>> OTOH, if you've expecting a maximum wave hight of X, you design to 
>> withstand 2X, and then suddenly a 12X have hits... hmm, Not Good(tm).
> 
> It's impossible to design anything to be 100% sure of lasting a set time 
> when it's being attacked by an essentially random force.  That's why you 
> have models and can choose a chance of it being broken in its lifetime, 
> eg 0.1% over 10 years.  Of course that means there's a 0.1% chance of a 
>  >2X wave coming along in 10 years, but what else can you do?

My point being "what happens if the model says force X is the largest 
that's ever likely to occur, but it turns out the model is wrong and 
actually force 12X is fairly common"... Presumably that's a pretty bad 
thing?

>> PS. Don't you ever hit a point where it's not possible to design for 
>> 2x the maximum?
> 
> It's never impossible, just sometimes too costly or not practical.  Then 
> you might do some more detailed simulations to bring the 2X figure down 
> to 1.5X or something with a suitable level of confidence.  Or just think 
> up a different way to solve the problem.  Usually it comes down to cost 
> though.

OK. Cool.

-- 
http://blog.orphi.me.uk/
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