Construction Techniques and Protecting the Pipeline from Earthquake
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.
The Ultimate Test
More than thirty years ago, engineers designed the Trans-Alaska pipeline to withstand a potential earthquake of 8.5 magnitude on the Richter scale. To put this in perspective, the earthquake that leveled San Francisco in 1906 would have notched a mere 7.7 on the Richter scale. The Alaska engineers may not have expected that their design would get the ultimate test. But on November 3, 2002, a 7.9-magnitude earthquake shook the Denali Fault, which sits directly under the path of the pipeline. It was one of the largest tremors ever recorded in the United States, and it rocked boats as far south as Louisiana. The shaking was so violent it left a scar measuring about 200 miles long across the landscape.
Across Three Fault Lines
When plans for the pipeline were first drawn up, it was inevitable that the pipeline would have to cross the Denali Fault. In fact, the pipeline traverses a total of three active faults. It would have been impossible to build a straight pipeline and go around so many big obstacles.
Dealing With Strike-Slip Movement
The Denali Fault is one of the largest strike-slip faults in the world, equal in size to the California's San Andreas Fault, the culprit in the San Francisco earthquake. Strike-slip faults produce earthquakes when tectonic plates on either side of the fault move horizontally against another. But the Denali Fault was not the only challenge the engineers faced. The pipeline had to be built above ground because of permafrost. To protect elevated pipe in the event of an earthquake, the engineers came up with an ingenious plan.
A Moveable Pipe
As the elevated pipeline approaches the Denali Fault, it comes off its vertical support members, the H-shaped pilings that hold the pipe above ground in the permafrost zone. The pipe is placed in a steel shoe, which sits on top of a low-to-the-ground concrete beam. The bottom of the pipe shoe is covered in a slippery, Teflon-coated plate, allowing it to slide across the beam. At the Denali Fault, the pipe can slide up to 20 feet horizontally and five feet vertically. As an added precaution, in several of the pump stations, the engineers installed an earthquake monitoring system, which records and analyzes the severity of the shaking and the potential damage it could do to the pipe.
Put to the Test
When the 2002 earthquake struck, the pipe moved seven and a half feet horizontally and two and a half feet vertically — well within the range conceived by the original engineers. The shaking broke at least five above-ground cross beams that support the pipeline and two vertical support members, but there were no reported leaks. At the time, Jim Lusher, an engineer on the pipeline, said "Quake damage to the pipeline was right in line with what was expected."