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been subject of much research and development in recent years
with the result that failures now are less frequent.
Further progress surely will be made as time passes. Notable
improvements have been in thread form and treatment, stress-relief
grooves both on pin and in box and balance in design bending
strengths of pin and box. The bore through the pin is important to
joint strength so step bore or reduced bore for a foot or more of the
pin-end of a collar will permit increased pin strength without
significantly increasing hydraulic pressure losses.
Pin and box threads serve the primary functions of pulling
and holding together their respective shoulders. The shoulders
must be of sufficient width, strength and furnish to resist
deformation when subjected to bending forces and also provide an
effective fluid seal.
The conventional drill collar has a cylindrical surface. That
is, they are round and uniform OD shoulder to shoulder. There are
many specialize signs. One variation is inclusion, below the box,
of a slight recess on the OD to provide a shoulder for elevators,
thus making lifting subs unnecessary A second recess further down
can be provided for safe use of slips, thus making use of a safety
clamp unnecessary.
A second variation is spiral grooving of outer surface, except
near the connections. These spiral grooves, usually three in
number spaced 120° apart prove effective in prevention of wall
sticking caused by fluid pressure differential when collars are
opposite permeable zones.
A third variation is the square drill collar. Normally only one
square drill collar is used just above the bit when drilling in
crooked hole formations. Its purpose is to reduce rapid change in
hole angle. The square drill collar becomes a long stabilizer fitting
the hole closely yet allowing free circulation between the flats of
the square and the round walls of the hole. The square collar
assures a smooth hole free of doglegs. Usually the distance across
corners of the square is 1/16-in. less than bit size Frequently a

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