Mine shaft piping stress analysis 2

[DGStavlas]

The consulting firm I work for acquired a contract for the reconstruction of an underground mine pumping station. Because we are a small company we asked for the help of a piping stress analysis specialist. Although I don't know the subject so well so as to judge his work, I have some questions that I'd like to share.
- The specialist proposed anchors every 6m for the shaft's pipeline. As far as I know anchors should be placed in elbows, valves etc and not in straight pipelines. If anchors must be placed in straight lines then expansion joints are needed. When I asked him this question he said that due to the nature of the project (maintenance difficulties), any other type of support wouldn't work for the mine workers and that the only result will be a minor oversizing of the supports.
-I tried to check his results only for the shaft's entrance bend with this type

http://www.engineeringtoolbox.com/forces-pipe-bends-d_968.html

but the number's were totally different. I imagine that is the result from the placement of the anchors. As far as I know stress analysis is checked between anchors so the resulting forces from the pipeline's weight are not calculated. The other thing that seems rather strange is that when I asked him to change the operating pressure, the resulting forces at the sustained load case didn't change that much. Probably I'm missing something fundamental here.
-Last question pipe shoes are modelled as anchors at softwares or as an other type of support.

Thank you in advance.

 

[LittleInch]

Miningman,

I have utmost respect for those who chooses willingly to travel to the bowels of the earth to extract things for the rest of us and as it gradually becomes clearer what and where this is used, the design becomes easier to understand and as said, the devil is in the detail. In those locations the desire is for something strong and easy to fix / replace, so doesn't lend itself to detailed design in the same way as several kms of piping / pipeline might. You also don't get solar gain down there in the depths....

Whilst this pipe is a bit longer, this type of design is fairly common on risers and shafts used for oil and gas lines, but we probably finesse it a bit more and it doesn't tend to have cages whistling up and down in close proximity many times a day.

Rx etc refers to rotation about axes. The anchor refered to doesn't allow any movement or rotation.

In reality it will probably be under axial stress due to hoop stress and poisons ratio as 5 C delta T is really small.

I still think you could use longer pipes and space the supports out more, but the design is acceptable, just could be chaeaper that's all.

My motto: Learn something new every day

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

 

[DGStavlas]

Dear all, thank you very much for your useful information...practical & theoritical.
As I can understand, you all agree (more or less) that the design is at least acceptable.
Miningman, I would like to give you a better picture for the installation in order to gain as much as I can of your experience.
The shaft was built somewhere in the early 80s and its diameter is 4,3m. Inside you can find the infrastructure that you described (cables, pipes etc.). Now our goal will be, to install the 12" pipe somewhere there.
This means that we need to drill some new holes and of course that SNORGY's question is very good..." Can the concrete (and, *especially the concrete*) accommodate the loads?". Now regarding the installation difficulties to be honest I can't say really much...because as I understand you are the only who was already a pretty good picture.

 

[miningman]

Ok there seems to be considerable mutual respect developing here, so again , on the strict understanding that I will not be contributing to any discussion on stress analysis, based on the Op's description of the shaft at 4.3 metres in diameter.... which is awefull small by todays standards, plus the idea that this is a new proposed line...this will be very tricky to locate an area of shaft wall having the necessary room. I will also stick my neck out and suggest that perhaps this shaft is in Northern Saskatchewan. If so, I almost certainly have first hand knowledge of this shaft as I was sinking there in the late eighties, and there aren't that many small shafts in the world that I am aware of. If the Op confirms this I might be able to offer some more definitive suggestions again completely avoiding stress analysis considerations.

 

[DGStavlas]

Dear miningman, the shaft is not located in Northern Saskatchewan. As I understand you can offer valuable information at a later stage. Now regarding the stress analysis considerations the results that I have from the specialist are : loads on restraints are 15087 N and the displacement of 0,023m. Are these results acceptable? A minor correction, we are talking about delta T of 10°C. Now after reading miningman's post for two anchors per pipe length I asked the specialist to check this proposal. The results that I got was 98752 N and the displacement of 0,013m. To be honest I could use some help with the comparison of the two solutions.

 

[LittleInch]

If he has modelled full anchors, where is this displacement happening? - what direction? per 6 m pipe or the entire 350m of pipe?

Anchors in stress programs don't move regardless if you apply 1 tonne or 1000 tonnes of force so that took me by surprise - what is the max stress?

Those loads look very high for a 12" pipe support (1.5 to 10 tonnes force?) but is what happens when you "anchor" a straight piece of pipe and then add pressure or temperature difference

To be honest, without seeing all the details it's a little difficult to go much further, but perhaps if you post a "typical" support with loads, stress, deflection etc we might be able to see where this is going


My motto: Learn something new every day

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

 

[SNORGY]

15 kN is not of concern to me. 205 kN concerns me.

I have no idea how the analyst has come up with 15 kN as the computed reaction force between two anchors of full (X, Y, Z, Rx, Ry, Rz) restraints with dT = 5 C for 12" A106B pipe, but it sure is a different result than I get no matter what textbook I use (LC Peng's work among them) or CAESAR II model (yes, I did run it) I create. They are other nchors, or they aren't. I have no clue. But what I WILL say is, if the computed load of 15 kN is accurate, my concerns disappear.

 

[DGStavlas]

Dear SNORGY,
Let me share with you the results that I have in my hands. I will use the same terminology as it is described in the report that I have.
For the straight pipeline the computed force at an anchor node e.g 156 is, 590 kN (dT=10°C I have correct it that, at an earlier post),the load on restraint at the same node is 15 kN, the displacements are all 0.
At the two bends of the piping network (entering and exiting the shaft) the resulting force is around 750kN, the load on restraints is 700kN, the displacement is 0,17mm at the flanged joint of the bend and the straight pipeline.
The max stress intensity is 76083 kpA @ the exiting bend.
Is a modelling error possible? I mean maybe the bends, the straight pipeline should be checked separately. As I'm trying to read as much as I can these days, I see that eveyone is checking loads stresses etc. between anchors. Forgive me if I'm missing the fundamentals here but I'm starting to doubt about the specialist's abilities.
I can't understand how the anchor node resulting force is different than the load on restraint at the same node.
Maybe I didn't give him the right info?
Your thoughts...

 

[miningman]

Don't know if this helps or not , but is the shaft upcast or downcast. If upcast I would be very surprised if delta T was even 3 degrees C. If downcast , I would need to know the geographical location and virgin rock temperature to comment.

 

[LittleInch]

It is not easy to work out what is being presented, but it needs to be clear what is load on the pipe, force of the restraint / support and combined stress in the pipe.

What I think is being shown here is that the pipe is under a fairly high axial load resulting in axial stress. However apart from the bottom and top support, the rest are effectively fairly lightly loaded as all the pipe force from poissons effect / thermal expansion is being balanced by a similar force from the next pipe up the shaft - the 15kN. This force on the restraint is just for keeping it inline and allowing for the weight of the pipe between each anchor

However the last support before the bend (top and bottom) has no such balancing force being applied and hence has some high loads / forces associated with it and the restraint from the anchor to the pipe and will also include some significant bending moments absent from most of the other anchors.

Whether any of this is actually an issue should be calculated by the program for equivalent stress and reported as a %of maximum allowable stress according to your design code. This is a pretty hunky pipe for the size and pressure it is taking so has a lot spare for other stresses. It's the last support at the top and bottom which will see some fairly heavy forces and if they fail or allow any movement then the effect will ripple up the pipe.

does that make sense for you looking at the actual report / numbers?

My motto: Learn something new every day

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

 

[SNORGY]

That explains a lot - LittleInch, thanks for pointing out the obvious that I (obviously) didn't see.

The net force on each anchor is balanced by opposing compressive forces imposed by piping either side of it. The high loads are NODE loads, not ANCHOR loads.

I just should have asked for a rstraint report, that would have cleared things up. My apologies for the alarm I might have caused. Now the magnitudes of the NODE loads and the actual forces on the restraints make sense to me. It looks like the stress analyst is doing things correctly - now that I realize what I should have caught on to. My bad.

 

[LittleInch]

It took me a while to think about it - it's the load on the final anchors top and bottom which is quite impressive....

My motto: Learn something new every day

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

 

[DGStavlas]

LittleInch thanks for clearing things out.
As I understand the proposed solution (anchors every 6m) is oversized but yet acceptable. But still I have few more questions.
The displacement 0,17mm at the bends is negligible?
Between the two anchors shouldn't be there something like an expansion joint or a spring?
At a previous post LittleInch you wrote "I would have thought vertical supports every 15 to 25m and horizontal guides would have been enough".
So as to understand why the solution is oversized..which is the maximum permissible load per anchor or the maximum displacement at the nodes.

I want to thank all of you gents for your valuable help.

 

[LittleInch]

Displacement is negligible - it's the stress / loads which you need to see if you can actually supply - 70 tonnes force on the bottom support is quite a lot....

Between the anchors the pipe is acting like a spring - that's why the forces at the node / within the pipe as so high.

Permissible load will vary in each case as to what is practical / possible. I've seen anchor loads at pipelines coming into a plant in the several hundred tonne mark - needs an anchor v=block as big as a house. Totally ridiculous and unnecessary in most instances to prevent movements of 40-50mm. Allowable movement usually depends on how much pipe you can flex to reduce bending loads - depend son your piping configuration - 5mm might be too much or 50mm acceptable - it all depends....

At least with lots of anchors the pipe isn't going to flex or bow outwards which could happen if you have anchors too far apart.

My motto: Learn something new every day

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

 

[DGStavlas]

I believe that the 70 tonnes force on the bottom support will be the structural engineer's concern. LittleInch I got the general idea but I was asking for a number so as to be able to compare it at this certain case because you seem to have solved the problem from the start.

 

[LittleInch]

There is no maximum figure - it all depends on your circumstances.

If you are going for multiple anchors and guides then for a 12" pipe 20-25m apart has the right "feel" with guides to limit any deflection or "bowing" to 2-3mm every 5-6m. however miningmans point about the practicality of either installing this pipe as a retrofit or replacing it in sections is also important. You are probably limited also by what the height of your crane is to get the longest section vertical before lowering it down the shaft to be fixed to the wall and to the pipes underneath it - again 20-25m sounds about as long as would want to go, but I don't know how you're able to or planning to install it.

My motto: Learn something new every day

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

 

[miningman]

Sorry Littleinch, I cant resist.. What Crane ???? That's the thing about underground in general. We don't have the headroom for cranes. And then we get into clearances at shaft stations. Sometimes we can just sneak a 30 foot member out of the shaft onto the level buts its often 20-25 feet only.

 

[LittleInch]

The crane that's on top of your shaft to lower down the bits of pipe that make up your 350m long main vertical bit. That's the thing about these forums, I can't see what is in your head / desk / screen unless you tell me/us or attach a few sketches / drawings / photos. If this shaft is your only way in / out then it will become your limiting factor in getting anything built underground, but the vertical bit might be a bit different. That's probably why mining man quoted 6m lengths as being your standard sizes.

Have you though about some flexibles instead? There are plenty now in the 8" size which go to high pressure (i.e. 100 bar ish) that you could fix to the side and reel it down the shaft all the way to your pump? Usually very corrosion resistant inside and out. Search for RTP (reinforced thermo Plastic pipe manufacturers. Usually a bit more expensive than steel but you save on the installation and jointing issues.



My motto: Learn something new every day

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

 

[cvg]

the skip, headframe and hoist are in the way. no way to use a crane

 

[DGStavlas]

LittleInch...just for the record I didn't ask for the crane...miningman did. Just joking..
Your idea for RTP seems quite interesting..but for a future project because for this one the specs are finalized.

 

[LittleInch]

Don't I get a star??

My motto: Learn something new every day

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