CAESAR II VER. 4.5 & ASME B 31.3 EDITION 2004

[abhigarg]

Dear members,

I need your expertise opinion for subject issue.

According to ASME B-31.3 edition 2004, we are allowed to increase stress reduction factor (f) value from 1.0 to 1.2 for the material having specific minimum tensile strength less then 75 ksi & cold & hot allowable stress is less then equal to 20 ksi.

Currently I am using CAESAR ver. 4.5 which calculate allowable stress based on ASME b 31.3 edition 2002 not 2004 while latest version of CAESAR 5.0 is using ASME B31.3 edition 2004 for calculating allowable stress range.

Now my pipr routing is fail in the fatigue (EXP case) as per B31.3 code. If I increase the material (for A106-B) allowable by 1.2 times then my routing will pass.

Is this correct to increase the allowable stresses of material in CAESAR-II by multiplying CAESAR II program allowable stress by 1.2 or it has some limitations ?

I hope I will get your expert comment soon from you.

Regards

Abhishek Garg

 

[NozzleTwister]

abhigarg,

The most important question is, what edition of B31.3 is the code of record for your work. Usually it is the Code that was/is effect when your project started.

If the 2002 Code was in effect then the 2004 is not valid for you.

The Mandatory Date for B31.3-2004 wasn't until October 31, 2005.

Per the curve, you have to have less than 3000 cylces during the life of your system to take advantge of this 1.2 factor. I would get that documented.

My expert advice is that if your stresses are that high, reduce them by adding flexibility, even if they pass. I hope there are no flanges near this highly stressed area.

Good luck,

NozzleTwister
Houston, Texas

 

[abhigarg]

Dear Nozzle Twister,

Thanks allot for your expert comments!

Project ask to use latest code for the calculation and we got confirmation from client too to use latest edition on B31.3 if we have stress problem. The design have enough flexibility but the hull deflections are very high, which is the main reason of out of plan bending moment.

Regarding number of cycles, it is less then 3000 as I am talking about offloading cycles only & doing seperate analysis for fatigue due to waves.

Question is if I multiply allowable material values given by CAESAR II Ver 4.5 program by 1.2 then is it correct or not.

Regards

Abhishek

 

[NozzleTwister]

abhigarg,

You can't split up your fatigue analysis. You can't consider expansion only for one fatigue analysis and wave deflections only for a separate fatigue analysis. You need to consider ALL of your cycles in the same fatigue analysis.

You must be dealing with an FPSO or a tanker. You mention hull deflections as the culprit for your high stresses, is this from differential displacements between the vessel and a fixed platform or on land facility, between a mounted module and a hull connection or are all displacements from the hull itself. The reason I ask is that although the deflections on these vessels are quite large, they are not that large from one support to the next. Just make sure you haven't modeled a large displacement at one anchor or support and zero at the others if they are moving relative to each other. Your Navel Architects should be able to give you the displacements at all of the supports for each sea state.


NozzleTwister
Houston, Texas

 

[abhigarg]

Dear NozzleTwister,

Thanks once again for your valuable suggestions!

I am currently working on FPSO project. I am carrying stress analysis in most conservative cases. Further the hull deflection was calculated based on worst offloading volume which is creating high bending moment and that is main reason of high hull deflection.

I carefully input hull deflection to every support & anchor point. So the relative hull deflection value is actually high not due to input mistakes. We are requesting to the client to allow to use bending moment based on 1 million bbls (70 offloading cyles/year) instead of 2 million bbls (2 million bbls cycle/year). But client is worry if the piping fail duing offloading cycle of 2 million bbls even it happen once or twice in whole year. He want mathematical proof to show that piping will not fail during 2 million bbls offloading case.

Well I am spllited the fatigue in four cases like 100 year storm, (offloading, loading & thermal, wave (onsite) & wave (transit). And I give damage propostion of all this 4 cases. And considering expansion of all the four cases as well as input the hull deflection in all the above menyion 4 cases.

Currently my piping is fail in (offloading , loading, & thermal) cases due to combination of hull deflection, motion & accleration effect , & themperature. And the effect of hull deflection is maximum which crosses B31.3 stress limit.

So this is the whole story. Now I am looking for solution to reduce the hull deflection so I will keep the routing which is already installed.

Regards

Abhishek

 

[ranyallo]

thank you