The China National Offshore Oil Company (CNOOC) announced Wednesday that its net profits soared by 51.4 percent year-on-year to reach 39.34 billion yuan ($5.16 billion) in the first half of 2011, boosted by the construction of new oilfields and projects amid high oil prices.

In a statement posted on its website, China's biggest offshore oil producer said that its net oil and gas production amounted to 168.7 million barrels of oil equivalent (BOE) during the period, up 12.9 percent year-on-year.

The company's basic earnings per share reached 0.88 yuan with a proposed pretax interim dividend of HK$0.25 per share, according to the statement.

The company's realized oil price reached $108.16 per barrel during the period, 40.8 percent higher than a year earlier, while its realized gas price rose 15.5 percent year-on-year, according to the statement.

Stable production growth and higher realized prices lifted the company's oil and gas sales revenues by 45 percent year-on-year to reach 97.03 billion yuan in the first half of the year, according tot he statement.

Despite rising prices for oilfield services and raw materials, the company's production costs have remained low. Cost-cutting measures and more efficient company practices sent its operating costs down 3.8 percent from 2010.

The company acknowledged in its statement that recent oil spills originating from one of the company's joint venture platforms in north China's Bohai Bay have caused a "certain impact on the marine environment."

CNOOC pledged to assist ConocoPhillips China Inc, operator of the bay's Penglai 19-3 oilfield, to clean up the spills in a timely manner and minimize their environmental impact.

"We faced a challenge after the oil spill incident occurred in the Penglai 19-3 oilfield and we feel deeply sorry about the spills," said Wang Yilin, chairman of CNOOC.

Wang said the company has started inspecting facilities, equipment and production operations in all of its oilfields and reinforcing its risk management regulations in order to avoid similar incidents in the future.

The company has reduced its annual production target for this year to 331-341 million BOE as a result of the spills.