Water utilities, sewer systems and telecommunications companies are always looking to improve infrastructure, frequently implementing HDD services. Just the same, electric companies also need to expand their outreach. For various reasons, installations or extensions of high voltage transmission lines are often required to be placed underground, rather than above-ground. Directional drilling offers electric companies that option.
As many contractors and engineers would probably agree, HDD is often difficult in crowded, urban areas. It can also be tricky in non-urban areas due to a variety of soil conditions and high water tables. Simply put, HDD is never without its challenges regardless of the application. However, it is actually becoming more economical and beneficial to both electric utilities and the surrounding property to hide electrical units underground. While aesthetics is part of it, a big concern for electric companies is compromising the value of the property where above-ground transmission lines would otherwise be installed. An example of this is an HDD project that is currently in progress in Wilmington, N.C., by electric company Progress Energy.
Cape Fear River Crossing
Progress Energy is an electric utility that serves roughly 3.1 million customers in Florida and the Carolinas. In 2009, the company decided it needed to build an extension of an existing 230-kilovolt (kV) transmission line crossing the Cape Fear River, near the state port in Wilmington. According to Progress Energy, the project was needed to ensure the utility met its government-mandated responsibility to provide a continuous, reliable supply of electricity to homes and businesses throughout the region.
After in-depth evaluation and planning, Progress Energy concluded in August 2010, that the best method was to use HDD to build a new transmission line beneath the floor of the Cape Fear River, which included a 6,800-ft bore, 100 ft below the riverbed.
To arrive at that decision, Progress Energy enlisted the help of Haley & Aldrich to provide engineering consultation. Dennis Doherty, national practice leader for trenchless technologies at Haley & Aldrich, was involved with the original feasibility study that helped Progress determine HDD was the best option. They also completed the HDD design (plans and specs), assisted in selecting prequalified contractors, went through the bidding process and stayed involved during construction, with a full-time inspector in the field during HDD installation. Doherty said engineering consulting for electric companies is something Haley & Aldrich has been heavily involved with recently, assisting utilities with a number of feasibility studies with respect to HDD design and construction.
Before this project, there were already two existing 230-kV transmission lines that crossed the Cape Fear River from Progress Energy’s Brunswick Nuclear Plant. Those lines were suspended on the same structures. In case of an outage on one of those lines, the other line could fail, causing neighboring transmission lines to become overloaded, potentially affecting customers in Brunswick and New Hanover counties. Such an outage could also affect the company’s ability to operate power plants in the region. Adding the third 230-kV line underground was the best overall option for the river crossing because it eliminated future overloading concerns.
Bob Wojnarowski, project manager for Progress Energy, said installing the lines underground was also the better option because the alternative of building structures to support more aerial lines across the river could have compromised high-value property on both sides of the river.
“We are pleased with how it’s going,” said Wojnarowski, who has been with Progress Energy for 30 years. “Cost control has gone extremely well and I think we’ve gained a lot of good publicity from the local community.”
Progress Energy hired UTEC Constructors Corp., as the primary contractor for the project. UTEC, which specializes in high voltage underground transmission construction, then subcontracted Michels to perform the HDD work. With the Cape Fear River being about 6,500 ft wide, Michels drilled from both sides of the river, meeting each bore in the middle. The bore was 24 in. in diameter as the install included an 8-in. diameter high density polyethylene (HDPE) pipe that contained the high-pressure fuel-filled (HPFF) transmission cable and a separate 4-in. pipe for fiber optics.
According to Progress Energy, once complete, the project will enhance system reliability for electricity consumers throughout the greater Wilmington area and minimize any impact to operations at the company’s Brunswick Nuclear Plant near Southport and the Sutton Energy Complex in Wilmington.
Wojnarowski said although the company hasn’t used HDD much, the project opened the door for future opportunities to implement a similar transmission extension or installation.
“It broke a barrier, under the circumstances, and now it can be considered a viable option for Progress,” he said. “Initially, we never considered it.”
Progress Energy has built many miles of underground distribution lines (lower-voltage lines) throughout communities in the Carolinas, but few underground transmission lines (higher-voltage lines) using HDD.
Contrary to the Cape Fear River crossing, electric utilities are also beginning to use HDD in urban areas. Again, depending on the exact drilling location, there are often many factors that lead contractors to choosing HDD over traditional excavation methods. Within that, there are then several factors to consider in planning the bore. In the case of electric utilities, performing HDD has not been done extensively in downtown areas.
A recent example of this type of project involved an underground street crossing in heart of Savannah, Ga., that began in 2009. The project was for the electric utility Georgia Power, owned by Atlanta-based Southern Company — one of the largest electricity providers in the country, supplying power to more than 4.4 million customers in four states in the southeast United States.
The project called for 900 total ft of drilling to extend new pipe for electric cables underneath the 25-ft wide River Street in downtown Savannah, a highly congested area due to tourism, traffic and a rail trolley car. For this job, UTEC Constructors Corp. was also the prime contractor and subcontracted Mears Group Inc., to perform the drilling work. For the extension, Southern Company used 6-in. diameter ASTM A523 pipe, the standard pipe for electric installs that comes in sections about 42 ft long.
Mark Smith, senior engineer for Southern Company, has been with the company for 30 years, specializing in underground design for the last 12 years. He said due to the narrow road and other utilities underground, one of the biggest obstacles was finding enough areas to lay the pipe.
“Small horizontal directional drilling rigs have been popular for telecom in downtown areas and this is a little bit different because our pipe is a lot bigger, we’re going to drill deeper and the rigs will be larger, Smith said. “I don’t know many electric utilities that have done it in city streets.”
Before the drilling in any application can be done, engineers will always determine soil classifications, looking at the type of rock or water tables the driller might encounter.
“There are risks involved when you are dealing with city streets,” Smith said. “We try to do all the appropriate geotechnical engineering and make the best decisions based on the soil conditions.”
Of the preliminary geotechnical work that goes into the project, Smith said electric utilities also look at the soil thermal resistance, or TR. This is because soil can transmit heat energy easily. For example, more compact, dense soil will generally have a lower TR. Smith said it is important to look at the heat equation because electric cables inside a steel pipe will give off heat to the soil underground and rise to the surface. This is just another aspect of using HDD for electric installs that may be slightly different than installs for telecom or water utilities.
“This is the first job where we risked HDD in the city street,” Smith said. “It’s different to apply it to an urban area. Other utilities may look at this and consider doing it also.”
Andrew Farr is an assistant editor for Trenchless Technology.