STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

Drill String: Composition and Design 761
heavy-weight drill pipe is used to create weight on the bit if, for some reason
(e.g., excessive torque and drag or differential problem sticking), a long string
of drill collars cannot be run.
The best performance of the individual members of the drill string is obtained
when the bending stress ratio of subsequent members is less than 5.5 [38].
Bending stress ratio (BSR) is defined as a ratio of the bending section moduli
of two subsequent members, e.g., between the drill collar and the pipe right
above it.
To maintain the BSR at less than 5.5, the string of drill collars must frequently
be composed of different sizes. For severe drilling conditions (hole enlargement,
corrosive environment, hard formations), reduction of the BSR to 3.5 helps to
reduce frequency of drill pipe failure.
Geometrical and mechanical properties of heavy-weight drill pipe (Hevi-Wate@)
manufactured by Drilco are given in Table 4-90.
Example
Calculate the required length of 44 in. Hevi-Wate" drill pipe for the following
conditions:
Hole size: 94 in.
Hole angle: 40"
Desired weight on bit: 40,000 lb
Drill collars: 7 x 2z in.
Length of drill collars: 330 ft
Drilling fluid specific gravity: 1.2
Desired safety factor for neutral point: 1.15
Solution
Check to see if the BSR of drill collar and Hevi-Wate") drill pipe is less
than 5.5.
Bending section modulus of drill collar is
Bending section modulus of Hevi-Wate" drill pipe is
BSR = 65*592 - 4.26 e 5.5
15.397
Unit weight of drill collar in drilling fluid is
110 1-- = 93.181b/ft
( ;.s25)