In July 2001, the South Carolina Department of Transportation decided to construct a new bridge across the Cooper River between the cities of Charleston and Mount Pleasant.

The new bridge would replace two existing bridges to give more clearance over the river for container ships using the Port of Charleston. South Carolina Electric & Gas (SCE&G; Columbia, South Carolina, U.S.) had to replace a 115-kV overhead transmission line, which was attached to one of the old bridges, with a 115-kV underground high-pressure, gas-filled (HPGF) pipe-type cable system that would be placed under the Cooper River. The length of the river crossing was 7030 ft (2143 m). The coated-steel pipeline was installed 80 ft (24 m) below the surface of the river using the horizontal directional drilling (HDD) process.

This installation is believed to be the longest pipe-type cable system ever installed using HDD technology. The overall project consisted of a 7030-ft segment under the river and 3280 ft (1000 m) on land to connect the SCE&G Charlotte Street Substation in Charleston to an existing transmission line in Mount Pleasant. Construction on the project started in January 2004, and the line was energized on Aug. 3, 2004. The circuit is constructed with three 2250 kcmil copper conductors and is rated at 960 A or 191 MVA.

Working Around the Bridge Replacement

The old overhead 115-kV transmission line was installed on the older bridge spanning the Cooper River, the Grace Memorial Bridge. This line was installed in the 1970s and had been a vital tie line between Charleston and the high-growth area of Mount Pleasant and East Cooper.

The new bridge is still under construction but nearing completion. Once the new bridge is completed, the Grace Bridge and the old Silas Pearman Bridge paralleling it will be removed. The old bridges crossing the Cooper River are configured in two separate spans over the two shipping channels. The new bridge is configured into one span, which passes diagonally over a low point between the separate spans on the old bridges. As construction on the new bridge progressed over the low point on the old bridges, the overhead line had to be lowered and eventually removed from normal service.

Because the electrical tie line between the two service areas was so important, we began reviewing options in 2003 on how to replace it. An overhead line alongside the new bridge was considered but was rejected because of permitting issues and heavy commercial ocean ship traffic going to the Port of Charleston. Installing a power cable on the new bridge also was not considered feasible, because the new bridge would not be completed in time for the cable to be energized before the old overhead tie had to be taken out of service. We felt our only other choice was an underwater cable crossing downstream of the new bridge, which is in the narrowest section in the lower part of the river.

Although this is the narrowest section, the Cooper River is still more than 7000-ft (2134-km) wide at this point. Laying the cable directly on the river bottom or burying it in a shallow trench was considered but was rejected, as this area of the river is considered part of Charleston Harbor and is continuously being dredged to keep the channels open for large commercial container ships. River dredging is not an exact science, and there is great risk of a dredge damaging the cable. Also, as container ships get bigger, future dredging could get deeper. With the large volume of commercial marine traffic in this area of the river, dragging anchors or other objects falling in the river could damage any cable lying directly on the bottom or buried in a shallow trench.

Our only option was to go deep enough so that dredging or dragging anchors would not be a concern. We decided to go with HDD technology to gain the desired depth. SCE&G had used this method earlier to install pipe-type underground transmission lines in the Charleston and Beaufort areas, but those crossings were only in the 5000-ft (1524-m) range. We felt that the soil conditions were right for a longer HDD bore, but no one had ever tried a 7000-ft drill. Our other concern was whether the pulling tensions required to pull the 2250-kcmil conductors through the installed pipe would exceed the limits of the conductors and the contractor's tensioning equipment.

Our first project obstacle was to determine the feasibility of constructing an HPGF pipe-type cable system of this length using HDD technology. To do this, we assembled a team of industry experts to study the project and determine if the project was buildable. Jacobs Civil provided HDD pipeline expertise, and Power Delivery Consultants provided cable expertise. Three alignments under the river were studied and soil borings were taken to map the soil conditions under the river. Samples were taken from the 125-ft (38-m)-deep borings to determine the ambient soil temperatures and the soil thermal resistivity. A soil stratum of calcareous clay, locally known as Cooper Marl, was located about 80 ft below the surface of the river. It was determined that this soil had the right properties for both the HDD technology to work and for the cable to provide adequate ampacity.

Our next obstacle was to move from feasibility to actual design. The feasibility team was transformed into a design team with Jacobs Civil taking the lead. Design drawings and bid documents were prepared during the summer of 2003. SCE&G entered into an engineer, procure, construct and management contract with Jacobs Construction Services to expedite the project. In this role, Jacobs procured the coated-steel pipe, the cable and other long lead items while the construction contracts were being bid. The HDD contract was awarded to Michels Corp. (Brownsville, Wisconsin, U.S.) and the electrical work was awarded to UTEC Constructors (Boston, Massachusetts, U.S.).

The 8-inch (20-cm) diameter, 0.375-inch (0.95-cm) wall, coated steel began arriving on-site in Charleston in January 2004, and UTEC began welding the 40-ft lengths into 1800-ft (549-m)-long sections for the pullback under the river. Due to space limitations on the Mount Pleasant side of the river, the complete pipeline of 7030 ft had to be constructed in four sections. These sections were joined together during the pullback operation. The land work on the Mount Pleasant side was completed by UTEC in the fall of 2003 in conjunction with the completion of roadwork by the Town of Mount Pleasant. The land work on both sides of the river consisted of installing 0.25-inch (0.64-cm) wall-coated steel pipe in open trench-type construction.

Michels Corp. mobilized the HDD equipment on the Charleston side and began the drilling operation in March 2004. The welded product pipeline pullback was completed on March 26, 2004. Successful installation of the coated steel pipeline was the first construction obstacle that had to be overcome.

After the underwater portion of the pipeline was installed, UTEC installed the four splice manholes and completed the land portion of the pipeline on the Charleston side of the river. A traditional H-frame type termination/riser structure was used inside the substation on the Charleston side. A unique single-pole termination/riser structure was used to tie into an existing overhead transmission line on the Mount Pleasant side. Due to space limitations, buried trifurcator assemblies were used at both terminations.

Our second critical construction concern was pulling in the 2250-kcmil copper conductor cable under the river. The predicted pulling cable tension calculated to be 45,000-lb (20,412-kg) force. Although the calculations indicated that the pulling tensions were within the capacity of the conductor, some concern remained. The conductor for the river-crossing section arrived from The Okonite Co. (Ramsey, New Jersey, U.S.) on special 14-ft (4.3-m) diameter reels. Each reel weighed 85,000 lb (38,555 kg) and had to be hauled on special heavy-duty flatbed trucks from Okonite's New Jersey plant. UTEC constructed special reel stands to support the big heavy reels. After a couple of delays from weather, the underwater portion of the conductor was successfully pulled in on June 24, 2004. The entire conductor pullback operation took 21 hours to complete. The pulling tension was measured during the pull with the maximum tension recorded during the Cooper River pull of 44,200-lb (20,049 kg) force. The underground line was energized on Aug. 3, 2004, and placed into service the next day, approximately 18 months after the feasibility of such a line was first considered.

Reliability Enhanced

The construction of this underground line improved the reliability of the transmission system supplying electrical energy to SCE&G's Mount Pleasant and East Cooper service area. Only a couple of weeks after the line was energized, it supplied a major portion of the electrical load to the service area due to hurricane damage to the transmission system in the area.

Through the use of HDD technology, there was no environmental impact to the Cooper River or the wetlands area on the Mount Pleasant side of the river where the underground line passed.

The Seen and the Unseen

The installation of this directionally bored transmission line satisfied the necessary reliability requirements of the Mount Pleasant and East Cooper portions of SCE&G's electrical system. These areas represent some of the highest rates of growth and demand for energy in the Southeast. Through collaboration with a host of local entities, in conjunction with world-class companies that specialize in such engineering and construction activities, SCE&G was able to provide a higher level of service to these communities. Construction of this underground transmission line extended the “technical envelope” for both HDD technology and pipe-type cable systems.

While we did not intend to break any world records with the construction of this underground transmission line, one of our public affairs folks recently made the comment, “When the new bridge is completed, South Carolina will have two world-class projects in Charleston to be proud of, one over the Cooper River and another one under the Cooper River.”

Gerald Ruschkofski is a senior engineer/team leader in the Power Delivery Engineering Group of SCE&G. He has been with SCE&G for 22 years. He is a registered professional engineer in South Carolina, North Carolina and Michigan. GRUSCHKOFSKI@scana.com

John McAnany is an engineering technologist in the Power Delivery Southern Operations Region. He has been with SCE&G for 31 years. JMCANANY@scana.com