Another Question - Natural Pipe Sag

[spd60]

Hi All.

Another question for you.

Is there a rule of thumb for the natural sag that a steel pipeline can experience.

In otherwords if I have a 16" pipe what would be the minimum radius that the pipe could take as it contours to the trench bottom?

Thanks

 

[BigInch]

Natural sag, by definition, is the deflection between simply supported ends or 5 wL^4/384/E/I. A cantelivered equivalent would be wL^4/48/E/I, provided the supported end of the pipe doesn't rotate, which it probably will to some extent.

I think your question is best answered using the simple support beam formula which would cover most typical cases (assuming that rotation of the end when backfilled should basically be held to zero), hence you would want to limit deflection at center span to the 5wL^4/384/E/I value to be within the natural sag bend criteria.

http://virtualpipeline.spaces.msn.com

 

[GregLamberson]

spd60

BiugInch is correct.

Another way to look at it is take your calculated minimum radius of the bend (R) (based on dia, WT, design factor, SMYS, design press) and then use (360*30)/(2*3.1416*R), that will give you the amount of deflection in degrees per 40' joint of pipe.

For example, your 16", if it is X60, .500 and a design press of 1200 psi, your minimum radius is 575' (175m) at a design factor of .72 (assuming a gas pipeline). That would equate to 2.99 degrees deflection per 40' joint of pipe.

Greg Lamberson, BS, MBA
Consultant - Upstream Energy
Website:

http://www.oil-gas-consulting.com

 

[spd60]

Thank you both.

I thought that the beam formula would be the correct starting place but wasn't postive.

Greg,

They say there is no such thing as a stupid question so I'll go ahead and ask mine...

What forumla did you use to calculate the minimum radius for your example pipe?

Thanks

 

[GregLamberson]

spd60

Sorry, I've been on vacation, well still am sort of, but....

There are few questions that are stupid and this certainly is not one -

R = (E*D) / ((Sa)-(P*D)/(4t)), where,
R = radius in inches
E = modulous of elasticity
D = pipe diamter in inches
Sa = allowable stress (SMYS)
P = design pressure in psi
t = wall thickness in inches

This will give you R in inches, divide bt 12 to get it in feet.



Greg Lamberson, BS, MBA
Consultant - Upstream Energy
Website:

http://www.oil-gas-consulting.com