At 23:00pm, June 10, 2014, Huayuan Company successfully completed pulling back the prefabricated pipeline through the HDD borehole underneath the Yellow river in Zhengzhou city for the Boai- Zhengzhou- Xuedian Natural Gas Pipeline Project, which was actually Huayuan’s 8th time of successful Yellow River Crossing.
The φ 610 mm × σ 12.5 mm pipeline of the Yellow river crossing section was 2621.76m long, whose crossing strata was mainly medium sand layer. The entry point was located at eastern side of Lupo hill, Wancun Town, Xingyang City, Henan Province; daylight point (exit point) was located nearby Chenjiagou (which is the birthplace of one the Chinese traditional Kung Fu, i.e., Chen’s Shadow boxing), Wen County, Jiaozuo City, Henan Province. Most of the crossing route passed through unsteady sand layers, which was very permeable, giving rise to the difficulty of the HDD construction works.
In view of avoiding such problems as insufficient torque of drill rig, deficient stiffness of drill string, positioning difficulty of drill bit, which are usually caused by too much length of the pilot hole drilling, Huayuan gave up the traditional one-shot pilot drilling method; alternatively intersecting method was employed, where two sets of drill rigs were respectively deployed at the entry-site and exit-site, drilling forward simultaneously to the intersection point in the midway, steering under the guidance of downhole surveying device, DX-I, which was self-developed by Huayuan company, exclusively used for downhole survey and downhole interception. The drill bits, with Huayuan’s lately-developed Mag-Target Probe and sensor mounted in the rear, conquered such difficulties as disturbance to position the drill bit due to too much length of pilot drilling and river waves when crossing the river. Finally, Huayuan successfully intercepted the two drill bits underground, making a single consecutive pilot hole strictly complying with the designed trajectory. At the reaming stage, two drill rigs were synchronously maneuvered to make the borehole larger, resolving the problems of insufficient working torque and deficient drill-string stiffness if only one rig were used.
Because of the risks of borehole contraction initiated by the unstable sand layer and the ensuing unanticipated resistance, the pipeline being pulled back was very probably been locked in the very long borehole; moreover, under this circumstance, if only one rig was used to forcibly pull back the pipeline, the drill string was very possibly been broken off, making the pulling back work fail. In view of this, at the pipeline back pulling stage, Huayuan used the self-developed 500t pipe thrusting device to assist pushing the pipeline from the oppositesite. At last, under the cooperation of the pulling rig at one end and the thrusting device at the other, the pipeline was successfully installed into the borehole.