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CASING AND TUBING
- American Petroleum Institute (API) has developed guides for the petroleum practices and equipment. Casing and tubing are important in oil drilling and production.
- API’s Specification 5A states that casing, liners and tubing must be seamless or electric welded.
- Casing physical characteristics Grade Weight Range Diameter Burst Collapse Yield point and pipe body Biaxial effect
3.1 Grade
The pipe grade indicates the pipe’s yield strength and certain special characteristics.
The grade usually consists of a letter and a 2 or 3-digit number such as N-80.
Pipe N-80 has a minimum yield strength of 80,000 psi. The average yield strength is usually 10,000 psi greater than the minimum yield or 90,000 psi for N-80 pipe.
The minimum value is used in burst and collapse resistance calculations, whereas the average is used for biaxial evaluation.
Examples of API Grades: H-40, J-55, K-55, C-75, N-80, L-80, C-95, P-105, P110, V- C pipe is a controlled yield pipe used primarily in environments such as H 2 S (corrosive
environment).
L grade pipe has the same burst, collapse and yield values as N pipe. The hardness of the pipe is a critical value due to potential hydrogen embrittlement in H 2 S environment.
3.3 Range
Pipe range is a value for approximating the length of a section of pipe. The following table illustrates some of the API standards with respect to range definitions.
API Range Length Range (feet) 1 2 3 Casing Total range length, inclusive Range length for 95% or more of carload: Permissible variation, maximum Permissible length, minimum
16- 6 18
25- 5 28
34- 6 36 Tubing Total range length, inclusive* Range length for 95% or more of carload: Permissible variation, maximum Permissible length, minimum
20- 2 20
28- 2 28
_Pup joints Lengths – 2, 4, 6, 8, 10 and 12 ft Tolerance - F 0B 13 in.
- By agreement between purchaser and manufacturer, the total range length for Range 1 tubing may be 20 – 28 ft._
3.4 Diameter
The drilling engineer must consider 3 types of diameter data when planning the tubular program: outer, inner and drift diameters.
For drilling considerations, the drift diameter is usually considered as the bit size that can safely run through the casing.
- Burst
The burst rating of the casing is the amount of internal pressure that the pipe can withstand prior to failure.
The internal yield pressure for pipe is calculated using the following equation.
where P (^) B = Burst pressure rounded to the nearest 10 psi. Y (^) P = Specified minimum yield strength, psi.
t = Nominal wall thickness, in.
OD = Outside diameter, in.
philosophy dictates that pipe used in sour service will not have a yield strength greater than 90,000 psi. After the engineer reviewed the available, commonly used weight and grades of casing, he realized that the string must be specially rolled to meet his requirements. Determine the wall thickness requirements for the pipe. Use a yield strength of 90,000 psi and assume the API tolerance of 87.5% wall thickness. Round up the wall thickness to the nearest 1/8” in. Solution: The API equation for internal yield resistance.
t = 0.726 in. Round up to the nearest one-eighth is: t = 0.750 in. or. 3.6 Collapse
Collapse resistance depends on the D/t ratio. This can be divided into four categories:
- Yield strength collapse pressure
- Plastic collapse
- Transition collapse
- Elastic collapse
API provides tables and recommended appropriated equations for the four types of categories.
- Pipe body yield strength
This is the axial load required to yield the pipe. PY = 0.7854 (OD 2 – ID^2 )YP Example A section of 10.75 in., 55.00 lb/ft, N-80 casing is to be run into a well. It has a wall thickness of 0.495 in. Determine the pipe body yield strength.
Solution: The ID is computed as follows: ID = OD –2t = 10.75 – 2(0.495) = 9.76 inci The yield strength is: PY = 0.7854 (OD 2 – ID^2 )YP = 0.7854(10.75^2 – 9.76 2 )(80,000) = 1,275,797 lb F 04 0 1,275,000 lb 3.8 Biaxial effects Burst and collapse resistances of casing are altered when the pipe is under a tension (or 8 compression) load.
