100 year wave and 100 year current

[Deepray]

Hi,

Can anyone enlighten me this?

Will 100 year wave and 100 year current occur at the same time? I understand that some industrial codes specified that for operation condition we can adopt 100 year wave with 10 year current and 10 year wave with 100 year current but some states that 100 year return period must be use in design.

Please provide reference if available.

Rgds

 

[BrianBT]

Can you say what it is you are building. fixed installation, floating installation, vessel etc. For the UK sector the governing document will be your safety case. Elsewhere it is the international, flag state and classification rules.

If you can give a little more information

 

[Deepray]

Just a general environmental question. Will both the 100 wave and 100 yr current occur at the same time. That's my question.

Rgds

 

[IRstuff]

Why not? It's now understood that what were thought to be 100 yr, or longer, waves are actually quite common, see:

http://en.wikipedia.org/wiki/Rogue_wave

TTFN

FAQ731-376

 

[Deepray]

Hi thanks for your reply but your link doesnt show that its a 100 year wave plus 100 year current. Do you have other literature that specify very clearly that the above 2 can occur at the same time?

 

[IRstuff]

Why would there be such a thing? It's a statistical entity.

My point was that rogue waves exceed 100 yr wave heights and are actually quite routine. Therefore, the likelihood of a 100 yr wave in conjunction with a 100 yr current is just almost as likely as a 100 yr current by itself.

TTFN

FAQ731-376

 

[JRMacGregor]

The 100yr current and the 100yr wave CAN occur at the same time. However, the probability of this happening is very low.

Also think about directionality - the max current direction is probably not the max wave direction. So what do you combine;
a) the max wave (from direction X) with the current from direction X, or...
b) the max current (from direction Y) with the wave height associated with direction Y

What is probably more relevant is - what does the design code ask for.

If you are designing a mooring system, the classiifcation/certification rules might demand that the loads on the floating structure are derived from the assumed 100 yr environment (wind, waves and current). This is maybe conservative, but if that is what the rules/codes ask for - its what you have to do (you have to assume that the factors of safety in the code take account of this conservatism).

And so they should - the highest wave does not necessarily produce the highest loads. This is the case for mooring loads and also structural loads on vessel huls (these loads are wave period dependent).

Of course, depending on the application - you could argue to use the 50 year return period data. For some short field life applications, this has been accepted.

You must be designing to some sort of code ?!

 

[Ussuri]

Taking the extreme current and the extreme wave together is conservative. If this is a concern you can speak to the people who developed the metocean data you are working with (typically the client) and ask them to provide you with the joint probability current. This is the current you would expect to see coincident with your extreme wave.