Because earthquakes are relatively common in Alaska, the pipeline has to withstand stresses of seismic activity. Where it crosses the Denali fault zone, the pipeline is designed to move 20 feet (6 m) side to side and 5 feet (1.5 m) up and down.
The design of the pipeline also had to leave room for the pipe to "grow" or "shrink," since metals expand and contract with changes in temperature. The pipeline takes care of that movement by "bending" along built-in zigzags.
"Pigs" are mechanical devices pushed through the pipeline by the oil. They are named for the squealing sound they often make as they rub against the pipe. "Scraper pigs," like this one, clean the walls of the pipeline. Other, more complicated pigs inspect dents and wrinkles in the pipe, detect corrosion, and measure the pipeline's curvature.
