Dear All,

I have a question regarding the allowables for occasional loads, especially for the offshore piping systems.

According to B31.3, two occasional loads need not be considered as acting concurrently (Refer 302.3.6). But especially in case of offshore piping systems, the wind load can eventually lead to wave load, which means these two might be acting concurrently!

In that case, Is it still conservative to use the same allowable limit (1.33*basic allowable stress) for SUS+OCC code compliant case?

I also wonder about the loadcase OPE+OCC for the support loads in this regard. Can we assume, for example wind & wave, to act concurrently in order to calculate the support loads?

Thanks for your answer & Have a great day ahead

Cheers

To be more specific, B31.3 302.3.6 state "Wind and earthquake forces NEED NOT be considered as acting concurrently" and not two occasional load need not be considered as acting concurrently. Secondly, the term is "need not" and not "should not".
Therefore you should follow the first part "(a) Operation. The sum of the longitudinal stresses, SL, due to sustained loads, such as pressure and weight, and of the stresses produced by occasional loads, such as wind or earthquake, may be as much as 1.33 times the basic allowable stress given in Appendix A.


This answer also to your question. 1.33 times the basic allowable stress given in Appendix A. It is not a matter of conservative or not.

For your second question: yes.

Best regards,
Dan


P.S. More likely you will have accelerations due to waves + wind, isn’t it?

Thanks for your answer Danb & sorry for my delayed reply!

Probably I should have asked my question more elaborately in my first post. What I meant by SUS+OCC in that post is SUS+WIND+WAVE (since the wind eventually can lead to wave in offshore conditions)

As you said in your reply (& stated in the code), "The sum of the longitudinal stresses, SL, due to sustained loads, such as pressure and weight, and of the stresses produced by occasional loads, such as wind or earthquake, may be as much as 1.33 times the basic allowable stress given in Appendix A."

I am inclined to interpret this as SUS + (either WIND or EARTHQUAKE). If we replace Earthquake with Wave as the OCC load, then the 1.33 times basic allowable stress can be used only for either SUS+WIND or SUS+WAVE cases, not SUS+WIND+WAVE!!!

Can we have such a stress check combining SUS with two different OCC cases together? If we can, Can it be still conservative to use 1.33*basic allowable stress (from App A) as the allowable limit, if the code's intention is not to combine two different OCC loads (WIND+WAVE in this example) with SUS case for the code compliance?

I am not trying to argue against the code, but just out of curiosity, want to get a good view!

Hope my question sounds clear this time & look forward to hear from experienced people

Cheers

Im not a code expert, but..

The code specifically allow you not to combine wind with earthquake, but is no mention regarding the substitution with other occasional loads.

"The sum of the longitudinal stresses, SL, due to sustained loads, such as pressure and weight, and of the stresses produced by occasional loads, .........(here you have only an example)..........., may be as much as 1.33 times the basic allowable stress given in Appendix A."

Also note that the code give you the possibility to use 1.33 times the basic allowable stress given in Appendix A but do not force you to use it. As a designer you may chose not to use it. It is only a superior limit.

So you should consider all the occasional factors that are coincident: "and of the stresses produced by occasional loads"

I hope that is clear.


I also like to know the opinion of some code experts. By the way I have noticed that Mr. Luf do not post anymore. Maybe he can change his mind and write again on this forum.

Best regards,
dan

Thanks Danb for your answer!

Yeah, even I would appreciate the views of the experts in this forum on this regard!

Cheers

If you read the "scope" of any of the Offshore Codes, they state that the loading condition should be the worst combination of operational and environmental loads to which the system can be subjected. This means you want to evaluate "OPE Loads" + "wind" + "wave" all in the same load case.