Maximum Hookload When Two Grades Of Drill Pipe Are Used

Maximum Hookload When Two Grades Of Drill Pipe Are Used

When two grades of drill pipe are used, the higher grade (i.e. the pipe with
the higher load capacity) is placed above the lower grade pipe. The
maximum tension to which the top joint can be subjected is based on the
yield strength of the higher grade of pipe. Calculations similar to those
already dealt with may be used to determine the maximum length of both
grades of pipe.
Another consideration is the maximum hookload which can be applied
when only a few stands of the higher grade pipe have been added. Provided
the higher grade pipe is in the vertical section, maximum hookload (pickup
load) is calculated as the yield strength of the lower grade of pipe PLUS
the “air weight” of the higher grade pipe. This is because the surface
hookload includes the weight of the higher grade pipe; but that weight
(since it is supported from the surface) does not act on the top joint of
lower grade pipe.
Maximum Hookload = Yield Strength + Weight
Of Lower Grade Pipe Of Higher Grade Pipe
When a sufficient length of higher grade pipe has been added, the limiting
condition will become the yield strength of the higher grade pipe.
The air weight of the higher grade pipe is used because the buoy force
acting on the drillstring is acting on the bit and components of the BHA.
The hydrostatic pressure which the mud exerts on the drill pipe in the upper
(vertical) section of the hole does not create a resultant force acting
upwards.