Increasing Pipeline Install Temperature After the Fact 1

[302Hugo]

I am in Alberta Canada. We have come across a situation that we have identified a pipeline that was installed back in the 90's had an install temperature of 10C, which puts us at a max operating temperature of 75C, however our max operating temperature of the pipeline is 85C based on the external coating. We wish to increase the operating temperature up to 85C but still keep the MOP of the pipeline at 9930kPa. Does anyone have any experience with this and provide any help on what we can do?

My only thought is that we would have to remove the anchor blocks on the pipeline and heat the pipeline (from what I understand there is a service that provides air heaters to increase the install temp of pipelines) to an acceptable temperature, and then re-install the anchor blocks. Thus giving us a new install temperature at time of restraint.

 

[racookpe1978]

Don't know, won't say. Won't guess.

 

[SNORGY]

Whatever pipeline walking or settlement into a hot state that can be expected is likely to have already occurred; whatever soil areas that might be expected to yield have likely already yielded. Are you referring to the limitations imposed by consideration of combined longitudinal stresses on the minimum pipe wall thickness per the salient paragraphs in Clause 4? The Regulator might or might not impose the measures that you state; my hope would be "no" unless the original design premise was Limit States Design. I would be inclined to take the survey and pipeline alignment sheets and - if available - geotechnical properties and re-model the pipeline in CAESAR II to see if it "passes", if it does, then see if it is necessary to submit a license amendment application to the ERCB - or EUB / AEUB / AER or whatever they call themselves these days.

 

[BigInch]

It is nearly impossible to have restrained pipelines free from overstress at 80C+ temperatures, even using normal tie-in temperatures. You need to add some flexibility. You might try shutting it down, warm it up, make some cuts and re-weld, so you can raise the tie-in temperature. Include some extra expansion bends in the re-welded segments.

What I always say, If you'd take off the anchor blocks forever you would have no longitudinal stress due to temperature. Increase the flexibility of the connecting piping (presumably near the anchor blocks) where movements are truly excessive. Do not replace the anchor blocks. In fact remove "anchor blocks" from your vocabulary as well.

 

[LittleInch]

302Hugo,

I essentially agree with SNORGY.

I'm not familiar with the Canadian codes, but in them or something similar, you can find a process to deal with uprating. Most of them are to do with pressure, but you can use temperature in the same way. Hence you need to review the design code it was built to, but this process also allows you to use actual wall thicknesses and actual minimum SMYs from the steel used, providing you still have all the material certs. The view of the regulator is key to whether this is a "goer" or not.

For a pipeline of this age, you will need to have done some inspections or will need to do inspections to demonstrate that the steel you think is there is still there.

I am using this is a buried line and hence what you suggest will not work. The idea about blowing hot air through is to heat the pipe up and let it expand BEFORE backfilling and compacting the soil. Backfilling effectively then locks the tensile stress in for the entire pipeline, not just the last 40-50m from the end. doing what you seem to imply will do very little or nothing for the vast majority of the pipeline.

Given its age, it is unlikely to have been analysed for stress in the way you would do it now, so my recommendation is to model it in Caesar or similar to see where your overstress conditions are now and if you raise the temperature. This could actually demonstrate that the pipeline is not overstressed or highlight a limited number of areas which need some attention.

Pure axial stress caused by temperature increase is debatable as to whether this is a true locked in stress. I'm also not sure what your combined stress limits are 0.72S or 0.9S. It varies between codes.

I am curious about your phrase "which puts us at a max operating temperature of 75C". Care to explain this a bit more or provide the calculation which give you this quite precise figure?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[BigInch]

If you have fully restrained underground pipe with an SMYS of 52,000 psi and your wall thickness is sized exactly for what is needed for hoop stress, 85C will put you at an Effective Combined Stress of 1.01 SMYS. Grade 60 pipe will be at 0.94 SMYS. 1% under the typical limit. That is for fully restrained (buried) pipe. Heating underground pipe any farther than a hundred meters from an above ground, unrestrained point, without first excavating all the backfill and allowing expansion to freely occur, will just overstress it in place. Even Caesar isn't likely to disagree.

Keep in mind that those stresses are WITHOUT any bending loads.

Providing enough flexibility to allow just 1/2 of the expansion to occur will reduce the effective combined stress to 0.72 SMYS which will allow some of remaining allowable stress to be used to resist small bending loads. The more flexibility you add, with expansion loops, etc. in any unrestrained, or semi-restrained sections of pipe, results in lessening any bending loads that ultimately exist. Trying to add flexibility to underground pipe isn't usually convenient and many times unreliable long term, so I'd suggest above ground expansion loops, or a few well placed above ground long lengths of pipe and a couple of 90deg ells. Be careful about making any unwanted leverage points. Check the above ground pipe configuration using Caesar, if you have to.

 

[302Hugo]

Thanks guys.

I wasn't too sure how much restraint the soil loads would actually add. My first assumption was that it's not much and the anchor blocks are the main contributor, hence if you use expansion loops all that stress is relieved through there. But after thinking about it the soil would give a lot of restraint (essentially a horizontal pile). I determined the max operating temp of 75C based on CSA's limit that Hoop - Longitudinal shall be less than 0.9 SMYS*Temp derate factor. And since longitudinal stress (from CSA) takes into account the temperature difference between restraint temp and install temp, this is the limiting factor.

 

[BigInch]

Anchor blocks are only effective in stopping movement within the transition zone of a buried pipe. The transition zone is the length over which pipe to soil friction force sums up meter by meter to where it finally equals the 100% fully axial restrained force, the point of virtual anchoring, and equals (thermal + Poisson stress) x cross sectional area of pipe. That is on the order of 200m more or less, obviously depending on soil type and temperatures. Between virtual anchors, adding anchor blocks have no additional effect, as the pipe is already 100% restrained. Anchor blocks only stop the expansion between virtual anchor points and their corresponding end-of-pipeline. If you remove one, the expansion at the end of line will equal 1/2 * [α] x virtual anchor length x [Δ]T.