No discussion of water well casings and screens can be considered complete without giving proper attention to the connections through which they are joined in the field. The tensile strength of any column is limited by the strength of the connections between its components. Observation of well failures shows that most involve casing or screen rupture, collapse or deformation. Frequently the problem originates in the connecting joints.
In addition to mechanical strength requirements, factors that should be considered in connecting joint design include smoothness of internal wall, minimization of external diameter, alignment, ease of installation and economy.
The four major types of connections used in the water well industry are threaded and coupled joints, joints with square or beveled plain-cut ends, bell and spigot joints for lap welding, and joints with welding collars for lap welding.
Threaded and CoupledThreaded and coupled connections commonly are employed in 4-inch and smaller diameter wells where they provide relatively inexpensive, fast and convenient connections. Strength requirements in such domestic low production wells are not critical. In larger diameter, the cost of threaded and coupled joints increases, and they generally are not available in sizes larger than 12 inches.
Plain EndsCasing and screen joints prepared with square ends for welding generally are satisfactory up to 0.1875-inch wall thickness. With heavier wall thicknesses, the ends should be beveled to facilitate weld penetration, leaving approximately 0.125 inch flat. Advantages of these connections are economy and smoothness of the external diameter, which minimizes the tendency of gravel to bridge in gravel-envelope wells. Disadvantages lie in greater assembly time and the difficulty of properly welding casing in the vertical position. A further problem occurs if removal and reassembly is required. The connection must be cut with a torch as withdrawn, and the joint prepared for proper reassembly by machining, if possible – a time-consuming and expensive process.
Bell and Spigot EndUse of bell and spigot joints overcomes a number of problems inherent in plain-ended or threaded and coupled joints. A downhand filet (lap) weld is used to connect the casing and screen sections. Lap welds are easier to make in the vertical position than horizontal butt welds. These joints also are very economical. Their chief disadvantage concerns proper alignment, which requires more installation time.
Welding CollarsAnother lap weld connection type that best meets all requirements for 6-inch and larger casing and screen is the welding collar. Welding collars are factory-installed on one end of the joint. Width of the collar ranges from 2 inches to 6 inches, with the casing end extending approximately midway through the length of the collar. A properly made welding collar connection is as strong – or stronger – than the casing. Removal of casing or screen sections requires only removal of the field weld at the top of the collar. Such sections are easily re-installed, because the original faces of the tubes have not changed. Finally, transportation and handling damage are reduced. The importance of having a field connection equal in strength to the casing or screen material cannot be overemphasized. Experience has demonstrated this frequently to unwary drilling contractors.
Stainless to Carbon ConnectionsSome concern has been expressed regarding the connection of stainless steel material to regular carbon steel in the field. A common thought is that galvanic action between the two dissimilar metals will cause the “less noble” carbon steel to deteriorate rapidly and fail. While galvanic action does take place initially, the carbon steel rusts and polarizes rapidly, effectively inhibiting further deterioration. If stainless steel is welded directly to carbon steel, the carbon steel section should be at least two times the thickness of the stainless section. Because threaded couplings usually are heavy, no difficulty results when stainless and carbon steel threaded connections are employed together.
Compression SectionsIn many arid and semi-arid regions of the world, a growing cause of well failure is the rupture of casing from subsidence of the ground because of depletion of water from the surrounding aquifer systems. Where the drop in artesian head greatly exceeds the water table decline, stresses are developed by the resulting hydraulic gradient that often are sufficient to cause one or more breaks in the well casing and screen. These breaks occur as the casing shortens by rupture. The broken sections usually divide into segments that slip past each other to produce a telescoping compression break. Experience has shown that employing stronger, heavier wall casing cannot prevent such breaks and deformations. Use of special telescoping joints – known as compression sections – installed in the casing and screen string, has solved this problem in many cases.
Compression sections are composed of three 6-foot lengths of casing, two of which are the same diameter and wall thickness as the well casing. These two joints are equipped with beveled steel rings welded to the lower end of the upper section and upper end of the lower section. Thus, the joints are free to move or telescope within the length of an outer section of shell, which is similarly equipped with rings at each end acting as stops and stabilizers. This shell usually is 2 inches larger than the parent casing. The rings are manufactured from 1⁄2-inch-by-2-inch steel, and beveled to a 45-degree angle.
Location of compression sections should be given some study, as compression failures have been known to occur in intervals between the joints. The user is advised to check the history of the area. Sections should be located at the depth or formation where breaks in surrounding wells have been obtained by locating a compression section at the bottom of the pump housing casing. If the well exceeds 1,500 feet in depth, another section may be placed in the middle of the screen string.
Clay is able to exert greater compressive force than coarser sediments, particularly in strata thicker than 8 feet. This should also be considered when locating compression sections.
Landing ClampsWhen casing and screen is installed in an open hole to be gravel-packed, it should be suspended from the surface by a heavy-duty clamp. This clamp may be supported at the ground surface by beams, or it may rest on – or be notched into – the surface protective casing. Surface protective casing, in turn, must be supported by beams or grouted in place. The purpose of suspending the casing and screen is to ensure that it remains straight and centered in the borehole. If a string rests on the bottom, it bows, preventing proper pack installation.
Casing GuidesCasing guides are used to center the screen within the borehole. They should be of sufficient strength and surface area to provide support, yet not impede the installation of gravel. These are conflicting requirements, and some compromise is required. A simple guide, which has proven effective, is manufactured from 5⁄16-inch-by-2 inch steel, 30 inches long, bent to provide the proper centering distance. The guides are attached to the screen by welding. Three or four guides are placed equidistantly around the screen at 40-foot intervals. Normally, they are not installed on the upper or pump housing casing.
Float PlatesFloat plates may be installed in casing strings, where the weight of the casing and screen exceed the safe lifting capacity of the installing rig. The plate, which must be manufactured from a frangible or breakable material, is installed between two joints of pump housing casing at a predetermined depth, where the collapsing strength of the casing is not exceed and the rig is not overloaded. The weight of the casing is reduced by the weight of the fluid displaced. Cast iron plates – machined to provide a watertight fit between the casing joints – have been found satisfactory.
The use of welding collars simplifies the installation of float plates considerably. To ensure that hydrostatic forces on the empty casing above the float plate do not exceed its collapsing strength, the casing may be partially filled with water during installation as the buoyancy increases. Once the casing and screen is installed, the upper pump-housing casing is completely filled with water and the plate removed by striking with a bailer, drill pipe or tubing. Float plates must be used with great caution, and under no circumstances should they be considered as safe a procedure as direct installation with equipment of adequate weight-bearing capacity.
Diameter ReductionsOccasionally, reductions are made in diameter between casing and/or screen sections. Regular bell pipe reducers have been used, but they are expensive, and the reduction is rather abrupt. Reduction cones have been fabricated from pipe by torch-cutting segments from the pipe, bending the remaining segments inward, and welding the new seams together (orange peeling). This is a fairly expensive operation and results in a non-uniform, frequently weak structure. A better solution consists of the use of a fabricated tapered cone with a short stub joint of each diameter casing welded at each end. These stubs may be machined for greater alignment accuracy.
While no supporting data exists, it has been suggested that the length of the cone of the reducing section should be at least 10 times the difference in diameter of the two ends for hydraulic efficiency and strength. It may be added that a longer tapered section or cone mitigates bridging at that point when gravel is introduced from the surface.
Bottom PlugsA bull nose or plug always is attached to the bottom of casing and screen strings installed in gravel envelope wells. These may be fabricated by orange peeling a short joint of casing to a 1-foot to 3-foot taper, depending on diameter. Semi-elliptical tank ends – readily available and inexpensive – provide a convenient fulfillment of this requirement.
This article is provided through the courtesy of Roscoe Moss Co., a leading manufacturer of water well casing and screen products.