A tapered string contains more than one OD, tooljoint, weight, grade, or levels of wear.
The problem with tapered strings is ascertaining how much weight can be placed on the weight indicator without failing any one of the sections of the tapered string.
COMBINED TENSION, TORSION, BENDING, & PRESSURE LOADS
Combined tension, torsion, bending, and pressure loads are frequently placed on drillpipe. The normal drilling operation of making hole does this. However, the damaging combined loads will most likely occur while fishing. The tension is created by the hook pulling on the drillpipe; the torsion arises while twisting the pipe with either tongs or the rotary table; the bending is derived either from doglegs in the drill hole or the misalignment of the crown block with the rotary table; and pressures are created by surface pressures or different fluid weights on the inside and the outside of the drillpipe.
Vonr Mises triaxial equation depicts the combined stress. This stress is also known as general stress. If failure is forecasted by the equation it is known as general failure, because the mode of failure is not forecasted. For example if drillpipe is subjected to tension and pressure simultaneously and it fails, it would not be possible to ascertain from Von Mises equation whether it was the tension or the pressure which caused the failure.
The fundamental approach to drillpipe selection is to compute all the loads separately to determine if the strength of the drillpipe is exceeded by the individual loads and then combine the individual loads with Von Mises equation to ascertain if general failure is likely.
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