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A Pipeline, An Earthquake, No Problem
polarfleece writes "November 3 is the first anniversary of the Denali Fault Earthquake that rocked Interior Alaska. America's greatest engineering marvel, the Trans-Alaska Pipeline just happens to cross the Denali Fault and, as described in Dan Joling's AP story "Alyeska engineers anticipated the effects of a bruising quake" the line came through just fine."
Depends. How often do they have to [partially] rebuild as a part of basic maintance vs how often earthquakes occure.
Earthquakes in most cases happen a bunch at a time [several months], and then nothing for a long time [years], while pressure builds up.
The pipelines needs maintance. Each pipe can be replaced, and they inspect it regularly to make sure all the pipes are holding up. When a pipe fails inspection (which if done right means it is still fine, but failure is expected after a time, they have to repair/replace.
So if the total movement from an earthquake cycle is less than 20 feet, and they will have to replace that section anywhere between cycles, there is no hurry to do it now, and no need to redesign for more margin because they won't go through a second cycle before normal maintance already fixes the problem.
OTOH, seeing 18 feet of movement when you planed for 20 seems too close for comfort. My gut feeling is they should redesign for at least 30 feet of movement when they repair that section, just for margin of safety. However I'm not a geologists, nor a pipeline engineer, so I don't know what is a reasonable margin.
The quake that struck was a 7.9, nearly the design maximum. I think they got lucky that the pipe only moved 20 feet, not the 30 that they estimated it could move in the event of a 8.0.
I can't see doubling the average as good engineering practice, especially if doubling the average doesn't get you outside the expected maximum range of movement. I think they were allowed to use unconservative numbers, and they got away with it (for which I'm glad).
> I can't see doubling the average as good engineering practice, especially if doubling the average doesn't get you outside the expected maximum range of movement. I think they were allowed to use unconservative numbers, and they got away with it (for which I'm glad).
Yeah, if that's what they actually did then you have to wonder about their engineering qualifications. If the design case could result in 30', they should have allowed for 30'. No ifs, buts, maybes, or averages.
Consider, for example, a system that fails when you get more than twice the average daily rainfall. Where I live that kind of system would fail every time it rains.
Or consider a program that is supposed to work for inputs of a million records, and some PHB rationalizing that on average it will only need to handle a thousand records, so if you implement it to handle two thosand records you have actually satisfied the design requirement...
Design by doubling the average works if-and-only-if the peak does not exceed twice the average.
BTW, nice post.
Sheesh, evil *and* a jerk. -- Jade
Oh sure, just double everything. Riiiight. You've obviously never worked in construction.
The whole skill here is balancing cost vs. risk. If it costs 10X to design for 40' of movement vs 20' of movement, then it's obviously not practical in the least. It's a much better choice to design for 20', and invest part of the massive savings in a "rainy day" insurance policy that covers the slim chance of a larger-than-expected earthquake. Financially this comes out way ahead of your fanciful "double everything" policy, regardless of the outcome.
Just because you say it's good to double everything doesn't mean that it's practical. Often times the right solution isn't the one that's the nicest or prettiest, it's the one that balances all of the options against each other and arrives at the appropriate compromise. You will not get very far in construction if you have such an unwavering "fuck everything, damn the cost" viewpoint. It's just not how the world works.