Inertia acceleration-FPSO

For a FPSO topside engineering, the inertial accelerations in normal operation (rolling, pitching & heaving) are considered by structural design of all modules as 0.143g, 0.347g & 0.57g respectively. For extreme sea condition, it is 0.265g, 0.693g & 0.886g respectively.

I think due to structural dampening & friction at pipe support(except at anchor) the accelerations will be less on piping. If so, how to find out reduced acceleration for piping stress analysis. Please guide.
Regards,

ANY SUGGESTION PLEASE.

I am sorry that I cannot offer a "real" answer to your question.

Regards, John.

Dear Manoj,

How are you? If I have correctly understood your question then please note the following.
Ship as a whole will be in motion during rolling, pitching and heaving. So structure, equipments and piping all will be moving and will have same acceleration at some time. Pipe will also experience same acceleration as structure however there may be some phase lag in acceleration experienced by structure and pipe. Moreover piping should be properly guided in order to avoid relative movement between pipe and structure. Anchor, Guide and Friction will try to make pipe & structure as integral part of ship.

In order to simplify the problem consider the same acceleration for piping as accelerations considered by structure fellow for structure.

In my opinion don’t consider reduced accelerations.

With Best Regards

Rakesh Kumar
Piping
EIL Qatar
Email ID: rakesh8193@yahoo.co.in

Sir John,
Please try to offer me a "real" answer.

Mr Rakesh,
Thanks for the reply. I agree that it is safe to consider same acceleration. For our previous FPSO projects, there was no extreme sea condition analysis. Extreme condition accelerations are much higher, almost 5 times higher, compared to our previous projects. My intension is to reduce this accelerations without hampering safety of the system.

Regards,

Manoj,
I don't have a definitive answer but Rakesh's reply is the one I would use unless somebody could come up with a very good reason. You are effectively applying a seismic response reduction factor, aka behaviour factor to a marine situation. I would doubt you would design and support shipboard pipework such that it had a "response reduction factor" very different from unity. Or do you ?
Wouldn't the acceleration vary significantly depending on position or are your accelerations subject to a geometrical - roll/pitch rate correction factor ? From my very limited sea going experience, you are more likely to puke on the bow than at midships.