Two Platforms, Two Shore Approaches, Two Lines… and a Hurricane
By Angus W. Stocking Jan 21, 2014
The power supply for Virginia’s densely populated Middle Peninsula Area, on the Chesapeake Bay, is currently adequate but somewhat fragile; a single 115-kV line extends southward from the Harmony Village Substation of Dominion Virginia Power (DVP). The non-redundant power supply was at risk for long interruptions and local demand threatened to outstrip supply.
Accordingly, in 2010, DVP obtained approvals to serve the region with a new, 230-kV power line extending from its Yorktown Power Station. But approvals are one thing and actual achievement is another. Installing the line was a significant technical challenge.
For one thing, about 3.5 miles of the historic York River lies between the Yorktown Power Station and the Middle Peninsula. That mandated a very long river crossing and environmental conditions made horizontal directional drilling (HDD) the only feasible method. Actually, to supply sufficient power, with redundancy, two parallel crossings were needed. DVP elected to tender the project on a design/build basis, and HDD contractor Mears Group was selected based on experience and their innovative proposal — Mears commenced design work in 2010.
Conceptually, the solution proposed by Mears Group was simple. Two platforms, 7,300 ft apart, would be established. The north platform would be about a mile from shore, and the south platform about half that — both were to be located within the bends of the S-shaped right of way (ROW), so that the longest crossings of the project were straight. Line A would then be run from north and south shore staging areas to the nearest platforms and Line B, offset 20 ft from Line A, would be run next. HDD equipment was then shifted as needed, and the long platform-to-platform HDD crossings would take place. The proposed product to be installed was actually a bundle of two pipes — 8-in. steel conduit for the copper cable and the high-pressure dielectric fluid needed to keep it cool, and 3-in. HDPE for a fiber-optic communication line.
Getting Ready to Drill
Project design took six months. Working with side-scan sonar surveys and geotechnical information provided by DVP and factoring in tidal flows, designers worked out drill paths that avoided buried structures and stayed within the ROW established by DVP — this ROW had been established years previously, based on a design for a conventional dredged crossing and was relatively narrow for HDD. Mears designers also worked out construction schedules and staging area requirements and created contingency plans for evacuations due to hurricanes.
The length of the river crossing dictated that intersect drilling be used on the platform-to-platform and north shore-to-platform crossings. And ‘platforms’ meant actual, stable structures in 20 to 25 ft of water, built to support 150 tons of equipment and supplies and to withstand the pull loads of a 7,300-ft crossing. The platforms were built in three weeks, and Corman Marine Construction provided platform construction and general marine support including tugs, cranes, and divers.; they also installed ‘goalposts;’ twin pilings, with a cross member, that supported the drill string and held it in place from the drill rigs to the river bottom.
At 2,955 ft, the south shore crossings were the shortest of the project, the first to be completed and they were the only non-intersect crossings. The entry points were in a high-security area next to the DVP power plant.
But there were some complications even here. For one, the south shore crossing included the 45-degree bend, which wouldn’t have been a concern for the conventional open-cut dredged crossings envisioned when the ROW was acquired. But since the ROW is only 120 ft wide, and two lines were being installed offset 20 ft from each other, staying within ROW was a concern for an HDD crossing. To make it as easy as possible, Mears used a ‘racecar’ alignment. “Basically, we started the drill path curve well before the bend, just like a racecar uses the full width of a track to start turning before it gets to curves,” explains Dennis Doherty of Haley & Aldrich, who was part of the Mears design team, “This allowed us to use flatter radiuses and avoid any issues in that area.”
Meanwhile, after the 12-in. casing was established for the south shore approach. The north shore-to-platform crossings were 5,845 ft long with a 32-degree bend in the ROW. These were drilled as intersect crossings, with the intersection point about 3,500 ft north of the platform, just on the platform side of the horizontal curve. An interesting dual guidance system was used to achieve intersection on the north shore Line A crossing. Working from shore, Para-Track II was used with a surface steering coil. But passive magnetic ranging wasn’t a good solution in the York River’s main shipping channel, since large ships and barges could cause magnetic anomalies. So a gyro steering tool (GST) was used instead; this system combines optical gyroscopic sensors and sensors with World Geodetic System (WGS) latitude and longitude. The two methods successfully brought the two drill bits within a few feet of each other and passive magnetic ranging was used to close the gap. With connection achieved, the drill strings were pulled back and advanced to the platform until a continuous drill string stretched from shore to platform.
This completed the shore-to-platform Line A crossings.
The Long Crossings — 7,300 Ft from Platform-to-Platform
Though difficult, and technically accomplished, all four shore-to-platform crossings were achieved as planned. But the 7,300-ft platform-to-platform intersect crossings were even more challenging. Not only were they extremely long for HDD water-to-water crossings, Mears Group also had to contend with installation difficulties, an abrasive river bottom and a tropical storm that forced evacuation mid-project. But ‘unexpected’ difficulties actually are expected in projects of this size; Mears Group’s project team was well aware that challenges would arise, and when they did they reacted calmly and rose to the occasion.
On the Line A platform-to-platform crossing, gyro guidance was again used to get the drill bits within a few feet of each other. But GSTs can’t detect other GSTs; therefore, the south platform drilling was put on hold while the north platform drill head was equipped with a Para-Track II guidance system. After a few ‘pass bys,’ passive magnetic ranging was used to touch bits and achieve a successful intersect.
The drill strings were now tripped back and advanced to the south platform in order to create a continuous drill string. But the north platform drill string was not able to negotiate the angle of the south platform casing. To flatten the angle and make it traversable, the south platform crew installed another 300 ft of 12-in. casing.
With a continuous, 7,300-ft drill string now created, the pilot hole could be expanded to final dimensions. The existing 12-in. casings were used as guides for the placement of new 24-in. wash-over casings and then removed.
Meanwhile, Corman installed a series of wooden pilings to guide the steel pipe portion of the product from the north shore to the north platform; the pilings held the steel pipe in place laterally as it was pulled along the river bottom to the platform by crane barge and winch. After a 29-hour continuous pull (extended due to necessary reapplication of weld coating) the middle Line A crossing was accomplished successfully.
That left the Line B platform-to-platform as the final HDD crossing of the project. Equipment was shifted and, as with the north shore-to-platform crossing, a steering wire was run through the Line A HDPE so that Para-Track II-equipped bottom hole assemblies could successfully complete the intersect.
But then Hurricane Irene rolled in. A contingency plan that had been in place from the beginning of the project became effective, shore-based equipment was chained down and all platform equipment was offloaded onto barges and pushed upriver to a sheltered inlet; the entire procedure took two days. When the storm passed, it took an additional two days to reset equipment; in total, barely a week was lost to Irene.
Mears’ plan was to drill an additional pilot hole for installation of an individual 2-in. HDPE line, which would be sufficient for the fiber-optic line needed.
The 2-in. line installation was made possible via a technique that was unlike anything Mears Group had ever attempted. After a pilot hole was drilled using the same methods described above, the 2-in. HDPE wasn’t pulled back by itself. Instead, it was pulled back through the drill string, using a 3/8-in. cable and a custom-fabricated drum adapter on the north platform’s 140-drill spread. After the 2-in. line crossed successfully, the drill string was removed a joint at a time by pulling toward the south platform. This was an innovative and successful conclusion to a long and challenging HDD project. The HDD portion of the DVP Transmission Project was officially completed on Dec. 15, 2011, comfortably in time for Christmas.
Because they are literally invisible, HDD crossings don’t capture the public’s imagination in the same way as skyscrapers or dams or sophisticated new bridges. But the technical achievements are just as spectacular. To cross 3.5 miles of river without disrupting the environment or shipping, and to do so on schedule and within budget, is a major infrastructure accomplishment, one that will enable the Middle Peninsula to be powered up for decades to come.
Angus W. Stocking wrote this article on behalf of The Mears Group.