There are many situations where pipe can float. Sometimes this is intentional, as in a floating dock built on sealed sections of plastic pipe. At other times, this is unintentional and can cause serious problems. For example, imagine a large pipe that is laid along ground normally above water level. If the water level rises due to an unexpected flood and submerges the pipe, it will be subject to a large buoyancy force. If this force was not taken into account in the design of the pipe supports, the pipe can move and rupture.
Calculate the pipe weight. Flotation is the result of a balance of forces acting on the pipe. Pipe weight is counteracted by buoyancy force. The formula for pipe weight per foot of pipe is:
Wp = 3.14/4 * (D^2 - d^2) * Rp
where Wp is the pipe weight in pounds per foot of pipe.
D is the outside diameter of pipe in feet.
d is the inside diameter of pipe in feet.
Rp is the density of pipe material in pounds per cubic feet.
^2 indicates the square of the number. D^2 = D * D.
Calculate the buoyancy force. The formula for buoyancy force is:
Wb = 3.14/4 * D^2 * Rf
where Wb is the buoyancy force in pounds per foot of pipe.
Rf is the density of the fluid surrounding the pipe in pounds per cubic feet.
Calculate if flotation can occur. Subtract the buoyancy force from the pipe weight. If the result is greater than zero, the pipe will not tend to float because its weight is larger than the buoyancy force trying to lift it. If the result is less than zero, the pipe will tend to float. If the pipe is not intended to float, the supports must be designed to withstand this net lifting force.
For example, say you have a length of 20-inch diameter steel pipe sealed at both ends, and it is lowered into a container of water. Will it float? Assuming the water is at 15.6 degrees Celsius, Rf is 283 Kilogram per cubic foot. From the references and the equations given:
D = 20.0 inches, which equals 1.67 feet.
d = 18.8 inches, which equals 1.57 feet for standard wall thickness.
Rp = 222 Kilogram per cubic foot, for carbon steel.
Wp = 56.5 Kilogram per foot of pipe.
Wb = 620 Kilogram per foot of pipe.
Wp - Wb = -54.4 Kilogram per foot of pipe.
The result is less than zero, so the pipe will float.
Buried pipe can also float if the fill material becomes flooded. Additional calculations are needed to account for the weight of material above the pipe. If the pipe is filled with a fluid with significant weight, such as water, it must be accounted for as well.
These simplified calculations are intended for illustrative purposes only. A professional engineer should be contacted to evaluate actual situations.