Page 115 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:56 PM Page 91

1.4.21
Casing Design [ ]

may need pupjoints to be made up or to recover joints where the
connection has been damaged.

5. Clearance. If an external coupling is used, there may sometimes be
limitations due to wellhead design, especially with a subsea well-
head or mudline suspension system. Couplings can be turned
down and/or the shoulders beveled to ensure passage through such
a restriction. This may affect the connector strength in tension or
burst and must be checked out before a final decision. In cases of
restricted clearance, also bear in mind the possibility of fracturing
weak formations if the casing is run too fast when couplings pass
through the restriction.

6. Practicality. In severe conditions it may be hard to run a premium
threaded connection or a fine thread. In areas with regular sand-
storms or high winds, you may want a well-protected seal area or a
robust connection that is hard to mis-stab. For instance VAM
threads have the seal area exposed at the very end of the connec-
tion, where it is most likely to be damaged. NS-CC threads from
Nippon Steel have the seal area well protected by a torque shoul-
der where it is not so likely to be damaged.

7. Cost. Of the connections that meet your criteria, the lowest cost
can be chosen.

1.4.21. Casing and Liner Accessories

Centralization. Effective mud removal is essential to a good
cement job. In an eccentric casing annulus, mud will preferentially
flow through the large side instead of in the narrow part of the annu-
lus. Good centralization is needed at the shoe and anywhere that a
good bond is important (such as for zonal isolation).
The restoring force of a spring centralizer is quoted by the manu-
facturer. Given this, the correct centralizer placement can be calculat-
ed for a particular casing string, mud weight, and hole deviation.
Service companies can recommend centralizer placement, but these

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