China National Petroleum Corporation commenced construction of the $US23.3 billion project in late 2008. The scope of works involves the construction of an 8,704 km gas pipeline that will connect Horgos, in the Xinjian Uygur Autonomous Region, with the Hong Kong Special Administrative Region after traversing 14 provinces, autonomous regions, and municipalities. The trunk pipeline is estimated to cost $US10.3 billion, while the network is expected to cost $US13 billion.

The project has been divided into eastern and western sections with Zhongwei, in the Ningxia Hui Autonomous Region, designated as the pipeline’s midpoint.

The western section, running 2,461km from Horgos to Zhongwei, commenced construction in February 2008 and the majority of the welding is now complete.

The eastern section, running 2,477 km from Zhongwei to Guangzhou, and with a design pressure of 10 MPa, commenced construction in December 2008. The eastern section will connect to the Turkmenistan – China Gas Pipeline, which is also currently under construction and is scheduled to be commissioned in 2011.

When completed in 2011, the Second West-East Gas Pipeline will have a capacity of 30 Bcm/a of gas.

HDD in Tianjin

The Tianjin Pipeline Bureau planned the installation of pipe near the Tianjin West Outer Ring No.10 Bridge district. The crossing started on 16 May 2009, with the pipeline pull-back completed on 6 June 2009.

Horizontal directional drilling (HDD) was chosen for the pipeline installation under approximately 400 m of green belt land, as well as under a canal and neighbouring high-tension electricity pylons. The length of the directional drilling crossing was 510 m, with a design depth of 15 m.

Project contractor Mr Xing identified one of the major challenges of the crossing. “The main difficulty was overcoming the strong electric field interference from the high-voltage lines, which changed the magnetic declinations we use to guide the drill head.”

The river banks in the green belt area were smooth, and the river is approximately 110 m wide and 4 m deep. Ground conditions along the crossing path consisted of clay and sand at varying depths.

In order to place precisely the drilling rigs, the entry and exit points of the drill path needed to be set along the route’s central axis. Mr Xing said that it was very difficult to set out the central axis due to the dense vegetation of the green belt, and GPS was used to overcome this challenge.

The completion of the pilot hole, the key to the entire crossing, used a ZT-150 HDD rig. The bore was guided under the river with the SST system, using the shore as a reference point. After the river crossing, ground tracking was used to guide the bore path.

Accuracy of the entry and exit points is fundamental to the success of HDD crossings. In this case measurements were taken frequently and trial holes were dug to avoid interference with existing utilities. The drill bit broke through along the centre-line with a 6° angle – an indication of the project’s success. The pilot hole was completed in a straightforward manner, in part due to continuous adjustment of the drill pump volume and accurate control of the mud’s viscosity with bentonite drilling fluid.

A number of passes were completed with the reamer, increasing the diameter of the pilot hole to a final diameter of 1,000mm. Throughout the reaming process, adjustments to the mud displacement, control of the pull-back speed, and close attention to the pump pressure, torque, and the back-drag force ensured the successful completion of the crossing. The pull-back of the product pipe similarly depended upon careful observation of the drill rig tension, and control of the torque, mud back pressure, and the pull-back speed.

Conclusion

Mr Xing said that lessons learned for future construction projects include paying close attention to the drilling rig, mud back pressure, and promptly adjusting the mud.

The Second West – East Gas Pipeline is expected to increase the share of natural gas in China’s energy consumption by one to two per cent. It is expected to play a significant role in boosting the country’s domestic natural gas demand, facilitate the improvement of China’s manufacturing industry, improve the nation’s energy structure, and promote economic and social development in the regions through which the pipeline route passes.