Safe lifting of pipes with corrosion at pipe supports

[Thisforumishacked]

Hi all pipe stress engineers!

I have a general question regarding safe lifting of pipes in operation at an offshore oil&gas installation (FPSO). A typical problem is corrosion at the contact point towards pipe supports, where access is restricted for both inspection and maintenance. The preferred way to handle this is to lift the pipe a bit to perform inspection and maintenance/coating, and since production normally cannot be stopped everything needs to be done safely while the piping is in operation. I'm looking for a general way to justify safe lifting based on good engineering practice, formulas and standards (not just "gut feeling"), for example something that can be put inside an Excel sheet where we put in material, diameter, wall thickness, etc.

The lifting location will be completely free to move (support removed), but I assume we will need different formulas for different neighboring pipe support types, e.g. anchor, guide, axial restraint. I will be able to find piping isometrics (i.e. diameter, wall thickness, material) and design code for the pipes to be lifted (normally ASME B31.3), but unfortunately not stress isometrics or computerized models.

What we typically need to know is either:
[ul]
[li]We need to lift e.g. 40 mm to perform maintenance, what is the minimum required free length required before the next fixed location (support)?[/li]
[li]We have a distance of 4 m to the next pipe support, what is the maximum allowable lifting heigth?[/li]
[/ul]
For straight pipe lengths I hope this will be fairly easy to calculate, but I'm not sure what formula to use or what to specify as maximum allowable stress in the pipe (e.g. 90 % of SMYS?). And also, what can we do to simplify calculations for pipes with branch connections or changed geometry (typically one or two 90° bends) before the next support? As a start, we can live with some conservative simplifications if that makes calculations simpler. For example consider all supports as fixed, or even also consider all bends or branch connections as fixed. Then if the calculations cannot justify lifting, we can look more into each case separately to see if it's too conservative.

I would greatly appreciate input from someone with experience in this subject, since it's not easy to find much information on this online

 

[Inspectengr]

Why lifting for inspection is a preferred in your opinion.

I think if you find a good inspection company with LRUT skills you can atleast do reliable inspection. Do share all your concerns with LRUT operator and also share all the data like isometrics,welds details, support details (welded not welded) etc. and get detailed shots of defects like orientation and depth (percentage).

But with orientation of defect you would be able to differentiate the defect.

Please be aware, not every LRUT operator can provide detailed results.

From experience point of view, you can lift the lines without much issue, alternatively you can install temporary structure upstream and downstream of the support and remove the support for inspection. (This will be costly approach)

I hope it helps.

 

[Thisforumishacked]

Hi Inspectengr! The main reason we want to lift is when we want to perform maintenance at the same time. I agree many cases can be managed by inspection only and we do that as well, but most of the cases that come to my desk are when we have reached the anomaly stage, i.e. typically somewhere between consumed corrosion allowance and minimum allowable wall thickness. So at that stage sooner or later we will have to perform surface preparation and coating to stop the ongoing corrosion before we breach the MAWT.

Also in many cases we are dealing with pipe racks on top of large structural beams, so unfortunately it won't be an option to remove the support only – we will have to lift the pipe to get access for maintenance.

 

[0707]

Try to use EMAT ultrasonic guided waves and maybe we will not need to lift your pipes.

https://www.researchgate.net/publication/257288496_Emat_Pipe_Inspection_with_Guided_Wave

luis marques

 

[Thisforumishacked]

Hi, 0707. Yes, I know it is possible to inspect and like I said we already do that sometimes, but that is not what I'm asking for in this thread. My question is for cases where we have to lift to perform coating or other maintenance. If anyone has information on how to calculate this, it would be much appreciated

 

[0707]

Go to

https://patents.google.com/patent/US20090297317A1/en

maybe it helps

luis

 

[LittleInch]

This is simply a static beam problem.

I don't really understand why this is seen as something different.

Now in general most people don't bother calculating it unless you have some seriously corroded pipe. You just look at your system and identify where the constraints are,e.g. anchors, guides etc and then lift gently. Smaller bore connections are clearly at greater risk of yielding so address those, but getting 40mm clearance shouldn't be an issue for the vast majority of supports.

Something more complex, just create a simple string to assess in a stress analysis program and insert a point lift.

Clearly the more flexible you can make it the better it is.

Piping is generally too complex to come up with standard guides like you're looking for in the OP.

But you can try inputting a beam model with fixed vertical guides as a worst case and lift the middle of your span and see what happens.



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

 

[Thisforumishacked]

Thanks LittleInch! I've done some thinking since my original post and have also ended up with some beam calculations. However, I still have some questions regarding these since my mechanical theory is quite rusty... Here is how I tried to solve this:
[ol 1]
[li]Starting with simply supported beam with centre load and use the formula for deflection (δ = FL³/48EI). To find required free length on each side of lifting point I'm trying to solve this to find L/2[/li]
[li]Finding F: I don't know F, but I know the desired deflection δ. I assume lifting will lead to mainly bending stress and try to insert F = 4M/L[/li]
[li]Finding M: Use formula for bending stress, M = σ_bI/c [/li]
[li]Combining the above: F = 4σ_bI/Lc[/li]
[li]Finding c: Distance from neutral axis to most extreme stress, this would be outer radius of pipe (d_o/2)[/li]
[li]Finding σ_b: I assume the lifting mainly leads to bending stress, and use the "allowable stress" value I can find in the pipe design code ASME B31.3.[/li]
[li]Combine the above values for F and c, and solve for L: L = √(6 δ E d_o/σ_b)[/li]
[li]Required free length on each side of lifting point: L/2 [/li]
[/ol]
My main concern here is if I can go from F via M to σ_b like this. Another concern is if I can disregard shear stress or if I will have to include this as well. The pipe design code allows us to go 33 % above "allowable stress" for occasional loads like this will be, so I can disregard small stress contributions. My third concern is that I lose moment of inertia I in the calculations.

I tried calculating an example, 10" carbon steel pipe (d_o 273.1 mm, 6.35 mm (0.25") thickness which I don't get to use in my calculation, and 138 MPa (20 ksi) allowable stress) which resulted in required free length of 4.9 m (16 ft) on each side of the lifting point to allow 40 mm (~1.6") lifting height. It seems reasonable, but could also be quite wrong... All relevant input would be very appreciated