Would double walled pipelines help prevent spills?

We are often asked why pipeline companies wouldn’t simply use a double-walled system, to prevent pipeline spills.

It’s an interesting question, because the technology does exist to build double-walled pipelines, and they are already used in certain circumstances. We asked Dr. Alan Murray (an adjunct professor in the Schulich School of Engineering at the University of Calgary) why double walled pipelines are not routinely used to help prevent pipeline spills. Here’s what we learned:

When double-walled pipelines are most commonly used

Alan explained that double-walled pipelines may be used when there are large differences in temperature between the flowing liquid and its surroundings. For instance, some offshore pipelines carry a waxy crude, which would solidify if it were exposed to the colder temperatures of seawater. The insulation afforded by using aerogel (which has a very low thermal conductivity) in the gap between the layers of pipe keeps the crude at an appropriate temperature.

“Closer to home,” said Alan, “the issue of large temperature difference can be found with steam-assisted gravity drainage (SAGD) facilities, where the product could be at say 160 degrees Celsius while the ground temperature is often below zero. That temperature difference will cause the metal to expand or contract, but if the pipe is relatively free to expand inside the outer pipe jacket then it is not subjected to very high longitudinal stress. There are several such pipelines in northern Alberta and Alaska.”

Double-walled pipelines as a spill prevention technique

“The double skin concept is widely used in ocean going oil tankers – essentially to prevent the inner skin being breached in the event of a collision or running aground,” Alan explained. “The equivalent for a pipe would be external damage and, yes, a second pipe is likely to afford protection. However, there are other, more cost-effective, solutions such as burying the pipe deeper or, in areas with a higher risk of interference,  by placing slabs over it to prevent a line strike.”

Unfortunately, there have been incidents of double-walled pipelines leaking, which is why leak containment is not considered their primary purpose.

The disadvantages

So let’s take a look at why double walls are not more commonly used in new pipeline construction:

  • They are extremely complex, and costly, to design and construct because of the extra pipe and welds required, and the technology needed to centralize the two concentric pipes.
  • It is more difficult to monitor a double-walled pipeline. For instance, inline inspection tools are unable to evaluate the condition of the outer pipe, to confirm that it is still in a fit condition to act as a leak containment device.
  • The outer pipe interferes with maintenance efforts and integrity digs.
  • If the inner layer were to leak, then the airspace between pipes would have to be large enough to contain the escaping fluid, adding further to the cost.

“In situations where double-walled pipelines have failed,” said Alan, “the contributory factors are likely to have been the same as in conventional  pipeline failures.” In other words, any additional protection they might provide does not make them a viable option given today’s technology and economic environment.

To learn more about the leak detection and prevention technologies more commonly used in pipelines, check out the following posts:

The Canadian Energy Pipeline Association represents Canada’s transmission pipeline companies who operate approximately 119,000 kilometres of pipelines in Canada. In 2015, these energy highways moved approximately 1.2 billion barrels of liquid petroleum products and 5.4 trillion cubic feet of natural gas. Our members transport 97 per cent of Canada’s daily natural gas and onshore crude oil from producing regions to markets throughout North America.