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."

Comments

Doubling design tolerances

[henrygb]

This makes the case nicely for taking the number you think of first, and then doubling it. In this case the requirement was for 20 feet of landslip, and in the event there was 18 feet of slip - above average but below the maximum.

But as the article says: Though there was minimal damage, the earthquake may have one potentially expensive effect: the pipeline is now out of compliance with original design criteria that require it to be able to survive 20 feet of horizontal motion.

Perhaps next time they will specify 20 feet as the safety requirement, but build to allow 40 feet, so that they do not have to rebuild after every landslip.

Re:Doubling design tolerances

[bluGill]

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.

Re:Doubling design tolerances

[WayneConrad]

Right. They got through it, but I think someone allowed them to use foolish design numbers. It's foolish to design for the average if an above-average number causes disaster. From the article:

The best guess of Alyeska's seismic experts was that in a magnitude 8 earthquake -- the largest expected -- the ground could shift up to 30 feet, but the average would be 10 feet, along the fault. Alyeska engineers designed for a number in middle.

"We doubled the average and said, 'Let's design it for 20 feet,'" said Lloyd Cluff, a consultant on the pipeline and manager of the geosciences department for California-based utility Pacific Gas and Electric.

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).

Re: Doubling design tolerances

[Black Parrot]

> 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.

Re:Doubling design tolerances

[ls -lR]

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.

Permafrost More Fearsome

[4of12]

IMO, that's not the greatest engineering feat associated with the pipeline.

I'd reserve that honor for the resilience of the pipeline to a much slower amplitude shaking.

Namely, frost heaves from permafrost, ground that is normally frozen year-round. Scrape off a little ground cover to build a house, a road, or plant a utility pole and suddenly there's a difference freeze/thaw cycle that will do real Bad Things.

You have to either keep all frozen all the time, or largely unfrozen and fairly dry soil.

There's a reason that roads have 6 ft of gravel on them for insulation to protect the underlying permafrost.

Permafrost, Money, Pointless Ranting

[mahonri5]

On the note of permafrost, the pipeline (when it is above ground anyways) has heat pipes attached to it, that, for lack of a better word, suck the heat out of the ground, keeping the permafrost nice a frozen. Check out Alyeska Pipeline's page on pipeline facts for details.

On another note, while the pipeline might not be the engineering marvel some would expect, consider it's roughly 800 miles long, above ground, below ground, below ground and refigerated, and built back in the 70's. This thing puts up with a lot. And you can't quite tell from most pictures, but the thing is huge. Most sections that are above ground are designed to let animals pass right under without noticing it.

Of course the big benift of the pipeline has to be the money involved. It brought a whole bunch of people up to Alaska to work on the pipeline and still (if I remember correctly) oil is the number one source of income. Most of my friends growing up either worked for the state, or the oil companies. (or contracted for both) Makes for a very interesting time when oil prices drop and the oil companies start letting people go. Not the funnest times around.

But that's my rant, I've been out of the state for the better part of four years anyhow.

Which average, exactly?

[peacefinder]

The best guess of Alyeska's seismic experts was that in a magnitude 8 earthquake -- the largest expected -- the ground could shift up to 30 feet, but the average would be 10 feet, along the fault. Alyeska engineers designed for a number in middle.

It sounds odd and insufficient to design for twice the "average" movement. However, the definition of "average" is notoriously loose in journalistic writing.

Perhaps they meant: "In places, the fault could move 30 feet, but the average movement over the entire length of the fault is 10 feet. And at this point here, where the pipeline is, we expect it to move no more than 20 feet."

On the other hand, maybe they just got lucky. :) either way, it's a nice piece of engineering, to come through without even a dent in the actual pipe.