> True.
>
> 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?

> 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.

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