Reduction in operating pressure during works on a pipeline 2

[LittleInch]

My first time writing a post so I hope I get all the info in.

I can't actuallly give all the info I would like but hope that the info below is enought to allow the forum to comment.

The situation
I have works planned on a 42" natural gas transmission pipeline, design pressure 85 bar, located in open countryside some considerable distance from occupied buildings. The works involve removing earth from all around the pipe over a length of about 3 joints - no powered excavators within 2m of the pipe. There would be some lifting of items above the pipe at a height of about 2m. Overall stress checks at design pressure show combined stress levels of less than 70% SMYS. Pipe is new and has no known defects either internal or external and has factory applied FBE coating. Pipe is X 70, Df 0.72, wt 14mm (0.55"). The pipeline operator is imposing a standard pressure reduction of 15% of Design pressure which has significant commercial implications due to reduced capacity, i.e. no free lunch here.

My challenge to them was to show that such a reduction in pressure was justified in terms of reduction of risk versus cost and effectiveness. My thoughts are that in this location, the people at risk (the workers) are all within the consequence area regardless of operating at 85 bar or 72 bar and hence it doesn't really impact on the overall risk to the actual population at risk. If the pressure was reduced to below a DF of 0.3, then I could recognise that this eliminates the chance of a leak developing into a full bore rupture, but we're a long way from that.

The question to the forum is: - What is your experience (and please identify the country or region) of operators imposing a fixed reduction in pressure in similar circumstances regardless of the particualr circumstances. Do you agree or not with me that this reduction in pressure has no real impact on the risk or indeed any other comment.

My motto: Learn something new every day

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

 

[brimmer]

One of the industry best preactices we use in Canada as described by CEPA (Canadian Energy Pipeline Association) is to reduce the operating pressure by 20% when excavating (maintenance) etc. It is coming from more of a safety perspective. Anyway, I have attached a paper explaining/justifying the process.

 

[BigInch]

How do you evaluate a risk like that? That would be one of the most risky operations that can happen in pipeline engineering, next to a hot tap at full pressure.

At least 20%?, I prefer 40%-50%. DOT statistics of gas pipeline reportable indicents over the last two years shows that most pressure loss accidents occur at 80% OR LESS of MAOP. Surprizingly very few accidents happen anywhere near MAOP, or over.

You did not mention the reason for the work. It might have some bearing on whether pressure reduction to even lower limits might be appropriate.

Independent events are seldomly independent.

 

[BigInch]

Here's my analysis of DOT "Gas Pipeline Incident" statistics covering 181 incidents occurring during the years 2010-2012. The raw data is available at PHMSA.

http://www.phmsa.gov

As you can see there is a marked increase of incidents precisely at the 50% MAOP level.
Interestingly enough there is a 50% probability of an incident occuring at 0.8 MAOP or less.

To reduce the probability of an incident to 10% or less, THIS data would suggest you should reduce pressure to at least 50% of MAOP. To be 99% sure, reduce to 10% MAOP.

Independent events are seldomly independent.

 

[LittleInch]

Big Inch and Brimmer - Thanks very much for the info. I'm sorry for being coy about the reason for the work, but it is very sensitive at the moment.

Big Inch - That data kind of proves my point, which is that a 15% reduction in MAOP is neither here nor there in terms of reducing risk or consequece. The 50% reduction in MOP equates well to the 30% of SMYS limit which prevents a hole becoming a rupture and clearly gives a good level of reserve strength, but is not practical for this pipeline as there would be no flow.

The key point to emphasise is that this line is relatively new, has no known defects on it, is very well located and is being worked on by the operating company itself. The point, as made in the discussion section of the paper supplied above, is that unless there is good evidence of a pre-existing damage "... the risk reduction benefit associated with a pressure reduction is generally considered insufficient to justify the negative consequences of the disruption in service caused by a pressure reduction."

Clearly the ideal is to reduce pressure to zero, but commercially this cannot be justified for the operation being considered (no hot work, no tapping, careful hand excavation, no blasting, no ground movement (actually in very rocky ground) and a well trained and experienced contractor).

I am just trying to see what other operators around the world do or don't do and how they justify any pressure reduction that isn't zero pressure compared to the commercial impact of loosing capacity, especially for natural gas transmisison lines. Any experiences gratefully received and acknowledged.

My motto: Learn something new every day

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

 

[brimmer]

So as far as simply excavating the pipeline for maintenance, such as an integrity dig (removing the soil around the pipe, coating removal, blasting, inspection), crossig, etc. I have been involved in thousands of excavatins like this. For the most part, operators in Canada and the US do not shut in, or stop flow. I have worked with many operators who continue operations as normal. Many do take the 20% reduction from say a 60 day maximum oeprating pressure as described. Always check for anomalies on inspection logs as well, and calculate a maximum safe operating pressure (burst pressure with safety margin) in the anomaly is severe enough. On many pipelines they are operating at lower SMYS levels already (<20%) for the at-site pressure they are digging and a pressure reductoin won't really do much. Ideally 0 is great, but if we followed this we simply wouldn't be in business, as we complete hundreds of these excavations every year. They key is having proper procedures, supervision, and an experienced crew that understands what they are doing and the consequences. I have worked with operators in Australia, Mexico, various in South America as well, none of them took any pressure reduction. Not saying this is right or wrong, just industry experience I have seen. Most of the "transmission" type companies had procedures and experienced crews around this, most of the "upstream" only oeprators had no idea what they were doing when it came to this it was dangerous to be around the work.