22 SOURCE fall 2013
Pipelines, continued from page 21
Although pipes in open areas, such as the trans-Alaska oil pipeline,
have been installed in a zigzag pattern to accommodate large fault
movements, this project’s proximity to roads, residential areas and
other utilities made a more compact design necessary.
The pipeline diameter in the articulated vault will be reduced
to 72 inches and two 72-inch ball joints will be installed to allow
pipeline rotation, along with one 72-inch slip joint to allow pipe
compression, making it possible for the pipe to move in response to
movement on the fault. The 72-inch ball joints for the project are the
largest ever built and were specially commissioned for the project.
Given that the pipeline design requires a slip capacity of nine feet to
absorb the force of a large earthquake and because most commercially
available slip joints have capacities for approximately two feet
of movement, SFPUC is working with Houston-based Stress Engineering
Services to design and manufacture a special slip joint for
the project with a much greater range of movement. Both the ball
joints and slip joint will undergo rigorous factory testing to verify
their performance before installation.
Existing BDPL No. 3 will be abandoned between the two shutoff
stations and modifications will be made to connect it to a local flood
channel to collect and divert potential water leakage from the area.
BDPL No. 4
At Trace A approximately 460 feet of BDPL No. 4 will be
sliplined with a new 80-inch diameter one-inch-thick steel pipe
and an expansion joint will be installed in a new vault adjacent
to I-680. At Trace C, a 420-foot section of the existing 96-inch
diameter pipe will be replaced with new 96-inch diameter oneinch
thick steel pipe.
Since BDPL No. 4 is not needed to maintain service immediately
after an earthquake, the 96-inch diameter pipeline has been engineered
to fail within an existing vault at Trace B, where the expected
fault movement is 6.5 feet. Portions of the 96-inch diameter pipeline
on either side of the existing vault will be encased in concrete
to strengthen the pipeline at those locations. The earthquake forces
will then be directed to the relatively weaker segment of the pipeline
within the existing vault where breakage is desired. The resulting
leakage from a potential pipe break would accumulate within the
existing vault and be drained through the abandoned existing
BDPL No. 3 to a local flood channel, minimizing the leakage onto
streets and preventing undermining of the new BDPL No. 3.
Traffic management during the project has required planning and coordination
between Cal Trans, the City of Freemont and other local government agencies.