The normal drill string design practice aim is to avoid abrupt changes in component cross sectional areas.
Abrupt changes can lead to concentrations in bending stresses which in turn can lead to a twist off (Refer to figure 8.h).
The ratio I/C between the moment of inertia (I) and radius (C) of the pipe is directly related to the resistance to bending (Refer to Section 8.4.2). The following are used to determine the section modulus I/C:
I = Moment of inertia
C = Radius of the tube
At a crossover from one tubular size to another size, the ratio (I/C large pipe)/(I/C small pipe) should be less than 5.5 for soft formations and less than 3.5 for hard formations.
table 8.c illustrates some possible drill strings and their acceptable use.
ENI S.p.A. Agip Division
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