Half of the 800 mile pipeline is above the ground. This was done to keep the warm oil (approximately 140 degrees Fahrenheit) from melting the frozen ground (known as permafrost). The pipeline is held up off the ground on a small platform supported by two 16-inch diameter vertical support pipes that are buried between 15 and 60 feet into the frozen ground.
These vertical support pipes are cooled by refrigerant coils which help to keep them from transmitting heat into the ground and consequently melting the frozen ground which supports the pipeline. These refrigerators, usually two and sometimes three in each vertical support, are completely passive; that is, they work automatically, requiring no power, whenever the surrounding air temperature is lower than the ground temperature.
"Specially designed vertical supports were placed in drilled holes or driven into the ground. In warm permafrost and other areas where heat might cause undesirable thawing, the supports contain two each, 2-inch pipes called "heat pipes," containing anhydrous ammonia, which vaporizes below ground, rises and condenses above-ground, removing ground heat whenever the ground temperature exceeds the temperature of the air. Heat is transferred through the walls of the heat pipes to aluminum radiators atop the pipes."

http://www.alyeska-pipe.com/Pipelinefacts/PipelineEngineering.html

Keep in mind that the pipeline was built with the ultimate in expediency in mind. The Aleyska Pipeline Consortium was more than willing to pay extra and sacrifice long term durability in exchange for immediate profits.  It had already been delayed for over 4 years by legal challenges, and the estimated 600 million barrels a day it would carry would more than make up for any shortcuts taken.  The fact that it has lasted this long is a testiment to luck and good quality construction on the part of those who braved the elements to assemble it.