China National Offshore Oil Corp - the country's largest offshore oil producer - said on Thursday it has completed construction of Asia's biggest deepwater platform in the South China Sea, marking a breakthrough for the company.

The project, known as the Liwan 3-1 natural gas central offshore platform, will have an annual processing capacity of 12 billion cubic meters after completion and will be put into operation by the end of the year, CNOOC said.

"The processed natural gas will be mainly supplied to power plants and other industrial users in Guangdong province, which will contribute to diversifying the natural gas supply in the region," said Yang Zhuangchun, deputy director of the project, who is with the South China Sea deep water gas development department of CNOOC.