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."

Re:Greatest engineering marvel?

[GigsVT]

It's still good to see some media stories when engineers plan for something disasterous and it turns out right.

If all you did was watch the Discovery channel, you'd think all engineers had a bad problem forseeing natural disasters. :)

Re:Greatest engineering marvel?

[/dev/trash]

Or the Horseshoe Curve. NOW that was impressive.

Re:Greatest engineering marvel?

[Chris Y Taylor]

my vote for America's greatest engineering marvel: Panama Canal

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

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 s

Re: Doubling design tolerances

[R2.0]

"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."

I assume you live in Lake Woebegone, where ALL of the kids are above average.

Re:Doubling design tolerances

[BurritoJ]

While I agree that what you're saying sounds good, I disagree with it in actual practice. The only products that are engineered to withstand the conceivable worst-case events are Radio Thermal Generators which use radioactive material to provide electricity to satellites. For instance, do you think your car, or anyones, is designed to keep you alive in the worst case collision? Think tanker truck full of gasoline t-boning you at 80mph... Doesn't seem very survivable to me, but that is a low probability

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 le

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 everythi

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 [alyeska-pipe.com] 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

Re:Permafrost, Money, Pointless Ranting

[ForestGrump]

Alaska's state income.
Something like 80% oil
5% tourism
5% other

The state gets so much income from big oil that there is no state tax, and the residents are even given a yearly check "for being residents".

Trust me, alaskans are big oil people. They have reaped many benefits from big oil.

Heak, without oil, nobody really wants alaska. (does russia ring a bell?)

-Grump

At risk of reinforcing a stereotype

[SimJockey]

Stuff like this makes me really dig being an engineer. Nice to see some recognition being given when an engineer gets it right and disaster is averted. This happens all the time, but no one really ever notices.

I work in the oil business (refining) and I am thinking of safety every day. Reviews are done throughout the design process that question what are the consequences if X happens. And if the consequences are unacceptable, we have to design them out or we can't go any further. It is very humbling t

Remarkable

[SloWave]

This was pretty remarkable considering that the earthquake was something like the third or forth largest in the world that year. Alaska routinely gets earthquakes that would level cities in other parts of the world. I've set though a couple 5 plus ones that people there hardly mentioned the next day. Now if they could only stop the drunks from shooting holes in the pipeline [solcomhouse.com]...