Scottish and Southern Energy (SSE) is buying a 25.1% interest in the Walney offshore wind farm in the Irish Sea for up to £39m.The 367MW (megawatt) wind farm, being built 14km west of the Isle of Walney, is owned by Danish state-controlled oil and gas group DONG Energy, which will keep a 74.9% stake.SSE says about £17m of the money is subject to the operational performance of the wind farm and it will pay its pro rata share of the construction costs.Walney is being constructed in two phases of 183.6MW each with the first phase due to start in the spring of 2010 and the second phase a year later.It's expected to enter commercial operation in two phases, beginning in the first half of 2011 and then towards the end of the same year. Today's acquisition takes SSE's portfolio of onshore and offshore wind farms in operation, in construction or with consent for development in the UK and Ireland to over 2,100MW.'The potential for harnessing the energy of the wind off the coast of the UK is vast and the combination of Walney and Greater Gabbard give us a very good offshore wind construction portfolio for the next few years,' SSE boss Ian Marchant.Earlier today, British Gas owner Centrica said it has agreed to sell a 50% equity stake in its 270 megawatt (MW) Lincs offshore wind farm for £50m in cash.The buyers, DONG Energy and Siemens Project Ventures, part of Germany's Siemens Financial Services, are expected to complete the deal in January.