Plunging global crude oil prices crimped the 2015 earnings of China's big-three oil companies,according to their annual reports.The net profits of PetroChina,China's top oil and gas company by production at home,shrank 66.9 percent to 35.52 billion yuan in 2015,the lowest profit since 1999.Sinopec Corp.,the country's the largest oil refiner,saw its net profit decline 30 percent year on year to 32.4 billion yuan in 2015,the worst year for Sinopec since 2009.CNOOC Limited,China's largest offshore oil and gas developer,made 20.2 billion yuan (US$3.1billion) in net profit last year,plunging 66.4 percent year on year.