A Guide To Water Well Casing and Screen Selection   -   32

9.3  APPENDIX III

Analysis of Driving Casing with Cable Tool Equipment

Unfortunately water well drilling history is filled with examples of casing that has failed under the stress on installation by driving. Driving is accomplished through the impact of the weight of the drilling tools on the casing. The top of the casing is equipped with a drive head which acts as an anvil. Clamps are attached to the tool string to serve as a hammer face. Blows may be struck singly through raising or dropping the tool string under the control of the operator, or repeatedly by using the motion of the walking beam.

Since friction restraining the casing usually originates at the bottom or shoe, it appears as though driving would not create stresses that would cause the casing to separate, either in the parent tube or at the joints. Observation suggests that driving forces are compressive forces which would tend to hold the casing together. This is not the case, however, and an analysis of what actually occurs to a casing string under these stresses may help to avoid operating conditions which cause failure.

When a casing that is free at both ends is struck at one end, a compressive pulse travels downward. As this pulse travels, it sets in motion the particles at any point in its travel. This motion continues in the direction of the pulse. When the pulse reaches the end of the casing (shoe), it returns in the opposite direction, but changes to a tensile or pulling apart force rather than a compressive one. This process is repeated until the pulses are dampened by formation material surrounding the casing.

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