More than 40 Years Ago, 138-kV and 69-kV Pipe-Type Cables were Installed from Miami to Miami Beach and Key Biscayne, Florida, U.S. The cables were initially installed by plowing the pipes in some areas and laying the pipes on the water bottom in others. These circuits pass in the vicinity of Lummus Island, which, in 1980, was surrounded by sheet piles and topped off with fill so the Port of Miami could accommodate container ships.

In the 1970s, the U.S. Army Corps of Engineers approached Florida Power & Light (Juno Beach, Florida) with plans to dredge a major shipping channel near the port in the vicinity of FPL's cables; the Army Corps reinitiated contact with FPL in the late 1990s, stating its intent to move forward with the project. The dredging project would increase the depth of the channel to accommodate large ships, and the expected after-dredging channel depth would be below the elevation of the existing pipe-type cables crossing the area.

The 138-kV and 69-kV cable circuits are integral to the utility's extensive underground transmission network, so FPL needed to relocate the portions of the circuits under the water channel before dredging could occur.

Horizontal directional drilling (HDD) was the only viable means to install new sections of cable pipe at the required depth below the channel, and to avoid disrupting shipping traffic at the port or disturbing aquatic plant and wildlife preserves in the vicinity. In 2001, FPL contracted with Jacobs Civil Inc. (St. Louis, Missouri, U.S.) to perform the needed design and environmental studies for installing new cable pipes well below the channel along a 2297-ft (700-m) route.

Power Delivery Consultants Inc. (Schenectady, New York, U.S.) worked closely with Jacobs to develop a cable electrical design that was compatible with civil design constraints to meet FPL's desired power transfer. Specifications were prepared for contractors — UTEC Constructors Corp. (Boston, Massachusetts, U.S.) working with directional driller Mears Group Inc. (Rosebush, Michigan, U.S.) — to build the circuit. Many challenges existed during the design stages and construction, which made this an exciting project. Okonite Cable Co. (Paterson, New Jersey, U.S.) supplied the cable for the project, and MAC Products (Kearny, New Jersey) supplied the splice kits.

CABLE DESIGN

The cable system design was done in close coordination with the civil design, because several factors were influential to both. The electrical design — mainly to achieve FPL's desired ampacity — was critical. But in some ways, the requirements of the cable design were counter to the civil design requirements, which made for an interesting project.

No consideration was given to alternative cable system types, because the project only involved sections of existing 138-kV and 69-kV pipe-type lines. Completely rebuilding either line would have added significant cost and caused further community and environmental impacts. Thus, the initial step in many cable projects — determining which cable type to use — was predetermined at the onset.

FPL wanted to match the existing pipe-type infrastructure, including installing two cable pipes for each voltage, one containing cables and one a spare. The circuit routing generally traveled south across port property and under water. There, the 138-kV cable circuit continued south to Key Biscayne and the 69-kV cable circuit headed west to Miami. Using directional drilling, the relocated sections would traverse from a point on port property to an area in the water.

Circuit-current rating was a critical design challenge. The utility wanted 970 A on the 138-kV line and 945 A on the 69-kV line. However, there were civil design constraints. Specific depth of sheet piles were unknown, so the civil design required boring well under the piles, increasing the pipe bore depth to about 120 ft (37 m). The thermal resistance to heat leaving the cable would increase with bore depth, so for cable-rating purposes, the cable engineer wanted to maintain as shallow a bore as possible while still satisfying the civil installation requirements.

The required bore depth was hardly shallow. The drill path was carefully considered to minimize the chance for unintended drilling mud returns (often called frac outs) and to account for the limestone rock through which some of the directional drilling would be done below the shipping channel.

The fill material used to construct the port on Lummus Island posed another ampacity challenge. Years earlier, sheet piles were placed around the perimeter of the island, and various types of fill material was used to bring the port area up to grade. Geotherm Inc. (Aurora, Ontario, Canada) conducted thermal testing that showed that much of this material had a very high thermal resistivity, particularly when the material was above the water table.

To mitigate the effect of those poor soils, the design team needed to install a low thermal resistivity grout around the cable pipes below the normal trench depth of about 8 ft (2 m). An outer casing was used for each pair of cable pipes. The interstitial space between the cable pipes and the 24-inch (610-mm) steel casing was then filled with a low thermal resistivity grout.

Various cable designs were considered to maximize ampacity. An 8-inch (219-mm) cable pipe is commonly used for 138-kV cable systems and provided adequate internal clearance for the cables, so this pipe size was considered for the project. The design team specified a -inch (9.5-mm) wall thickness rather than the more-typical ¼-inch (6.4-mm) wall, because the installation was going to be under water and essentially unavailable for repair if corrosion successfully attacked the cable pipes.