The Thanet OFTO is the high-voltage electricity transmission system connecting the 300MW Thanet Wind Farm, off the coast of Kent, to the onshore transmission grid.
The deal puts Balfour on the major acquisition trail again after selling Parsons Brinckerhoff and rejecting bids from Carillion and John Laing Investments.
Balfour will invest over £20m of equity in the Thanet scheme which was previously operated by energy giant Vattenfall.
The assets include one offshore and one onshore substation and over 50 kilometres of subsea cables.
Balfour Beatty’s Services division is responsible for their operation and maintenance under a licence granted by Ofgem with a 20 year revenue stream.
This is Balfour Beatty’s second OFTO project – in November 2013 it achieved financial close on the £317m Greater Gabbard scheme in Suffolk.
Balfour also remains preferred bidder on the Gwynt y Môr OFTO in North Wales, which is expected to close in Q1 2015.
Steve Marshall, Executive Chairman, said: “For the Investments business, the offshore transmission market is an important part of its strategy to diversify into new markets and achieving financial close on our second project is a great success.
“This project, and this market, is also an example of how Balfour Beatty Services can derive revenues from the Investments business.
“Today’s announcement supports our wider strategy of developing our delivery capability in the offshore renewables industry.”