Monday, 15 January 2018

CASING PIPES DESIGN, THEORY AND PRACTICE - SPECIFICATIONS

Casing Pipes Specifications

steel casing pipes grade specifications P110
Casing Pipes Specification

What Is the Parameters Which Specify Steel Casing Pipes

  1. Casing Size.      
  2. Casing Length Range      
  3. Casing Nominal Weight / Weight per unit length
  4. Steel Casing Grade 
  5. Casing Coupling Types

The First Casing Specifications Parameter is The Casing Sizes

There are three types of Casing Size designations
  • Outer diameter : is the diameter of the casing measured from outer wall across to outer wall and is the diameter measurement casing is identified with.
  • Inner diameter: is the diameter of the casing measured from inner wall to inner wall.
  • Drift diameter :  is the guaranteed minimum diameter of the casing, the drift diameter is important because it indicates whether the casing is large enough for a specified size of bit to pass through.
steel casing pipes sizes tables

steel casing pipes sizes tables

steel casing pipes sizes tables

The Second Casing Specifications Parameter is Length Range  

API has established three length ranges for casing pipes with limits and tolerances as shown below. API specifications for casing pipes and tubing pipes designate the length range of each joint. There are three length ranges for casing pipes (R1, R2, R3):

casing pipes specifications length ranges

Casing Pipes are mostly run in R-3 lengths. These longer lengths reduce the total number of threaded connections needed for the casing string. Since casing is usually run in single joints (instead of doubles or triples), the longer R-3 lengths are easier to handle. 

API Casing Pipe And Tubing Weight Designation 

Casing and tubing weights are one of specifications which are expressed in lb/linear ft and are designated as either plain end weights or nominal weights. 
  • Plain-end weight per foot is the weight per foot of the pipe body excluding the threaded portion and coupling.  
  • Nominal weight per foot is the weight per foot that is reflected in casing tables and is an approximate average weight per foot of the pipe with API connections, including upsets, threads, and couplings. 
  • Average weight per foot is the total weight of an average joint of threaded pipe with one coupling divided by the total length of the average joint. 
The plain end weight of casing pipes can be calculated by knowing the outer and inner diameter of the pipe and the density of steel (489 lb/ft3):

equation nominal wt casing specification

The difference between nominal weight and average weight is generally small and most design calculations are performed by using nominal weight per foot.

The Casing Pipes Specification Parameter Called Casing Grades 

Steel Casing pipe grades are identified by letters and numbers which indicate various characteristics of the Casing pipe steel ( as example P110). It is a specification according to its yield stress, ultimate tensile strength, chemical composition, heat treatment or other characteristics. There are many grades of casing steel pipes that make up oilfield tubulars.

API Casing Pipes Grades  

The steel grades of common oilfield API-5CT casing pipe include J55, K55, N80-1, N80Q, and P110. Casing pipe is mostly used for oil well drilling.

To understand API  casing pipe grades, it is important to understand the terms minimum yield stress, maximum yield stress, and minimum ultimate strength. To explain these terms, two popular grades of oilfield tubulars will be used as an example: L-80 and N-80.

The Steel casing pipes grade is denoted by a letter of the alphabet followed by the minimum yield stress of the particular steel. For example, the API P-110 , has a minimum yield stress of 110,000 psi, the API grade L-80, has a minimum yield stress of 80,000 psi as shown by point "A" in Figure 2.  In other words, it can support a stress of 80,000 psi with an elongation of
0.5%.

STRESS STRAIN CHART for casing pipes specifications

The 'L' is a distinguishing prefix to avoid confusion between different steel grades. The letter in conjunction with the number designates such parameters as the maximum yield strength and minimum ultimate yield strength.

In L-80 the maximum yield strength is  shown by point "B" as 95,000 psi which is 15,000 psi higher than the minimum yield stress. The minimum ultimate strength is shown by point "C" as 95,000 psi. Note that there is no maximum ultimate strength specified.

N-80, another API grade (see Figure 3), also has a minimum yield stress of 80,000 psi, but is different from L-80 in that the former has a greater maximum yield stress of 110,000 psi (shown by point "B").
This is 30,000 psi higher than the minimum yield stress and twice the tolerance of L-80. The minimum ultimate strength of 100,000 psi is also higher as shown by point "C". Whereas N-80 has no hardness specification, L-80 has a hardness specification of 23 HRC. The tight tolerance on yield strength and hardness allow the L-80 to be more suitable for H2S service than N-80 grade tubulars.

Non-API Casing Pipes Grades 

In addition to API grades, there are many proprietary steel grades which may not conform to the API specifications, but which are used in the industry. These extensively used special grades are often run for various applications requiring such properties as very high tensile strength, high collapse strength, or steels resistant to sulfide stress cracking.

This pipe is manufactured  to many, but not all of the API specifications with such variations as steel grade, wall thickness, OD, threaded connection, and related upset.

As a result of these changes, the ratings of internal yield, collapse, and tension for both the pipe and the connection are non-API.

The rating of these proprietary products are generally calculated using API formulas or are consistent with API methods. Also, such parameters as drift diameter, wall thickness tolerance, length range, and weight tolerance are kept the same as, or are consistent with API specifications.

Now, The Last Casing Pipes Specification Parameter which is Casing Connections
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