March 2019, Vol. 246, No. 3

Tech Notes

Technology Provides Steering Accuracy for Tough PG&E Replacement

The United States’ infrastructure is crowded with underground pipelines that will need to be replaced. When the time came to replace a critical but vintage, PG&E 30-inch gas line in the East Bay Area of Oakland, Calif., there were issues that Senior Gas Engineer Joshua Hampton needed to resolve. 

The new line needed to go under two large and deep culverts. Closing the extremely busy San Leandro Street, directly between the BART Coliseum Station and the Oakland-Alameda County Coliseum, home to both the Oakland Athletics and the Oakland Raiders, and the Golden State Warriors’ Oracle Arena would have inconvenienced millions. The best answer seemed to be horizontal directional drilling (HDD), but that option presented its own difficulties.

Safety was the primary issue. To that point: How to accurately drill more than 2,800 feet, beneath one of the areas busiest streets, crossing two wide and deep culverts. Additionally, the tunneling would parallel an existing, live 30-inch gas line, crossing 25 to 30 known utilities, while nearly passing under the Coliseum Station. This would also occur next to the Bay Area Rapid Transit (BART) DC-powered mass transit line. The conversation on the project started and stopped with the need for pinpoint accuracy during the entire length of the drill.

PG&E selected ARB Underground (A division of Primoris Services Corp.), head-quartered out of Lake Forest, Calif., as the HDD contractor. Jody Parrish, ARB’s HDD division manager has more than 30 years HDD experience, having installed lines via directional drilling around the world, as does his General Superintendent Ronald Rhodes. 

It’s not likely PG&E could have found a more experienced contractor. Parrish immediately started conversations with Don Chaddock, president of Horizontal Technology, an industry leader in HDD navigational tools and services.  

A look down from the rig, down on San Leandro Street. Photo courtesy of PG&E

“We work on complicated projects on a near daily basis,” said Chaddock.  “Working with professionals such as Jody and Ronald of ARB, who have personal experience on just about every type HDD situation imaginable makes a big difference.  They know when it’s time to drill ahead but they also know when it’s time to plan and set things up properly.  This was one of those times.” 

All discussions revolved around safety, and every option was analyzed thoroughly. The new 30-foot gas line’s pilot hole would have very tight tolerances, including the vertical and horizontal radius.  Not only did every foot of this project need to be drilled accurately, the underground location needed to be verified in real time. 

PG&E Senior Engineer Joshua Hampton wanted to know what steering system provided the most accurate guidance verification.  The drill, under San Leandro Street, needed accurate verification and location confirmation each survey, before drilling ahead. No guesses, only documented accuracy.  

Walk-over guidance was out of the question, leaving the two versions of wireline steering, magnetic or gyro? Which matched the requirements? Safety was paramount with accuracy being the key. As either calculated steering systems progresses, magnetic or gyro based, the elliptical error-range widens.  That error factor wasn’t acceptable.  

It isn’t uncommon for gyro steering tools to be combined with a walk-over system to try and determine its general location as it approaches exit. Even with the gyro’s auxiliary GPS surface locate, it’s still just a walk-over receiver attached to a surface mount GPS. With this method, the drill bits location would have the same accuracy variables as a standard walk-over system which didn’t meet the steering accuracy required. 

This is where HDD technology saved the day. HDD veterans understand that magnetics is used to enhance the accuracy of magnetic steering systems. The magnetic field steers and guides the bit’s location. In fact, the accuracy of the magnetic steering system, with the DC tracking, was the catalyst for HDD tremendous growth.  

Magnetic steering tools have an ever-increasing error factor, just like the gyro, but DC tracking brings that error factor back to zero at every survey. To this date, nothing comes close to the accuracy of a magnetic steering system verified with a DC surface tracking system.  

Yes, in a magnetically clean area, a magnetic steering system can very accurately guide an HDD pilot hole. Beach approaches are a good example.  So, too, wide waterways like the Mississippi River. Both have been routinely drilled with the magnetic azimuth alone. Yet many HDD newcomers, some in important management positions, due to the difficulty in placing a surface coil, assume a gyro-based tool must be used.   

This same mistake applies to congested urban areas, where steel structures, in the mind of less experienced HDD workers, supposedly eliminate the use of a magnetic steering system due to interference. Wrong. This is the exact situation for which DC tracking was invented and it remains HDD’s safest, most accurate steering method.  

The only way to safely track and verify the Oakland PG& E, 30-inch HDD project was to establish HT’s DataTraX surface coils and verify the drills precise location, from entry to exit. ARB and Horizontal Technology established a plan, knowing they would adjust it as the situation required. Accurate navigational data was to be provided to PG&E as the drill progressed, and the pilot-hole must remain within tolerances.

A view illustrating the importance of keeping the street open.

HTI’s on-site navigator, worked closely with the ARB crew, specifically Shane Smith, driller for ARB’s Vermeer 330, setting up the project; tedious work, locating and marking all utilities. Additional utilities were discovered when digging the entry pit, substantiating the meticulous complexity of this HDD project.  

PG&E turned off the cathodic protection on the existing 30-inch line to minimize magnetic anomalies and surface coils were created and then adjusted to overcome the significant magnetic interference. Everyone knew high intensity was needed for the down-hole probe to locate itself below the coil.  Even so, the extent of the interference affecting the accuracy of the surveys varied. The navigator and ARB crew knew they could do better.

The third rail of the BART system works off 1,000-volt DC electricity. Providing each axle of each train 150 hp, four motors per car. There were north and southbound tracks parallel to the entire drill with trains leaving the station about every five-minutes. In efforts to overcome this, coils were shortened, and the surface coils doubled (up to four strands). Additional DC welders were added.  The bore was broken down into six surface coils between 400 and 700 feet in length, mostly resulting in good, accurate surveys. 

Still, good wasn’t enough, drilling was continuously stopped anytime the surveys had a spread outside the specified requirements. It was noticed that survey accuracy improved when taken between train movements. The BART schedule was reviewed, and drilling was limited to breaks in the BART schedule. 

Placing safety and accuracy first, the drilling schedule was further adjusted to match a small window every morning when the trains stopped running. The surface tracking over came all the utilities, pipelines, rail roads and steel structures, providing pinpoint accuracy.

DataTrax's documented "perfect exit."

Every foot drilled was closely monitored and as the drill bit approached the exit, it was 0.45 feet left of the planned drill path, looking good. Pilot-hole completed, the ecofriendly surface coil was rolled up, leaving zero indication it ever existed.  ARB reamed the hole to the required diameter and pulled in PG&E’s new pipeline

“This was a prime example of HDD technology being used to match the situation.  Too often, HDD managers are making the wrong decisions, selecting the less accurate, less safe steering method.  Not on purpose, but just because they don’t have the needed experience. This risk isn’t good for the industry,” Chaddock said. “Replacing these older pipelines will become an increasing part of HDD. Most are in congested areas. Safety should always come first. The ability to know the precise location of the drill bit, and produce good accurate as-builts, greatly affects the safety of our industry. Not knowing which steering system is most accurate will result in unneeded incidents.”

HTI navigator Robert Barnett added, “It was a pleasure working with so many experienced professionals. The project and I benefited from their background and knowledge. Everyone from PG&E and ARB offered their full cooperation, understood the need for accuracy and put safety first. P&GJ


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