STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

where DF =
W=
Wd‘ =
%=
K, =
r, =
r,, =
Drill String: Composition and Design 721
design factor (DF = 1.1-1.2)
weight on bit in lb
unit weight of drill collar in air in lb/ft
buoyancy factor
1 - Ym/Ysc
drilling fluid density, e.g., in lb/gal
drill collar density, e.g., in Ib/gal (for steel, y, = 65.5 lb/gal)
a= hole inclination from vertical in degrees (”)
Design factor (DF) is needed to place the neutral point below the top of the
drill collar string. Some excess of drill collar weight is required to take care of
inaccurate handling of the brake by the driller. For an “ideal driller,” the design
factor should be equal to 1. The excess of drill collars also helps to prevent
transverse movement of drill pipe due to the effect of centrifugal force. While
the drill string rotates, a centrifugal force is generated that may produce a lateral
movement of drill pipe and, in turn, bending stress and excessive torque. The
centrifugal force also contributes to vibration of the drill pie. Hence, some
excess of drill collars is suggested. The magnitude of the design factor can be
determined by field experiments in any particular set of drilling conditions.
The pressure area method (PAM), occasionally used for evaluation of drill
collar string length, is wrong because it does not consider the triaxial state of
stresses that actually occurs. It must be remembered that hydrostatic forces
cannot cause any buckling of the drill string as long as the density of the string
is greater than the density of the drilling fluid.
Example
Determine the required length of 7 by 2t-in. drill collars if desired weight
on bit is W = 40,000 lb, drilling fluid density y, = 100 lb/gal, and hole deviation
from vertical a = 20”. From offset wells, it is known that a design factor DF of
1.1 is satisfactory.
Solution
From Table 4-75 the unit weight of drill collar WdC = 117 lb/ft. The buoyant
factor is
10
K, = 1 - - = 0.847
65.5
Applying Equation 4-50 gives
(1‘1)(40,000) = 376ft
L,, =
(147)(0.847)(cos20)
The closed length, based on 30-ft collars, is 390 ft or 13 joints of drill collars.
Actually, drill collar sizes and lengths should be considered simultaneously.
The optimal selection should result in the maximum penetration rate. Such an
approach, however complex, is particularly important when drilling formations
sensitive to the effect of differential pressure and also in cases where the amount
of hydraulic energy delivered at the rock bit is a controlling factor of drilling
efficiency.