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Home arrow Archives   The American Surveyor     

Survey Says Drill Here Print E-mail
Written by Mary Jo Wagner   
Friday, 18 September 2009

A 2.919Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE

When Boart Longyear, a leading drilling services provider and products manufacturer, was awarded a critical upgrade project at Arizona's Navajo Generating Station (NGS) near Lake Powell, it had all the essential technology and expertise it needed to perform the work except for one very important detail­the precise start point for the drill. Tasked to directionally drill five new water-intake shafts, the challenge for Boart was to align their drill rig in the correct position and inclination at the surface to accurately drill 48-inch diameter holes at a 53-degree angle to a depth of 500 feet.

"The compound angles and tight position tolerances of each shaft made it difficult to calculate exactly where to start each shaft to ensure the drill remained online and hit each target location," says Rusty Otto, a drilling supervisor with Boart, headquartered in Salt Lake City. "Without an accurate start point as well as an `invisible' azimuth line to guide the drill, we would have been drilling blind and hoping for a positive result. And `hindsight' isn't exactly an ideal project strategy."

Fortunately, this same challenging project introduced Boart to today's survey technology, which provides nothing but foresight to help resolve the "where" question.

"With my controls set up specifically to align the drill rig, I could use my Trimble total station and survey controller to not only determine the exact start point for each shaft, I could also check the status of the hole at certain depths and calculate in real time whether the hole will be on target at 500 feet," says Darren Yellowaga, survey manager and assistant vice president with Project Design Consultants (PDC), a Phoenix-based professional design, engineering and survey firm. "Trying to figure that out without the capabilities of the Trimble S6 would have been extremely difficult."

Otto, in fact, is even more direct. "This multimillion dollar project would have been a non-starter without Darren's measurements," he concludes. Indeed, with a firm start point and monitoring scheme to ensure field teams would hit their end point, the NGS­a critical base load generating station for millions­can now ensure an uninterrupted power supply well into the future.

A Powerful Position
Constructed in the early 1970s, the NGS is a 2,250-megawatt coal-fired, steam-electric power plant located about three miles northeast of Page, Arizona near the shore of Lake Powell. Operated by the Salt River Project (SRP), a public/private organization consisting of the Salt River Project Agricultural Improvement and Power District and the Salt River Valley Water Users' Association, the NGS serves more than one million electric customers in Phoenix and Tucson as well as residents in Nevada and California.

NGS was built near Lake Powell to ensure its water-intake shafts had a dependable supply of cooling water for the plant's three coal-fired generating units. The original five intakes were installed about 230 feet below the lake's full pool level, but a persistent several-year drought began dropping the intake level to alarmingly low levels­in 2004, the intakes were only 100 feet below the lake's surface­which could force the NGS to shut down. In 2003, NGS' six owners, including SRP, began devising a plan to sustain the plant's operations against the backdrop of falling water levels.

Spearheaded by SRP, the redesign presented a number of technical challenges and "geo-sensitivities," such as the permeability of the lake's sandstone cliff face and the tight environmental restrictions the National Park Service (NPS) imposed on the upgrade project. Mindful of those requirements, SRP worked with design firm Hatch Mott McDonald (HMM) to develop a suitable redesign for the NGS. In late 2007, they completed a design that involved drilling five new slant holes aimed at an upstream portion of the cliff face about 280 feet from the plant's existing pump house and about 140 feet deeper than the original pipes. With PDC's guidance, Boart began drilling the first shaft in May 2008 and completed the final shaft in March 2009.

Trouble at the Surface
Moving HMM's design concept from paper to the ground presented obstacles before the drill rig even rolled onto the site. In preparing the site for drilling, SRP commissioned bathymetric and two topographic surveys to both map the lake's waterbed and establish base control points for the site. In combining the topographic survey sources together, project managers realized the positioning data was inconsistent. HMM contracted PDC to provide an independent land survey to ultimately resolve the datum errors.

Using Trimble® R8 GNSS systems, Yellowaga and a colleague rectified the datum issue within one day. Setting the GNSS base unit on one of the existing control points on site, they collected data for about four hours and then uploaded the data to the National Geodetic Survey website and received an OPUS solution report indicating the accurate coordinates for the data. They entered those values into their Trimble TSC2® Controller and then checked into the same control points, an efficient exercise that indicated the vertical elevations were off by three feet. Yellowaga then reestablished site control using a Trimble S6 Robotic Total Station.

Though rectifying and reestablishing ground control was routine procedure for PDC, it turned out to be a very significant day's work because Boart realized that same efficient and accurate survey technology could also serve its precision needs for aligning its drill rig to a start point on the ground. And with that, Yellowaga's Trimble technology became bolted­sometimes literally­to the drilling operations at NGS.

X Marks the Spot
With the main site control set, Yellowaga's first task was to establish secondary control specifically for drilling the five shafts. Since the design plan called for Boart to drill underneath an existing pump house, Yellowaga used the Trimble S6 to set control on the roof of the pump house, providing him with a solid, stationary base and a better aerial perspective on the drill site.

The next tricky challenge was to align the drill rig. "Because of the sloping cliff face topography, the new shafts needed to be aimed upstream of the original shaft's positions to hit the required elevation," he explains. "That vertical slope created a very tight target window and compound drilling angles so we needed to have extremely accurate northings, eastings, elevations, azimuths and inclinations. And that meant we had to maneuver, center, and angle a 140,000-pound drill rig over a small target and keep it there while it churned through the sandstone."

To position the rig, Yellowaga uploaded a table of surface and shaft coordinates provided by HMM into his Trimble TSC2 and set the Trimble S6 on a main control point. Holding a 360-degree prism pole, he recorded the angle and distance to a second control point and then climbed to the top of the pump house. Using the total station and controller, he set a point that was exactly in between the top surface coordinate and the bottom shaft coordinate (the end-target position for the shaft) and recorded the coordinate value in the controller. After establishing this new control point on top of the pump house, Yellowaga moved the Trimble S6 to this location, oriented it, and selected the starting point coordinate. The Trimble S6 automatically turned and locked on to that point.

Using the Trimble S6 Direct Reflex (DR) technology, which allows surveyors to measure points without a prism, Yellowaga recorded two fixed points on the center of the drill rig­one above the cab and one near the back­to acquire exact deltas for proper alignment of the rig.

With those measurements set, they were ready to back the drill rig into position. Yellowaga looked through the Trimble S6 scope and directed the drill rig driver in real-time until it was exactly centered on the Trimble S6 "cross hairs."

Once the rig was in place, crews needed to set the correct inclination for the rig's 19,500-pound, 43-inch hammer to ensure it would drill the slant holes at the right angle. To guarantee the right inclination, Boart crews fabricated a small bracket and fastened a small prism in the middle and attached it to the center of the hammer. A team set the rig's mast to the given design inclination and with the total station, Yellowaga had them raise and lower the hammer to allow him to measure a bottom point and a top point to measure and verify the exact alignment for the hammer to hit the drill start point set on the ground. Using the real-time measurements of the Trimble S6 and TSC2, Yellowaga could then instruct the team to maneuver the mast and continue to remeasure until it was precisely online over the point. Boart teams then welded the mast in place and Yellowaga measured its position again to certify it was still in the right location. Then the drilling began.

Yellowaga says that dynamic back and forth process would have been a completely different experience without his survey technology. "The S6 automatically turns angles on its own and its tracking speed is phenomenal," he says. "I couldn't imagine trying to turn all those angles manually. It would have not only been far more time consuming but there would have been a lot more room for error."

Scanning in the Drill Strategy
As Boart would be drilling five holes, it was important for crews to establish a viable drilling strategy. Initially Boart planned to drill a 48-inch hole in one pass so at predetermined intervals they mapped the position of the hole using a GyroSmart tool located in the hammer. To complement this 3D map, Yellowaga used a Trimble VX™ Spatial Station to scan and create a 3D as-built of the first 100 feet of the hole.

Setting the scanner inside the mast of the drill rig, Yellowaga scanned the open hole, collecting about 12,000 points in about two hours. CAD specialists at PDC processed point clouds of the acquired data using Trimble's Realworks SurveyTM Software and provided Boart with a 3D model of the shaft at five-foot segments that were accurate to one-eighth of an inch. Based on that detail, Boart revised its initial strategy to a two-pass method, requiring them to drill a smaller hole first and then widen the shaft to 48 inches.

With the confirmed strategy in place, Yellowaga then repeated the rig-alignment process to begin drilling the first shaft. At a depth of 30 feet, Boart crews installed a steel surface casing tube and Yellowaga set up the Trimble S6 to check the tube's position and alignment. From the height of the pump house roof, he could view the top 15 feet of the tube. Using the total station's DR technology, he measured both the position and inclination and crews made any necessary adjustments. A smaller tube was then placed inside the surface casing and Yellowaga again measured its position by setting up from inside the drill rig mast.

For efficiency and better accuracy, Boart crews fabricated a four-legged, round steel plate, centered a simple prism on it and lowered it to the bottom of the tube with ropes. Yellowaga inserted himself and the Trimble S6 inside the drill rig mast, sighted his center point on top of the pump house and then turned the instrument down to sight the prism 30 feet below. Using the TSC2, he recorded those measurements and then projected them out 500 feet to verify whether they would hit their target window. Guided by that survey precision, Yellowaga says each shaft hit its target within one foot every time­a feat he doesn't believe could have been achieved without his advanced survey equipment.

"The sheer volume of calculations I needed to run would have been extremely difficult to do without the speed and accuracy of the TSC2 survey controller software," says Yellowaga. "Using the controller's software, I knew in seconds whether the shaft was online or not. And without the Direct Reflex, I would have needed more crews and more prisms and their safety could have been at risk."

Otto also credits the survey technology for the project's success. "We completely lived off Darren's survey numbers and they never steered us wrong," he says. Indeed, should Boart find themselves again asking the "where" question, they can take comfort in knowing that today's survey technology will steer them in the right direction.

Note from Yellowaga: I'd like to pass along a special thank you to HMM from PDC for giving us the initial call on this project and for all of their help and guidance along the way. It was greatly appreciated and we look forward to working with them again in the future.

Mary Jo Wagner is a Vancouverbased freelance writer with more than 10 years' experience covering the geospatial technology.

A 2.919Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE

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