Reading Selection, Lesson 5

The Trans-Alaska Pipeline: Meeting Nature's Challenges

It was 1968, and the United States was concerned about its oil supply. With war brewing in the Middle East and an oil embargo threatening, where would the United States get the petroleum it needed? How could the country become less dependent on oil imports in the years ahead?

Just when concerns were getting serious, geologists discovered the largest oil field in this country—in Prudhoe Bay on the northern slope of Alaska. Part of the problem was solved.

But during the winter, the waters of Prudhoe Bay are frozen solid. For much of the year, they cannot be reached by sea-going oil tankers. How could those billions of gallons of oil be transported to the lower United States? The answer: Build a pipeline!

The people who took on this problem would find themselves involved in one of the most difficult engineering challenges of this century. To solve it, they had to focus on three features of the Alaskan territory: permafrost, earthquakes, and temperature extremes.

Watching Out for Permafrost
At first, the engineers assumed that the pipeline would be buried underground. That’s how most pipelines are built, after all.

But no one had ever built a pipeline in a place like Alaska, where it gets so cold that in many parts of the state, the subsoil is permanently frozen. This deep soil, which never thaws, is called permafrost.

Planners realized that the pipeline couldn’t be buried in the permafrost, because the heat of the oil could cause the icy soil to melt. If the icy soil melted, the pipe would sag and it might leak. In winter, the soil around the pipe would freeze again. This freeze-thaw cycle could cause the pipe to move enough to cause serious damage.

Fiberglass insulation is being wrapped around the pipeline to reduce heat loss.

To avoid these complications, the engineers made an important decision: About one-half of the pipeline (about 700 kilometers) would have to be built above ground. They supported the pipe with refrigeration posts that are topped with aluminum radiators. The posts conduct heat away from the soil. The pipeline is also wrapped in 10 centimeters of fiberglass insulation. Both of these measures help to keep the permafrost solid.

Blowing Hot and Cold
A second challenge was Alaska's temperature, which ranges between -60 °C and 35 °C. Because the metals from which the pipeline is made expand and contract with changes in temperature, the pipeline had to be built to accommodate changes in length. The engineers estimated that a 304-meter segment of pipeline could shrink by as much as 0.3 meter in the coldest weather and expand by an equal amount during the warmest season. That doesn't sound like much of a change, unless you remember that the pipeline is nearly 1500 kilometers long! If the pipeline were straight, even a small change in each segment of the pipeline would be disastrous. The pipeline would either snap if it contracted too much or buckle if it expanded.

To prevent the pipeline from breaking, the designers used a zig-zag configuration. These bends help relieve the effect of contraction and expansion.

Accounting for Earthquakes
As if these extreme temperatures weren't enough, engineers had to deal with another big problem: earthquakes. Earthquakes are fairly common in Alaska. In fact, the largest earthquake ever to occur in the United States (measuring 9.2 on the Richter scale) took place in southern Alaska. The engineers had to build a pipeline that could survive such an event intact.

The pipeline was mounted on posts above the frozen ground. The aluminum radiators on top of the posts conduct heat—lost from the pipeline—away from the soil.

They designed a two-part system of "shoes" and "anchors" that hold the pipeline in place at weak areas (faults) where earthquakes have occurred, yet allow it to move enough so that it does not fall off its supports if the ground moves. At the Denali fault zone, where earthquake activity has been heavy, the pipeline is designed to move up to 6 meters side to side and 1.5 meters up and down.

The zig-zag in the pipeline allows it to expand and contract without breaking.

QUESTIONS

1.  How did engineers overcome the challenge of a 95 °C temperature range when designing the Trans-Alaska Pipeline?

2.  What is the difference between conduction and radiation? Use a dictionary or other references to help you answer this question.