Natural gas pipelines are typically designed to have a useful life of about 50 years. The safe working lifetime of a gas pipeline is limited by the inevitable corrosion of steel buried in the ground and by the extreme stress of continuous high-pressure operation (a 40-foot section of steel pipe pressurized to 1480 psi will have a total force of over 93 million pounds, or 46,800 tons, continuously pressing outwards on its inner wall).
As was demonstrated by the Williams pipeline failure near Appomattox, VA in 2008, pipeline operators face a challenging and uncertain task in determining at what point a given pipeline can no longer be used safely. Once a pipeline has been determined to no longer be safe to operate, the two choices are to replace it or abandon it.
Pipeline replacement is the same as pipeline construction, although conditions may allow sections of a long pipeline to be replaced at different times.
If a pipeline reaches the end of its safe operating life and is not replaced, it will be abandoned. This usually involves removing all surface structures that supported operation of the pipeline and leaving the pipe in the ground. Pipes that are abandoned in place may be left unfilled, in which case eventual collapse of the pipe wall through further deterioration becomes a concern.
Should additional capacity be required in the future, it is reasonable to expect that laying another pipeline alongside the currently proposed one would be the preferred plan, for all of the same reasons that Williams Transco prefers the currently proposed Central Penn Line South route over alternatives. As the FERC strongly encourages constructing new pipelines along existing rights-of-way, it is reasonable to expect that they would look favorably upon such an expansion.
If, in the future, an additional pipeline is run alongside the currently proposed pipeline, the pipeline construction process used to install the first pipeline would again be employed. Williams Transco indicates in its FERC filings to date that additional pipeline would be installed at least 25 feet from an existing pipeline, so it is reasonable to expect that the right-of-way would grow by at least 25 feet if a second pipeline were to be installed.