Page 136 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:56 PM Page 112

[ ] Well Design
1.5.2

between surveys, “minimum curvature,” assumes a perfect arc between
survey points. In practice the actual dogleg severity will be greater in
some places than others, imposing a point loading at those places. If
the limit for dogleg severity were 2.05˚/100 ft, you could plan on an
average 1.5˚ dogleg severity to allow for this variation.
There is also a practical solution to allow higher dogleg severities
than the limit calculated above. If drillpipe protectors were to be posi-
tioned at the midpoint of each joint of drillpipe, and if the OD of those
protectors were similar to the tool joint OD, you would effectively
halve the length of the drillpipe joint. The load would be taken by the
protectors that would reduce the load on the tool joints. As the factor
related to the length of the drillpipe joints L is on the bottom half of
the formula, halving the length would double the allowable dogleg
severity. Therefore, by using drillpipe protectors, one per joint on
drillpipe being rotated through the build section, the allowable DLS
will double to just over 4˚. Two protectors per joint, equally spaced at
one-third and two-thirds inches along the pipe, will further reduce the
load and allow a larger DLS.
Drillstring fatigue. The area of the drillpipe subjected to the sever-
est cyclic bending stresses when rotated in a dogleg is where the
drillpipe body joins the tool joint. Here the stiffness of the drillpipe
changes very quickly between the rigid tool joint itself and the flexible
pipe body.
Calculation of fatigue is fairly complicated. Calculations for fatigue
limitations of dogleg severity gives greater dogleg severities than the
maximum found by calculating for preventing tool joint damage, except
at very low drillstring tensions (below about 75,000 lbs or lower).
Therefore, as long as doglegs are limited by the 2000 lbs lateral force for
tool joint damage, pure drillpipe fatigue is not likely to be a problem.
Reference can be made to the graphs in Section B4 of the IADC
Drilling Manual and also in API RP7G These graphs show the maximum
dogleg severity for commonly used drillpipes. Preston Moore’s Drilling
Practices Manual also has some graphs illustrating fatigue limitations of
dogleg severity. The most commonly referenced paper on the subject is
“Maximum Permissible Dog-legs in Rotary Boreholes,” by A. Lubinski.
Fatigue failures can occur at other areas on the drillpipe. If the pipe
is not sufficiently torqued up so that the shoulders are compressed
together, fatigue failure of the pin will occur very quickly. Also, if the

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