Identification of Risks

An important part of the planning and design phase of a HDD project is to identify and avoid or at least reduce possible risks. If potential risks cannot be avoided or reduced, the crossing alternative should be closely evaluated and other alternatives considered. A decision has to be made as to whether to accept the risks or select another crossing option.

Even with the best planning and design efforts unexpected risks can occur. Experienced and qualified contractors are well equipped to solve many of these problems as they occur. The problem-solving process can be assisted during the planning phase by developing contingency plans for any anticipated risks. Listed below are some of the risks that are often of concern with HDD projects:

  • Failure to complete the bore
  • Damage to environmental, natural, historical, and cultural resources
  • Surface collapse of heave
  • Hydrofracture or inadvertent drilling fluid returns
  • Damaging other utilities

Drill-Path Design

The information gathered during the planning phase is used to develop the best possible design for the drill path. The designed drill path should meet all the location and depth control points while keeping the drill length as short as possible. All designed drill paths are made up of a series of straight lines, called tangents, and curves, which are typically sag bends, over bends, or side bends depending on their axial plane. It is not uncommon for HDD drill paths to have compound bends even though they are generally avoided if possible. The location and configuration of a drilled profile are defined by its entry and exit points, entry and exit angles, radius of curvature, and points of curvature and tangency. The relationship among these parameters is shown in Figure 4-l. The values of these parameters should be called out in the contract plan and profile drawings. Often the design drawings will call out maximum or minimum parameter limits and allow the contractor to vary the bore path as long as he or she meets the limits and maintains all the design control points, such as depth at a particular location.

As previously described, the entry and exit points are the end points of the drilled profile. Confusion often arises concerning the location of the points. It is important to understand the terminology and identify the point correctly, because it has an impact on the pull load and stress calculations. The drilling rig is positioned at the entry point. The pilot hole is drilled from the entry point to the exit point, and the pipeline is pulled into the ground at the exit point and then pulled back to the entry

Horizontal Drill Path Design
FIGURE 4-1 Drill Path Definition

point. When designing the drill path, keep the number of bends to the minimum required. This reduces pullback loads and extends drill-rod life. The best bore path starts with a straight tangent section at the prescribed entry angle to gain the depth required for steering control and the depth of cover. At the required depth the drill head is steered upward with a curve, then transitions to a horizontal segment, and again turns upward with another curve before transitioning to another straight tangent section at the desire exit angle.

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  • fatima
    How to get tangents from a drilling curve?
    2 years ago

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