Enhanced Pre‑Qualification (EPQ) stage for bidders for the Sofia OFTO licence, publishing TR12 documentation, and setting key dates including bidder clarification requests by 22 April 2025 and EPQ bid submissions by 13 May 2025.

Ofgem launched Offshore Transmission Owner (OFTO) Tender Round 12 with the Sofia transmission link as the featured project, holding a launch and preview webinar on 25 February 2025 to outline the Sofia OFTO assets, the competitive tender process and upcoming Enhanced Pre-Qualification and Invitation to Tender stages for prospective bidders.

In June 2024 the completed offshore converter platform for Sofia departed the fabrication yard in Batam, Indonesia, bound for the UK, marking the transition from manufacturing to overseas transport for this key HVDC transmission asset ahead of installation at the Dogger Bank site.

In November 2023 the Sofia onshore converter station site in Teesside received its transformers, demonstrating that these major HVDC components for the onshore end of the Sofia OFTO system had completed factory manufacture and been delivered to site for installation.

On 5 September 2023, offshore construction for the Sofia transmission system officially started with works to install essential subsea HVDC export cable infrastructure between the UK’s north‑east coast and the wind farm site on Dogger Bank. Prysmian’s cable‑laying vessel Leonardo da Vinci was mobilised from the Port of Middlesbrough to begin this three‑year offshore construction phase for the Sofia OFTO export link.

In early September 2023, Prysmian’s vessel Leonardo da Vinci commenced laying the first sections of Sofia’s ±320 kV HVDC export cables, marking the start of subsea cable installation for the Sofia OFTO. Operating from the Port of Middlesbrough, the vessel began installing two 130 km cable sections in parallel from just off the Teesside coast between Redcar and Marske‑by‑the‑Sea along the route towards the offshore wind farm, with cables pulled through pre‑installed landfall ducts.

The official Sofia Offshore Wind Farm offshore construction timetable states that the 'First phase of export cable installation' occurs in Mid 2023, indicating the start of export cable installation for the Sofia OFTO transmission assets around mid-2023.

By January 2023 all decks of Sofia’s offshore converter platform had been installed at the Batam fabrication yard, indicating advanced progress in manufacturing the offshore HVDC converter platform for the Sofia OFTO link.

On 20 January 2023, roof cladding work on the valve hall of Sofia’s onshore converter substation (an OFTO asset) had commenced, as shown in a drone overview of the onshore converter station construction site.

By October 2022, the structural framework for the onshore converter station’s valve hall and control building at the Sofia OFTO site in Teesside had been completed, marking a key civil construction milestone for the onshore HVDC substation.

In August 2022 Sofia Offshore Wind Farm Ltd completed its unexploded ordnance (UXO) campaign for the Sofia project area on Dogger Bank, safely finalising UXO investigations and clearance to enable offshore construction to begin in 2023.

In April 2022, a ‘first spade in the ground’ event formally marked the official start of construction of Sofia’s onshore HVDC converter station in Teesside. The ceremony, attended by senior managers from Sofia, GE’s Grid Solutions and Kier Infrastructure, signalled the transition from enabling works to full-scale construction of the onshore converter facility forming part of the Sofia OFTO assets.

In January 2022, Jones Bros Civil Engineering completed the load‑bearing stone platform and associated civil works for Sofia’s onshore HVDC converter station in Teesside, preparing the Offshore Transmission (OFTO) site for GE Grid Solutions’ main construction contractor to begin installing the converter equipment.

A ceremony in September 2021 marked the first steel cutting for Sofia’s offshore HVDC converter platform at the Batam yard in Indonesia, signalling the start of fabrication of the offshore converter platform that will form part of the Sofia OFTO transmission assets.

In June 2021, onshore construction for Sofia’s HVDC transmission system began in Teesside, with enabling works starting at the onshore converter station site and along the cable corridor. These works marked the start of physical construction of the Sofia OFTO onshore infrastructure that will connect the offshore converter platform to National Grid’s Lackenby substation.

On 24 March 2021, RWE achieved a positive Final Investment Decision for the 1.4 GW Sofia Offshore Wind Farm on Dogger Bank, authorising the multi-billion pound investment in the project's development and construction. The FID covers the entire Sofia project including the offshore wind farm and the associated transmission assets that will form the Sofia OFTO — the offshore converter platform, HVDC export cable system, onshore converter station, and HVAC connection to National Grid's Lackenby substation in Teesside. Under the UK's developer-build-transfer model for offshore transmission, RWE (via Sofia Offshore Wind Farm Limited) takes the investment decision and builds the transmission infrastructure, which is then transferred to a licensed Offshore Transmission Owner (OFTO) following commissioning via Ofgem's competitive tender process. The FID was followed shortly afterwards by the signing of contracts with key component suppliers including Prysmian Group for the HVDC export cable system and GE Grid Solutions (now GE Vernova) with Sembcorp Marine (now Seatrium) for the HVDC converter stations. Onshore enabling works at the Teesside converter station site commenced in June 2021, marking the transition from development to construction. The FID was taken on the basis of Sofia's Contract for Difference (CfD) award and the project's Development Consent Order granted in August 2015, providing the revenue certainty and planning consent needed to commit to the investment.

In late 2020 Sofia concluded further preferred supplier agreements that introduced three key installation vessels, including Prysmian’s new Leonardo da Vinci to install the HVDC export cables, while Van Oord’s Aeolus and Nexus were lined up for monopile foundation and array cable installation, thereby securing installation capability for the project’s transmission and generation infrastructure.

In July 2020 Sofia announced that GE’s Grid Solutions and Sembcorp Marine had been selected as preferred suppliers for the project’s HVDC transmission system, covering the offshore and onshore converter stations and associated HVDC equipment required to transmit 1.4 GW from the Dogger Bank array to Teesside.

Sofia Offshore Wind Farm Limited stated in its Supply Chain Plan submitted to BEIS (dated 15 April 2020) that it "will issue a formal ITT for the export cables asset contract in mid-2019". This document is forward‑looking, not a post‑event report, so it does not prove the ITT actually issued on that schedule. No later public corrections or notices were found, but there is also no direct evidence of the actual ITT release date. Given the lack of clear confirmation, this projection is kept as approximately mid‑2019 when the ITT was planned to be issued.

In August 2015 the UK Secretary of State granted development consent for the project that became the Sofia Offshore Wind Farm, issuing a Development Consent Order covering both the onshore and offshore works for the Dogger Bank Teesside B site on Dogger Bank in the North Sea. This consent authorises construction of the wind farm and its associated onshore and offshore works, including the infrastructure that will form the Sofia OFTO transmission link.

Sofia Offshore Wind Farm Ltd (SOWFL) issued a global request for information (RFI) to a large and geographically diverse group of high-voltage subsea cable suppliers to stimulate greater competition for the project’s HVDC export cable scope, in response to expected supply constraints in the export cable market for far‑offshore wind projects such as Sofia.

SOWFL confirmed a strategic commitment to use HVDC technology for the Sofia export electrical system, determining that HVDC is more cost‑effective than HVAC for the project’s approximately 220 km offshore export cable route and noting that Sofia could be the first UK offshore wind project to demonstrate this technology.

Prysmian Group signed a preferred bidder agreement with RWE Renewables for a turnkey high‑voltage submarine and land export cable connection for Sofia, covering design, supply, installation and commissioning of an HVDC symmetrical monopole cable system linking the offshore converter station to the onshore converter station in Teesside, with offshore installation to be carried out by Prysmian’s Leonardo da Vinci cable‑laying vessel and an expected contract value of over €200 million, subject to final negotiations and RWE’s final investment decision.

Ofgem’s Sofia OFTO Tender Round 12 materials identify GE Vernova and Seatrium as the technology providers and HVDC contractor for the Sofia link, supplying a 1,320 MW "+/- 320 kV" VSC‑HVDC converter system with one offshore and one onshore converter substation, including the offshore converter platform and onshore converter station that will comprise core Sofia OFTO assets.

Project documentation for Sofia OFTO shows Prysmian Group as both the export cable supplier and onshore cable supplier, providing two ±320 kV HVDC subsea export cables with fibre‑optic communications from the offshore converter platform on Dogger Bank to landfall near Redcar/Marske‑by‑the‑Sea and a 7 km onshore HVDC plus 2 km HVAC cable network to the Lazenby converter and National Grid’s Lackenby substation, with subsea installation carried out by Prysmian’s Leonardo da Vinci vessel.