API 579 FFS-Flaw Size Criteria

[satrio.febriyanto]

Hi

I'm currently doing an FFS assessment for a pipeline project
can someone explain to me what is the reason that the API 579 coded states the minimum measured thickness-FCA has to be grater than 2.5 mm?
I'm using API 579 2007 edition.

Here it is the condition :
The minimum required thickness = 2.14 mm
Thickness on assessment (including FCA and metal loss, refer to Equation 5.3 on API 579) = 2.42 mm
RSFa = 0.9
RSF calculated = 0.995
Based on the acceptance criteria if RSFc >=RSFa , means that the pipeline can continue to operate
But in step 5, there is a statement if the tmm-FCA is less than 2.5 mm, the pipeline does not satisfy the FFS assessment

based on the minimum thickness, hoop stress calculation, RSF calculation, and remaining life, the pipeline is still in safe condition to continue operation, but why the API 579 code states the condition is not satisfy?

I've been searching some reference, but there is no any single article that can explain this condition
Hope someone can tell me the reason behind this

Thank You

[FATHI]

..I am also eager to know.
I please allow me to discuss a few points:

1-But I have the idea that API 579 ( lets talk about edition 2016 ..they are some new things there !!) in its level 1 and 2 does not consider buckling which have law allowable, a very thin local section could be a sort of hinge.

2-There is also another standard which recognizes API 579 , namely ASME B31G (Manual for Determining the Remaining Strength of Corroded Pipelines ) check also there.

3-I suggest to have a look in ASPEN site (by googlinaug) and find a document (title: The Structural Integrity Of Oil And Gas Transmission Pipelines) of Phil Hopkins an authority in this field , there is also an extensive literature there which could give you a lead ?

4-I suggest also to minimize FCA (future corrosion allowance) so you will get a large thickness and have t_min > 2.5 mm and you will be ok. This can be achieved by a proper working cathodic protection/ coating ??

5-Check also if original code design of your pipeline or Company policy/ DEP, or similar prescribe a Minimum thickness ??
Having a very thin wall , even if it is ok, means that you have to replace the line or a portion of this line..on the short term.
A calculation is not the whole engineering, in your decision you have also to be supported by Maintenance, Management and they are also legal and safety issue..so no unnecessary risks.

6-I believe also that you have used part 4 or 5 of API 579 ( local or large corroded areas) but you might also have other issues to check ( fatigue, other loads: wave, seismic, dents..) ?supplementary loads do need additional thickness

7-Finally if Level 1 and 2 are not satisfactory then one has to de-rate / replace or use level 3= finite elements,....this is costly but more accurate and I believe you are not limited then by restriction of level 1 and 2.

take care .