HAIJIAN 50, March 17, 2012 (Xinhua) -- China's patrol ships are seen in waters near the offshore oil and gas fields of East China Sea, March 17, 2012. The patrol by China Marine Surveillance, the country's maritime law enforcement authorities, was carried out by two patrol ships "Haijian 50" and "Haijian 66", which arrived in waters near the offshore oil and gas fields of East China Sea Saturday. The ships joined "Haijian 46", "Haijian 51", "Haijian 15" and "Haijian 17" to make a cruising formation, which held a collaborative drill with helicopter "Haijian B-7115" there after arrival. (Xinhua/Zhang Jiansong)