## Pipe Thickness Calculation for Underground Pipe

## Pipe Thickness Calculation for Underground Pipe

(OP)

Can anyone help me out with a detail procedure or may be a spread sheet to calculate the Pipe thickness for underground pipes as per AWWA need it very urgently please help pipe material API 5L

## RE: Pipe Thickness Calculation for Underground Pipe

## RE: Pipe Thickness Calculation for Underground Pipe

It is generally a fairly simple calculation once you have the relevant data. It can be complicated by site conditions and service and as rconner pointed out depending on the criteria can be large.

Greg Lamberson

Consultant - Upstream Energy

Website: www.oil-gas-consulting.com

## RE: Pipe Thickness Calculation for Underground Pipe

Case in point. A Pipeline was installed under a lake here a long time ago. While working on the end of the pipeline many years later the lines was blown down. 500psig gas line at the bottom of a 100' deep lake. When the pipeline depressurized for the first time in 20-30 years it collapsed creating a very big problem and big mess. External forces on the pipe were a very big factor here and they needed to be considered when the pipeline was at Atmospheric pressure.

This may not apply to your application but it brings to light things that need to be considered.

## RE: Pipe Thickness Calculation for Underground Pipe

WT (inches)=(P x D)/(2 x SMYS x design factor x T x E), where:

P = design pressure, psi

D = nominal diameter, inches

SMYS = pipe grade, i.e. X60 = 60,000 in psi

Design Factor = depends on your class location, Class 1 = .72, Class 2 = .60, Class 3 = .50, and Class 4 = .40

E = joint factor (normally 1 on standard cross coutnry pipelines)

T = temperature factor (normally 1 on standard cross country pipelines)

Again, the above is for piplines installed according to B31.4 and B31.8 using API 5L pipe. If you have any factors that are different from the above, you may need to do some more investigation or throw it back on the forum and we can see if we can help.

Greg Lamberson

Consultant - Upstream Energy

Website: www.oil-gas-consulting.com

## RE: Pipe Thickness Calculation for Underground Pipe

This is unless you are looking at a very long pipeline and then you can save some money on tons of pipe by requesting a special rolling at your required wall thickness but I suspect yours will be fairly small quantities? If so, stick with the standard wall thicknesses.

Greg Lamberson

Consultant - Upstream Energy

Website: www.oil-gas-consulting.com

## RE: Pipe Thickness Calculation for Underground Pipe

DeepBlue, you need the yield strength of the 5L pipe. There are a few grades, 5L-B, X42, X52, X60, X70 ?? What do you have?

Most engineers look at the design of steel pipe without evaluating all of the possible solutions.

Using AWWA guidelines for design, a 48” diameter pipeline with 0.240 wall thickness would withstand an earthload of 20 ft. of cover. To get to 27 ft of cover you would need to increase the wall thickness to 0.500” wall thickness.

C-200 Steel Water Pipe applies to 6 inches and larger.

AWWA Steel pipe wall thickness formulas for internal pressure design only is,

t = Pw*D/2/S/F

Pw = working pressure psig

D = outside diameter inches

S = yield strength of steel API 5L Grade ?

F = design factor AWWA usually is 0.50

Where surge pressure is > 1/2 Working pressure the formula changes and P = (Working pressure + Surge Pressure)

rconner's reference has what you need for burial stresses.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

## RE: Pipe Thickness Calculation for Underground Pipe

In your example, the pipe was not laid under the lake, the dam was built many years after the pipe was laid and the lake filled up on top of it--Gas Company of New Mexico had some defeciencies, but I don't believe they would have missed that one.

David Simpson, PE

MuleShoe Engineering

www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

The harder I work, the luckier I seem

## RE: Pipe Thickness Calculation for Underground Pipe

You are right the pipe came first and then the lake, and you could probably tell the story better than I did, but the moral is still there, and I guess it applies more to the builders of the Dam than the pipeline. Thanks for the correction.

## RE: Pipe Thickness Calculation for Underground Pipe

David

## RE: Pipe Thickness Calculation for Underground Pipe

i want to know the reason for the above statement. can u help me out.

## RE: Pipe Thickness Calculation for Underground Pipe

The way the above works with the wall thickness calculation is that the 50% safety factor can handle surge pressures up to where surge pressures are 50% of the normal operating pressure.

At surge pressures higher than 50% of normal operating pressure, AWWA no longer considers that the 50% safety factor is adequate, hence requires that additional wall thickness must be provided.

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

## RE: Pipe Thickness Calculation for Underground Pipe

## RE: Pipe Thickness Calculation for Underground Pipe

The line was pretty long with some block valves so that compressor problems didn't require a blowdown and new tie-in's happened upstream of the compressor stations. The type of gas that it carried wasn't prevelent in the middle of the line (i.e., they were picking up conventional gas in New Mexico and processing it in Colorado, the part of the line that failed was going through some very prolific CBM acerage with very little conventional gas). There just never was a compelling reason to blow that much high-pressure gas to atmosphere. It really isn't that uncommon.

David

## RE: Pipe Thickness Calculation for Underground Pipe

However, some contemporary and growing applications that should perhaps also be mentioned are horizontal directional drilling (HDD) or other very deep trenchless installations, where increasingly larger pipes might well be subjected to high external fluid pressures of drilling fluids, grouts, and or groundwater etc. by virtue of the profiles/equipment/procedures etc. sometimes used. I recall an ASCE Pipeline Division paper I attended several years ago (presented incidentally by the engineer who was posthumously awarded the Stephen D. Bechtel Pipeline Engineering award last year) wherein some buckling case-study experience was presented. In a very deep HDD crossing involving I think a few thousand feet of larger sized oil or gas pipes (I think with not great wall thickness as a result of the use of high yield strength steels), after much preparation, work and some fanfare associated with such work in that area then an assembled entourage and dignitaries were treated to the emergence of the line pulled out onto the receiving bank unfortunately inverted flat as a flitter. Apparently fearing that the line had been somehow creased bearing on some hidden piles that might have been buried in the area, the contractor repeated the whole shooting match with some new pipe and trajectory. The new line unfortunately also emerged flattened on the far bank. I think they eventually repeated the installation a third time with a little thicker pipe and were finally successful. The pipe was apparently crushed repeatedly somewhere in the deep profiles and continually from that point on until the pull stopped, by the combined effects of external pressure and whatever ovalization effects that I believe are inevitable in bending a long, circular cross-section rigid-welded pipe beam in a deep profile. I guess in presenting/publishing this paper, the engineer was doing what he could to avoid further repetition of such a fiasco.

It might also be helpful to mention that at least by some authorities, buckling or ring instability resistance is dependent on an elastic modulus and effective stiffness of the pipe (thus dependent on long-term elastic modulus, or at least a modulus appropriate/effective to the duration of buckling load application), not on the yield strength or shorter-term stiffness of a pipe material.

Everyone have a great weekend.

## RE: Pipe Thickness Calculation for Underground Pipe

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

## RE: Pipe Thickness Calculation for Underground Pipe

Geoffrey D Stone FIMechE C.Eng;FIEAust CP Eng

www.waterhammer.bigblog.com.au

## RE: Pipe Thickness Calculation for Underground Pipe

The design formula that is suggested for collapse of an (unburied) steel pipe with "External Fluid Pressure-Uniform and Radial", at least per M11 is:

Collapse Pressure = 50,200,000(t/d)^3 [with units in compatible psi and inches etc.]

[Note: This formula was presented by Stewart, is reportedly at least slightly more conservative than that developed by earlier Timoshenko, and thus accounts to some extent for some manufacturing variations etc. in steel pipe.]

I calculate for the input parameters this collapse pressure would be less than 61 psi or ~140 feet of clear water head. While that particular HDD crossing was not quite that deep (apparently it was slightly less than 120 feet at its deepest, middle point), unfortunately this pipe was apparently, as is the nature of at least some HDD business, surrounded by a significantly more dense fluid (drilling fluid and native soil cuttings etc., that increased the external pressure on the pipe). While the results by different forensic analyses with some different assumptions/procedures etc. were apparently at some point in discussion close enough to provoke some differences of opinion, let’s just say that with what kind of things go on pulling in a fully-welded string of pipe and subsequent operation there does not appear to be a whole lot of security/safety factor for such a gas pipe! They reportedly also did some kind of further research that showed that the collapses were beginning precisely at the deepest point in the line.

The Contractor involved was “provided” a new string of heavier pipe that was subsequently successfully installed, and the pain of the cost of new piping was reportedly off-set at least a little by the salvage value of the mashed steel piping.

However, I guess it is quite possible what you say is true with regard to external head capabilities if the same steel pipeline were instead buried in a good quality soil envelope, as it is known (I think found by notable researchers who followed stanier’s venerableTimoshenko) that pipelines are some stiffened against buckling/collapse by at least good, firm bedding. AWWA M11 provides a different formula for buckling analyses of buried pipes, that takes into account the quality of bedding support (and also provides for direct input of a specific level of vacuum if anticipated in service (that of course can have the tendency to increase external pressures beyond simple elevations etc.)

## RE: Pipe Thickness Calculation for Underground Pipe

## RE: Pipe Thickness Calculation for Underground Pipe

However, if your line is perpetually "aerial" and cannot be submerged etc., it sounds like the most differential external pressure you might have on same would be whatever level of vacuum if any you might develop on the line in all operational/non-operational modes (you would want to make sure whatever pipe thickness you choose perhaps for other criteria is satisfactory with the security level you desire for that level external collapse pressure condition, per the formula).

DEEPBLUE76, minimum AWWA M11 design criteria in fact has a 2.0 safety factor considering design operating pressure vs minimum yield strength of the steel. M11 also has a lesser (1.33) minimum safety factor on the sum of operating plus transient/surge vs yield strength of the steel as your numbers infer (perhaps with some thinking that a very brief transient event might be considered a little less needing of high safety factor than an inexorable load on the pipe exerted on same in perpetuity?)

This is however different (arguably less conservative) than the AWWA design method for ductile iron pipe, wherein a hypothetical net thickness is chosen that gives at least a safety factor of 2.0 on the sum of water working pressure plus surge, and then a further service allowance of .08" (additional safety factor) is added to that thickness to establish the minimum manufacturing thickness. For AWWA ductile iron pipe, the true safety factor vs total design internal pressure (including surge) is is thus substantially greater than 2.0.

## RE: Pipe Thickness Calculation for Underground Pipe

OK, makes sense. My estimate was made for offshore installations where X52 is usually a good maximum value to prevent exactly what happened above. The extra wall thickness needed for X52 helps collapse pressures and the high installation loads from the stinger tensioners often leaves about a 400 ft depth margin before collapsing stress is reached. With that poor D/t ratio, I can see where it could very easily have been an external pressure collapse. The LOWER D/t limit for most pipelines is around 100, (ASME B31.8 has a limit, but I don't recall the exact value right now) for which a thicker wall thickness is usually needed for common internal pressures. Using the X-65 would give even a thinner wall for pressure than X52 or X60, so it looks at first glance exactly like it was designed only for internal pressure and NO engineer checked it for external pressure collapse!

BigInch-born in the trenches.

http://virtualpipeline.spaces.msn.com

## RE: Pipe Thickness Calculation for Underground Pipe

As for aboveground pipe the external pressure will not be as for this original case and could not exceed that of full vacuum.

Many engineers out there select pipe based on hoop stress alone. I think that is pretty risky. This particulalry so as in there ignorance they dont do a surge analysis. But that is another subject.

Geoffrey D Stone FIMechE C.Eng;FIEAust CP Eng

www.waterhammer.bigblog.com.au