6.2.3

Coupling Procedures for Joining Casing

Only a limited number of methods are available for joining lengths of casing or casing

and screen together. The joining method depends on the type of casing and type of casing

joint. Flush joint, threaded flush joint, plain square end, and bell end casing joints are typical

of joints available for plastic casing; threaded flush joint, bell end, and plain square end

casing joints are typical of joints available for metallic casing.

Metallic Casing Joining

There are generally two options available for joining metallic well casings: 1) welding

via application of heat, or 2) threaded joints. Both methods produce a casing string with a

relatively smooth inner and outer diameter. With welding, it is possible to produce joints that

are as strong or stronger than the casing, thereby enhancing the tensile strength of the casing

string. The disadvantages of welding include: 1) greater assembly time, 2) difficulty in

properly welding casing in the vertical position, 3) enhancement of corrosion potential in the

vicinity of the weld, and 4) danger of ignition of potentially explosive gases that may be

present.

Because of the disadvantages of welding, it is recommended that threaded joints be

used with metallic casing and screen. Threaded joints provide inexpensive, fast, and

convenient connections and greatly reduce potential problems with chemical resistance or

interference (due to corrosion) and explosive potential. Wrapping the male threads with

fluoropolymer tape prior to joining sections improves the watertightness of the joint. One

disadvantage to using threaded joints is that the tensile strength of the casing string is reduced

to approximately 70 percent of the casing strength. This reduction in strength does not

usually pose a problem because strength requirements for small diameter wells (such as

typical monitoring wells) are not as critical and because metallic casing has a high initial

tensile strength.

Thermoplastic and Fluoropolymer Casing Joining

The most common method of mechanical joining of thermoplastic and fluoropolymer

casing and screen is by threaded connections. Molded and machined threads are available in

a variety of thread configurations including: acme, buttress, standard pipe thread, and square

threads. Because most manufacturers have their own thread type, threaded casing may not be

compatible between manufacturers. If the threads do not match and a joint is made, the joint

can fail or leak either during or after casing installation.

Casing with threads machined or molded directly onto the pipe (without use of

larger diameter couplings) provides a flush joint between inner and outer diameters. Because

the annular space is frequently minimal, casings that do not use couplings are best suited for

use in monitoring well construction. Joints should create a uniform inner and outer casing

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