A Guide To Water Well Casing and Screen Selection   -   10

5.2  Wall Thickness

A more difficult dimensional selection of well casing is wall thickness. As mentioned earlier, certain stresses such as the external hydrostatic force (draw-down) can be calculated, but others cannot. Fortunately, many years field experience under varied conditions provides useful information and guidance.

The tensile (ultimate) strength of a metal is defined as the load in the direction of pulling it apart which is required to break it. Compression or crushing strength is the load in the opposite direction (pushing it together) required to deform it. This value is equivalent to the yield strength.

Steel strengths vary according to chemistry and manufacturing techniques. Pipe tensile and compression strengths are directly related to the tensile and yield strengths of the parent material as well as its dimensions. Generally, wall thickness is the critical factor to consider.

The resistance of steel tubes to collapse under hydrostatic or similar squeezing forces is determined by outside diameter, wall thickness and ellipticity (out of roundness). In addition to these dimensional parameters, collapsing strength is influenced by some of the physical qualities of the material. These are Young's modulus which is a measure of material stiffness, yield strength and Poisson's ration. While several agencies, including the American Petroleum Institute and the national Aeronautics and Space Administration, have developed formulas relating these parameters to collapsing strength, the values shown in Table B, page 70,, are based upon Timoshenko's formula using 1% as a value of ellipticity and 35,000 psi as the steel yield point. Use of this formula provides conservative values which are justified, considering the critical nature of collapsing strength in water wells. An analysis of Timoshenko's formula shows that casing continuously deforms with increasing pressure until the critical pressure is reached. The casing is considered to be collapsed once deformation continues without additional pressure.

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