Trenchless engineering innovation saves the day for Dominion Power
To trench, or not to trench
Dominion Virginia Power planned to install twin 230 kV electric transmission lines from its power station in Yorktown, across the York River to its terminal station in Gloucester Point. An active shipping lane north of historical Yorktown, VA, the York River is actually an ocean inlet that varies in width between two and three miles wide in that location.
Submerging the new transmission lines using traditional trenching methods presented problems. In addition to disrupting marine traffic, trenching would generate environmental impacts by disturbing sediments, oyster beds and potentially impact historical shipwrecks from the American Revolutionary and Civil Wars, triggering cultural preservation issues should they fall in the path of the utility’s easement. Conventional trenching across the York River was a risky avenue with significant downsides both in costs and schedule completion.
Trenchless installation using horizontal directional drilling (HDD) offered an attractive solution, eliminating the impacts to shipping, sediments and cultural resources. But when Dominion Virginia began to investigate the HDD alternative, they soon found out that it was not without its own issues. The 120-foot wide easement across the river was acquired before trenchless installation was on the table, and contained two sharp turns. However, HDD installation requires gradual bends. Expanding the width of the easements to accommodate gradual curves would be costly and delay project start by months, or even years to obtain new easements from the Commonwealth of Virginia.
More than simply drawing a line on a map
Haley & Aldrich was well acquainted with the many questions and misconceptions surrounding trenchless technology. It was quite common even among experienced members of the construction industry to overlook the depth of engineering science required to perform flawless trenchless installations – so much so that the expression “There’s more to horizontal directional drilling than simply drawing a line on a map” had taken on a life of its own among the company's practitioners. And for them, the fact that other companies had called the project unfeasible was simply the starting point of an interesting technical challenge.
Haley & Aldrich conceived of the best case solution as akin to a racecar taking a turn at high speed: by entering the turn wide and then cutting the corner, the driver achieved the most gradual turn to preserve maximum speed. Given the tensile characteristics of the 8.625” steel carrier pipe that would be installed, could its minimum turn radius fit within the real estate of the easement? The calculations showed it could – but would require some unconventional drilling approaches. The team overcame these issues by placing the drills strategically to limit disruption to the power station, as well as to avoid encroaching on private property, and shipping traffic in the river.
In the role of lead HDD engineer, Haley & Aldrich managed the sub-consultant deliverables and coordinated and prepared contract drawings; provided consultation for permitting issues and aided in obtaining permits; and provided engineering consultation on HDD alignment, pipe stresses, buoyancy and potential for frac-out – a problem condition when the slurry used inside the bore accidentally escapes into the in situ environment.
The HDD alignment was performed using twin drills. Initial bores were done from east and west banks of the York River to pile-supported platforms in the river on the edge of the shipping channel, and then connecting the east and west legs by performing a water-to-water drill between the platforms. The total length of the transmission line was 21,300 feet, with 17,100 feet (3.24 miles) installed via HDD. The underground lines consist of a high-pressure, fluid-filled pipe-type cable system and two separate electric-resistance-welded, grade A steel pipes with minimum outside diameter of 8.625 inches and minimum wall thickness of 0.375 inches.
Designing to remove risk
Trenchless methods offer tremendous potential for minimally intrusive construction, and are vital in situations such as crossing highways or water bodies, or in urban work with significant infrastructure in place. Reduced cost and time to completion over open cut trenching also make it an attractive option for a wide range of applications. It remains, however, a very technical endeavor, and especially in projects of greater length, challenging logistical issues for support equipment, and difficult geology; success depends upon anticipating the full range of problems that are likely to occur and designing the project so as to prevent them from the outset.
For Dominion Virginia Power, Haley & Aldrich was able to engineer a solution that delivered the best-case results to the client despite challenges. The company was able to improve its transmission network without impacts to the environment or commercial shipping traffic or historical artifacts, and to meet its cost and timeline objectives.
For more information on this project, contact Dennis Doherty.