start of pullback

HDD Used to Install 69,000-volt Submarine Transmission Line Under Sturgeon Bay

Gabe’s Construction Co. (Gabe’s) of Sheboygan, Wisconsin, was recently involved in a three-year project to replace a half-mile, 69,000-volt submarine transmission line under the Sturgeon Bay Ship Channel that connects Wisconsin’s lower and upper Door County. The new transmission line replaces a single circuit with three cables that were installed under the channel in the early 1980s and initially anchored to an active water main.

Founded in 1942 to install drain tiles in local farm fields, Gabe’s has since expanded over the decades to include the construction of gas distribution; water, sewer, oil and gas pipelines; fiber-optic cable; a multitude of wireless services; and, for the past two decades, installing underground high-voltage infrastructure. This work includes large conduit ductbanks formed with thermal dissipating concrete and backfill, large precast concrete splicing vaults, fiberglass rebar reinforced thrust blocks, and horizontal directional drill (HDD) of multiple conduits.

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Rig with downhole assembly

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For the Sturgeon Bay Submarine Cable Replacement Project, which was completed in June 2021, Gabe’s was hired by Southwire Company to install all new below grade infrastructure, including an 1,886-lf HDD of (four) 8-in. and (four) 4-in. High Density Polyethylene (HDPE) conduits. Gabe’s performed the HDD drill and pull, as well as fusing the HDPE. Gabe’s finished with all the open-trench work that tied the HDD to the utility’s substation trifurcation structure and a new overhead transition pole. Finally, Gabe’s turned the completed and proofed infrastructure over to another contractor for cable installation.

HDPE conduits

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This project encountered multiple challenges that were overcome with early planning, precise coordination, and exemplary execution by the crews:

  • Compound Curve: Due to tight alignment and work area constraints, the HDD path included a compound curve of 50 percent of the HDD length and a 3,600-ft radius in the horizontal and a tight 2,000-foot radius in the vertical. The path still traversed below an active electrical substation and a boat maintenance building.

  • Rock up to 49 ksi: In conjunction with the compound curve, the extremely hard rock required a very precise pilot setup as fractions of a degree steering errors could result in major delays to scrub on the hole. Gabe’s used InRock Drilling Systems to furnish Steering Services and all rock tooling. Gabe’s ran InRock’s Gyroscopic Steering System, its ABIA (At Bit Inclination Assembly, which gives inclination visibility directly behind the bit), and Annulus Pressure Monitory system. It took a couple attempts to find the right bit to get the steer needed. Gabe’s started with a 12.25-in. and ultimately landed on a much smaller 10.89-in. Final tooling consisted of 10.89” HDX Bit with 7-in. mud motor, 24-in. XTR-W Rock Reamer (five) 16-in. XTR-W SB TCI Segments with hard formation inserts: IADC 6-3-7), 36-in. MXR/HDX Rock Reamer (five) 26-in. HDX Bit Segments)

  • Traffic: The pipe bundle string out was down the middle of a heavily trafficked local road, which is one of two roads into downtown Sturgeon Bay. Gabe’s worked closely with local city representatives and Wisconsin Department of Transportation to close the road to through traffic. Gabe’s also coordinated closures with local waste management, school bus services, and heavy industrial facilities, as well as dozens of homes, driveways, multi-family housing units. The main concern was a busy intersection that accessed the heavy industrial area. As full-size semis and trailers required continuous access, and each of the 8-in. conduits required the internal fuse bead to be removed via a 50-ft long debeading tool (to limit pull forces, potential for damage, and eliminate pressure points on the 69-kV cable), Gabe’s opted to bury the HDPE conduits under this intersection, instead of keeping them above grade on rollers. Gabe’s crews excavated and shored 60-ft long and 3-ft wide 1-in. steel road plates. Gabe’s then closely coordinated the pullback activities as the bundle was raised up onto rollers, which required a major WDOT bridge and intersection closure for two days.

  • Environmental: Portions of the work were in a Wisconsin Department of Natural Resources flood zone, so care and preparations were needed to keep work from impacting that area. Lake Michigan, and therefore the shipping canal, were at historic high-water levels in late 2020 when the work was conducted. The south side rig work area was only 18 in. above the groundwater level. At the end of each shift, Gabe’s had to put heavy weight mud in the exit pit to hold back the infiltration of groundwater. To handle the groundwater during subsequent open-excavation work up to 12 ft deep, Gabe’s hired Kelley Dewatering to install nine 30-ft deep dewatering wells and eight 13-ft deep dewatering wells. The water discharge was then treated before being discharged into the sewer system or the canal. There was also a layer of potential methane along the HDD path. Precautions, such as continuous methane monitoring above the pits, were employed. Finally, and thankfully, for being 45 minutes north of Green Bay, the weather did not drop below zero degrees until after the HDD was complete. The open excavation crews had to deal with the freezing temperature (including using ground thawing systems), but not the HDD crew.

  • Historical Granary: The Sturgeon Bay Historical Society Foundation was storing a wood granary, built in 1901, onsite. They were concerned about the impact of potential ground vibration on the granary. Gabe’s installed vibration monitoring equipment during all HDD activities, comparing the vibrations daily to data obtained prior to HDD activities. The distance from HDD path to the monitoring equipment was less than 100 ft and no increase of vibration was observed due to HDD activities.

  • Pull-head and Pullback: Gabe’s modified its standard 24-in. pull-head to taper it out to a 27-in. diameter pull-plate that all eight conduits were attached to. This resulted in a heavy and long pull-head. When the pull-head was passing through the tightest of the compound curve sections (a 600-ft compound radius), crews could feel the pull-head catching downhole. The concern was it could potentially get jammed on the sides of the rock hole. There were a few tight spots, but ultimately the pullback was completed with no damage to any conduits. Buoyancy modification was not used. A thinner, shorter pull-head will be used on future projects with tight radii.

Pullhead and HDPE stringout

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All in all, the project — including three and a half months of HDD, four and a half months for open excavation work following 18 months of construction pre-planning — was completed on time and on budget. Most importantly, every employee from each company went home safe to their loved ones.

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Nate Eastway, P.E., is the vice president-HDD and Specialty Projects at Gabe’s Construction Inc.

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