STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

766 Drilling and Well Completions
very deep drilling) that has sufficient strength to successfully accomplish
expected goals. Having in mind that the drill string is subjected to many loads
that may exist as static loads, cycling loads and dynamic loads, the problem of
drill string design is complex. Due to the complexity of the problems, some
simplifications are always made and, therefore, several decisions are left up to
the person responsible for the design.
In general, a reasonably bad working condition should be assumed and, for
that reason, a good knowledge of expected problems much as hole drag,
torquing, risk of becoming stuck, tendency to drill a crooked hole, vibrations,
etc., is of critical importance.
The person responsible for the design must know drill string performance
properties, data from wells already drilled in the nearest vicinity and current
prices of the drill string elements.
The designer should simultaneously consider the following main conditions:
1. The working load at any part of the string must be less or equal to the
load capacity of the drill string member under consideration divided by
the safety factor.
2. Ratio of section moduli of individual string members should be less than 5.5.
3. To minimize pressure losses, the ratio of drill pipe outside diameter to
borehole diameter, whenever possible, should be about 0.6.
Normally, based on hole diameter, the designer can select drill collar diameter
and drill pipe diameter. Next, specific pipe is chosen; the maximum length of
that pipe must be determined based on condition 1. For this purpose, the
following equation is used:
(4-71)
where Ldc = length of drill collar string in ft
Wdp = unit weight of drill collar in air in Ib/ft
Lhw = length of heavyweight drill pipe (if used in the string) in ft
Whw = unit weight of heavy-weight drill pipe in lb/ft
Ldp, = length of drill pipe under consideration above the heavy-weight drill
pipe in ft
Wdp, = unit weight of drill pipe (section 1) in lb/ft
K, = buoyant factor
PI = tension load capacity of drill pipe (section 1) in lb
SF = safety factor
Solving Equation 4-71 for Ldp, yields
(4-72)
If the sum of Ldc + Lhw + Ldp, is less than the planned borehole depth, the
stronger pipe must be selected or a heavier pipe must be used in the upper
part of the hole.
The maximum length of the upper part in a tapered string may be calculated
from Equation 4-73: