Pipe Sections replacement for 8" line 1

[gasoperations]

We have to replace about 8 section on a 8" NPS pipe with Wt 0.188". The available pipe joints are of slightly higher schedule #20 thk 0.25" and #40 thk 0.322.

Could some one please clarify if these sections may cause problem for intelligent pigging?

As intelligent pig is normally 95% of pipe bore, I think these replaced sections will not pose any problem as ID difference between pipe sections with different schedules will be 3.2% max.

Any comments will be appreciated.

 

[rneill]

Since there are so many different ILI tools on the market with different design capabilities, it is always best to check with the Manufacturer of the specific tools that you intend to run.

That said, a general rule of thumb that I've seen used is that the maximum variation in internal diameter of an 8" pipeline be limited to 6/16" (0.375").

Variations in wall thickness should be done with transitions that have a 1:5 slope in order to prevent hard impacts on tool sensors.

 

[gasoperations]

Thanks rneil

The problem is ours being a bid oriented award of job contracts, it can never be determined which of the ILI companiey may get the job.

Anyway is there any thumb rule to arrive at 6/16" limit for 8" pipe or for other different pipe sizes.

 

[rneill]

For a range of pipe sizes, the numbers are:

4" = 3/16"
6" = 4/16"
8 to 12" = 6/16"
14 to 20" = 9/16"
20 to 36" = 10/16"
36" and larger = 13/16"

Again these are rules of thumb but given they've been around for awhile and tools are becoming more flexible, they are probably conservative.

I know many vendors have specialized dual diameter tools that can take significant diameter restrictions but for a variety of reasons (including maintenance pigging), it is best to minimize your ID variation as much as possible.

 

[rneill]

FYI, without getting into too many details, when you are doing follow-up ILI runs in pipeline that have had prior ILI tool runs, there are significant benefits from running the same tool. If you go with the same vendor, the run to run comparision (looking at corrosion growth and changes) can be based on evaluating the actual difference in the raw unmodified data stream. This is typically much more accurate than comparing the interpreted results from one run to the interpreted results from the other which is pretty much the best you can get when you use different tools and vendors.

Taking this into account, many Operators "negotiate" rather strictly bidding their ILI tool runs. They go out for prices and make it clear that they will award based on best value rather than lowest price.

There was a paper published a few years ago (by an Operator) that analysed the total cost of an ILI tool run in comparision to the quality and accuracy of the tool used. In particular, they considered the number of digs they could eliminate by having a more accurate tool (with smaller error bands on the reported anomaly size) and the money they would save on digs vs. the extra money they had to pay for the tool run itself. Overall, they reported it could be significantly cheaper to pay for a more expensive ILI system.

 

[gasoperations]

Yes I agree with you on quality of data rather than price, but unfortunately our bids committee is no where near this agreement.

Anyway thanks for comments

 

[brimmer]

The schedule 20 is unlikely to cause problems with any ILI vendor, the schedule 40 may cause a problem with some of the ILI vendors, we have the same issue all the time...