Overreliance on Material Standards for Well Design

There are times when they can lead to potential pitfalls, so testing in the field is always needed.

By Marvin F. Glotfelty, RG

While preparing technical specifications for the drilling and construction of a water well, well designers
commonly reference national standards that define the minimum characteristics for materials and procedures to be used or implemented.

These standards have been developed by committees of qualified groundwater professionals with established levels of technical knowledge and reliability. As you may know, the commonly cited national standards used for water well design include standards published by the American Water Works Association (AWWA), the American Society for Testing and Materials (ASTM), and the American Petroleum Institute (API).

For drinking water wells, construction material and chemical additives are also required to be certified by the National Sanitation Foundation/American National Standards Institute (NSF/ANSI) for treatment chemicals (NSF/ANSI 60) or distribution systems (NSF/ANSI 61), as applicable.

Well designs must also comply with relevant national regulations, such as the U.S. Environmental Protection Agency’s guidelines under the Safe Drinking Water Act, state-specific regulations, and local ordinances.

Citation of national standards makes the preparation of technical specifications less burdensome and often provides well designers with a degree of confidence that the intended details of the well design have been addressed.

However, overreliance on material standards can introduce potential pitfalls with the well design. There are nuances within some commonly cited material standards that may diverge from what was intended in the well design, so consideration of the variability within some standards is worthwhile.

The materials used in water well construction generally consist of well casing, well screen, filter pack material, and annular seal material. AWWA Standard A100 recommends site-specific design for some of these materials:

• For filter pack material: “No two vertical water supply wells are exactly the same. Therefore, filter pack materials and methods of placement must be selected on a site-specific basis.”
  – AWWA A100, Section 4.3.6.11.
• For annular seals: “The listed materials are commonly used for sealing wells. The purchaser must select sealing material on a site-specific basis.”
  – AWWA A100, Section 4.3.7.
• For well casing: “Selection of casing wall thickness merits analysis and judgment by experienced, qualified engineers and drilling experts. Actual wall thickness should, in each instance, be based on an analysis of the anticipated stresses to which the casing will be subjected during each phase of construction and any pertinent state or local requirements.”
  – AWWA A100, Section 4.4.5.

AWWA Standard A100 also provides cross-references to standards ANSI/AWWA C200, API 5L, ASTM A53 Grade B, and ASTM A139 Grade B for steel well casing materials.

Variability in Well Casing Wall Thickness

Figure 1. Field verification of well casing characteristics. Measurements are made of the casing diameter (upper left), casing wall thickness (upper right), and casing joint lengths (bottom). Photos courtesy Geo-Logic Associates.

Let’s focus on steel well casing standards because contained therein are some of the pitfalls that may result from excessive dependence on published standards rather than site-specific well design as recommended by AWWA A100.

Of the four common standards for single-ply carbon steel well casing (ANSI/AWWA C200, API 5L, ASTM A53, and ASTM A139), only AWWA C200 requires the casing wall thickness to comply precisely with the wall thickness that has been specified. The three other standards allow variability in the wall thickness.

The casing wall thickness is permitted to be as much as 12.5% less than the specified amount in ASTM Standard A53 (Section 10.3), in ASTM Standard A139 (Section 11.2), and in API Standard 5L.

The 12.5% tolerance for the wall thickness of steel casing is not new to these standards and has been in place in some of them for decades (at least as far back as the late 1960s, and possibly earlier). The 12.5% tolerance for casing wall thickness reportedly results from practical limitations during the manufacturer’s
process of rolling and welding the steel casing. Thus, there will be some variability in the casing wall thickness from place to place, but the minimum wall thickness at any point will not drop below 12.5% less than the specified value.

That uncertainty in the specified casing wall thickness begs this question: Does a 12.5% reduction in the wall thickness make any difference during the construction and operation of a water well?

The primary reason that a particular wall thickness was specified is to account for the tensile strength and collapse strength that the casing will require during the construction and operation of the well. The casing wall thickness needs to handle the tensile load that the well casing will be subjected to due to the weight of the casing/screen that is suspended below that section of casing. Similarly, the casing wall thickness
needs to provide adequate collapse strength to withstand the compressive stresses it will be subjected to during installation of annular cement grout seals or other well construction/development activities.

Figure 2. Field verification of well screen slot size, using a spark plug gap gauge. Photos courtesy Geo-Logic Associates.

To calculate the collapse strength of a steel well casing, we use Timoshenko’s Formula, which is generally accepted to be the most accurate method for estimating the collapse strength of a well casing. Timoshenko’s Formula is based on Young’s Modulus for steel (3×107), Poisson’s Ratio (0.3), the outside
diameter of casing (inches), the casing wall thickness (inches), the yield strength of steel, and the ellipticity of the casing (assumed to be 0.01).

Online calculators are available on casing suppliers’ websites from which the Timoshenko’s Formula can be
performed.

So, we can apply Timoshenko’s mathematical formula to calculate the collapse strength for a series of typical well casings with the specified wall thicknesses and also with a 12.5% reduction in those wall thicknesses. The results of that exercise are shown in Table 1.

We considered steel well casings for smaller wells with 6-inch, 8-inch, and 10-inch diameters that had specified wall thicknesses of 3/16-inch (0.188-inch) or 1/4-inch (0.250-inch). We also considered steel well casings for larger wells with 14-inch, 16-inch, and 18-inch diameters that had specified wall thicknesses of either 5/16-inch (0.3125-inch) or 3/8-inch (0/375-inch). The steel type for all these wells was assumed to
be low-carbon steel (ASTM A53 Grade B), which has a yield strength of 35,000 psi. We also assumed the ellipticity (outof- roundness) of each casing to be 1%.

The calculated collapse strengths of the casing examples in Table 1 vary quite a bit due to the differences in casing diameter and wall thickness of the various well casings. However, we note that when the 12.5% reduction of wall thickness is applied, the drop in collapse strength is between 66 psi and 342 psi, which represents a drop in the casing collapse strength by 22% to 29%. Depending on the site-specific conditions
of the well installation project, those levels of reduced collapse strength can be problematic.

Although the 12.5% tolerance for casing wall thickness in the various standards is a manifestation of the practical manufacturing methods used for steel casing, my experience has been that the actual casing wall thickness provided by suppliers is generally very close to the specified value. However, the wall thickness uncertainties are stated in these standards for a reason, so verification of the casing wall thickness (as well as other physical and metallurgical attributes of the casing) is prudent.

Field Verification of Well Construction Materials

Whether or not the well design specifications include a reference to national standards, it is always a good idea to verify that the materials delivered to the drilling site are consistent with what has been specified. Field validation of the materials should be done at the work site after those materials have been delivered and unloaded, so that any items that may have been damaged during transportation will be identified and set aside.

Field verification should be conducted with all well construction materials, and that verification should be done with accurate measurement devices while both the hydrogeologist and driller representatives are present.

For steel well casing, the drilling contractor generally receives mill certification documentation from the casing supplier, which indicates compliance with any national standards (e.g., ASTM A53 Grade B), and the associated chemical composition of the metal that’s been delivered to the site. The physical attributes of the well casing (casing length, diameter, wall thickness) can be measured on location (Figure 1) and documented in field verification notes.

For wall thickness measurements, it’s important to keep in mind that the caliper devices used to verify the casing wall thickness can only be used at the end of the casing joints. Therefore, well designers should include a safety factor to account for the possibility of slightly thinner-walled casing in local areas that cannot be readily measured, so that the intended tensile and collapse strengths of the overall well will be met.

Other measurements are sometimes needed and can also be performed at the drill site. For example, confirmation measurement of the specified well screen slot size can be achieved with the use of spark plug gap gauges (Figure 2) to verify that the actual well screen delivered to the site complies with what has been specified.

Conducting field verification of well construction materials and documenting those field test results will provide confidence that the well was installed in compliance with the intended design.

Such field tests also provide confirmation that the materials were delivered to the site undamaged, and in the amounts required for the well project. These verifications cannot be provided by citation of national standards alone.

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Marvin F. Glotfelty, RG, is the principal hydrogeologist for Clear Creek Associates, a Geo-Logic Associates Co. He is a licensed well driller and registered professional geologist in Arizona, where he has practiced water resources consulting for more than 35 years. He is author of The Art of Water Wells (NGWA Press, 2019) and was the Groundwater Foundation’s 2012 McEllhiney Lecturer. Glotfelty can be reached at mglotfelty@geo-logic.com.