Shares sank more than 38% as the company revealed the sale of SeaEnergy's 80.13% stake in SeaEnergy Renewables (SERL) to Repsol for £30.7 million cash.
Repsol will also repay £8.1 million of debt owed to SeaEnergy by SERL and fund the repayment of some £3.2 million in other loans owed by SERL.
SeaEnergy said it will use some of the net proceeds of £29.1 million from the deal to develop its marine business providing vessels for constructing and servicing offshore wind turbines.
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Repsol has formed a consortium with EDP Renovaveis to develop two of SERL's projects.
SERL was first put on the market a year ago and the company said its disposal would mean it could SeaEnergy could focus on offshore wind farm business.
Steve Remp, SeaEnergy's executive chairman said: "This deal shows how SeaEnergy uses early mover advantage to create value from new opportunities. We created a business out of nothing, but a concept, and three years later it has been valued at £50 million."
He said that the time is right for the company to capitalise on its success. "We believe that our nascent Marine business has a unique vessel concept that will make a material difference to the commercial economics of offshore wind. We look forward to pursuing this exciting opportunity, and to seeking out other projects to leverage our expertise."
Brokers Ambrian said: "SeaEnergy's sale of its interests in SERL provides it with the much-needed capital to build out its Marine business, which we believe is likely to generate shareholder value quicker than the offshore wind assets.
"However, as per the chairman's comments, it is still a relatively nascent business and, therefore, it will require time to develop. We believe the transaction gives management a platform to continue to build the business model."