This summer, the Centennial Olympic Games will hold its rowing, kayaking and canoeing venues on Lake Lanier (north-east of Atlanta, Georgia, U.S.). Preparations for this event have had a unique impact on Jackson Electric Membership Corp. (JEMC), Jefferson, Georgia.

This electric cooperative serves about 110,000 customers in an eight-county area of northeast Georgia. Part of its service territory is around 38,000-acre Lake Lanier, which will contain the rowing lanes and finish lines for the Olympic events. This area fell right under a 1200-ft (366 m) three-phase, 24.9-kV overhead feeder lake crossing that JEMC had planned to construct.

This crossing was needed to provide a firm tie between two existing substations. During a long range system study in 1992, the tie was planned as the most economical method to accommodate the area's increasing load. The lake's popularity, the natural expansion of nearby Gainesville, Georgia and the proximity to Atlanta contributed to the growth.

The tie design began in 1994 when JEMC engineers began evaluating structures and the crossing path. Shortly after, the Lake Lanier Olympics venue was announced to be held at Clark's Bridge, right under the proposed line. Olympic officials and the Corps of Engineers made it clear that an overhead crossing with bright red balls over the finish line would not be approved.

The options at this point: Delay the crossing until after the 1996 Olympics. Relocate the span a distance up river to an existing transmission line crossing.

Because so much work was necessary to prepare for the Olympics, all the construction (including finish tower, sinkable lanes and new docks) was to remain and become the site of future events. And, the likelihood of a future overhead feeder crossing here decreased dramatically. Also during this period we met with consultants who were designing the facilities to house the media, officials and participants for the approximately one and a half years prior to and during the Olympics. The expected load demands demonstrated an even greater need for the tie line.

We then considered the second option. Because of the terrain and the necessary underbuilding (and replacement) of a number of transmission structures, the project's estimated cost quickly doubled. After considering everything, we came back to the original location and emphasized the urgency of the tie line. The only option remaining was a submarine lake crossing. Because of our service territory we have only a few overhead lake crossings and no need for the more typical armored submarine cable installation. The natural solution was to inquire to cable manufacturers and solicit information about prices and cable construction that would satisfy our needs. This solicitation was handled in a Request for Proposals format and was left open-ended to the cable manufacturers. The information received ran the gamut from a relatively simple design to a triplexed cable with armor wires at a cost of nearly US$80/ft. One of our goals was to have the cable match as nearly as possible our standard feeder cable, a 1000- kcmil, strand-filled, 260-mil, tree-retardent XLP, LC shield, jacketed conductor operated at 24.9 kV for main feeders. Southwire, Carrollton, Georgia, proposed a similar construction with a few modifications for water installations. The major item was adding a layer of lead tape impregnated between the insulation shield and a flexible fabric bedding tape. This addition was to create a "water impervious" barrier to the insulation.

The cable's outer jacket was 110 mils thick (ICEA requirements are 80 mils). This thickness, in conjunction with the protection provided by the LC shield, would provide adequate protection in the part of the lake we were considering. Also, because of the cable's water-impervious feature, Southwire offers a 40-year warranty on it. The price of this cable was roughly twice that of our normal feeder cable. This design had been provided by Southwire to several utilities whose service territories are near the coast or contain a number of waterways. Because this basic design had been used in the past and it would fit our standard components, we were more comfortable with the prospect of the submarine crossing. A submarine crossing had an additional benefit of not being subject to increasingly stringent Corps requirements on overhead clearance, which had become quite a problem for us in the past year or so. We contacted Jay Crofton of Crofton Diving Co., Portsmouth, Virginia, U.S., who had installed Southwire's "water impervious" cable for other utilities in coastal areas. In August 1994, Crofton Diving surveyed the lake bottom and provided a full proposal and cost of installation.

JEMC wanted to obtain a turnkey project until we received the estimated cost. Crofton's proposal consisted of all necessary equipment, permits and additional personnel for plowing the cable into the lake bed. This method required mounting the cable reels on a sizable barge with a crane and plowing equipment on board, then feeding the cables down the chute to plow them in.

To contain costs as much as possible, we negotiated with Crofton for options. He came back with an alternate solution that would cost much less but would require more JEMC labor. In the alternate proposal, the cable would be floated across the lake, sunk and then jetted into the bottom over about a week's time. This method was possible because of the relatively short (1200 ft or 366 m shore to shore) crossing distance. In addition, the original lake bed survey revealed that the bottom ranged from sand to clay to very soft bottom. Thus, we decided to proceed.

JEMC ordered 6000 ft (1829 m) of the water-impervious cable from Southern Electrical Equipment Distributors, Atlanta, around the end of October 1994. On Nov. 29, 1994, we gave Crofton a purchase order for the installation.

For the next few months we had to obtain Corps permits and easements before any other progress could be made. Here, Crofton Diving's experience was especially valuable. They had worked with the Corps in the past and were familiar with the requirements. They obtained the Corps permits while JEMC secured the Corps easements and other necessary easements.

Also, the design of the tie lines on either side of the crossing was necessary as well as the design for serving the Olympic facilities. This last issue was to become a major item as considerable grading was required at the location where the cable exited the lake. Construction Is Straightforward Crofton crews began moving equipment and personnel to the site on Monday, March 20, 1995.

Tuesday and Wednesday were spent assembling the diving and jetting structures and preparing for the installation. Thursday morning the cable reels were set up on the shore line and the barrels set for floating the cable into position (Fig. 1). A diesel tensioner was set on the opposite shore (Fig. 2). It was attached to a floating poly-propylene rope, which was pulled across in a boat to connect onto the cables. Each of the four cables had a Kellems grip attached to it and were joined together with duct tape. The four Kellems grips were attach-ed to a swivel on the polypropylene rope.

At about 10 am, Crofton personnel and JEMC construction crews began pulling the cable (Figs. 3&4). A pair of barrels were attached to the cable every 30 ft (9 m), and the cable was lashed together with duct tape about every 5 ft (1.5 m) to ensure that when the jetting was done the cable would stay together (Fig. 5). The Dept. of Natural Resources and a JEMC construction crew with boats spent most of the day diverting the normal traffic while the cable was on the surface. On two separate occasions, Olympic hopefuls training for the rowing event had to be turned around to avoid the floating cable. The cable floating was completed about 4 pm (Fig. 6). The next step was to sink it to the bottom, which required feeding out a little more cable as the wire was lowered to follow the contour of the bottom. During this step Crofton's experience was evident as the divers meticulously checked the cable as it reached bottom and ensured that it was not hung on any debris. This part of the lake consisted of the old river bed in the center and relatively level plains on either side. The old river bed had a maximum depth of about 50 ft (15 m) and the sinking process was sluggish until the main channel had been cleared.

A little after 7 pm, the cable sinking was completed. The final length of cable, installed from switch to switch, was about 1600 ft (488 m). The next morning, JEMC took care of the transition onto the shore. About 40 ft (12 m) of flexible conduit was slipped over the cables and a track hoe was used to reach out into the lake to the trench. The cable in conduit was placed in the trench and covered with sand and mud and eventually covered with riprap per the Corps' specifications for the shoreline.

The side of the lake with the finishing tower was critical because the grading contractor wanted us out of the way of his bulldozers and shoreline work. A padmounted switch was set on each side of the crossing to make the transition from the water-impervious cable to our standard cable and to act as a subfeed for the 200-A loops that would serve the facilities under construction.

The padmounted switches are standard vacuum-in-oil switches with dead front terminations. These switches are "9" configurations with two 600-A termination switches for the 1000-kcmil water- impervious cable and two fused 200-A termination switches for our smaller cable feeds to the transformers servicing the Olympic venue. After terminating the water-impervious cable in the switch, we continued the feed from the second 600-A switch northward to a pothead pole that tied the new circuit into our existing distribution.

The terminations were really no problem as both sides of the cable were made up on dry land, well off of the shoreline. The cable was 2000 ft (610 m) long and the distance between the switches took about 1600 ft (488 m) of cable, so no splices or terminations were made in the water. The fourth cable was simply covered with a heat shrink and rolled up in the cabinet.

Also on Friday, Crofton began jetting the cable into the lake bottom. This part of the job was the most time consuming, requiring one week to finish. Because of the condition of the soil on the lake floor, the divers were able to get the cable 4 ft (1.2 m) deep along the entire route. Crofton's original proposal guaranteed that the cable would be jetted between 3-5 ft (0.9-1.5 m) deep.

The cable was jetted into the lake bottom with a high-pressure water jet, powered by a gas engine on the diving platform (Fig. 8). The divers had quite a job because the water jet blew water into the murky bottom forming a trench into which the cable assembly simply fell. After the cables fell into the trench, the river bottom just covered them up when the mud flowed back onto the trench.

The pressure that created the trench tended to push the divers upward away from the river bottom. Secondary side jets lessened this effect somewhat, but the divers still required the use of metal weights to keep them in position. Another problem: with the pressure blowing the mud and silt around, the visibility was about zero.

The cable was energized on April 13. Load was placed on the circuit about a week later. Lessons Learned

As is the case with any project, there are some things we would have done differently. The lessons learned from our experience include: The cable would be mounted on steel returnable reels instead of our usual disposable wooden reels. With the cost of the cable and the frailty of the wooden reels, the extra cost is justifiable. Pulling a fourth security wire. A last-minute decision was made to include a fourth conductor (of our regular construction - not water impervious) as a hedge in case one of the other cables failed. We had debated about including this initially, but it was sacrificed at the budget altar. In retrospect, the cost is actually easier to justify than we thought. A fourth water-impervious cable would have added about 17% to the total project cost, and the peace of mind would have increased as well.

The final price tag on just the lake crossing was about US$70 per circuit ft for labor and materials. This price is in line with some concrete ductbank jobs that encountered rock. A concern we had almost after the fact was how we could ensure that the cable was not damaged during installation. Fortunately, Crofton was extremely careful in the way they handled the cable and our concerns were put to rest.

Conclusions Submarine cable installations are not limited to long-distance, high-voltage, armored-cable construction fed from ships or barges. With the advancements in solid dielectric compounds, medium- voltage cable construction and alternative installation methods, short-to-medium-distance underwater installations are now reasonably comparable to overhead crossings in cost and expected life. Submarine cable installations will never completely replace overhead crossings, but they do offer some advantages. The drawbacks are obviously a higher installed cost and a limited ability to repair the cable system. The advantage in our case was the ability to get the tie line installed. TDW

Joe L. Dorough is director, Engineering Services with Jackson Electric Membership Corp., Jefferson, Georgia, which he joined in 1988. He has the BEE and MBA degrees from Georgia Tech and Brenau University, respectively. His responsibilities include system planning, forecasting and overseeing line design. He is a member of IEEE, PES and is a registered professional engineer in Georgia.