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Maximum allowable of stresses during pipeline installation

Maximum allowable of stresses during pipeline installation

(OP)
Hi everybody. I have a question that's been bothering me for quit some time. An extra point of view on things would be appreciated. If a pipeline is installed in a trench it is going to be manipulated with cranes and the pipeline will be bent. The bending stress can be estimated but compared to what? Could the stress be categorized as a secundary bending stress? Most codes i have come across seem to limit the allowable to the 90% yield value... What would you chose to be the allowable when the design code is ASMEB31.3?

RE: Maximum allowable of stresses during pipeline installation

Cold bending is not recommended using cranes. Use a pipeline bending machine.

You can cold bend a pipe in yielding stress range, provided the minimum radius remains in accordance with the minimum cold bend radius permitted by your design code, B31.4 or B31.8. You can hot bend to tighter radaii, provided the resulting "pipe bend" is within what is allowed by your design code.

After cold bending, when the bend is released from the bending machine, there are no applied forces, so applied stresses within the bend are zero. That is the preferred method to install a pipe, since there are no stresses remaining from the installation itself.

If you do not use a bending machine and allow the pipe to bend under its own weight, or if you pull it into some kind of horizontal bend while in the trench, bending stresses are created. If the pipe is not returned to straight before it is buried in its final position, bedded on the bottom of the trench, bending stresses from any remaining curvature will remain in the pipe. Installation stresses (in this case, bending stresses) are created. Any installation stress retained will have to be considered when the pipe is pressured and put into service, meaning that the installation stresses will have to be added to hoop, thermal and all other stresses present under any and all loading conditions and the resulting total combined stress must remain less than the Allowable Combined Stress permited by your design code.

Richard Feynman's Problem Solving Algorithm
1. Write down the problem.
2. Think very hard.
3. Write down the answer.

RE: Maximum allowable of stresses during pipeline installation

(OP)
Thanks BigInch for your opinion. For clarity, the pipeline is welded above ground (on blocks) and large strings will than be lowered in the trench using cranes. When the pipeline is installed in the trench most of the bending stresses are going to be zero. The remaining welds are performed in the trench, connecting the pipeline strings together.
So the expected stresses (on straight pipeline sections) will only be present during the lifting/lowering and horizontal manipulation of the pipeline.
Looking at your reply, you would consider the stresses as primary limited to the yield value (or yield divided by a safety factor). I agree with you that it would seem logical to do so however is there any reason why the stresses could not be categorized as secondary? Is it not an imposed displacement?

RE: Maximum allowable of stresses during pipeline installation

B 31.3 is not a pipeline code, however as far as I can see para 320 and 302.3.5 c0 apply, i.e. don't exceed the max allowable S given by the tables.

S is in fact the lower of 2/3 SMYS or 1/3 UTS so already has a pretty good safety factor.

Installation there is of course no pressure stress or thermal stress, so in most case so long as you don't have a great big weight or valve in the middle of your lift you will be fine.

I think BI answered a different question, but you confuse the issue by saying "with cranes and the pipeline will be bent.". Now I think BI has assumed this is a permanent strain, but I think what you really mean is "...and the pipeline will be in elastic bending". Hence when in the trench the strain and bending stress will reduce to near zero.
which one?

I don't really understand your primary / secondary point - please explain.

~B31.3 doesn't do buried systems very well - you call it a pipeline, but you're using a piping code - why?

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

RE: Maximum allowable of stresses during pipeline installation

You don't know which design code to use? Why?
As LittleInch told you, the B31.3 code should not be used to design a pipeline.
As I said, Design code is B31.4 or B31.8
Your allowed primary stresses and allowed combined stress will be found within those pipeline codes.

Elastic bends stresses remain in the pipe while there is curvature, regardless of what machines do the lifting, lowering and installation. Pipe bending machines bend in the yield range, so when bending force is removed there is no remaining stress (other than some relatively low residual stress).






Richard Feynman's Problem Solving Algorithm
1. Write down the problem.
2. Think very hard.
3. Write down the answer.

RE: Maximum allowable of stresses during pipeline installation

(OP)
The reason why I did not specifically mentioned a design code is that I have actually the same issue for several design codes.
- design code ASMEB31.3: an aboveground pipingsystem was lifted for modifications on several supports.
- design code EN1594 : lowering a gas pipeline in the trench as explained in my original question.
- design code NEN3650-1+2 : HDD pipeline that needs to be bend to get the pipe in the borehole.

But using different design codes, it is (in my opinion) confusing, unclear, not mentioned, contradictory... Therefore I decided to make the question referring to primary and secondary stresses explained in ASMEVIII div2. The philosophy for defining the allowable stress is for most design codes (I expect) somewhat the same. Primary stresses PL+PB are always bellow the yield (< k*1.5*S with S=allowable stress as defined by LI and k= occasional load factor). Secundary PL+PB+Q stresses are allowed above the yield (<SPS=3*S=2*SMYS).

Also referring to "elastic bending" would be stupid (and I hope my question was not) as it would imply that that the maximum allowable is bellow the SMYS.

The doubt I have is that why should it be limited to the SMYS. Between the lifting points, you would have a sustained or primary stress induced in the pipeline/piping. But the higher stresses are induced due to the displacement at the lifting points. And if I'm correct imposed displacement are categorized as a displacement/secondary stress (limited in ASMEB31.3 §302.3.5 (d)to maximum 1.66*SMYS with SL=0, Sc=Sh=SMYS/1.5).
Or am I wrong to make the link between ASMEVIII div2 and ASMEB31.3, because than why a maximum of 1.66*SMYS and not 2*SMYS?


RE: Maximum allowable of stresses during pipeline installation

I think you're confusing yourself (and me) by looking at all these codes.

Your OP states " What would you chose to be the allowable when the design code is ASMEB31.3? "

My response was use Sa as listed in table A-1
ASME VIII is a pressure vessel code and will just confuse you.

When you lift a pipeline or section of piping for installation purposes, whether off supports or into a pipeline trench, you should not create stresses which yield the pipe. How much below that figure you go is your safety factor and usually 0.9 is accepted, if a little high. Once you've completed your operation the resultant loads and stresses are either created by the span between support (yr first bullet) or hopefully near zero (your second bullet). If you create stresses > SMYS you risk bending and buckling the pipe leading to potential failure of your pipe (most times you will then be removed from the project....).

The stress induced in an HDD is different because that bending stress is permanent (in some sections at least) and hence needs to be added into the equivalent stress calculation in whichever pipeline code you are using. The sections above ground during installation will normally need support, but this is the same as the installation stresses noted above.

I looked at 302.3.5 d) and can't see where your figures come from, nor how Sl is zero noting that d) is really looking at fatigue and stress cycles.

LI

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

RE: Maximum allowable of stresses during pipeline installation

You need to check stresses in accordance with the pipe design code. NOT ASME VIII div2

The bending stress induced in an HDD and during offshore pipeline installations as well, are elastic, so they remain after installation, becoming part of the combined stress calculations.

Allowable stresses are always less than SMYS. PLEASE STOP YOUR WORK AND FIND SOMEBODY THAT KNOWS WHAT THEY ARE DOING BEFORE YOU GET SOMEONE HURT.

Richard Feynman's Problem Solving Algorithm
1. Write down the problem.
2. Think very hard.
3. Write down the answer.

RE: Maximum allowable of stresses during pipeline installation

(OP)
Little Inch,
You state use Sa as listed in table A-1, I suppose you mean Sh as per §302.3.5(c) or just plane old S per §302.3.1 . Sa is a combination of Sc, Sh and Sl as per §302.3.5(d).
I know ASMEVIII is a pressure vessel code but it is the design by analysis section (=§5) I'm referring to, so when choosing FEM you will end up using it's rules.

In some cases the biggest bending stresses are due to the lifting operation other bending stresses are from dead load. The case I'm keen to receive your reply is when the dead load stresses are minimal compared to the lifting stresses.

I'm also well aware of the fact that buckling etc requires verification if above yield.

Big Inch,
Sa=f*(1.25*(Sc+Sh)-SL))
- Suppose no meaningful temperature difference => Sc=Sh
- Suppose SL is minimal (near zero).
- Suppose f=1 but can also be 1,2 according fig. 302.3.5.
As already stated S=min(Sy/1.5;UTS/3), so lets suppose the Sy value determines the allowable stress.
Sa=1,2*(1,25*((Sy/1,5) +(Sy/1,5))-0)=2*Sy.
So thank you very much for your refreshing input which helped me to solve at least one of my questions.

Before being verbally attacked, I agree it just doesn't feel right, but is there any real argument why it should never go above the SMYS.
Consider a piping system where due to thermal loadings (the impossible happens) and the stress value goes above the yield. Local yielding occur and stresses drop. A little later the product is redirected causing the piping temperature to drop and so does the thermal stresses. Is it not somewhat similar. Thermal stresses are secondary but so is imposed displacement...

I just keep on trying peace

RE: Maximum allowable of stresses during pipeline installation

KVdA,

You're looking at the wrong section in B 31.3

302.3.5 (d) is all about fatigue. I don't agree Sl is zero and Sc and Sh are limited to 20 ksi anyway.
Sl is Stresses Due to Sustained Loads, SL. The sum of the
longitudinal stresses due to sustained loads, SL, e.g.,
the pressure and weight in any component in a piping
system (see para. 320), shall not exceed Sh,


You want to use 304.7.2 d)

(d) detailed stress analysis (e.g., finite element
method) with results evaluated as described in
Section VIII, Division 2, Part 5. The basic allowable stress
from Table A-1 shall be used in place of the allowable
stress, S, in Division 2 where applicable.

It shouldn't go beyond yield because you bend the pipe....

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

RE: Maximum allowable of stresses during pipeline installation

Just keep your stresses below allowable stresses unless you're doing a controlled cold bend with a pipe bending machine. I have nothing else to say about this.

Richard Feynman's Problem Solving Algorithm
1. Write down the problem.
2. Think very hard.
3. Write down the answer.

RE: Maximum allowable of stresses during pipeline installation

(OP)
Well that was the hole point of the thread, to discuss what the allowable stress should/could be (as it is not mentioned in the ASMEB31.3).

Please notice that §302.3.5(d) is also valid with 1 load cycle, ASMEB31.3 surely does not exclude it.
Sl can be zero, if no pressure, no dead load (or piping fully supported) and the only load (causing stress) is thermal temperature difference.
But the example was just to show that there is a link between ASMEB31.3 and ASMEVIII div2 part5. And of coarse that under certain loading conditions it is allowable to exceed the yield value of the used material.

§304.7.2 d) is for unlisted material but actually that doesn't matter. The only thing I whish to discus is why ASMEB31.3 §302.3.5(d) is not applicable (or why can the stresses not be considered as secondary).

But ok, thanks anyways for your insights.


RE: Maximum allowable of stresses during pipeline installation

I have some experience in designing lifts using cranes for pipe (both pipe to-be-buried and pipe-rack situations). Unfortunately, it appears that our resident pipeline experts don't fully understand the issue(s).

First, in my experience, localized buckling (kinking) of the lifted pipe at the lift location(s) is the biggest threat. You're inducing not only longitudinal compressive stresses, but also radially-inward stresses. I'm not aware of a hand-calc method that can save you. And you would need to examine a non-linear buckling analysis from Part 5 in VIII-2.

Secondly, you need to understand the purpose of the 3S limit - it is a ratcheting limit to ensure shakedown to elastic action. It is not applicable to a one-time-only load. Nevertheless, some stresses may exceed yield on a single load application. However, classifying and categorizing the stresses from an elastic analysis may be quite challenging, because most pressure vessel engineers with experience, have that experience for pressure vessels and not piping or pipelines. I would recommend the services of an expert in this regard.

Finally, your service may play a large part here. If you are in sour service and require a low level of residual stresses, then your answer will change. Otherwise, yield-level residual stresses will be the likely result.

RE: Maximum allowable of stresses during pipeline installation

(OP)
TGS4,

What do you use for allowable stress for the design of the lifts? Looking at your reply you would also stay bellow the SMYS/SF correct?

Do you always perform a buckling analysis per ASMEVIII-div2 in those cases?

I thought that once above the yield value the system shakeddown (or is it not a verb)to a stress strain function perpendicular to the original. But I think I start to realize why introducing additional compressive stress and strain to a system is perhaps not that healthy for piping, because it may limit the use of the system after installation. Do you agree?

Thanks

RE: Maximum allowable of stresses during pipeline installation

Thank you TGS for the barb. Atually this time it's simply that I would rather not encourage the OP to do work that IMO he isn't qualifed to do. My choice. Your statment, "I would recommend the services of an expert in this regard." indicates you apparently agree. You don't actually offer any real help anyway, so did you come here today just to toss a few barbs or what?

Richard Feynman's Problem Solving Algorithm
1. Write down the problem.
2. Think very hard.
3. Write down the answer.

RE: Maximum allowable of stresses during pipeline installation

(OP)
BigInch,

If your replies on questions are only going result in insulting people than I would rather have no reply of you at all in this thread.
Also understand that stating incorrect things in combination with questioning peoples qualification damages your credibility.
I can understand that you might have misunderstood the question, or perhaps I didn't do a good job in expressing myself. In any case please let us focus on the question or discuss (without being offended if people don't agree on certain topics). I'm still more than eager to hear a well argumented point of view of an expert.

RE: Maximum allowable of stresses during pipeline installation

I don't see the insult. It's more like a fact that several people can apparently agree on.
Better to do as TGS says and hire an expert.

Anyway I'm more than happy to leave this alone. Just wondered what TGS' problem was. Usually he has pretty good advice. Oh yes, there it is. "hire an expert".

Richard Feynman's Problem Solving Algorithm
1. Write down the problem.
2. Think very hard.
3. Write down the answer.

RE: Maximum allowable of stresses during pipeline installation

Ok, this thread has bounced around a bit. Let's reset here.

primary question as far as I could see was about lifting pipes for installation. Pipeline design codes restrict equivalent stresses at 0.9 SMYS, what was the limit in B 31.3. was the original question.

I still maintain its the allowable stress value in table A1 and provided a few references.

Now my deep understanding of buckling and pressure vessels isn't massive, but one thing I like about ET is that it forces you to go and look things up. So my somewhat simplistic view of primary and secondary stresses is that primary is the overall stress inputs (pressure, bending, axial etc), whilst secondary is more for discrete local areas where the stress is self relieving or limiting if the material yields.

Lifting pipes for installation should not result in reaching yield or deformation / buckling of the pipe.

When you calculate that using FEA or similar I don't believe lifting loads are secondary stresses as if it starts to yield at the lifting point it will just keep on yielding, plus the stress in the pipe are not local. Hence my point that Sl cannot be considered as zero until there is no load or lift on the pipe. That's not the point here.

Now when the pipe is in it's final position, you can analyses the stresses based on the initial loads and stresses, if any. To me piping and buried flat pipelines should have no or very low residual stress levels.

HDD pipes in the curved zone will have residual stresses and these need to form part of the start point for any analysis.

comments?

LI

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

RE: Maximum allowable of stresses during pipeline installation

BigInch - no barb intended. It's just that looking at lifting a piece of steel is much more complicated than evaluating an operating pipe/pipeline. For starters, the concept of design margin for the specified failure modes is something that needs to be considered in the context of a risk assessment. Since the pipe/pipeline is not actually operating, I would have a difficult time justifying using only the Code-mandated operating design margins without any other information. That, an LI's understanding of stress categorization is, by their own assessment, lacking.

RE: Maximum allowable of stresses during pipeline installation

TGS4 - Ok, but what's your opinion on what the design margin should be for a pipe installation evaluation?

why would you have a difficult time with 0.9 SMYS? Like I said, the aim is surely to install the pipe without damaging it?

sincerely - in simplistic terms for stress categorization was I right or not? I like to learn as well...

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

RE: Maximum allowable of stresses during pipeline installation

(OP)
TGS4, please clarify what other information you would require? The limit state I'm specifically interested in here is stress.
The applied loading is, as stated earlier lifting (imposed displacement which in normal OPE condition is categorized as secondary) during construction (it may/ may not be classified as secondary???) and dead load that's it (which is always primary).
Could you also give me your insight on the questions I had posted earlier?
LI, I agree 0.9*SMYS is in any case safe, and I believe the only correct answer looking alone at ASMEB31.3 . Perhaps I should have not dragged ASMEB31.3 into the question. But by doing so I hoped to play a better advocate of the devil by referring to an imposed displacement.
Secundary stresses are not necessarily discrete nor local, for example thermal loadings can induce stresses which can be local but also general (membrane and bending). Therefore I must agree with TGS4.

Thanks guys for your time and effort,

RE: Maximum allowable of stresses during pipeline installation

My last effort on this is simply to disagree that a crane lift for pipe installation can be classified as an imposed displacement. It's simply a force equal to the weight of the suspended pipe located essentially as a point load.

There is though a significant difference between vessel design and analysis and pipe, though they share common elements it's not the same thing.

good luck

LI

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

RE: Maximum allowable of stresses during pipeline installation

Yes installation stresses and operation stresses are different. But yielding in either case is not allowed, or severely limited when it is.

Offshore installation is probably the best example to make.

Evaluating INSTALLATION stresses, for example when lowering the pipeline into the ditch using sidebooms, or cranes, or towing a submerged pipe string out to its offshore route location, stringing off the stinger from the end of a offshore construction barge in a near catenary shape, or maybe dropping a vertical string in a J-lay and evaluating RESIDUAL INSTALLATION stresses that might have been retained from any of those installation methods in combination with OPERATING stresses when in the OPERATING CONDITION are not the same. Yielding is not allowed in any of those installation operations, as all bends are kept within elastic ranges, except when making a cold bend to appropriately limited radaii. Reel lays do use plastic bending to reel and let out the pipe, but pipe is limited to small diameters and radaii are large and yielding well controlled. When the pipe is let out, the process becomes similar to the S-lay technique.

Offshore laying from a stinger in relatively deep water is done in a very long "S" bend. The pipe can easily buckle from excessive bending stress in what can be 1000 ft or more of pipe hanging from the barge down to the mud line as the pipe is strung off the stern of the barge. The pipe can ovalize if bending becomes excessive and yield stress is reached, during which the pipe may collapse and flatten as the top arc and bottom arc are pulled together. Pulling with a very high tension load, usually 100,000 lbs or more, applied by moving the barge forward will prevent the pipe collapse by reducing the compressive bending stress such that local buckling and collapse does not occur. Essentially a hanging string shape is adopted. A string hold in a catenary form is pure tension and it won't buckle. In fact, the compressive bending stress may be reduced so much that the entire cross section is actually put into pure tension. All catenary curvatures are ideally held to elastic radaii that prevent yielding in bending.

B31.4
402.6 Longitudinal Stress
402.6.1 Residual stresses from construction are often present
for spanning, elastic bends, and differential settlement.
Designers should determine if such stresses need to be
evaluated.

403 Criteria for Pipelines
403.1 General
Pipelines within the scope of this Code may be subject to conditions during construction and operation where the external pressure exceeds the internal pressure. The pipe wall selected shall provide adequate strength to prevent collapse, taking into consideration mechanical properties, variations in wall thickness permitted by material specifications, out-of-roundness, bending stresses, and external loads.



Depending on friction between pipe and mud on the bottom of the ocean and the lay stresses developed during installation, some axial tension might not completely relax and be held within the pipe as the barge moves well ahead. Axial tension can remain in the line after construction has been completed. Of course the pipe will retain any elastic horizontal, or vertical bend stresses into the operation phase as well. Those must be combined with stresses from pressure (internal and external), currents, waves, thermal excursions, mud subsidence, seismic loads and/or any other stresses that might become present during its operating lifetime. To evaluate the effects of those with operating stresses, you use the combined stress limits in the pipe design codes.

403.3 Criteria to Prevent Yield Failure
403.3.1 Strength Criteria. The maximum longitudinal stress due to axial and bending loads during installation and operation shall be limited to a value that prevents pipe buckling or otherwise impairs the serviceability of the installed pipeline. Other stresses resulting from pipeline installation activities such as spans, shall be limited to the same criteria. Instead of a stress criterion, an allowable installation strain limit may be used. Stress values for steel pipe during operation shall not exceed the allowable values in Table 403.3.1-1 as calculated by the equations in this Chapter.


Maximum Tension stress is often limited to 90% of SMYS to prevent yielding in tension. Where yielding has occurred or is used in a particular installation method, strains are limited to 2%

403.3.3 Strain Criteria for Pipelines. When a pipeline may experience a noncyclic displacement of its support (such as fault movement along the pipeline route or differential support settlement or subsidence along the pipeline), the longitudinal and combined stress limits may be replaced with an allowable strain limit, so long as the consequences of yielding do not impair the serviceability of the installed pipeline. The permissible maximum longitudinal strain depends upon the ductility of the material, any previously experienced plastic strain, and the buckling behavior of the pipe. Where plastic strains are anticipated, the pipe eccentricity, pipe out-of-roundness, and the ability of the weld to undergo such strains without detrimental effect should be considered. Maximum strain shall be limited to 2%.

The problem is not limited to pipelines.
Effect of bending on Wind turbines.
http://www.sciencedirect.com/science/article/pii/S...

Richard Feynman's Problem Solving Algorithm
1. Write down the problem.
2. Think very hard.
3. Write down the answer.

RE: Maximum allowable of stresses during pipeline installation

KVdA:
If you find something in a code someplace, I suspect it will be... ‘during lifting, thou shalt not cause yielding or buckling of the pipe, elst you will kink it..., and that’s a big big sin.’ You guys know the ASME and API codes (or whichever other codes) you use every day much better than I do. But, it seems to me that we have become so dependant upon codes and stds. to do all our thinking for us that we’ve lost the use of all common sense and engineering judgement. And, I’m not particularly throwing darts at anyone. If I were lifting and installing your pipe spools, et.al. into a trench, I would not want to leave any residual stresses after my lifting operation. That is my obligation to you. You just don’t know how to design your pipe line around these, because I could do so many different, damaging things, in almost infinite ways and locations, if I were not making every effort to avoid same. The lifting loads (primarily pipe dead loads plus some impact, maybe wind) and stresses are primary loads and stresses to me and for my part of the project. And, while some of your codes for pressure vessels or pipe lines may allow primary design stresses (pressure, temp. changes, pipe spanning, etc.) of .9Fy, I would probably limit my design considerations (allowable stresses during lifting) to .75-.8Fy, and no buckling or concentrated wall kinking. This is because there are a fair number of uncertainties in my part of the project, not the least of which, we have a bunch of crane and tractor jockeys and line foremen who are of the get-er-done mentality to gain a few minutes at lunch time.

My guess would be that anyone who’s done this a few times and has a bit of experience, will literally have a flow chart, spreadsheets, etc. for this design phase of their work. It will involve pipe material, mechanical props., section prop., pipe dia., wall thick. and thus weight/ft. Now, I could calc. my max. span length btwn. lift points for a plain straight run of pipe (spool?). I would look at the end pipe section, a cantilever, and the first couple interior spans; with span length as the primary variable to arrive at a max. allowable bending moment and lifting reaction for the given pipe section props. This leads to a tabulation of max. distance btwn. pick points. Then, I would look at pipe curvature which might add unusual loading/stresses (torsion, etc?) to the string, and also added heavy loads, like heavy valves, etc. Normally, these would be applied by superposition to my basic tabulated stresses, span lengths, etc. when they occur.

My max. lifting reaction would be based on the types of things TGS4 brings up in his 13JUN17, 05:51, para. 2. And, I think all wounds could be healed if he changed his para. 1, from “Unfortunately, it appears that our resident pipeline experts don't fully understand the issue(s),” to ‘unfortunately.... resident pipeline experts (due respect intended).... misunderstand the OP; we are not talking about cold bending, except to prevent it during pipe spool handling.' I suspect he will agree with this rephrasing. My max. reaction will be based on the type of lifting equip. we are using, and the newer roller/cradle lifting devices which spread the reaction over 4,5-6' of pipe length, on several sets of rollers, and allow some longitudinal pipe or cradle movement are far superior to the older simple sling. Once you get a pipe dia., or so in length, away from this lifting point you should have no pipe stress/strain or permanent deformation problems, under normal conditions. Any permanent deformation will induce residual stresses, my fault, infinite possibilities, which again, you don’t know how to account for in your pipeline stress analysis. But, if you haven’t preped. your trench properly, to grade, and with a nice clean base, you can be introducing pipe stresses every bit as significant as some of the things I could do in lifting. I also agree that this kind of problem is not something which should be done, the first time, without some good, experienced local mentoring.

RE: Maximum allowable of stresses during pipeline installation

Yes that phrasing helps a lot.

I think my just prior to post identifies B31.4 code sections that detail the exact requirements you mention... but in "code language".

Richard Feynman's Problem Solving Algorithm
1. Write down the problem.
2. Think very hard.
3. Write down the answer.

RE: Maximum allowable of stresses during pipeline installation

Due to the challenges in categorizing the stresses in such a situation, I would generally not perform this type of analysis using a linear-elastic approach. I would simulate the actual true stress-true strain curve. Understand that some plasticity is inevitable, since the proportional limit of many pipeline steels is quite low, even compared to the pseudo-elastic "allowable stress" limit.

I would evaluate the buckling failure mode, ensuring that a margin of at least 2 is achieved (using an LRFD-like elastic-plastic buckling approach, while imposing the maximum of the out-of-roundness tolerances).

Otherwise, the residual stresses would be examined and compared to a reasonable limit based on conditions such as SCC, or Cl-SCC. If the pipeline is in sweet service, I probably wouldn't really care about the residual stresses - the weld residual stresses would be high anyways. Unless you PWHT your welds, in which case I would leave the residual stresses equivalent to what you have after PWHT (generally 0.2*Sy).

RE: Maximum allowable of stresses during pipeline installation

Sure. The nonlinear property of the steel above proportional limits, (70% +/- of pipeline steel's SMYS). The allowable stress methods used in pipeline design in the USA are not used everywhere. Norway, Holland and U.K. behavior. I think ASME B31.4 and 8 use linear properties for the usual reason, simplicity for the no spreadsheet era. The Offshore Part of B31.8 allows design to be based on strain. Using the nonlin method, I would guess that the required wall thickness decreases a bit.

Richard Feynman's Problem Solving Algorithm
1. Write down the problem.
2. Think very hard.
3. Write down the answer.

RE: Maximum allowable of stresses during pipeline installation

So for pipeline steels API 5L type, what is the limit of proportionality?

I had a search and it looked like ~80-85%, but would be good to see some reference or typical graphs. Given that actual stress / strain and "yield" is often higher than SMYS, indicates a good reason to limit such stresses to a certain percent of SMYS??

I realize some situations require this level of investigation and analysis but the vast majority you want to not get so close to the limit that some transient event (wind, jerking of the lift etc) causes the pipe to deform or buckle. IMO.

Been an interesting discussion though

LI


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

RE: Maximum allowable of stresses during pipeline installation

(OP)
Thanks again everybody. We all have different backgrounds, and possibly therefore somewhat surprised of each others reaction/point of view on things. I'm starting to realize that my question was formulated wrong, I'm now also aware that this is quite a dangerous subject as no clear guidance is provided in most codes (so left to the interpretation of the engineer) hence the first reactions I received (which I now beginning to understand). However if one is in the situation where it is obvious that certain damage is done, it is less obvious to simply go to the owner and say " you will need to buy 70 new pipes because your contractor has gone above 0.75 the SMYS". The owner is going to ask "on what basis are you refusing, is there any code requirement not fulfilled....", in any case he is going to do the extra mile to ensure that the piping/pipeline is going in service (as they generally are more interested in time schedules and being as cost effective as possible).
So BI I certainly hope you realize my intention was not to send out a message letting contractors do whatever they want during installation. At the contrary, in my opinion to little people are aware of how a pipe flex's (in my case) and are unaware of how easily they are trespassing the great unknown. I also find it very surprising that B31.4 allows 2% strain, that seems a lot... (or am I missing something).

RE: Maximum allowable of stresses during pipeline installation

2% strain limit can most likely be safely applied as a tensile strain limit.
The onset of wrinkling or buckling typically occurs for compressive strains ranging from 0.3% to 0.6%

Page 3
https://www.researchgate.net/publication/272488696...

C 8.2
https://www.researchgate.net/publication/232405640...

Ask for the Tensile Test Diagram to see the Proportional limit.
It is the highest stress at which the curve in a stress-strain diagram is a straight line

Richard Feynman's Problem Solving Algorithm
1. Write down the problem.
2. Think very hard.
3. Write down the answer.

RE: Maximum allowable of stresses during pipeline installation

(OP)
Apparently 2% strain is not that uncommon see §833.5 of ASMEB31.8(I'm not that familiar with using strain as limit state).
also :
A842.1.4 Allowable Strains. Instead of the stress
criteria of para. A842.1.3, an allowable installation strain
limit may be used. The maximum longitudinal strain
due to axial
and bending loads during installation shall
be limited to a value that prevents pipe buckling and will
not impair the serviceability of the installed pipeline.
BI, Could the strain calculation you are referring to be found in API RP 1111?

RE: Maximum allowable of stresses during pipeline installation

(OP)
Something just doesn't feel right here. Allowing 2% of strain during installation would seems like a whole lot of allowable stress. And also residual stress.
I'm used to 0.2->0.5% strain (hence my original reaction). I know there's also buckling which will define the upper strain limit but still? Am I missing something, perhaps everything becomes clear after reading the references.

RE: Maximum allowable of stresses during pipeline installation

KVdA Note that the section you refer to is for offshore pipelines and not cranes etc onshore.

Offshore installation analysis is normally much more rigourous and in depth than land based installation due to the cost of getting it wrong and also the much higher stress and potentially strain levels involved in laying pipe, especially in deeper waters. If someone buckles a pipe lifting it in by a crane or sideboom, it's much easier to see by all the supervisoirs and inspectors.

The 2% limit referred to in the codes refers to loss of support or e.g. ground movement or other single cycle events, but doesn't talk about installation.

2% also covers cold pulled bends of 40D.

Usually the owner is telling the construction contractor that he needs to replace things and its the CC who is trying to say it's all OK...

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

RE: Maximum allowable of stresses during pipeline installation

All of these strain numbers being tossed around are starting to make my head spin.

First off - buckling is a phenomenon that does not require any plasticity - elastic (Euler) buckling can occur in the elastic regime. So, it is foolhardy to think that some pseudo-elastic (or plastic) strain limit somehow protects against buckling is just wrong.

Secondly, the proportional limit (which I typically define as 1e-6 plastic strain, but there isn't much difference if you use 1e-4 plastic strain) depends on the ratio of yield to ultimate. So, for API 5L pipe, it really depends on the Grade.

The magnitude of the strain (unless you approach the ductility limit) is generally irrelevant. It's the residual stress that is important.

RE: Maximum allowable of stresses during pipeline installation

(OP)
Buckling might be an important parameter (or even the most) to check. But I believe that we will never correctly predict it, as it is to depended on so many different parameters which are for most (actually all) constructions unknown. You can spend so much time at it, that it would be far easier and more reliable to just send an intelligent pig down to actually measure the ovalisation, dents and all other information you want.
So in my opinion buckling doubt = inspection.
Stresses or strains are more difficult. The slightest change during OPE might cause failure, and for underground piping there are a great deal of parameters that are unknown.
TGS4- Did I just noticed a contradiction in your replies? "It's the residual stress that is the important". (Which I completely agree with). Than earlier post:" If the pipeline is in sweet service, I probably wouldn't really care about the residual stresses - the weld residual stresses would be high anyways. Unless you PWHT your welds, in which case I would leave the residual stresses equivalent to what you have after PWHT (generally 0.2*Sy)." Or am I missing something here?

RE: Maximum allowable of stresses during pipeline installation

I'm not much interested in predicting anything correctly accurately (at least to more than 3 significant figures). Practicality usually means knowing how to stay well away from the number to the conservative side anyway, whatever it might be and whichever side conservatism might actually be on.

That's right. Residual stresses flip. I noticed that too. But for the most part, they are ignored in pipeline design because they are low. Those flip-floops might be one of the reasons why. I think they are typically of more concern when considering H2S corrosion.

If you're interested in preventing buckling during construction, obviously an intelligent pig isn't going to help you much with that problem.

Buckling is best prevented during construction of pipelines by keeping the bending moments small and when they are not small, keep cross section in tension. Bring enough cranes and sidebooms to the party to keep unsupported lengths of installation strings short. In operation you restrain the pipeline by burial at a depth sufficient to prohibit (upward) movemet should compressive loads near buckling loads. Predicted buckling stresses are limited to 60% of Euler bucking load. Full lateral support from soil weight above the pipe keeps the potential columns very short and Euler buckling loads above those that might develop when a fully restrained pipe reaches design temperature.

OK, now that we have the stresses relatively boxed in, I've lost sight of the reason you came here. would you mind trying to re-explain, if you can, what it is that you are actually trying to accomplish? A diagram representative of the general problem could be quite helpful.

Richard Feynman's Problem Solving Algorithm
1. Write down the problem.
2. Think very hard.
3. Write down the answer.

RE: Maximum allowable of stresses during pipeline installation

(OP)
BI, Well I totally agree, that with buckling it's better to be on the conservative side in order to prevent it during construction. Sometimes however the possible damage is already done. Hence the solution that can be provided using an intelligent pig after the construction phase.
Therefore I'm not that interested in predicting buckling (as I also have some criteria that can be used for that).
As stated earlier, I'm more interested in the stresses (after installation). I don't understand that you can say that generally they are ignored. Perhaps you can, if you are staying bellow the yield, but actually (and I'm almost scared in saying this)I'm interested in the case where during construction you are above. I'm still convinced that they (the imposed displacement)are secondary. But they should be taken in account as you bring an additional compressive (residual) stress in the pipe which should be taken in account for all other following loading conditions for which you are designing the pipeline/piping for.
Perhaps simulating the true stress true strain curve is a better option (as brought to my attention by TGS4), but for the moment I need to do some research on that topic.
I also did not yet check how the strains come in the equation (in detail), but I would find it strange that if you are respecting/correctly categorizing all involved loads you would have a problem with that. But please do correct me if I'm wrong.


RE: Maximum allowable of stresses during pipeline installation

"As stated earlier, I'm more interested in the stresses (after installation). "

Ah ok, that got lost in the noise of the debate we've been having. In that case (your pipe has yielded), as opposed to the actual lifting operation, then that is an imposed displacement.

The key to me is then whether that displacement is accommodated within either vertical or horizontal support from the earth or whether it is under permanent load / stress to try and force the pipe back to a straight condition. Otherwise you would have trouble at every cold formed 40D bend (< 2% strain) in every pipeline ever built.

As a very junior engineer I'll never forget seeing an excavator on full weight ( tracks off the ground) forcing a pipe down so that it would tie-in to a replacement section. I did think at the time that there would be a certain residual stress level there, but no one else seemed to be concerned...

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

RE: Maximum allowable of stresses during pipeline installation

If the displacement is within the elastic range, the stresses caused by that load remain present as do the forces applied to the pipe that caused them and these stresses cannot be ignored. They are carried into the operating phase. They might still be nearly at elastic limit stresses.

If the displacements were in the plastic range and the displacements remain after the loads causing them have been removed , the stress caused by active forces have been reduced to ZERO and any remaining stresses are the residual stresses resulting from the previous yield. They seldom exceed 20% of SMYS and are usually ignored.

This is why pipe bending machines that produce plastic range bends are used to shape pipeline bends in the field during construction and they are preferred to methods that may only result in elastic range displacements, which can leave high stresses trapped in the pipe's installed configuration. These stresses can be significant.

CLEAR?

Richard Feynman's Problem Solving Algorithm
1. Write down the problem.
2. Think very hard.
3. Write down the answer.

RE: Maximum allowable of stresses during pipeline installation

(OP)
BI, yes clear - thanks, now I understand the point TGS4 was trying to make. Very difficult discussion (for me that is) formulating real-life situations into words. The fact that I had different scenarios in my head also didn't help much.

RE: Maximum allowable of stresses during pipeline installation

Great!

Richard Feynman's Problem Solving Algorithm
1. Write down the problem.
2. Think very hard.
3. Write down the answer.

RE: Maximum allowable of stresses during pipeline installation

BI - excellent and clear explanation at 15 Jun 17 11:08. Apologies again for the earlier "sniping". I think that we've provided some worthwhile advice - and unfortunately there are no easy answers to this, if there were, we wouldn't need smart engineers...

RE: Maximum allowable of stresses during pipeline installation

Thanks TGS! I do think we finally got on the right track. Communication is 50% of problem solving, of which the technical aspects is often the easier part. Plus the old 3X rule, "Tell them once, twice, three times."

It is also said that residual stesses left over from the pipe fabrication process itself can be lowered, neutralized, or at the least redistributed-neutralized during the yield-strain process, so the combined result of all residual stresses might be +/0/-.

It was for that reason the Flatlander policy of the largest gas transmission company, in terms of length of pipelines, whom I worked for a long time ago was to hydrotest all their new pipelines to 105% yield pressure. To their credit they did of course keep an eye peeled on the P-V curves when approaching yield. I started working for them after they opened an offshore office in Houston, where I convinced them that the risk-reward to repair costs of that test policy just wasn't as great as it might have been for grassland work. Actually I did't want to spend any more time than necessary on a construction barge making repairs for something what really wasn't broken. Least until they played around with it too much. smile Actually that policy wasn't really great for onshore city work either.

Richard Feynman's Problem Solving Algorithm
1. Write down the problem.
2. Think very hard.
3. Write down the answer.

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