你的句子有误，改正的办法有两种：
一是将 its 改为 it’s (=it has)：
The China National Offshore Oil Corporation has announced it's (=it has) entered into a 20-year deal with British Petrolium for liquefied natural gas supplies.
二是将 entered 改为 entering：
The China National Offshore Oil Corporation has announced its entering into a 20-year deal with British Petrolium for liquefied natural gas supplies.