Reduce pressure of a pipeline temporarily

[mendoza7788]

Dear all,

I’m working for an oil and gas operator, and I need to define the magnitude of the operational pressure I have to reduce because some dents haven’t been repair.
DOT and ASME B31.4 establish a reduction of the operational pressure to 80% of the maximum operational pressure recorded in recent history, but I have two questions regarding this concept:

1.How long is recent history, my pipeline has 30 years of service, and I have records from at least 10 years back from now?

2.For how long time should this maximum operational pressure had to be maintained in the SCADA records, 1 hours, 4 hours?

Thanks all

 

[BigInch]

I don't believe that there is any particular guidance on that subject, but I would think that 5 years should be more than sufficient and the duration should be at least 24 hours, but that's just my feeling about that.

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[LittleInch]

I would go for 1 year and 1 hour.

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Also: There's usually a good reason why everyone does it that way

 

[BigInch]

I could agree that one year should be sufficient, but not the 1 hour.
B31.4 requires minimum hydrotest hold times of 4 hours, of which, if not offshore or near populated areas, can be conducted using product in the pipeline as the test medium.

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[BigInch]

Remember to take the SCADA pressure at the pressure sensor position and back out the calculation to get the MINIMUM pressure at the highest point on the pipeline segment of interest.

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[brimmer]

If your line is liquid with some elevation change remember as "BigInch" mentions to take this into account, you will need to try to get an "at site" pressure and reduction here as opoposed to just lowering the the discharge pressure at the pump (if you have a target of 20% reduction at the defect location). As far as time frame to get SCADA records, I can tell you I have worked for a couple of operators that use exactly this procedure when excavating, and the time frame ranged between 30 to 60 days. One year may be too long in my opinion as a defect could have grown during this time frame (so the idea is there could be a crack in the the dent), there could have been a presusre spike or higher pressure throughout the year at some period, and a 20% reduction from this may still be high, whereas a more recent pressure is demonstrating the line is not failing in recent operation.