drilling tool, a bent subassembly, and a steering/tracking tool. Pilot-hole directional control is achieved by using a nonrotating drill string with an asymmetrical leading edge. The asymmetry of the leading edge results in a steering bias. When a change of direction is required, the drill string is rotated so that the direction of the bias is the same as the desired change of direction. The drill string may also be continuously rotated when directional control is not required. Normally, the leading edge will have an angular offset created by a bent subassembly or motor housing. The most common types of down-hole drilling/steering tools used in the HDD industry are compaction tools and down-hole mud motors.
Compaction heads consists of a wedge-shaped drilling bit, which is used for cutting and displacing the soil as well as for steering. To bore a straight hole, the drill string is rotated and pushed simultaneously. When a correction in direction is required, rotation stops and the drilling head is preferentially oriented in the bore hole. Then the drill rig pushes the entire drill string forward. As the slant on the face of the wedge is pushed against the soil, the entire assembly is deflected in the desired direction. After the steering correction is completed, rotation is resumed until another correction is needed. Compaction-type drilling tools are most often used in mini and midi-size drill rigs to drill through soft to medium consolidated soils as well as loose and dense sands. When gravel or hard clay is encountered, compaction heads tend to wear rapidly. They are not suitable for drilling in rock formations.
When drilling with compaction heads, steering difficulties are often encountered in very soft soils. This is caused when the resistance to the deflector plate is not sufficient to offset the tendency of the drill string to drop vertically under its own weight. To solve this problem, use a larger deflector plate. Steering can be improved by increasing the flexibility at the head of the drill string. A common method is to add a length of a smaller-diameter, more flexible drill rod behind the drill bit.
Mud (down-hole) motors are used in ground conditions ranging from hard soil to rock. Mud motors convert hydraulic energy from the drilling mud being pumped from the surface to mechanical energy at the drill bit. This allows for the bit to rotate without drill string rotation. Positive-displacement motors are typically used in HDD operations. These motors generate torque and rotation at the drill bit from the flow output of the mud pump. Directional control is obtained by a small bend in the drill string just behind the cutting head. As with compaction heads, once the correction is made, the complete drill string is rotated to continue boring straight in the new direction. This method costs more than compaction heads and is less common in the utility industry.
The advantage of mud motors is that they cut the formation, reducing drill string rotation requirements and making it possible to drill long bore holes to substantial depths. The main disadvantage of mud motors is that they are more expensive in comparison to compaction heads and require hundreds of gallons of drilling fluids per minute.
16 Chapter 1 ■ Introduction to Horizontal Directional Drilling
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