The Trans-Alaska Pipeline System is one of the largest pipeline systems in the world. It starts at Prudhoe Bay on the Alaskan North Slope and then winds it way south transecting the state for a distance of 1,287 km, until it arrives at the Marine Terminal at Valdez. The pipeline was built between 1974 and 1977 after the 1973 oil crisis caused a sharp rise in oil prices in the United States that made exploration of the Prudhoe Bay oil field economically feasible. To transport oil from Prudhoe Bay on the Arctic Ocean to the ice-free port of Valdez on the Gulf of Alaska, the pipeline had to cross the Denali Fault, a major cause of earthquakes in North America and a source of great worry for the engineers who built it. The Trans-Alaska Pipeline is a vital lifeline that transports about 17% of the domestic oil supply for the United States. Breakage to this pipeline would cause damages worth hundreds of millions of dollars.

Trace of the Denali Fault after the 7.9 magnitude earthquake of 3 November 2002, Alaska, USA. The fault trace passes beneath the pipeline between the 2nd and 3rd slider supports at the far end of the zone. Photo credit

To prevent such a disaster, some of the country's top seismologists and geologists were brought together to study the Denali Fault. Those studies located the fault within a 1,900-foot corridor crossing the pipeline route and estimated that the pipeline could be subjected to a magnitude 8.5 earthquake in which the ground might slip 20 feet horizontally and 5 feet vertically. To accommodate the projected fault movement, a section of the pipeline where it crosses the Denali Fault was laid on sliders rather than on rigid pillar supports. The pipeline rests on Teflon shoes that are free to slide on long horizontal steel beams, such that the pipeline moves when the ground moves.

When the team of structural and geo-technical engineers came up with this design in the early 1970s, they didn't expect it to be tested in their lifetimes. They were wrong.

On November 3, 2002, the Denali Fault ruptured over a distance of 336 km, producing the largest earthquake from a continental strike-slip fault in North America since the 1906 San Francisco earthquake. Measuring 7.9 in magnitude, the earthquake caused ground to shift beneath the pipeline 14 feet horizontally and 2.5 feet vertically. The violent shaking damaged a few of the pipeline's supports near the fault, but the pipeline itself did not break. It was the first significant quake to test the pipeline's mettle.

The survival of the pipeline in the Denali Fault earthquake was the result of careful engineering to meet stringent earthquake design specifications based on meticulous geological studies. The studies when they were done in the 1970's cost only about $3 million. Had the pipeline ruptured in the quake, the lost revenue and the cost of repair and environmental cleanup could have easily exceeded $100 million, perhaps many times more.

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The sliders on which the Trans-Alaska Pipeline rests. Photo credit

The Tok Cut-Off highway was offset 23 feet (7 m) by the earthquake. Photo credit

Richardson Highway offset 8.5 feet in right-lateral sense. This location is near where supports to the Trans Alaska Pipeline sustained damage. Photo credit

Richardson Highway on right, looking north. The Alaska Pipeline is on the left. Road offset reveals Denali fault location. The part of the pipeline designed to withstand movement along the Denali fault is in the middle part of the photograph. The fault runs beneath the pipeline near the left edge of the photograph. Photo credit

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Sources: Wikipedia / ASCE Library / USGS.gov / Seattle Pi