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.

"The project's production capacity of 12 billion cubic meters accounts for about 10 percent of China's current natural gas output, and will ease the gas shortage in southern China," said Li Lingxuan, a natural gas industry analyst at Sublime China Information Co Ltd, which provides information on the commodities industry.

The South China Sea's oil and gas reserves take up about one-third of China's total oil and gas resources.

"However, up to 70 percent of the reserves in the South China Sea are in deep waters which have high environmental and technological requirements for exploration," she said. "Thus, CNOOC's Liwan 3-1 project will help boost China's deepwater oil and gas resources exploration activities."

Yang said the biggest challenge for the project is that the construction process has to be done in deepwater, which is highly susceptible to bad weather.

After two and a half years of construction, engineers from CNOOC installed the topside of the central platform - the last step in the construction process - on Thursday morning while it was raining and under huge waves.

"The topside weighs more than 30,000 tons, which made it the largest floatover operation in the offshore oil industry in recent years," said Nie Min, a manager at CNOOC's project control department.

The project combines a central platform, an onshore natural gas plant, seven underwater production facilities and a 419-km underwater pipeline.

"The platform was designed, built and installed domestically," said Nie. "That has reduced costs significantly. It also shows that China has the advanced technology to make equipment for offshore deepwater exploration activities."

Many steel parts were made by domestic companies including the Baosteel Group in Shanghai, he said.

The quantity of steel used for the platform is four times that used in the Eiffel Tower in Paris, CNOOC said.

The gas field, which was discovered in 2006 jointly by CNOOC and Canada's Husky Energy Inc, is 1,500 meters deep.

CNOOC's $1 billion deep-sea rig - known as Offshore Oil 981 - was brought in for the construction of the Liwan 3-1 platform in March.