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Pipe Stresses due to tunnelling activities
2

Pipe Stresses due to tunnelling activities

Pipe Stresses due to tunnelling activities

(OP)
First post here folks. Struggling a little bit and need to pick some brains.

Have been tasked with demonstrating that any settlement caused by our pipe tunnel crossings is not having a detrimental affect on existing services.

We were given a copy of some calculations by P.B. Attewell to demonstrate the forces exerted due to the bending curvature. But for settlement amounting to 1% of the existing pipe diameter, we get stresses amounting to 10% of the permissible strength. We think we should be getting figures much less than this.

Are there any other methods that can be used to work this out or does anyone have any ideas how best to approach this?

Many thanks.

James  

RE: Pipe Stresses due to tunnelling activities

Here are a set of calculations for buried Spiral Welded Pipe. The formulas are all there, Mortenon, Spangler, and others so all you will have to do is change a few values to match your pipe.

http://www.acipco.com/aswp/pdfs/ASWP2-Design.pdf

RE: Pipe Stresses due to tunnelling activities

If the settlement occurs over a short length of line, stresses can be much higher than 10%.  Depends on the radius of curvature, not the absolute settlement.

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: Pipe Stresses due to tunnelling activities

(OP)
Thanks for those gents. Unfortunately I don't think either deal with the problem. We realise that the forces can be above 10% when we install pipes, but these are only temporary loads, and the pipes are tested thereafter. The problem arises because these are existing services we are crossing operate within their allowable operating limits, and if we add 10% on to that it puts them over the limit and I'm sure the owners would soon put a stop to our works if that was the case.

Thanks unclesyd for the doc. We've used similar calcs for calculating the loading on our pipe, but we want to try to factor in the permanent axial stresses that will be put upon the pipe due to say 4mm deflection over a trough width of 12m.

Our example at the minute churns out figures of nearly 40N/mm2. When I calculate this as a percentage of the Ultimate tensile Stress of 420N/mm2 I get the 10% figure. Should I even be using the UTS figure?

It may be the case that we already have the answer here, but it just seems way too high for what we're doing. Can you think of anything else that we might be doing wrong?

Thanks again

RE: Pipe Stresses due to tunnelling activities

You might try this search on Google quite a few hits.

marston theory pipe axial forces

Question:
Did you do your calculations with the pipe flexible or rigid?

RE: Pipe Stresses due to tunnelling activities

(OP)
Thanks. We have used the Marston Theory elsewhere for other pipe loading calculations, but they don't account for the settlement affects that the tunnelling activities create beneath the existing services.

The calulations we've been using are for a rigid joint pipeline and I've attached a copy so you can see what we've been working with.

We've downloaded some software and got similar results so are ready to admit defeat.

We're getting around 40N/mm2 for our stress when a 380mm pipe deflects 6mm over 12m. At the minute we're using the Ultimate tensile Stress (420N/mm2) to determine how this relates to the pipe's operating capacity.

Is this even the right way to go about it? Are there factors that should be accounted for relating to additional stiffness if the pipe is operating at high pressure? Should we even be accumulating these stresses with the stresses that the pressurised pipe is already subjected to, or is the affect only a fraction of the stresses already being exerted?
 

RE: Pipe Stresses due to tunnelling activities

I don't think ultimate stress relates to maximum allowed operating stress.  That's bursting strength and maximum stresses are well below those.  

MAOPs have a base calculated using either the "permitted stress", as in ASME B31.3, or "allowable stress" as in B31.4 & 8.  You need to check the design codes of those pipelines in order to know what the limits are.  The codes will also give you the load combinations, but they don't consider construction and operating loads simultaneously, which is what you may be looking at here, if pipeline operating pressure is not reduced when the construction activities are taking place.  In that case you must use the full operating pressure of combined construction loads and then calculate the maximum combined stress for the most critical point in the span.  A typical limit for that would be less than or equal to 0.72 * YIELD_STRESS, not ultimate stress.    

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: Pipe Stresses due to tunnelling activities

BTW, it would be extremely unusual for any pipeline company to allow tunneling below their pipelines.  Most crossings of existing services are done by overpass.  I assume you now know why.

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: Pipe Stresses due to tunnelling activities

(OP)
Thanks for those BigInch. I had a look at the ASME calcs. The allowable stress levels are similar to those here in the UK. These were the levels we were worried about exceeding.

I did start to look at summating all the stresses that the pipe might be subjected to (i.e. ground loading, traffic loading and operating stresses) and then added these with the bending stresses, but the actual stresses involved with these other three components have little / no effect on the outcome of the calculation. They amount to a total of about 0.5N/mm2 whereas the bending forces are 42N/mm2 and the total amounts to about 15% of the yield stress of the pipe which is will below the minimum level of 50%. It's just that these sets of figures seem to be worlds apart.

Most of our pipework is installed below ground and trenchless techniques are a common way of traversing either a service, road, rail or river crossing here. We do however have to submit proposals to the owner and protective measures (such as ground monitoring) are put in place at the request of the owner as insurance measures. The proposals contain all the settlement calculations and pipe loading calculations normally, but this is the first time we've been asked for anything like this.

If we were to lay a pipe in a trench, it would have more than 6mm deflection along it's length in places and the stresses would be permanent due to the undulations in the ground so we know that the pipework can handle deflections of this nature. It is just a case of producing corroborating calculations. The client is hapy with what we have produced, but we are unsure of whether or not the values are correct so we would like to verify these for our own piece of mind before we start submitting incorrect calculations.

Obviously these the results of these calculations are theoretical values using 'worst-case' figures, so in reality would the bending stresses be significantly lower in operation?  

RE: Pipe Stresses due to tunnelling activities

(OP)
Thanks unclesyd. We did come across this when we were doing the calculations originally. It all seems to be very similar to the Attewell calcs, even referencing it in places, but the conclusion says that the modulus of subgrade reaction is not suitable to complete the Winkler calculation and they don't give you the appropriate equation.

The problem we found was that because this is an unusual request as far as the submition of standard calulations is concerned, there isn't really much support material. We've produced the calculations in a spreadsheet and tested the worked example and we are getting the same results.  

When looking for other info, we did find research papers, not things put in best practice, and may just be documents produced by under graduates or post graduates. It's becoming increasingly frustrating as I've got to the stage now where I'm not even sure if we're not just creating work for ourselves.

RE: Pipe Stresses due to tunnelling activities

Just a thought as over time I've seen several references to University of Iowa and wondering if an enquiry to their library or maybe through the Linda Hall Library might be useful.

http://www.lindahall.org/services/document_delivery/


Anecdotal:
The is major project in my area of relocating the Waste Water Treatment Plant that involves many miles of force mains.  I watched a 20" D1 pipe being installed near my house.  The pipe is about 6 foot deep in sandy soil with occasional outcropping of hardpan.  The trench is excavated and the pipe dropped in on a bottom that looks like a catapillar as it moves.  I watch 8 joints being laid and 3 of these had to adjusted to where they would make up.  

RE: Pipe Stresses due to tunnelling activities

Pipe following terrain undulations is bent in a machine and once removed and carried to the trench, axial bending forces return to zero.  Secondly, the pipe when rested on a 12" bed of sand in a trench "theoretically" would not have a 6 mm bending deflection and axial bending stress would therefore again "theoretically" be zero.  That's why the sand bed is important and why the lay-in inspector continuously watches the contact of the pipe and the sand bed to assure no spanning is taking place.  Where it does inevitably occur is a question of still being within the safety factor.  
Don't understand, "It's just that these sets of figures seem to be worlds apart."  Apart from what?

I also don't think you can look at it as "we know it will work" so it must be OK to do it kind of question either.  Most of the time, and in fact, only under rare conditions; perhaps only during hydrotest, a pipeline is nowhere near full pressure for its entire length, so safety factors can cover a lot of installation errors, as far as the average piece of pipe is concerned.  As long as you've got no big bending stresses in the first kilometer or so, or at bottoms of valleys, there are very few places where pressure hoop stress are likely to be anywhere near design levels.  Well over half of a flat pipeline might not be at even be at 50% of hoop stress allowable.  (Another reason why they seem to always blow under the middle of a river.)
 

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: Pipe Stresses due to tunnelling activities

That lift bend in that 42" will be straight when its on the sand bed 2 meters down... theoretically.

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: Pipe Stresses due to tunnelling activities

(OP)
Again thanks a lot both of you. We install our pipes in strings on S4 grade bedding for the same reason. I just think that over a 12 m length there might be areas where the grade is not truely flat/level. I imagine that even though the grade is checked with a laser level, there are places missed due to the fact that it is checked at intervals, and obviously depending on the date that the pipe was originally installed. Obviously as best practices are put forward, industry starts to follow these to prolongue a life span and remove the need for repairs / maintenance.

By worlds apart I'm just referring to the fact that the figures I'd calculated for hoop stresses from the loadings and the operating pressure were on the whole negligible when compared to the bending stresses. So much so that the bending stress amounts to 15% of the yield stress, regardless of whether I decide to include these additional stresses or not.

The other two engineers that have looked at this with me were the ones that were originally dubious as to the validity of the results due to the relatively high figures encountered.

I appreciate the safety factor consideration and how the areas where levels of static head are greatest may require heavy wall pipe to extend hydrostatic test sections. And how 10m equates to approx 1bar of pressure. I have just taken a standard operating pressure of 75bar at the minute rather than the 135bar test pressure that may be used for a high pressure gas main, and as the one I'm dealing with currently is only 125mm and not one of the 1220mm ones.

Sorry if I appear not to be grasping something quite obvious here. This is the first time since I finished my degree I've done any engineering so to say I'm rusty is an understatement.

RE: Pipe Stresses due to tunnelling activities

15% isn't really that much bending, but what I don't understand why you say hoop stress is insignificant in relation to that 15% bending stress.  I think that's what your saying, but because there are two possible bending directions, axial bending and ring bending, and I don't know if you're talking about one or both.  Maybe you're talking about ring bending being 15%.  Actually you getting me confused now when you say bending is 15% and hoop stresses are negligable, but you have a problem with reaching a failing condition.  How do you reach failure stress if bending is 15% and hoop stress is negliable?

Your hoop stress at max pressure should generally be much more significant, normally limited at 40%, 50%, 60% or 72% of yield stress (not ultimate), the % depending on B31.8 class location factor of the pipe segment.  And ring bending might be 15% or so, with axial bending near zero for good bed contact.  That must be a very very thick pipe wall at a very low operating pressure, if hoop stress is negliable.  Normally a waste of material for a typical pipeline design.    

Surface load effects on ring bending drop fast with clear cover.  You can assume that surface point loads distribute laterally up to the width of the trench (more or less in a 45 deg cone) as depth increases.  With a 2 m clear you could easily drive most heavy trucks over a line at full pressure, that perhaps without pavement of any kind, if the backfill is good.

And directional drilling isn't done in gravel or sand where cave in and bridging of overloads isn't possible, so there should be little effect of subsidence on a line above when the soil is right for HDD. Directional drilling depths are usually planned to be well away from any foreign structures to avoid any subsidence at all, as that's the real advantage, right?



 

**********************
"Pumping accounts for 20% of the world's energy used by electric motors and 25-50% of the total electrical energy usage in certain industrial facilities."-DOE statistic (Note: Make that 99% for pipeline companies) http://virtualpipeline.spaces.live.com/

RE: Pipe Stresses due to tunnelling activities

A little off topic but why would you want to make two 90 degree bends to cross a road with the 20" sewer line mention in my post of 10 Nov, 13:21?
The crossing was on a long sweeping curve where a straight shot would have been easy.

RE: Pipe Stresses due to tunnelling activities

(OP)
Thanks a lot both of you for your help. We've generated out calculations through calculating the total stress from the internal and external loadings on the pipe. I think that the problem previously was that I had not been correctly calculating the hoop stress from the internal and external loading. I was calculating them to be a very small percentage, which didn't seem realistic when the bending stress was a much larger figure. I've reviewed both using the Barlow formula and we have now got results which to the best of our knowledge are correct. The total stresses are calculated to be well below the 50% minimum levels which we will use regardless of the area or location of ther services, giving us conservating figures where the operating stress limitations are greater than 50%.

You've both helped me a lot and definitely helped me to think about it a bit more logically, and stimulating memories of long forgotten lectures. Many thanks to you both.

As for the 90deg bends unclesyd, I never mentioned anything about them and our crossings are all via SBU or TBM machines. The bends we use are may be required to return the pipe to the normal buried level before and after the crossing.

Thanks again gents.

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