TDW Case Study: Onshore Transmission 48-inch Pipeline Repair Alaska

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CUSTOMER SITUATION:
In 1991, a pipeline operator in Alaska discovered an 8.5 mile stretch of its 48-inch pipeline to have
excessive corrosion. The 800-mile pipeline stretches from Prudhoe Bay to the ice-free port of Valdez. This
section, located in the Atigun Pass of the Brooks Mountain Range at Mile Post 160, would have to be
replaced. The north end of the section was above ground while the southern end was underground.
Replacement of this section would be the largest construction project undertaken by since the pipeline
was installed in 1977.
TDW SOLUTION:
The customer wanted to incorporate design changes that would reduce future corrosion, replace the
corroded section with the least environmental impact and complete the job at a reasonable cost with
minimum impact on throughput. Options included a) shutdown and drainage of the bad section into existing
tankage while the line was being replaced, or b) TDW’s hot tapping and plugging method with only limited
reductions and shutdown of flow. After the new 8.5 mile section was laid parallel to the old section, TDW
Model 2400 tapping machines were used to make 48-inch taps at both ends to accommodate STOPPLE®
Plugging Machines in a double block and bleed fashion, with installation of temporary bypasses. About 6
hours were required for each tap. Although the STOPPLE Plugging heads and bypass method would divert
flow through bypasses without a shutdown, the operator used a more conservative approach and scheduled
two static (no flow) mainline conditions for two hours to set the STOPPLE Plugging heads and 20 minutes to
retract them. An added safety feature developed jointly by TDW and local personnel used special proximity
ultrasonic probes to confirm that the “shoe” of each plugging head was in proper position on the bottom of
the pipe. TDW Services technicians and local personnel worked jointly to tap and plug the line.
CUSTOMER BENEFIT:
The tie-in of the new line was accomplished on schedule and the 8.5 mile section of corroded pipeline was
replaced with only minimum loss of throughput. STOPPLE Plugging heads were set within the time allotted,
and all plugging heads were removed in 19 minutes. TDW Services’ many years of experience plus several
additional safety features enabled the job to be completed on time with no environmental impact.
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