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-   -   Using springs (http://www.cfd-online.com/Forums/star-ccm/108554-using-springs.html)

amol wayse October 26, 2012 06:35

Using springs
 
I AM USING LINEAR SPRING FOR MY OFFSHORE STRUCTURES , I WANT TO KNOW THAT WHAT BOUNDARY CONDITION IS REQUIRED .EITHER WE HAVE TO MAKE A DOMAIN UP TO THE SEA BED OR WE CAN MAKE UP TO SOME HEIGHT BELOW THE STRUCTURE.AND IN THE SPRING PROPERTIES WE HAVE TO GIVE BOTH THE SPRING COEFFICIENT AND EQ.LENGTH (Ls) OR WE CAN GAVE ANY ONE OF THEM .
IF ANY BODY HAVING SOME GUIDE LINES FOR ATTACHMENT OF SPRING THEN IT WILL BE A GREAT HELP .MY OFFSHORE STRUCTURE IS A TENSION LEG PLATFORM (TLP).

abdul099 October 26, 2012 17:20

Did you already have a look in the user guide? Section "Modelling dynamic fluid body interaction"?

And PLEASE don't write anything in uppercase letters: It doesn't hurt to stick to the common spelling rules. But writing all in uppercase letters makes it hard to read your post.

amol wayse October 27, 2012 03:07

thnx for reply, ya i read the tutorial and proceeded according to that only but facing the above problem

abdul099 October 27, 2012 18:54

For attaching springs, just follow the user guide and / or tutorial.

You need to specify spring constant AND relaxation length, otherwise the spring wouldn't be fully defined. (That has nothing to do with CFD but simple mechanics).
Just imagine, you'd have a spring with a spring constant of 1000KN/m and an actual length of 10m. What force does act on the spring? The answer is: It is unknown. It could be huge when the relaxation length of the spring is 1m and hence it is elongated for 9m. Or it could be tiny or even zero when the relaxation length is 10m.
For your problem, I would calculate the expected lift of your platform under initial conditions. I'd calculate the actual length of the tension legs from the given water depth and depth where the platform is floating. The put the coordinates of attachment points on the sea bed and on the platform. And calculate the relaxation length from the current length, the force (lift of the platform) and the spring constant.

There is a pretty simple answer existing for the question how big your domain should be: That depends on your case.

You can cut your domain when there's no significant influence from the sea bed on your structure.
For most TLP, I would assume you can omit the sea bed due to the usually big water depth. When you're interested in the behaviour of the platform under storm conditions with a lot of waves etc. and the platform is located in 200m deep water, you will not miss anything by omitting the sea bed.

You need to model down to the sea bed when the sea bed has an influence on the structure, currents, waves etc.
This is the case when you have very shallow water or when the water depth rapidly changes over your domain while having some currents (tides, gulf stream etc.)


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