Hoover Dam

Trusted to help create a new Hoover Dam Visitor Center in complex circumstances.

Situation

The Hoover Dam and its popularity started to pose some very real issues in its early years.  Visitors were generating major traffic problems as well as concern for the safety of pedestrians moving along Highway 93, the dam's two-lane road that lead to the visitor center. With ever-increasing tourism, it was decided that a new visitor center needed to be built, located and constructed to provide awe-inspiring views of the dam, Lake Mead and Black Canyon below the dam. The location of the new visitor center required rerouting several electrical cables - a job requiring the installation of a utility conduit from the top of the dam to a tunnel lined with trays of power cables deep inside the canyon's west wall.

To accomplish this, a 3.5-inch diameter pilot hole had to be drilled to intersect the switchyard control conduit tunnel located a distance of 320-feet away. Once drilled, the pilot hole would be backreamed to a diameter of 21-inches to accommodate the utility conduit. To hit the breakout target, the pilot hole had to be drilled at a precise angle and direction. To intersect the target within the allowable tolerance, plus or minus two feet of planned hole center, required implementation of a variety of straight-hole drilling techniques.

Solution

IDS was charged with creating a solution utilizing measures that included employing special drilling tools, controlling penetration rates, frequent borehole surveys, and employing directional drilling techniques for correcting hole alignment.

One method with a reputation for drilling a straight hole is to stabilize the drill string. Before drilling could begin, IDS designed special stabilized drill tools including an impregnated diamond core bit that would hold up to the type of igneous rock encountered at Hoover Dam; this and other products were employed for precision and accuracy. A Surface Reading Gyro (SRG) survey was selected to watch the pilot hole, primarily for the SRG system's ability to provide real-time orientation data of hole angle and bearing and because it is unaffected by naturally occurring or induced magnetic interference.

IDS chose to use a truck-mounted angle core Accu-dril™ with its mast centered over the borehole collar and aligned with the pre-set surface casing. The 7-foot long by 21-inch diameter steel casing was collared into the rock and cemented at the prescribed angle of the pilot hole. The drilling plan called for the pilot hole to be within 2.5-inches of planned hole trajectory at a point 53-feet from the surface. For this reason, surveys were recorded every 10 feet of drilled depth. As drilling advanced the hole, overlapping survey stations provided a quality control check of the data for the preceding survey. At a vertical depth of 235 feet, a survey was recorded, computed, and projected to total depth. The projection indicated that the hole, continuing its present course, would be at the maximum allowable deviation tolerance. Based on this, a decision was made to realign the pilot hole, dropping hole angle while maintaining hole bearing. A downhole motor was used to realign the pilot hole from 230 feet to 260 feet dropping the hole angle from 12.45 to 10.5 degrees. After reestablishing hole alignment with the planned trajectory, the directional motor was pulled and core drilling resumed.

At a depth of 314.5 feet, indications of cement cuttings in the drilling fluid provided the first evidence that the bit was in the proximity of the tunnel. Continued drilling produced interspersed rock and cement cores, and finally solid cement at the 316-foot level. Sounds of the approaching drill bit were heard inside the tunnel near the planned breakout point. To confirm the location of the borehole relative to the inside of the tunnel, the drill string was pulled back and a sensor lowered to the bottom of the hole where its position was marked inside the tunnel. A final gyroscopic survey was recorded, and its bottom hole coordinate marked inside the tunnel wall. From these two reference points an air-driven jackhammer was used to drill a series of small diameter exploratory holes on 3.5-inch centers into the tunnel wall. This was in attempt to intersect the drill hole. The bottom of the pilot hole was located when drilling fluids gushed into the tunnel, less than 18 inches away form the target center, well within the specified alignment tolerances

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