GeoInsighter Summer/Fall 2006 Newsletter

SHOULD YOU CONSIDER HORIZONTAL DIRECTIONAL DRILLING FOR YOUR NEXT PROJECT?

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Horizontal directional drilling (HDD) can be a cost-effective alternative for installation of utility lines under a surface water body or surface transportation obstacles or for reaching otherwise inaccessible areas; that is, if everything goes according to plan. Before it is decided to include HDD in your next project, it is critical to understand the potential pitfalls, which can be daunting.

HDD is no different than anything else in the environmental and engineering business--the risk is quantified and managed with proper planning, sound design, selection of the right team members (site civil engineer, water supply engineer, environmental engineer, geotechnical engineer, steering contractor, drilling coordinator, drilling contractor, regulators and client representative), and patience in implementing the project elements. For projects to be successful, however, even the best planning needs to include a well thought out Plan B (and perhaps a Plan C, D, …). The reason for additional contingency plans is because if you are drilling from point A to point B, there will be unexpected conditions and events that will occur along the way.

The formation that will be drilled dictates the equipment and tooling, drilling methods, and overall project cost. At drilling speeds ranging from 5 to 20 feet per hour for unconsolidated material and 1 to 5 feet per hour for rock drilling, the cost to drill through unconsolidated material can be several times less than rock drilling. The presence of environmentally sensitive areas, limited physical space, and buried utilities are just some of the conditions in the launch and landing areas that can impact project costs.  

Perhaps the best way to describe some of the challenges is to provide an example. In protecting sensitive wetlands, casing was driven to a depth where a sufficient confining pressure was achieved to eliminate the possibility of a "frac-out" (release of drilling fluid either into the formation or the water body). The casing was aligned as it turns out dead center on a row of timber piles that were part of an historic ferry dock (the portion above the water surface was gone). The chances of hitting a single pile is relatively low considering pile spacing and deterioration; hitting an entire row would have been difficult. The timber piles added days to the project schedule as drilling bits designed to go through soil turned timber piles into toothpicks in a painfully slow process.

Knowledge of the formation is always a balancing act--how many design phase borings to install versus the associated project development cost. When making decisions, be certain that the formations are defined with enough horizontal and vertical latitude to allow some variance from the original bore path design. There may be reasons to move left or right or go deeper. Instead of a drill path, visualize a drill tube with the diameter of the tube set by the confidence of your geotechnical information.

What do you do if the drilling tool is lost in the borehole due to either equipment failure or human error? Be prepared to call on the collective experience of the entire team. Drillers have experience with the drilling equipment and tooling, but it could be the 'out of the box' idea that one team member has that yields the design of the fishing device. Fishing is a game of patience whether in the water or in the borehole. In the case of retrieving a drilling tool, you may be hundreds or perhaps even thousands of feet from the lost tool. However, successful retrieval of tooling, although not guaranteed, occurs more frequently than you might think.

Selection of HDD, or any technology, should not be made solely on the basis that it is the cool "new tool." Rather, site conditions for each project need to be evaluated thoroughly. Then, based upon the findings of this evaluation, the project team may choose to select HDD or an alternate route to complete the project. The realities of the site condition must be paramount in the decision-making process. Remember, if you choose HDD, make certain your team has a contingency plan(s) in their back pocket.

Donald P. Iannicelli, P.E.

Eric C. Watters

Timothy O. Myjak, L.E.P.
info@geoinc.com