NV Energy and contractors to surmount 235 miles of diverse challenges to build one of the nation's largest transmission lines.
Construction crews on a Nevada transmission line project are confronting incomparable and diverse challenges. For example, this project is being built exclusively on federal land, uses an innovative tubular-shaped design for its transmission structures and requires linemen to watch out for wildlife, cultural artifacts and special plants. In addition, this unique project can result in linemen stringing power lines in a mountainous snowstorm at the same time that other lineman are sweating under a blistering desert sun to erect towers on the other end of the line.
Known as the One Nevada Transmission Line (ON Line), this line also will link up two remote service territories for the first time, will improve system reliability and will carry 600 MW of power from concentrating solar thermal energy, wind capacity, geothermal energy and even renewable energy derived from landfill gas.
The 235-mile-long line, which will cost about $510 million to construct, is jointly owned by Great Basin Transmission South, an affiliate of LS Power, and NV Energy. It will connect the Harry Allen substation north of Las Vegas, Nevada, with the planned Robinson Summit substation located 20 miles west of Ely, Nevada.
This new line will transmit renewable energy to where it is needed. In addition, this first-ever interconnection will improve the utility's reliability and energy independence by enabling southern Nevada generating resources to cover northern Nevada resource shortfalls and vice versa.
Crews are assembling and erecting towers and stringing conductor over a distance that is longer than the distance from New York City to Washington D.C. As a result, they must build or upgrade more than 1,200 miles of roadways to transport crews and equipment to the necessary locations. Additionally, the workers are using five laydown yards to stage the construction equipment and receive and distribute materials such as precast foundations, poles, hardware, conductor, guy wire and anchors.
Whenever possible, construction crews use approved existing roads. In many cases, however, the Bureau of Land Management (BLM) has authorized the construction of new access roads that provide right-of-way to the route from existing county roads.
The crews are performing a wide variety of concurrent work in numerous locations along the line. Unlike some transmission line projects, the construction activities do not simply begin at one point and end at another. Rather, project leaders are faced with making numerous schedule adjustments and road and line alterations to avoid cultural sites, protected vegetation and wildlife, and seasonal restrictions.
The 235-mile span of line begins at an elevation of about 7,500 ft above sea level at the northern portion of the line and eventually drops down to 2,400 ft near Las Vegas. These vast differences in elevation result in dramatic contrasts and experiences with weather, terrain, working conditions, vegetation and wildlife.
The project team is striving to reduce possible construction impacts and to protect sensitive wildlife, vegetation, water resources, cultural resources and any existing uses of federal land. An environmental monitoring team constantly observes construction activity to assure compliance with construction, operation and maintenance plans approved by federal regulators.
For example, in the winter months, the workers will avoid the northern portion of the line, which winds through mountainous and high plains areas of Nevada, to avoid affecting mule deer migration and winter feeding.
All ON Line workers are trained to comply with the environmental protection measures. Everyone on the project is required to complete environmental training, which helps them know what to do and what not to do when they come across wildlife, protected plants, mining artifacts or Native American sites. Once they complete this training, they earn a hardhat sticker that helps to remind everyone of the importance of being environmentally vigilant.
For example, crews need to work around mule deer, sage grouse and coin-sized desert tortoise hatchlings. Because these animals live near the construction site, the workers need to keep their eyes open at all times and protect their habitat.
Another way that NV Energy and its project team is protecting the local wildlife is through the development and installation of a new type of transmission line structure. Tall lattice structures can provide an excellent perch for raptors that threaten desert tortoise and sage grouse. To minimize predation, the project team considered such options as self-supporting steel lattice structures, guyed steel lattice structures and steel pole H-frame structures with an integrated perch deterrent. Ultimately, the project team selected an innovative tapered steel pole guyed-V structure. The installed cost of the tapered steel pole guyed-V structure was similar to the installed cost of the least expensive guyed steel lattice structures.
The workers are also installing vertical metal fins in strategic locations to deter raptors from perching and hunting for the vulnerable sage grouse. The U.S. Fish and Wildlife Service plans one day to grant these large, ground-dwelling birds protection under the Endangered Species Act.
Original Structure Design
In addition to installing a new type of transmission line structure, NV Energy is also using a unique tubular structure for most of its ON Line towers. The structures, which are as tall as 164 ft, sit on a 6-ft by 6-ft by 6-ft precast foundation.
Four main 7/8 -inch diameter guy wires support these structures. Workers also attach four additional smaller diameter guys at a different point on the tower to provide stability in wind conditions. The guy wires are attached to one of four anchors at each tower site. The structure's legs are also tapered at the top and bottom to reduce weight and provide strength where it's needed.
Thomas & Betts fabricated these tubular structures using Cor-Ten weathering steel. Unlike other types of steel, a layer of rust is intended to coat the steel and provide a protective layer. The towers start off a rusty red and then become a dark brown color over time.
The utility will construct 766 tubular guyed-V structures and an additional 90 three-pole dead end and regular lattice angle structures at various locations in the line.
Structure Site Preparation
Before constructing these towers on the federal land, however, NV Energy and its contractors had to work with the Department of Interior's BLM to secure nearly 235 miles of federal right-of-way. These companies also had to consult with entities such as the National Park Service, the Nevada Department of Wildlife, nearby communities, various environmental groups and cattle ranchers who were leasing federal lands near the project.
The collaborative process and Environmental Impact Statement procedures took several years to complete. The result of this National Environmental Policy Act process is that NV Energy and Great Basin Transmission South are building ON Line safely and in compliance with several environmental regulations and BLM-approved environmental mitigation measures.
To minimize environmental impact on federal lands, crews must maintain all equipment within a small 200-ft by 200-ft tower site work area. After the environmental team has swept the area, crews mow any vegetation to enable access and safety. The mowing process does not disrupt root systems, which will enable the ground vegetation to grow back after crews move on.
Stantec Inc. surveyors then use GPS survey equipment to determine the exact location for each structure. Afterwards, the field crews use a large backhoe to excavate a depression for the precast foundation. They then place special leveling rock in the bottom of the excavation, set the precast foundation in place and backfill the excavation. The workers then use boom-mounted plate compactors to compress the soil around the foundation to construction specifications.
The crews are also using GPS equipment to locate the four anchor bolts. On a normal power line, linemen dig a hole, drop in a plate with an anchor rod, bury it and then connect the guy wire. On this project, the guy wire anchors are required to handle a significant amount of load. Therefore, the teams are using grouted soil anchors. Each anchor is carefully evaluated and drilled at the proper angle, depth and location, which varies according to the terrain.
Construction crews drill 6-inch-diameter holes from 20 ft to 40 ft deep using a hollow-threaded shaft and drill rig attached to a backhoe. During the drilling operation, crews pump grout though the center of the hollow shaft. The grout and drill bit connected to the end of the anchor rod are left in place. A few days later, a separate crew tests each of the anchors to 80,000 lb of tension. Remarkably, field crews have now tested more than 1,000 anchors and only one has failed.
Erecting the Towers
The tapered steel pole guyed-V structures can be assembled relatively quickly because they consist of only eight pieces. Linemen lay the individual structure components on the ground, and a small crew will arrange and assemble the components using a forklift and other small equipment.
Sturgeon Electric Co. crews had not previously assembled and erected a tapered steel pole guyed-V structure. It took the initial crew an entire day to erect the first 110-ft structure. Now that the process has been repeated and improved, crews can assemble up to nine structures in one day.
Before work begins each day, the workers conduct a brief safety tailgate meeting to ensure that everyone is on the same page. Sturgeon has an aggressive schedule, so they must take care to meet their deadlines without any accidents, incidents or violations. For NV Energy, as well as its contractors, safety is first and foremost.
The guyed-V structure erection consists of the following:
Contractors use a 200-ton, 180-ft-tall all-terrain crane to lift the pieces into place on the concrete foundation.
The assembled tower is set onto two 6-inch-long by 1.5-inch-diameter pins embedded in the foundation.
The structure is plumbed.
Linemen connect the four 7/8-inch guy wires.
Crews simultaneously pretension all guy wires using truck-mounted wenches.
The structures have been tested for more than a year in a variety of strong wind conditions. To inhibit wind-induced vibration, Power Engineers added cross-braces and smaller-diameter guy wires that attach to the same anchors as the main guys.
Work in Progress
The One Nevada Transmission Line is progressing steadily. During the winter months, many crews will focus their efforts in the warmer, more southern portions of the line. And by next summer, when temperatures near Las Vegas can often be well over 110°F, some crews will gladly work in the 70s or 80s on the northern portion.
The companies hired Sturgeon to provide construction services for the transmission line and Wilson Utility Construction Co. to construct the Robinson Summit substation. The new line and new substation are expected to be operational by late 2012.
Thanks to strategic planning, a well-trained workforce and a solid design, the ON Line project soon will be providing renewable energy and reliability benefits to more than 1.2 million NV Energy customers and the nearly 40 million tourists who visit Nevada each year.
Starla Lacy (firstname.lastname@example.org) is the executive over environmental services and safety and has been with the utility nearly six years.
Steven Payne (email@example.com) is a project engineer and has been with NV Energy for 10 years.
Construction crews on the ON Line project are working alongside environmental experts at all times. About 20 biologists and archaeologists are assigned to survey permanent and temporary work areas, including access roads and the 200-ft by 200-ft work areas for each tower. The biologists search for desert tortoise, nesting birds, special plants, cultural remains and fossils.
Because the federal land was relatively undisturbed and previously had only light recreational usage, project archaeologists have discovered numerous new cultural artifacts, petroglyphs, ranch remains and fossils.
One of the more significant discoveries was the fossilized remains of a 300-million-year-old lepidodendron plant. A sharp-eyed environmental monitor found the fossil near one of the tower locations. This fossil clearly shows the diamond-shaped pattern of the lepidodendron bark, and each diamond represents the point of attachment of a fallen leaf from this Carboniferous Period plant. These plants grew to more than 100 ft, and remarkably were not known to occur in Nevada. This discovery provides evidence of a prehistoric swamp which extended further north than previously thought.
Transmission Line Facts
- About 850 total tower structures
- More than 3,000 anchors and 6,000 guy wires
- Nearly 11 million feet of conductor
- About 25 million pounds of steel
- 60,000 gallons or more of grout to drill and set the anchor bolts
- 400 workers are expected to be employed at the peak construction period.
- 694 drilled piers
- 3,000 cubic yards of concrete
- 220,000 pounds of reinforcing bar
- 4 million pounds of structural steel and anchor cages
- 2 miles of cable trench
- 30 miles of ground wire
LS Power www.lspower.com
NV Energy www.nvenergy.com
Power Engineers www.powereng.com
Stantec Inc. www.stantec.com
Sturgeon Electric Co. www.myrgroup.com/myr-subsidiaries-sturgeon-electric
Tadano America Corp. www.tadanoamerica.com
Thomas & Betts www.tnb.com
Wilson Utility Construction Co. www.wilsonconst.com