Elastic Bending Radius 3

[jonjose123]

Hi All,

Our Pipeline Details are 219.1 mm OD, WT 7.04 mm CA=3mm, DP 70 barg Tdesign = 83 deg and T min = -5 deg c. Pipe material API 5L X60

Based on the above The minimum elastic bending radius calculated is 250 m. However, if I use the WT as 6.35 mm the minimum elastic bending radius required is 360 m. But the same 360 m if we uses in the Hydrotest calculation, the equivalent stress criteria fails where as with WT 7.04 mm it satisfies the criteria .

If we increase the min bend radius in 6.35 mm from 360 m to 4000 m it satisfies the equivalent stress criteria. Does this is alloweed to increase the bend radius to 4000 m?

 

[saplanti]

During the selection of the wall thickness of pipe, the elastic bending radius or cold bending allowance should be added onto the calculated thickness for the internal pressure (design pressure) to select the nominated wall thickness of the pipeline.

In your case, you have the pipe wall thicknesses available and you want to use the thinnest for some reason on the bend radius area. If this is true and your calculation requires minimum 4000 m for the bend radius, you cannot reduce it if you want to comply with the code.
But the radius of 4000 m will look like a straight line unfortunately. Why do not use the thicker pipe instead if it satisfies the all criteria?

 

[jonjose123]

Thanks Splanti for the quick response.

We will increase the wall thickness to 7.04 mm which is higher of the two to avoid this situation.

 

[LittleInch]

jonjose,

Can you tell us what design code you're using, why you're so interested in minimum bending radius and what you mean by " in the Hydrotest calculation, the equivalent stress criteria fails " You don't have an allowable equivalent stress for hydrotest.

I disagree with sapplanti - I've never added anything to a wall thickness to deal with elastic bending radius, which is different from "cold bending".

In extreme cases, the pipe wall thickness is such that other stresses such as thermal expansion can mean your elastic bending radius is rather big, but you don't normally add anything specific for this.

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

 

[BigInch]

Elastic bending creates longitudinal bending stress, which adds (and subtracts) directly to any other axial stresses already present, such as those from pressure and thermal loads, so extra wall thickness is required whenever total equivalent stress would exceed allowable. That is precisely why onshore pipelines should use cold bends, not elastic bends, as cold bends do not add to axial stresses. Once the bending machine force is removed, bending stress returns to zero. That is not the case with elastic bends where bending stress remain in the pipe forever.

Offshore it is not possible to avoid elastic bends due to required methods of installation, so extra wall is required whenever bends cannot be made with radaii great enough to keep combined stresses below allowable.

 

[saplanti]

It seems I cut my comments too short. I agree with LittleInch and BigInch on the elastic bending stress additive onto the axial stress. However the cold bending is a plastic operation, and the wall thickness reduction occurs during operation therefore it is direct additive to the wall thickness.

From my experience you normally select the wall thickness for internal design pressure (it seems there is dispute with my first comment) and check the allowable bending radius with the remaining allowable. However, this does not always give you adequate elastic bending radius for the laying of piping that you may use. Therefore you slightly increase the wall thickness to be able to achieve the logical elastic bending radius that you are after. It seems that this gives heavier wall thickness for the straight lines, however I do not see laying out the straight lines as straight in practice for small diameter pipes if you are not in a flat environment.

 

[BigInch]

Saplinti,

If you cold bend within the radius limits given in B31.4 and .8, no reduction in wall thickness need be considered, presuming your design code is B31.4 or .8.

No argument until you reach this bit. Perhaps I don't understand what you mean by, "It seems that this gives heavier wall thickness for the straight lines,", because you would not necessarily use the heavier wall thickness on pipe joint lengths that were to remain straight.

And I admit that I don't understand this part at all, "however I do not see laying out the straight lines as straight in practice for small diameter pipes if you are not in a flat environment."

I thought that I was correcting LittleInch, as IMO and apparently Saplinti's as well, elastic bending will require additional wall thickness should the combined stress exceed allowable, however as I read LI's comment, he seems to say that he doesn't add wall thickness for elastic bending. LI?

 

[saplanti]

Hi BigInch,

My Australian experience shows me that the pipeliner companies do not provide adequate geotechnical report for the small diameter pipelins, and some of them are satisfied by the geotechnical report at the start and stop fields only for buildings. Therefore there is always a concern you may find different soil or even rock under the soil. So pipelinesr will lay out the pipe within allowable route tolerance to be able to pass the trouble location. So they use the the elastic bending radius (pipeline engineer's solution) to do that. I hope this is clearer and please do note that I do not mention large diameter pipelines.

Again the cold bending radius needs to be controlled for the plasticity. I have asked many cold bending machine producers/fabricators about the calculation of wall reduction during the cold bending, there was no answer. The code that you mention has the table for the angles you operate as you mentioned. How about the large angles? If you have a reference other than the BS 8110 (which has recommendation to consider the wall reduction) to see the relationship between diameter, wall thickness, cold bending angle steps etc... and the calculated wall reduction I would like to know. I guess this will be part of the reliability calculation whatever the code is. Small diamater pipelines do have sharp change of directions, therefore they use smaller radius for the turns, and the pipeliners/owners ask for the hot formed bending instead with large tolerances for manufacture. This process is time consuming process and expensive, and the bends need to be ordered a lot earlier. I suppose, at least, following BS guideline for the wall thickness reduction and adding them to the pipe wall is a good practice. I hope this is clearer now.

 

[saplanti]

It is a typo, the BS standard should be read as BS 8010, not BS 8110.

 

[jonjose123]

Thank you all for the responses.
its really helpful. As little inch asked, our design code is ASME B31.8

The reason for interest in bending radius is that as I mentioned if I uses a wal thickness of 6.35 mm the min bending radius becomes 350 m whereas if I uses 7.04 mm WT the minimum bending radius becomes 250 m.

total equivalent stress would exceed allowable if I uses this minimum bending radius if 350 (WT 6.35 mm) and this satisfies only when I increase the bending radius (WT 6.35 mm) to 4000 m.

However with 7.04 mm WT this doesnt happnes (total equivalent stress not exceeding allowable.)

This is where I get confused if I can use 6.35 mm WT or 7.04 WT is more safe and realistic.

 

[LittleInch]

saplanti - apologies if y response was a little "snappy" - wasn't meant to be.

What I'm saying is that the process, in my experience is as saplanti explained, i.e. chose your wall thickness for pressure then check for other stresses including bending.

I don't understand why you're so interested in such elastic bending - Sure onshore and offshore epipelines follow the ground elevation, but horizontally and for smaller beding, you need to cold bend it.

The wall thinning on cold bending (40D) is about 1% - negligible and included in the B 31.4/8 allowances.

Hot / Induction / formed bends (5D) the wall thinning is about 8% so often a thicker mother pipe is used.

jon jose, you're now saying 6.35 (with a 3mm CA(!) gives you 350m. That sounds like a good deal to me. What are you including in your equivalent stress? It shouldn't be hydrotest ( no thermal effect) and hydro has a higher allowable.

Only you can judge whether your pipe line needs more and how much an extra 0.7mm costs you.

You might want to look at the CA. 3mm sounds like a "standard" piping CA - 0.1mm/yr for 30 years. For pipelines which can be inspected much more than pipework is, there is an argument to reduce this or eliminate it all together and rely on internal and external protection. Even reducing this to 2mm will save you a fair amount. ( anything less than 5mm becomes difficult to construct).

What thickness of pipe did you use for the bending assessment?

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

 

[BigInch]

We're back on the same page. Good.