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The Alafia River runs east and west through central Hillsborough County, Florida, U.S., separating the primary load centers in and around the city of Tampa from Tampa Electric Co.'s largest generation point at Big Bend Power Station in Apollo Beach. Inspectors identified significant corrosion-caused deficiencies on the structures supporting four of the utility's seven transmission circuits crossing the river. With the ever-present threat of severe weather during hurricane season, Tampa Electric faced potentially catastrophic dangers if the situation was not resolved quickly, including major interruptions - or worse. The clock was ticking, and the utility had one of the biggest projects in its history to execute.
The three 230-kV circuits and one 69-kV circuit in question are in close proximity along a 1,300-ft (396-m)-wide stretch of the Alafia River. A variety of wood, steel and concrete structures supported the circuits to the north and south of the river, with steel lattice towers and tubular steel H-frames used to span the river itself. This included a set of structures on the tiny Key West Island, a spoil island in the river that reduced the longest span to about 1,000 ft (305 m). Adding to the project team's challenges, the island was sinking into the river.
Project Requirements and Goals
The initial driver for this project - to address corrosion-related structural deficiencies at the river crossings and eliminate their threat to reliability - soon gave way to other considerations: How could Tampa Electric use this opportunity to provide the best value and performance for its customers and shareholders? The utility assembled a project team to develop options, perform analysis and provide a recommendation to management.
In addition to the previously cited challenges, the geography of the project area proved to be extremely important. The Alafia River is an environmentally sensitive area and a popular recreational waterway. To the east and west, nearby bridges limit horizontal and vertical clearances. Tampa Electric made it a priority to minimize the project's impact on the environment and local community.
The utility had to consider other factors including future system upgrades and access to the project site for large equipment and many workers, both during the initial construction and later for possible restoration activity, while, at the same time, balancing its construction...