Diameters and Wall Thickness
As discussed previously, casing diameters range from Ц to 24 in. so they can be used in different sections (depths) of the well. The following tolerances, from API Spec. 5CT (1992), apply to the outside diameter (OD) of the casing immediately behind the upset for a distance of approximately 5 inches:
Casing manufacturers generally try to prevent the pipe from being undersized to ensure adequate thread run-out when machining a connection. As a result, most
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1.05 — |
+ A T 32 |
1 "" 32 |
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4-5 |
+ 64 |
— 0.75% OD |
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+ 1 |
— 0.75% OD |
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|
>9§ |
+ A 1 32 |
— 0.75% OD |
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Table 1.1: API manufacturing tolerances for casing outside diameter. (After API Spec. 5CT, 1992.) |
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(in.) (in.) |
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Outside diameter Tolerances |
casing pipes are found to be within ±0.75% of the tolerance and are slightly oversized.
Inside diameter (ID) is specified in terms of wall thickness and drift diameter. The maximal inside diameter is, therefore, controlled by the combined tolerances for the outside diameter and the wall thickness. The minimal permissible pipe wall thickness is 87.5% of the nominal wall thickness, which in turn has a tolerance of -12.5 %.
The minimal inside diameter is controlled by the specified drift diameter. The drift diameter refers to the diameter of a cylindrical drift mandrel, Table 1.2. that can pass freely through the casing with a reasonable exerted force equivalent to the weight of the mandrel being used for the test (API Spec. 5CT, 1992). A bit of a size smaller than the drift diameter will pass through the pipe.
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Table 1.2: API recommended dimensions for drift mandrels. (After API Spec. 5CT, 1992.)
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The difference between the inside diameter and the drift diameter can be explained by considering a 7-in., 20 lb/ft casing, with a wall thickness, t. of 0.272-in.
Inside diameter = OD — 21 = 7 — 0.544 = 6.456 in.
Drift diameter = ID — |
= 6.456 — 0.125 = 6.331 in.
Drift testing is usually carried out before the casing leaves the mill and immediately before running it into the well. Casing is tested throughout its entire length.