Operating at 1% above MAOP, down stream of compressor stations 1

[09091960]

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.

 

[rneill]

I don't know where you are located but in most jurisdictions, operating above MAOP is considered a no-no.

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.)

 

[zdas04]

Just an additional data point, one of my clients requires hoop stress (disregarding any corrosion allowance) to be less than 20% of SMYS at MAWP. I designed a pipe project for him that came in at 22% and he wouldn't even talk about a variance, I had to go to the next pipe size since he also wouldn't allow any grade above X60. The next pipe size allowed me to go back to X42 and stay under 20% so everyone was happy.

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

 

[09091960]

Hi All

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.

 

[brimmer]

My opinion, keep the 1.39 X MAOP or simply yhe 10191 kpa throughout the line (or class 2 area you appear to be in with the 1.39 safety factor) in this over MAOP area. My question would be, the 1% increase equates to a 100 kpa increase after the 1.39 applied safety factor(I assume your MAOP is 7332 kpa nad not mpa), are you really going to have that many corrosion anomalies that would suddenly fail at 100 kpa more? I suspect not. You also have to consider normal operating conditions, not pressure anomalies. I would not change the rules if one of our pipelines suddenly had an overpressure. The other reason for leaving it the same, more conservative approach, especially if you are in a class 2 area with population nearby (or your own compressor station operators). So you could keep the 10191 kpa as upper limit no matter where, or back calculate the corresponding ERF as suggested which would be less than 1. You may also consider asking the regulator their opinion, though I suspect you won't get a definitive answer. The other question, the MAOP of 7332 is what % of SMYS? Is it the registered licensed MAOP, or internal company set point? You could simply increase MAOP slighlty to make sure your always operating below MAOP rather than up to 1% over, assuming you are below the regulatory class 2 limit for %SMYS.

 

[09091960]

Many thanks for all the replys.
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?

 

[corrosionguy]

09091960

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.

 

[brimmer]

Remember KAPA is calculating a factor of safey based on MAOP only (not SMYS), simply MAOP / burst pressure, so what your safe operating pressure is according to KAPA depends on your MAOP. KAPA does not assume your MAOP is equal to 72% SMYS, as does B31G & RStreng. Your pipeline MAOP may not be equal to 72% SMYS, but lower, so KAPA & RStreng are going to give you different values as far as potential safe operating pressure, but you should get the same actual burst pressure of the corrosion feature. RStreng uses the 1.39 safety factor based on 100% of SMYS, KAPA does not.
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.