The capacity will be split, with 235.2MW in the Mermaid concession area and 252MW in Seastar.
The Otary consortium will remain the majority shareholder, with a 70% stake, alongside Engie Electrabel at 17.5%. Eneco's Belgian wind unit has also joined the partnership with a 12.5% holding.
Previously, Otary owned 100% of Seastar and 65% of Mermaid, with Engie Electrabel owning the remaining 35%.
Mathias Verkest, CEO of the new Seamade joint project development company explained why they decided to bring in a new investor: "Typically, we were able to finance wind power projects of about 300MW in Belgium. 500MW has never been done before."
Mermaid and Seastar will be considered as one project (pic: Otary)
Combining the two projects will help create economies of scale and make the projects viable under the much-reduced strike price of €79/MWh agreed with the Belgian government in October 2017.
"Synergies and scale effects are needed to be able to proceed on a substantially lowered levelised cost of energy," Verkest added.
Siemens Gamesa is lined up to supply 58 of its new 8.4MW turbines installed on monopiles, with two offshore substations connecting to Belgium's modular offshore grid.
Deme, a member of the Otary consortium, will be the marine contractor.
Before giving the final go-ahead, the developers are waiting for the European Commission to approve the new support mechanism and for the necessary legislation to be passed.
All being well, financial close is expected by the end of 2018 and start construction in 2019, for Seamade to be fully operational in 2020.