China National Petroleum Corp. (CNPC), the country's largest onshore oil and gas producer, continues to gear up for expansion into offshore gas plays.

The company has commissioned Dalian Shipbuilding Industry Offshore Co. Ltd. (DSIC Offshore), a unit of state-owned China Shipbuilding Industry Corp. (CSIC), for two DSJ300 series jack-up drilling rigs, CNPC announced Wednesday.

CNPC previously ordered and received its first jack up drilling rig from DSIC Offshore last August. A second rig, under the same earlier contract, is currently under construction and is expected to be delivered by year end, according to CNPC.

The companies signed a strategic cooperation agreement in Dec. 2009.

CNPC is slowly building up an offshore gas portfolio, traditionally an area dominated by China National Offshore Oil Corp. (CNOOC). In February, CNPC acquired exploration rights to the offshore Yanchengdong Block in the South Yellow Sea Basin, Interfax previously reported. The state-owned energy major has allocated RMB 718 million ($114 million) for similar projects over the next three years.

The DSJ300 unit is capable of operating at depths of up 91.44 meters, drilling to 9,000 meters. Design of the initial vessel took approximately nine months while construction of each rig will take around 12.5 months, CISC said last August upon delivery of the first rig to CNPC.

"What differentiates jack-up drilling rigs from submersible rigs is depth of water," Han Huawei, an engineer with offshore vessel manufacturer Yantai CIMC Raffles Offshore Ltd., told Interfax today. "Usually jack-up [rigs] can operate at a maximum depth of 100 meters, while submersible can operate up to 3,000 meters underwater."

Han added that new orders are made up primarily of submersible drilling rigs.

Shanghai Waigaoqiao Shipbuilding Co. Ltd. began construction of a jack-up drilling rig for CNC capable of operating at depths of 121.92 meters and drilling up to 9,144 meters, Interfax previously reported.