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API REPORT 80-30A Document Information:
Title
Collapse Performance of HC-95 Casing
American Petroleum Institute
Publication Date:
Nov 1, 1982
Scope:
I. Introduction and Background
This report will describe the procedures and results of a program
performed at Southwest Research Institute (SwRI) for the purpose of
assessing the collapse resistance of High Collapse 95,000-125,000 psi
yield strength casing (Grade HC-95). Over two hundred casing
specimens,
manufactured by seven different companies, were evaluated for their
collapse resistance characteristics. These ranged in diameter from
4 1/2 in. to 9 5/8 in. and in D/t (diameter to thickness) from
approximately
15 to 23, representing the most popular sizes currently in use.
The program was funded by the American Petroleum Institute (API) under
the direction of the Production Research Advisory Committee (PRAC),
Data obtained will be presented in reduced format but a minimum of
data
analysis and interpretation is given as this was not within the scope
of
the contract.
Historical Background
HC-95 is a popular grade of casing within the industry as evidenced
by the more than 138,000 tons shipped by domestic and foreign mills
during the year preceding the initiation of this contract. This
tonnage
is expected to increase as oil and gas wells are drilled to deeper
depths
and the need is increased for casing having high collapse resistance
characteristics.
Despite the interest in this grade of casing, two previous
efforts to develop an API performance specification for its use have
failed
due to questions over the validity of collapse data collected. These
questions centered around the following concerns:
• Collapse data was supplied by the steel industry using
individual
mill collapse testers. This raised the question
of whether these individual testers, which have dissimilar
methods for mounting, sealing, pressurizing, and indicating
failures of specimens, produce the same results.
• The different collapse testers within the industry use varying
lengths of collapse test specimens or differing L/D
(length to diameter) ratios. A number of research studies have
shown this to be a possible source of variance in collapse
data. The magnitude of this variance will depend primarily on
specimen end loading conditions (sealing method).
• There has been concern over the possible lack of uniform
calibration of pressure gauges among the various collapse
testers.
• In most mill programs, test specimens were selected from mill
stock instead of from user field inventories and this did not fully
represent casing at the drilling site which had been subjected to
shipping and field handling.
The current program was designed to eliminate these questions.
The Current Program
Quality control of the data collected was very important to this
program and much of what was done centered around these
considerations.
The program was designed, first of all, to eliminate the questions
raised in previous programs. Among the steps taken were the following:
• All test specimens had a L/D ratio of eight.
• All tests were performed using the same equipment which
eliminated errors associated with the use of different
testers ("inter-equipment error").
• All tests were performed using the same test operators which
minimized errors associated with the personal idiosyncrasies
of the different testing personnel ("inter-operator error").
• The same test gauges were used for all tests, and these were
calibrated to the National Bureau of Standards.
• Test specimens were randomly selected from user field
inventories
so that they would be representative of what is being used in
actual wellbores.
With these problems eliminated or minimized, the problem of "standard
testing error" was addressed. This is a measure of the difference in
results
between two or more theoretically "identical" tests and represents
idiosyncrasies within the test system itself. Specific steps taken to
minimize standard testing error included the following:
• Only proven operators with several years of experience were
used to perform tests.
• Specimens were sealed using a slow, expensive, but highly
effective end sealing method which SwRI has developed over
many years of performing these types of tests. The SwRI
method is designed to seal the ends with only a few ounces
of radial pressure instead of the high loads in some testers.
• All handling and testing procedures were rigidly standardized
and controlled.
• Specimen collapse was indicated by a four point check.
Past tests performed by SwRI using uniformly machined steel cylinders
have shown that the system testing error is negligible when these
procedures
are employed.
All procedures used during the program were in accordance with API
Bull. 5C3.
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