The government of the Gulf state of Abu Dhabi will invest about $1 billion in the world’s biggest offshore wind farm, in the United Kingdom, stepping in after Royal Dutch Shell backed out last spring. The $5 billion London Array will consist of 271 turbines to be built in the Thames Estuary, generating 1,000 megawatts of electricity — enough to power 750,000 homes. The project is key to the British government’s goal of building enough offshore wind power to supply all residential electricity by 2020. London Array’s first phase is expected to be completed in 2012. The Danish energy group Dong owns half of the project; the German company E.ON now owns 30 percent, after selling 20 percent to Abu Dhabi’s Masdar clean-energy fund.