Operating at 1% above MAOP, down stream of compressor stations
Operating at 1% above MAOP, down stream of compressor stations
(OP)
Pipe line with MAOP of 7332 Mpa with a DF of 0.72 got a burst pressure of 7332x1.39=10191Mpa.
Which means pipe line defects with burst pressure lower than 10191 Mpa requires repairing. In terms of estimated repair factor for a corrosion defect define as ERF=MAOP/ P where P define as safe operating pressure for the pipe line. Because of the safety factor standard says that for a short intervals we could operate the pipe line 1% above MAOP for e.g just after a compressor station. Can I relate this extreme case to the section of the pipe with corrosion defects and to the ERF values of the defects? How we should approach to the defects in this section of pipe line?
Do we have to define the ERF value for e.g 0.95 and above should be repaired by considering the issue related to operation of 1% above the MAOP.
Which means pipe line defects with burst pressure lower than 10191 Mpa requires repairing. In terms of estimated repair factor for a corrosion defect define as ERF=MAOP/ P where P define as safe operating pressure for the pipe line. Because of the safety factor standard says that for a short intervals we could operate the pipe line 1% above MAOP for e.g just after a compressor station. Can I relate this extreme case to the section of the pipe with corrosion defects and to the ERF values of the defects? How we should approach to the defects in this section of pipe line?
Do we have to define the ERF value for e.g 0.95 and above should be repaired by considering the issue related to operation of 1% above the MAOP.





RE: Operating at 1% above MAOP, down stream of compressor stations
I'm not sure I agree with your analysis as to what defects require repairing. You assume it is okay to have a defect that puts you at 99.9% of the specified yield stress. I don't believe that most codes and standards would accept this approach.
You might want to look at some industry accepted standards for defect assessment and evaluation of corrosion defects. First place to look might be ASME B31G but there are many others as well (RSTRENG, DNV, ASME B31.8S, etc.)
RE: Operating at 1% above MAOP, down stream of compressor stations
The 20% of SMYS is pretty restrictive, but I see 30% pretty often. That says that at MAWP after the corrosion allowance is used up the pipe will still be a long way from SMYS. Even at that, I have never seen anyone document a decision to ever run a pipeline above MAWP, and certainly have never seen a vessel PSV set above MAWP (and the PSV calcs say that you should size your PSV so that the system never exceeds 110% of MAWP but most everyone still sets the valve at MAWP or lower).
I was curious as to why you said that the piping immediatly after a compressor station was "a short interval"? Are you taking the word "interval" to mean "a length of pipe" instead of a time interval? I think you'd have a difficult time selling that in court.
David
RE: Operating at 1% above MAOP, down stream of compressor stations
Under Australian standards 2885.1 (Design & Construction)
section 7.2.1.2 MAOP under steady state conditions - For pipelines intended to be operated at a set point equal to MAOP, the control system shall controll the maximum pressure within a tolerence of 1% further the standards comment on transient pressure at any point in the pipeline shall not eexceed 110% of MAOP.
RE: Operating at 1% above MAOP, down stream of compressor stations
RE: Operating at 1% above MAOP, down stream of compressor stations
As we are in the topic of defect assessment, our company uses
Kiefner (KAPA) for corrosion defect assessments. I understand that KAPA gives theoretical burst pressure based from 100%SMYS.
Which gives 1.39 safety factor for the subject pipeline under Rstreng calculations. Furher from the failure equations we can show that the defect will have ERF value of 1.Say this pipe line is hydroested to
90% of SMYS which gives a safety factor of 1.25. Can this be incorporated to KAPA? How can I explain this difference in safety factors when it comes to the burst pressure of the pipe line?
RE: Operating at 1% above MAOP, down stream of compressor stations
I would like to answer your original question.
If your burst pressure for an indication is less than MOP of the pipeline then it is considered as a defect and needs to be replaced.
Lets take this example MOP of the pipeline = 9000 kPa
Burst pressure calculated at defect = 10, 000 kPa
which means you don't have to replace or repair this indication.
However, in Canada/USA we usually go very conservative by multiplying the burst pressure with safety factor(design factor)0.72. which means for the above indication Burst pressure is
burst pressure X 0.72 = 7200 kPa.
According to the code, this indications has a burst pressure (with safety factor) is 7200 kPa which is less than the MOP (9000 kPa)and this is a defect and needs to be repaired.
I hope I explained it clearly.
corrosionguy.
RE: Operating at 1% above MAOP, down stream of compressor stations
Theoretically you could incorporate 90% SMYS into KAPA, by entering what the pressure is at 90% SMYS as the MAOP, the only problem is you can only operate a pipeline to 72% SMYS as per regulations. What you want is a safety factor based on your MAOP for corrosion. Think of the fact you can only operate a pipeline to 72% SMYS as an inherent safety factor you must always have, and the other safety factor is to this pressure (MAOP) equal to 72% SMYS. So your corrosion anomaly safety factor is applied to them bursting at MAOP, not 100% SMYS. The safety factor you use (not design factor already built in) is up to your company, regulations do not specify. You may choose not to go with any safety factor, but right to MAOP, which wouldn't be good practice in my opinion. Bottom line is if your burst pressure of corrosion anomaly is greater than MAOP, you have to repair, (or apply for an increase in MAOP assuming you are not already at 72% SMYS).
Remember also, design factor changes for class location, 72% for class 1, 60% class 2, 50% class 3.