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dynamic load factor during pipeline lowering in operations

dynamic load factor during pipeline lowering in operations

dynamic load factor during pipeline lowering in operations

(OP)
Hi all, can anyone inform me of a suitable dynamic load factor experienced during pipeline lowering in operations. I'm trying to calculate the stress and strain in a welded steel high pressure gas pipeline as it is being layed in the trench. The pipeline is 6 km long and the lowering in process is continuous using a number of side booms or excavators with lifting points that have a roller assembly (dolly). As the pipe is lowered and the side booms moves along, the pipe bounces slightly, hence the need for a dynamic lifting factor. The pipeline will be hydrotested after it has been buried in the trench. The reason for the calculation is to specify to the contractor the maximum distance between lifting points (dollies). I want to limit the stress in the pipe to 90% specified minimum yield stress (SMYS) as this is stress that the pipe undergoes during hydrostatic pressure testing in the pipe mill. By limiting the stress to this maximum I can be confident that the strain experienced during lowering in, is elastic and within a reasonable range. The reason for limiting the strain during lowering before hydrotest is because the weld joints are coated with a high build epoxy which has limited flexibility. The pipeline is coated with fusion bonded epoxy which is much more flexible.
I'm confident with performing the calculation but I just don't know what dynamic lifting load factor to use.
I hope my explanation above makes sense. It'll probably make most sense to people that work with high pressure gas transmission pipelines. The link shows a lowering in operation from a previous project.
Any advice is appreciated.

RE: dynamic load factor during pipeline lowering in operations

Lower-in should be controlled sufficiently such that the need for a dynamic load factor should not required.  If you want to use one, I would think 1.1 would be reasonable, 1.2 would be too much, easily fitting inside the increased margin for installation stresses, hence nobody worries about it.  It is a dynamically slow operation... drying spaghetti.  Nobody should be seeing massive sags, or dropping anything.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

RE: dynamic load factor during pipeline lowering in operations

(OP)
Thanks BigInch that helps. I realise that the operation is a dynamically slow operation I just wasn't sure how to quantify the dynamic load factor for the slight bounce from time to time. Now with your response I can sleep a bit easier tonight.

RE: dynamic load factor during pipeline lowering in operations

Don't sleep too easily.  You do know that it was WAG.
I'd look at it simply as a static deflection problem and limiting single pinned end span max bending stress at centerline during carying to 50% SMYS.  If you used 90%, the distance between lift points would probably be way to long and just the sight of the sag deflections would keep you awake.

If you don't mind; A general spec writing comment. It is better not to tell the contrator how to do anything.  If you give him specific instructions and he follows them to the letter, but for any reason it doesn't work, you're looking at fixing it on your paycheque.  If it does work, you're paycheque isn't going to be any larger anyway.  Where's the risk-reward factor?  

When writing specifications, ONLY tell the contractor what will define the acceptable results he must achieve in the final product.  If you say a maximum of 20 meter spacing and he does it, what have you really said?  If he puts them at 20 meters and the pipe breaks, even because it wasn't good pipe, you'd probably have to prove it was bad pipe before you got completely off that hook.  And technically there still isn't a specific reason for him to space at 20 or even 10 meters.  Did you give him a maximum stress criteria?  No, so he still has no idea what the acceptable product is.  

In this case, (if you're still worried about it) you might tell him something like, "The maximum installation stresses shall not exceed X % of yield stress.", or whatever other condition you like, and let him work out the spacing between sidebooms.  He's far more likely to have enough sidebooms available.  If you tell him 3 sidebooms at 20 m spacing "maximum" and its not enough, you might not only have to pay for any pipe he damages, but even be out the additional mobilization costs and schedule delay of getting 2 or 3 more out there.  See what I mean?

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

RE: dynamic load factor during pipeline lowering in operations

(OP)
I'm assuming that the acronym WAG stands for Wild --- Guess. Hmmm. Not sleeping well now. ;)
I do appreciate your comment regarding spec writing. I will definitely take that into account in my spec writing but also how I deal with Technical Queries from the Contractor.
I really appreciate your guidance. Thankyou.

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