Pipeline Ovality

[brimmer]

There are 2 main methodologies in industry for calculating ovality, API & DNV.

In API 5L out-of-roundness is defined as the difference between the maximum (orminimum diameter) and the nominal diameter, expressed as a percentage of the nominal diameter. In DNV-OS-F101 and ISO 3162, out-of-roundness is defined as the difference
between the maximum and minimum diameters, expressed as a percentage of the nominal diameter. Assuming symmetric ovality, the API 5L definition gives half the percentage ovality of the ISO 3162 definition.

Given one method gives half the value of the other, which methodology would you use to assess ovality in steel pipelines and why? Looking for any background / insight in experiences and why these 2 methods are different? Does acceptance criteria generally differ then when using one method or the other?

Thanks.

 

[rconner]

You may wish to draw yourself a nominal (tape) diameter circle, then for the sake of argument deflect it into say an ellipse, with the the same perimeter but a vertical dimension reduced 1% from the true nominal circle. Then very carefully apply the different definitions from the standards you reference to measuremnents of your deflected figure. [I don't think you will find "the API 5L definition gives half the percentage ovality of the ISO 3162 definition" as your inquiry states, but instead I believe you will get basically the same result.]

Yet another standard ISO 2531 for ductile iron pipe defines “out-of-roundness” I suspect to avoid confusion of words instead with an algebraic expression and also as a percentage:

100%(A1-A2)/(A1+A2)

Where A1 and A2 are the maximum and minimum measured diameters. Again I believe you will get very similar result, with yet this third way of expression!

All this sort of makes sense to me, and while these standards are expressed differently, all would appear to me to basically give similar result!

 

[brimmer]

Hmmmm. Lets use some numbers, say 6 inch pipe (Dnominal = 168.3mm). Symetrical ovality, Dmax 170.3mm, Dmin 166.3mm.

API 100% Dmax – Dmin / Dmax + Dmin
= 1.2% with this method

ISO 100% Dmax – Dmin / Dnom
= 2.4% with this method

 

[rconner]

For whatever it is worth (maybe not much!), per the API definition/words you provided, and your example:

I get Dmax 170.3 mm - Dnom 168.3 / Dnom 168.3 = ~0.012 or 1.2%

 

[rconner]

I guess I stand corrected - I would be curious, however, if you could restate here for this forum the precise layout of the words, of the actual section, talking about this requirement in ISO 3162 (it is some odd that there would be two markedly different determinations for the exactly same term in two differnt ISO pipe standards, and I don't have the 3162 standard).

 

[BigInch]

Which one to use? If your pipe was ordered to API 5L, use the API definition. If you are doing calculations according to DNV, F 101 use the DNV definition. From the mechanics of material and structural perspectives, the only thing important is that you can calculate the moment of inertia and the radius of gyration using the information you have and arrive at the critical axial buckling load, or collapse pressure. The method you're using should specifically state what geometry it is based on.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[waterpipe]

Brimmer,

API 5L defines out-of-roundness as:
"the out-of-roundness shall be determined as the difference between the largest outside diameter and the smallest outside diameter, as measured in the same cross-sectional plane."

The unit is distance (mm) and the tolerances shown in API 5L are also in mm given such as 0.015D
there is also no mention about the word "ovality" in API 5L.

I don't have the DNV but I can not understand the comparison of API 5L (mm) and DNV (%). Where does the API formula in your calculations come from?

At any rate, what we use as the pipe vertical deflection (ovality) in water sector is:

percent deflection= (change in diameter)/(nominal diameter) x 100