The project honored as the 2021 Trenchless Technology Project of the Year for New Installation is one that successfully applied horizontal directional drilling (HDD) to place underground a critical pair of 12,200-ft parallel power cables under the Rappahannock River near White Stone, Virginia — a hot topic and the type of crossing that is recently making national headlines.
Undergrounding telecom/power lines has been a part of HDD’s repertoire for many years but a national spotlight was shown on this particular type of application in August with PG&E’s announcement that it plans to underground 10,000 miles of power lines in northern California. This year’s winning New Installation project was selected prior to this news.
Traditionally, power lines are installed overhead, but this option leaves them vulnerable in times of severe weather and other events such as wildfires — costing owners even more money and time to replace and/or repair. HDD piques owners’ interest as a long-term protection for their investment when looking at environmental, aesthetic and cost factors.
The Greys Point to Rappahannock Station-Rappahannock River Crossing provides a great example of how HDD is utilized in undergrounding power lines. This project installed two parallel power and cable lines, replacing an overhead 115kV line, giving Dominion Energy Virginia (DEV) customers more capacity, as well as protection from potentially future downed lines.
This project was incredibly challenging for the contractor and its engineers designing it, with several issues requiring tremendous planning and teamwork to overcome. These issues included logistical planning, working during the early months of the COVID-19 pandemic, creating a design that would be least disruptive to the waterway, resulting in the successful undergrounding of the lines; the project also included intricate overboarding and post lay burial of intermediary tie-in sections using a unique jetting machine to maintain the depth of the new lines 10 ft below the river’s bottom without the use of any mechanical dredging.
The project featured the Engineer, Procure and Construct (EPC) delivery process, which requires critical planning and communication to put together a design to install the two 8-in. steel pipe cables, with two fiber-optic cables tethered to each.
“It’s truly an honor for this project to be selected recipient of this year’s Project of the Year,” says JB Brown, Mears senior vice president and project director for this project. “This could not have happened without a lot of dedicated people working together…It’s great to know the industry sees and recognizes what we are doing.”
Project Nuts and Bolts
The Greys Point to Rappahanock Substation project required installation of two parallel 230kV underground pipe cables underneath the Rappahannock River. These underground cables would replace the existing overhead line across the river. Also included with the scope of work was installation of two fiber-optic cables lines. DEV retained Mears Group as the project’s primary contractor for the EPC project and Mears, in turn, retained the services of Brierley Associates, POWER Engineers and Verrified Engineering to handle the engineering portion of the work; UTEC was hired for the electric/pipeline support and Corman Kokosing for marine support.
The width of the river at the crossing location is 10,300 ft, with approximately 12,200 ft between the two HDD work pads on either side of the river. One of the challenges for Mears was that neither of the crossings could be done in one single, continuous shot due to the limitation of the cable, which was only available in 7,500-ft lengths, hence requiring at least one splice to be done. Corrosion was also a potential roadblock to the single continuous shot methodology as the installation would have likely resulted in a steel pipe inside a steel pipe scenario. DEV and Mears started working together to solve this problem, however, once it became apparent that the cable could not be fabricated in the required length, this process was abandoned. “It wasn’t possible to split the crossing into two crossings because this would require a temporary work platform to be built at the middle of the river channel, obstructing traffic,” Brown explains. “Therefore, two platforms were built in the river, dividing the alignment into three HDD crossings for each alignment, totaling six crossings.”
Two segments from the shores to the platforms were approximately 2,700 ft and 2,900 ft long. The middle segment that went from platform to platform was approximately 6,600 ft long. The geotechnical conditions consisted of very soft soils, which dictated use of wash-over casing for the pilot hole installation to mitigate risks of inadvertent returns. The two alignments were HDD-installed with one 8-in. pipe cable casing and one 4-in. HDPE fiber-optic conduit pulled into a single reamed hole. Shore-to-platform crossings (one alignment on each side of the river) also consisted of additional 6-in. HDPE conduits that would be used for transportation of water from shores to platforms.
“The marine component always adds a little bit more complexity to [any project],” Brown says, noting how this component impacts everything about the project, from splicing to welding to transporting crews and equipment to the actual HDD application.
The platforms were constructed about one month prior to the HDD intersect work beginning. Mears used two American Augers DD440 rigs (operated from the platforms), as well as an American Augers DD140 rig, used from the shore. An American Augers MCD100 mud system and a Tulsa Rig Iron MCS1000 mud system were also used. ISCO provided the HDPE pipe and UTEC the steel pipe.
This project presented several challenges, such as the geotechnical makeup of the soil below the river, drill path constraints and the overboarding of the splice location within the river. “The soils beneath the river consisted of very soft clay and silt with weight of hammer N-values to a depth of over 100 ft below the river surface in some locations,” says Brierley Associates senior associate Jim Williams, P.E., PMP. “These conditions resulted in the center water-to-water HDD crossing being installed at a depth of 200 ft.”
Brierley Associates served as the lead design and engineering firm for the project, responsible for the overall management of the HDD engineering and electric components, the latter subcontracted out to POWER Engineers.
An early challenge for the team involved the pipe stringing area for the water-to-water crossings. “Getting the pipe string from shore to over 1,000 ft to the offshore platform to install two 6,600-ft water-to-water crossings was not only challenging from a logistical standpoint, with a 30-ft bluff at the shoreline, but also from permitting restrictions with significant areas not accessible because of vegetations, oyster reefs, etc.,” says Brown. “We solved this issue by installing a casing pipe on one of the shore crossings and using this casing as conduit to transfer the 8-in. steel pipe from shore to the platform and then inside the reamed hole.”
One of the biggest challenges to the project involved jetting the overboarded pipe under the river bottom, as mechanical dredging was not permitted. Jetting was also challenging due to the curved alignment of the overboarded pipe. Instead of the traditional mechanical bucket to remove the soil from the river bottom where the pipes would be placed, which is the commonly used technique, crews used a jetting machine with high-pressure hydraulics that jets the soils under the pipe as the machine glides on top of the pipe. This process was done in 600-ft sections in four separate areas.
“The process of splicing the cable and welding the steel pipe on the platforms after the HDD work was completed, then lifting and placing the pipe onto the riverbed, termed ‘overboarding,’ is a complex process,” explains Williams. “Factors such as the weight of the pipe and cable, crane capacity, rigging configuration, water depth, tides, currents and others are carefully considered when developing the procedure for this process. Three-dimensional modeling and finite element analysis are used to determine the stress state of the steel pipe during this process.”
While Mears and its team planned and mapped out the known challenges, an unexpected wrinkle brought the project to a temporary halt: the COVID-19 pandemic. Work was already in full swing when the pandemic forced the shutdown of work of all types around the United States and the world in March 2020. While this project was deemed an essential service, Mears and DEV together decided to halt the HDD work for about six weeks to determine how to safely move forward. The pause in work did not negatively impact the overall schedule or project completion. During the stoppage, crews were able to continue welding and coating the pipe.
“We had finished up the first of the six shots and were ready to start the next but decided to wait. We were concerned that we’d be halfway through the next shot and possibly have an [COVID] outbreak and have to stop and [potentially] lose the hole,” Brown says, noting that when work resumed, COVID protocols were assessed and implemented, such as handwashing stations, sanitizing work areas, social distancing and masks.
Both Williams and Brown emphasize the importance of teamwork and planning, which made this difficult project a success. “This project is an outstanding accomplishment due to the diverse expertise that was needed to overcome the challenges,” Williams says. “HDD construction, HDD engineering, structural engineering, finite element modeling and marine construction were all required to collaborate to develop the successful strategy to complete the project.
“It’s a great honor to be part of a project like this and shows what can be accomplished when the project team, from the owner and all other parties, work together toward a common goal,” he concludes.
Brown concurs with Williams’ sentiments and makes special note of one team member’s contributions: Ron Halderman, who unexpectedly passed away before the project was completed. “Ron was important to Mears for many years and especially on these big, complex projects,” Brown says. “We always leaned on Ron to help us think through potential methodologies and brainstorm through issues and this one was no different.”
Owner: Dominion Energy Virginia
EPC Contractor: Mears Group, Inc.
Engineer: Brierley Associates, POWER Engineers and Verrified Engineering
Contractors: Mears Group Inc. (prime), UTEC (electric) and Corman (marine)
Manufacturers/Suppliers: American Augers, Tulsa Rig Iron, ISCO, Brownline