API RP 1102 Road and Rail Crossing Calculations for Buried Pipelines

[mch22112]

Hello,

I am doing some API RP 1102 calculations to confirm adequate natural gas pipeline wall thickness for external vehicle loading at road and rail crossings. One of the 1102 inputs is the design factor.

I am uncertain if I should apply the same design factor in 1102 as I used for internal pressure wall thickness calculations (done in accordance with ASME B31.8). I tend to think that I shouldn't, and the following extract from the Austrlian pipeline standard, AS 2885, backs me up. Am I correct in considering the design factor in 1102 as basically a design factor for resistance to external vehicle loads, which can be considered independent of the design factor for resistance to internal pressure? Thanks for the help! Mark




EXTRACT FROM AS 2885:

API RP 1102 formulae include a design factor the value used shall be as follows:
(A) At designated road and rail crossings 0.72
(B) Elsewhere 0.9. (Applies to locations where there is no formed road or track but a vehicle may nevertheless cross the pipeline on rare occasions, such as farm paddocks used infrequently by agricultural vehicles.) The hoop stress check to Clause 4.8.1.1 of API RP 1102 is not required. The design for internal pressure and wall thickness shall be in accordance with Clause 5.4 of this Standard.

 

[mch22112]

Sorry, I think I found the answer. For the "total effective stress" calculated in 1102 I think it is best to apply the design factor for combined stress in accordance with the applicable pipeline design code. For B31.8 this is 0.9. This complies with wording in 1102, Section 4.8.1.3: "The designer should use values forthe design factor, F, consistent with standard practice or code requirements."

For the 1102 Barlow and fatigue stress checks the same design factor used for the pipeline internal pressure wall thickness calculations can be used.

AS 2885 is more conservative than this approach (using either 0.72 or 0.9 for all stress checks).

Please correct me if I'm wrong!

 

[Orefunto]

Hi,
I'm not sure I get the gist of your question and I'm quite unfamiliar with Australian codes, but I've used API 1102 quite a bit and have always used 0.72 as my design factor, but then I was designing according to ASME 31.4 for liquids

It may be that 0.9 may be O.K for gas , but I very much doubt it because the only time I've come across 0.9 is when considering equivalent stress for shear failure in restrained lines when calculating longitudinal stresses in anchored lines.

Hope I haven't confused you and hope more knowledgeable members weigh in