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Unformatted text preview: rotation and
reciprocation reduces the strength of casing.
Wear models such as Maurer Engineering’s
“CWEAR4”, DEA-42 Project, can effectively
predict the degree of wear to be expected in
a casing string. The wear is estimated as the
average percentage of the casing wall
thickness wore away.
220 Copyright(c) 2005 by Ali Ghalambor & Boyun Guo 110 Consider 9 5/8” ppf, P110 casing with a
predicted wear of 10% of the casing wall
thickness. Worn by 5”, 19.5 ppf, H-90 drill
pipe.(Tool joint OD=6.5”)
The API Barlow equation for burst strength is :
for 10% wear, t’=0.9t
and Burst(Worn) = 9810 psi
Burst(Unworn) = 10900 psi
Difference = -10%, the burst strength is directly
proportional to wear. Collapse Strength:
The API procedure of r determining collapse
strength involves D/t comparisons to determine the
failure mode. As t’=t(1-wear %), the failure mode
for collapse might be different for worn casing than
it was for the same but unworn casing.
For the example, the failure mode remains plas...
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- Fall '14