Page 137 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:56 PM Page 113

Directional Design [ ]
1.5.2

pipe body has internal corrosion, external slip marks, or other damage,
the effect of these stress raisers will lower the fatigue resistance of the
pipe substantially, and a failure of the body may occur there rather than
at the upset to the tool joint.
Casing wear. In addition to tool joint damage and fatigue consid-
erations, wear on the casing as a result of lateral forces also has to be
considered. Wear from tripping is much less than that from rotating
and in medium drillstring tensions in doglegs below 6˚/100 ft, the pipe
body does not touch the casing. Therefore, wear arises mainly from
rotating tool joints that are pushed against the casing.
Wear is affected by many different factors:

Contact pressure between the tool joint and casing (depends
on lateral force and contact geometry)
Type of fluid in the hole
Number of rotations
Presence or absence of hard facing, whether it is smooth or not
and whether it stands proud of the tool joint or not
Presence or absence of tong marks or other sharp edges that
would cause abrasion
Materials in contact

and may occur as a result of three different mechanisms:

Two-body abrasive wear—sharp edges on the tool joint act like
a file on the casing. Produces small cuttings, shiny on one side,
like file or lathe cuttings, and very high wear rates.
Two-body adhesive wear—a galling mechanism where the two
bodies become friction-welded together momentarily. Produces
flakes of metal and moderate wear rates.
Three-body abrasive wear—solids in the mud become embed-
ded in rubber protectors and act as a fine abrasive. Produces
fine metal powder and very low wear rates.

In new casing on the first bit run, the contact area between tool
joint and casing is very small. Wear rate is very high. Since the inside
of the casing is worn, contact area will rapidly increase and for the
same lateral force, the lateral pressure (in psi) will decrease. The initial

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