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Alaska is an enormous state with an incredible variety of terrain and weather. When the original engineers sat down to design the pipeline, federal geologists and environmentalists noted that unique obstacles, including permafrost and earthquakes, would challenge the idea that the pipeline could be constructed underground in the usual way.
"We had a group called Mile by Mile Design. There are a lot of pipelines where they do it typical. They say, okay, it's going to be buried six feet deep and you're going to go across Kansas and that's all you have to do. This thing [the pipeline] was being invented as you went along."-- Bill Howitt, Alyeska engineer
The Tale of the Concrete Horseshoe
The first challenge came on the very first day pipe was laid in the Tonsina River. An empty pipe is buoyant, so it had to be weighed down with a 7,000-pound horseshoe. If it isn't done right, the pipe comes floating to the surface.
Bill Howitt: "The first pipe went in and there were dignitaries all around and everybody clapped and kind of walked away and they were almost gone when the concrete weight slipped off and she came up."
The Tonsina was not the only waterway the builders had to cross. The engineers counted 34 rivers and 800 streams. Ralph Jackson, an engineer for the oil company SOHIO, said, "We counted until we got to 800 and called it that." In some cases the pipe was buried underneath the body of water. But in most cases, the pipe was elevated. In all, fourteen major bridges were built. The bigger rivers --- the Yukon, Tanana and Tazlina Rivers -- got suspension bridges, including the 2,290-foot Yukon River Bridge. Engineers put steel plate and girder bridges over a myriad of streams. Most bridge construction took place in the winter so the disruption wouldn't disturb migrating fish.
The pipeline was designed with two types of safety valves. Seventy-one "gate" valves were designed to shut down the pipe within four minutes. Most were installed on flat terrain and downhill slopes near stream crossings, environmentally sensitive zones and towns. They can withstand temperatures as extreme as minus 70 degrees Fahrenheit. In addition, eighty-one "check" valves were installed on uphill sections of the line to prevent oil from flowing backwards in the event of a break upstream.
The pipe can withstand a wide range of temperatures, from minus 70 degrees Fahrenheit, when the pipeline is empty in the harshest winter cold, to 145 degrees Fahrenheit when it is full of oil. To accommodate these two extremes, the pipe can expand 18 inches lengthwise. When pipes are buried beneath the ground, the weight of the soil keeps the pipe from expanding and contracting. But 420 miles of pipe -- over half of the total line -- are sitting above ground. That pipe needs someplace to go when the temperature changes. Engineers built the pipe in a zigzag configuration, which converts the lengthwise expansion into a sideways movement. Wider zigzags were added in the Denali Fault area to anticipate earthquake movement. If it were not for the zigzags, the 800-mile pipeline would have only been 789 miles long.