# How to calculate buoyancy for a pipeline

Written by melissa sandoval
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When laying pipeline in areas that may become inundated, such as areas with a high water table, you need to make sure it will not float. Otherwise, heavy rains can make the pipeline become buoyant, causing breakage. The factors that determine whether a pipe will float are the weight of: the pipe; the water displaced by the pipe; the backfill; the liquid inside the pipe. For maximum safety, assume that the pipeline will be empty when you calculate buoyancy so it will not float regardless of how little fluid is inside it at the time.

Skill level:
Moderate

### Things you need

• Manufacturer's product data sheet for pipe material
• Calculator

## Instructions

1. 1

Obtain the pipe's weight per linear foot from the manufacturer's product data.

2. 2

Investigate local conditions of the project area to learn what type of water will be near the pipeline. If unknown, assume fresh water.

3. 3

Use the following values for water density: 28.3kg. per cubic foot for fresh water and 29kg. per cubic foot for seawater. The density of brackish water falls between that of fresh and seawater.

4. 4

Find the square of the outside pipe diameter. For example, for an outside pipe diameter of 5 feet, 5 x 5 = 25.

5. 5

Divide pi by 4. For example, 3.14 ÷ 4 = 0.785. Use your calculator's ∏ button if available for greater precision.

6. 6

Multiply your result from Step 4 with your result from Step 5. For example, 25 x 0.785 = 19.625.

7. 7

Multiply your result from Step 6 by the density value for water. For example, 19.625 x 62.4 = 1,1019kg. This is the weight of water displacement per linear foot of pipe.

8. 8

Subtract your result from Step 7 from the pipe's weight per linear foot. If the resulting number is negative, the pipe will float. If the result is positive, the pipe will not float.

#### Tips and warnings

• The formula for displaced water weight is WW = (∏ ÷ 4) (BC2) 62.4, where WW is displaced water weight and BC is the outside pipe diameter in feet.
• If your calculations show that the pipe will float, calculate how much backfill you will need with the formula: wI = w - [(w ÷ (SG X 62.4)) X 62.4], where wI is the average unit weight of inundated backfill in pounds per cubit foot, w is the average unit weight of dry backfill in pounds per cubit foot, and SG is the specific gravity of the backfill material. For sandy soil, w = 110 and SG = 2.65.
• If the pipe will still float with the backfill in place, consider using a thicker pipe, switching to a heavier pipe material or add concrete collars around the pipe.
• These calculations are accurate only for circular pipe.

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