November 2017, Vol. 244, No. 11
TechNotes
Inline Isolation Provides Decommissioning Benefits
When a multinational energy services company planned to decommission an onshore gas terminal in northwest England, it needed a fast, total solution that would avoid depressurizing and blowing down a 25-mile (40-km), 36-inch pipeline.
The pipeline runs from an offshore field through north and south sub-terminals prior to feeding into the United Kingdom’s gas transmission network. A key concern was to complete the line decommissioning safely and quickly to minimize potential disruptions to the supply schedule.
The solution was the proprietary SmartPlug® inline isolation tool from T.D. Williamson (TDW). The tool is a remotely controlled, piggable pipeline pressure isolation device that allows pipeline sections to be isolated at or close to their operating pressure. This reduces downtime during both planned maintenance and unexpected repairs and also keeps pipeline inventory in place.
The operator initially contracted TDW to provide isolation services using a 36-inch SmartPlug tool, providing double-block isolation against the gas pressure in the pipeline. This would enable the safe removal of a 36-inch full bore tee as part of the decommissioning process. During operational discussions, however, the operator requested a pipe end barrier. The goal was to provide a safe vapor barrier behind cutting and welding near 0.62 miles (1 km) of open, water-filled pipeline potentially containing a low content of dissolved hydrocarbons.
After considering the options, TDW delivered a second 36-inch tool configured to provide a single block isolation as a risk mitigation measure. The single-module tool acted as a pipe end barrier to contain the pipeline contents once the pipeline was cut. As time was of the essence, extreme focus and effort were required to deliver the designed, assembled and tested second single-module SmartPlug tool on site within three days.
The operation at the south sub-terminal began by launching, pigging, and setting the first tool to create double-block isolation against the 16 bar (232 psi) gas pressure in the pipeline. Once the first tool was set against full line pressure, the line was depressurized. The second tool was then launched and pigged to the designated location 40 miles (131.23 feet) upstream of the primary isolation tool. This tool acted as a vapor barrier to the pipeline from the north terminal.
Both tools were tracked and remotely activated by the proprietary TDW SmartTrack™ system, which uses extremely low-frequency signals from the built-in SmartPlug transponder. After both tools were set, TDW monitored the integrity of each isolation during the full decommissioning work activity.
Upon completion of the decommissioning activities, the tools were pigged back to the receiver, using residual gas pressure from the pipeline.
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