Seaway will install up to five platforms at the Burbo Bank Extension, the Walney Extension projects in the Irish Sea and Race Bank in the North Sea.
The Cyprus-based firm will use its Stanislav Yudin and Oleg Strashnov crane vessels to install the platforms. Seaway previously installed the substation at Dong's 210MW Westermost Rough project, which is also located in the North Sea.
Bob Dunsmore, senior vice president of renewable energy for Seaway, said the combined contracts would help bring down the cost of offshore projects by increasing standardisation and "operational efficiencies".
With these orders, Seaway has either installed or is due to install 25 offshore substations for the industry.