Buoyancy & Hookload - Overpull

Overpull

In tight holes or stuck pipe situations, the operator must know how much
additional tension, or pull, can be applied to the string before exceeding the
yield strength of the drill pipe. This is known as Overpull, since it is the
pull force over the weight of the string. For example, in a vertical hole with
12 ppg mud, a drillstring consists of 600 feet of 7.25-inch x 2.25-inch drill
collars and 6,000 ft of 5-inch, New Grade E drill pipe with a nominal
weight of 19.5 lbs/ft and an approximate weight of 20.89 lbs/ft.
First, the hookload is determined
Hookload = Air Weight x Buoyancy Factor
= [(6,000 x 20.89) + (600 x 127)] 0.817
= 164,658 pounds
Referring to the API RP 7G, the yield strength in pounds for this grade,
class, size and nominal weight of drill pipe is 395,595 pounds. Therefore:
Maximum Overpull = Yield Strength In Pounds - Hookload
= 395,595 - 164,658
= 230,937 pounds
The operator can pull 230,937 pounds over the hookload before reaching
the limit of elastic deformation (yield strength). Obviously, as depth
increases, hookload increases, at a certain depth the hookload will equal
the yield strength (in pounds) for the drill pipe in use. This depth can be
thought of as the maximum depth that can be reached without causing
permanent elongation of the drill pipe (disregarding hole drag as a
consideration). Practically, an operator would never intend to reach this
limit. A considerable safety factor is always included to allow for overpull
caused by expected hole drag, tight hole conditions or a stuck drillstring.
In practice, selection of the drill pipe grade is based upon predicted values
of pick-up load. For a directional well, the prediction of pick-up load is
best obtained using a Torque and Drag program, as well as including the
capacity for overpull. Some operators include an additional safety factor by
basing their calculations on 90% of the yield strength values quoted in API
RP7G.