Pipeline Cut Spring Back Effect Calculation! Please advice!

[bippedibopp]

Hi!

I need some help with calculating the possible spring back effect of cutting a 20" subsea pipeline. Prior to cutting it will (off course)be cooled down after which the pipepline should start to contract. I have the means of calculating the expansion (from when production was started) and the anchor length. However, I don't have any idea on how to calculate the contraction of the pipeline as it cools down prior to cutting. I need to know the location of the cutting point compared to its initial location so that I can estimate the possible spring back distance. It is critical to know the worst case scenario as the spoolpieces to tie in to will already be fabricated at the cutting time.

So anyone that can help will get a big gold star in my book;-)

 

[BigInch]

Expansion and contraction for steel is 0.0000065 in/in/ºF.

The key parameters are installation temperature and repair temperature.

But I'd recommend not trying to calculate this. It will only give you what the misalignment might be.

Operating temperature stress due to expansion upon startup might have all been resisted by increased compression in the pipe. If there was residual lay stress, maybe the pipe has zero axial stress right now. If lay stress did not relax, cooling back down to installation temperature will simply return the compression stress to zero and there will be no spring. Depending on age, creep stress redistribution might also have occured. Cooling down to some temperature other than installation temperature, now may still result in residual tension or compression stress in the pipeline which could be released when cut. Don't forget that it is also possible that frictional and cohesion forces and the setup of virtual anchors may have caused some uneven stress redistributions during operation temperatures, so its difficult to tell what will result unless you just happen to know the exact magnitude of residual stress at the cut point after its cooled off. Not too likely right? So you should always have a misalignment fitting and sufficient pipe lengths on each side included in your drop-in assembly that will allow enough make-up slack for any likely dimensional gaps in both length and angles that might develop after cutting. You really shouldn't build an offshore drop-in without some kind of misalignment handling strategy. Cut to fit could be very very expensive.

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain

 

[rconner]

Was just curious is this "calculating" related to (diver etc.) safety concern, some sort of design pieces for insertion/maintenance, of an old or new line, or what? Do you have enough information now to kinow that the line will in fact to some degree "spring back" (as opposed e.g. to binding inward or bucking sideways from the cut etc.)? How or with what method/equipment etc. are you thinking the line should be cut?

 

[bippedibopp]

First of all, thank you for your replies! I am not very experienced in pipeline engineering as my field of expertice is petroleum technology.

The pipeline to be cut is almost 30 years old. It is ca 36 km long and the cut location is almost at the end. The pipeline is buried (approx 2m) at this location. (It can also be mentioned that it was cut once before, even further towards the end and a 90 m repair spool was installed).

The entire operation will be diverless so the main concern is that the spoolpieces will actually fit the cut location. I guess for now (as the design is in the early phase) the most interesting is to know the magnitude of the potential spring back...will it be metres or cm...

I have consulted my colleagues and there are various assumtions as there are a lot of unknown factors; the documentation from 30 years back are not exactly overwhelming

So any other considerations that you might have I

 

[BigInch]

What's the water depth?
What's the water temperature now?
What pipe wall thickness?
What coatings were used? Corrosion and weight?
How far from the end of the line is it?
How much length do you plan to excavate and to what depth below the mud line?
2 meters is a pretty good burial depth if its 2 meters over the top of the pipe, so maybe not much spring will occur, if you have reasonable friction and cohesion going on.
What kind of sea bottom material, sand %, course, fines, clay%?
Do you have a cohesion value of any clay as it is today?
How are you planning to take up any dimensional mismatches?
Will you use a hydraulic type clamp on repair fitting at each end of the spool?
Are you planning to cut on the bottom, lift each end up to the barge, weld on flanges, lower then measure the distance between flanges, cut to length, drop and remote-bolt up the new spool to the flanges?

http://virtualpipeline.spaces.msn.com

"What gets us into trouble is not what we don't know, its what we know for sure" - Mark Twain