The causes of accidents in horizontal directional drilling of overhead insulated steel pipes can be roughly divided into broken drilling tools, drill pipe burying and sticking, flat and immovable pipes when the pipes are pulled back, and damage to the original underground buildings. The reasons for the breakage of drilling tools: ① The drilling tool has excessive wear after long-term use; ② The drill pipe is thin and worn, and when drilling guide holes, the curvature radius is too small. The drill pipe breaks under repeated shear forces. Preventive measures: Regularly inspect drilling tools. No need to wear and exceed the limit of drilling tools. When elastic bending is used to lay pipelines, the radius of curvature should not be less than 1o00~1500 times of the outer diameter of the pipeline outlined by the horizontal Directional drilling construction method. The main reason for burying and sticking the drill bit is due to the collapse of the hole. The reason for the flattening and inability to move of the pipeline during the return process is mostly due to the improper use of mud during construction in non viscous formations, resulting in preventive measures. In non viscous formations, the quality of mud performance directly affects the success or failure of construction.
In the construction of overhead insulated steel pipes in non viscous formations, it is required that the mud has a certain viscosity, good rheological properties, high ability to carry drilling debris, and strong wall protection ability, in order to maintain the stability of the hole wall and prevent mud loss. Mud has good lubrication performance, ensuring the minimum friction resistance coefficient for construction and backhaul pipelines. High quality bentonite and appropriate mud additives and treatment agents shall be used for construction of non cohesive strata 1.4.1 The accident of sticking in the construction of pilot hole The river surface of a horizontal Directional drilling river crossing project is about 150m wide, the upper part of the formation is composed of loam soil, and the lower part is composed of loam and silt. The construction equipment is 21T drilling machine, 250/40 mud pump, and the pipe diameter is 3mm and the length is 3m. The accident happened: after the equipment was installed, the pilot hole was drilled and more than 60 drill pipes were drilled, When starting to raise the direction, it was found that the drill pipe could not be pushed forward. The operator drilled more than 20 drill pipes forward, but they still couldn't move forward. At this point, they had already crossed the river centerline and could no longer drill forward. They had to retract the drill pipe in order to construct according to the design trajectory.
Unexpectedly, only 5 or 6 drill pipes were unloaded, and the drag back pressure increased in a straight line. After forcefully pulling another drill pipe, there was a phenomenon of being unable to pull or rotate. At this time, there were still 73 drill pipes stuck in the borehole. The reason was that no mud was used during the drilling, causing the borehole to collapse and cause sticking. The idea to deal with such accidents: use high-quality mud to punch holes parallel to the drill pipe in the original hole to remove the stuck drill 1.4.2 The horizontal length of the crossing section of a river horizontal Directional drilling crossing project is about 1600m, and 10 engineering geological layers are divided within the exploration depth of the crossing: ① plain fill; ② Silty silty clay; ③ Silty clay; ④ Clay; ⑤ Silty clay; ⑥ Clay mixed with gravel sand clay; ⑧ Clay; ⑨ Silty sand: 10% angular gravel. After successfully completing the communication casing crossing and main pipeline expansion project, the construction unit encountered obstacles while dragging back for 160m. Temporary measures taken by the construction unit; Straighten the pipeline without dragging it back, and after failure, inject water into the pipeline to increase the weight of the pipeline and reduce the upper friction force to continue dragging it back. Failure analysis: Due to the long crossing distance, there was an increase in resistance during the dragging process, and the location of the obstruction point was the head section of the dragging back. The possible situation of crossing and dragging back and encountering obstruction was hole collapse or incomplete hole cleaning. Accident handling: The plan of excavating a 14m deep sunken well first and then lowering it to a depth of 8m is adopted. The equipment is used to detect the end of the pipeline and drag it back. After finding the obstructed pipeline, the remaining pipelines are buried in trenches to complete the crossing project. According to the determined plan, the construction unit invited the local water conservancy department to carry out excavation. Upon reaching the predetermined location, it was found that the geological conditions were good and the groundwater volume was low. The plan for lowering the sinking well was changed and the excavation was carried out directly to the point of obstruction. After more than a month of construction, the crossing work was completed.