Pipeline buoyancy

[Gene1964]

When determining if a pipe will float in seawater, can you simply compare the weight of the pipe with the weight of the displaced water (Archimededs Principle)? I am convinced that 4" sch.80 (.337" wall) will not float but some formulas for buoyancy that I have found do not agree with the simple weight comparison. (Ex: D^2 * .35 - pipe weight)

While doing this exercise, I also noticed that if you calculate steel density from the pipe weight and volume (calculated) listed in a standard pipe data table, the densities vary from 464 lbs/ft to 416 lbs/ft for 4" sch 40-160. I would think that the density would remain constant. Anybody know a reason for this?

 

[Ussuri]

Yes, the Archimedes Principle is applicable. The submerged weight of the pipe is equal to the weight of the pipe in air minus the weight of water displaced by the pipe.

Can you tell me where you found these other equations, I would be interested at looking at them.

I also had a check on your calculated density problem (using SI units, just coz I can) and for a 4" SCH 40 got a density of 7886kg/m3, for a 4" SCH 80 got a density of 7808kg/m3 and for a 4" SCH 160 got a density of 7836kg/m3. That is a variance of less than 1% which I would say is probably due to rounding. So all in all they tie in reasonably closely.

 

[StephenA]

Two things to remember.

Does the pipe have a coating? This affects the weight and the buoyancy. A lining only affects the weight of course.

Also the density of seawater varies with salinity and temperature.

The weight/ metre (ft) published may vary due to the tolerance on wall thickness. Some companies may use maximum wall thickness and others the mean wall thickness.

If it is critical (for surface towed pipelines for example), we usually weigh a percentage of pipes and measure their girths and then compare with the mill certificates.

 

[Gene1964]

Ussuri: I got the equations from "Pipeline Rules of Thumb" 5th Edition by E.W. McAllister, p. 104.

 

[Ussuri]

Gene1964

I had a look at the equations you mentioned and they look correct to me. Just a different way round.

I look at like this:

Pipe weight (in lb/ft) is your weight in air - [1];

The force due to submersion is ((pi * OD^2)/4) * water density (62.4lb/ft^3) -[2];

The submerged weight is then [1] - [2]. If this is greater than 0 the pipe will not float.

McAllister looks at it the other way round. He does [2] - [1], so if this is less than 0 there is no buoyant force.

The equation 0.341D^2 is just a simplification of constants, including water density, and altering the units of D to inches.

e.g.

(pi/4)*62.4*(1/(12^2))=0.34