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| API Bulletin 5A2 ("Bulletin on Thread Compounds for Casing, Tubing and
Line Pipe"), presents a
thread compound formulation for use on API round-thread connections
that will meet certain listed
performance objectives. These objectives include: prevention of
galling and thread damage during
make-up, ability to seal the leak path at pressures to 10,000 psi and
temperatures to 300°F,
and material stability in a range of conditions. The original
composition was developed in 1951 by
the Mellon Institute and subsequently modified in 1955 to the present
formulation. This compound
formulation came to be known as "API Modified Thread Compound".
API Modified has been the primary compound used for LP-C&T (line
pipe, casing and tubing)
connections for over forty years and by and large has proven
satisfactory in a wide range of
conditions on API threadforms. Over the years, however, concern was
raised over the apparent
difference in performance properties of API compounds made by
different thread compound
manufacturers and even in different lots from the same manufacturer.
Various controlled laboratory
tests by several oil field companies, seemed to validate this concern
and led to an evaluation of
existing commercially available API compounds by the API Committee 5
Task Group on Thread Compounds
and Leak Resistance. The results of this study were presented in
January 1982. Particle size and
component analysis showed several compounds that did not conform to
the compositional requirements
of Bulletin 5A2. Even those compounds that did conform showed
significant differences in both
sealing and frictional performance.
At that time, it was realized that there was a need to reevaluate
Bulletin 5A2. There were several
shortcomings that were apparent:
1. Thread compound manufacturers were not required to demonstrate
compliance to the compound
composition specifications or to the control and performance test
2. Although there are eight performance objectives listed in the scope
of the Bulletin, there are
no standardized tests available to demonstrate that those objectives
have been met. In fact, the
original testing by the Mellon Institute did not address all of these
3. The performance requirements listed in the Bulletin deal mostly
with the properties of the
grease base and are easily met by virtually any grease currently
In 1984, a Technical Advisory Committee (TAC) was appointed by the
Task Group on Thread Compounds
and Leak Resistance to supervise a test program on thread compound
performance. This program (PRAC
84-51) was to include:
1. An evaluation of sealing performance as a function of component
particle size variance within
Bulletin 5A2 requirements.
2. An evaluation of alternative compounds including specifically lead
and zinc-free compounds, due
to increasing environmental concerns.
3. Development of standardized test equipment and procedures to
measure compound performance.
4. Recommended changes to Bulletin 5A2 and possible alternatives based
on test program results.
The compounds to be tested were solicited from various thread compound
manufacturers and were
classified into three categories as follows:
CATEGORY I - API Modified compounds made in accordance with API
Bulletin 5A2. Compounds were
requested to be made in the midrange of the allowable component
percent. Particle sizes were
requested in each of the particle size distributions as follows:
CAT. I-L - Compounds with particle sizes at the large end of the
CAT. I-M - Compounds with midrange particle size.
CAT. I-S - Compounds with particle sizes at the small end of the
CATEGORY II - Compounds containing no lead or zinc.
CATEGORY III - Compounds, other than API Modified, that contain lead
The thread compound manufacturers were asked to furnish the
composition and particle size
distribution of the compounds submitted. Compounds from a particular
manufacturer were assigned an
arbitrary letter and a number to identify multiple compounds submitted
in each category. The total
number of compounds received was 73: 29 CAT.I, 17 CAT.II and 27
The TAC designed a set of precision ground, heavy wall 4½"
buttress threadform test
fixtures, with thread flank clearances of .001", .005" and .009".
These clearances represent the
minimum, median and maximum tolerances within the API specifications.
The buttress thread profile
was chosen because the initial contact during make-up occurs at the
root and crest of the thread.
The clearances are at the thread flanks and change very little with
make-up. The buttress
threadform also presents a more severe test of a compound's
sealability compared to the smaller
leak path cross section in a round-thread connection.
The first phase of the program consisted of ambient temperature
sealing tests of the 73 compounds
using the test fixtures. The pressure medium was nitrogen gas.
Pressure was applied in 500 psi
increments to a maximum test pressure of 10,000 psi. In the initial
tests, the attempt was made to
make-up the fixtures to a constant engagement. Because of the
substantial variance in particle size
and type, high torques were required for a number of compounds to
achieve the desired test
position. In subsequent tests, the fixtures were made-up to a fixed
low torque (100-150 ft-lbs),
rather than a fixed position, to prevent damage or distortion of the
fixtures. Because of this
limitation, the stand-off (i.e. crest to root clearance) and
consequently, the leak path cross
section of the fixtures varied considerably depending on the type
compounds being tested. A total
of 170 pressure sealability tests were performed.
The second phase of the program (PRAC 86-51), initiated in March,
1986, consisted of:
1. Elevated temperature (280°F) sealing tests using the special
buttress test fixtures.
2. Full-scale sealing tests (ambient and 280°F) using 4½"
N-80 buttress connections.
Pressure medium was nitrogen with a maximum test pressure of 7,780
3. Full-scale make-up and break-out tests on both the 4½"
connections and 2-7/8" N-80 E.U.E.
4. Laboratory friction/galling evaluations. Tests included the Drilco
Relative friction Tester
which uses a precision machined 1" shouldered bolt to generate torque
vs. turn data and the Shell
Four-Ball Weld Point Test.
Seven new compounds were formulated by the TAC for evaluation on the
special test fixtures in the
second phase. These compounds were made to the API Modified
composition with only the particle size
of the primary sealing constituents, lead and zinc, varied. Three of
these compounds were
formulated with lead and/or zinc particles larger than specified in
API BUL. 5A2. The compounds
were called "Blended Component" compounds and were identified as BC-1
- BC-7. Ten compounds were
selected from the phase one and BC compounds for full-scale testing on
the 4½" buttress
connections. Seventeen compounds were selected for the 2-7/8"
make/break tests and the laboratory
friction/galling tests. The compounds were selected based on
performance that was felt to be either
representative of the category or, in the case of alternative
compounds, the ones that demonstrated
superior performance. The compounds with the best overall performance
for both sealability and
galling protection were N-III-5 (lead/Teflon™/graphite) and
The conclusions from the first two test phases can be summarized as
1. Variation in component particle size within the specifications of
Bulletin 5A2 has a significant
influence on the sealability of API Modified and to lesser amount,
also affects the make-up
2. There are alternate compounds, including non-lead and non-zinc that
performance equal to or better than API Modified.
3. To effectively seal the buttress threadform at high pressure (>
7,000 psi) and/or elevated
temperature (280°F) requires large, easily deformed particles such
as lead or PTFE.
4. Threadform variables (pitch, lead, taper) can cause sealing and
make-up performance to be
5. Frictional/galling tests currently in use do not adequately model
LP-C&T connections and are
not useful for performance evaluation.
Based on the results and conclusions of PRAC 84-51 and 86-51 the TAC
redefined the objectives of
the test program. Rather than develop test procedures to be used to
recommend specific compound
composition, the TAC determined that it would be more useful to
develop a set of standardized tests
on the laboratory scale, that would define the primary requirements of
thread compound performance,
2. Frictional/make-up characteristics
3. Galling resistance
These tests would be designed to eliminate or minimize threadform
influence and would be used to
measure relative thread compound performance. Minimum performance
standards would be set using
average API Modified performance as a baseline. Any compound,
regardless of composition, that met
these minimum performance standards would be acceptable for use under
any applicable API
specification or recommended practice. Thread compound manufacturers
would be required to provide
users with standardized test results on compounds intended for use on
This proposal was approved by the Task Group on Thread Compounds in
1988 and was designated PRAC
88-51 as a continuation of the previous program and was initiated in
April, 1988. The TAC designed
a sealing test fixture that would eliminate threadform variables. The
fixture consists of two flat
circular steel plates with a pressure cavity machined in the center of
each plate. The upper plate
has a spiral groove machined from the central pressure cavity to the
outer edge of the plate. There
are separate grooved plates that model the "worst case" leak path
cross sectional area of 8-round
and buttress thread forms (Fig. 3-3). The leak path or groove length
of each plate is 200 inches.
The grooved plate is coated with the test compound and pressed against
the lower "seal" plate with
a hydraulic ram (Fig. 3-9) to achieve bearing pressures equivalent to
a fully engaged connection.
The fixture is pressured in 500 psi increments until a leak occurs or
maximum test pressure (10,000
psi) is attained. The fixture can be heated with thermal electric
platens to elevated temperature.
Preliminary tests were run using selected compounds from the prior
test phases. An additional
"Blended Component" compound (BC-8) was formulated with a particle
distribution chosen to represent
a "nominal" API Modified compound. This formulation was to be used to
establish a baseline
performance criteria. Initial results with the fixtures were promising
and demonstrated a
correlation with the prior tests. Subsequent testing showed some
anomalies, mainly with the
buttress groove plates.
A small-scale test apparatus to determine the frictional or make-up
characteristics of thread
compounds had been developed and assembled by the APl Committee 7 Work
Group on Thread Compounds
for Rotary Shouldered Connections (described in Section 4.0). This
tester, based on the design and
test procedure developed by Drilco, is operated by a SCR controlled
D.C. motor and utilizes a
torque transducer and a rotary encoder to obtain torque vs. turn data
using a small threaded test
specimen, a precision machined 1" shouldered bolt. The TAC initiated
the design and development of
a test specimen that would model the thread interference of an
LP-C&T connection. Initial tests
with a modified, tapered NPT thread hydraulic fitting showed some
promise but did not generate
torque curves as consistent as anticipated.
The evaluation of various galling tests that would model the high
bearing stress, slow rotational
speed, sliding surface contact of a tapered threaded connection during
make-up and break-out, was
initiated. These included modifications of three ASTM galling/wear
tests and a test procedure
developed for a joint industry project by Texas Tech University.
Proposals for test specimen design
and test procedure were developed but no actual galling performance
tests were performed during
The interim report on PRAC 88-51 was presented at the Annual
Standardization Conference in June,
1989 and included the following recommendations for the completion of
1. Sealability Test
• Establish the number of test runs required to determine, with
reasonable confidence, the
sealability rating of a test compound.
• Evaluate several mixtures of the current API Modified with the
particle size distribution
varied within the specified limits. This would determine the range of
sealing performance and be
used to set minimum performance standards.
2. Friction Performance Test
• Develop a test specimen that would:
- Model LP-C&T thread interference with a standard API thread.
- Allow constant thread length engagement during make-up.
- Allow typical bearing stresses without yielding.
• Develop a reference compound with consistent frictional
properties that can be easily
formulated. This compound would be used to normalize test data and to
compensate for any minor
physical changes in the test specimen.
• Evaluate representative API Modified samples to demonstrate
repeatability of the test
procedure and to establish a baseline performance.
3. Galling Performance Test
• Perform galling tests, with representative API Modified
compounds, based on proposed
modifications to existing ASTM standard test procedures.
• Determine bearing stresses required to produce galling.
Evaluate the suitability of these tests for LP-C&T thread
PRAC 89-51 initiated in July, 1989 and a status report was submitted
to the API in July, 1990. In
order to conserve funding, it was decided at that time to delay a
formal report until the
completion of the test program. The continuation, PRAC 91-51, was
initiated in January, 1991 with
status reports submitted in June, 1991 and June, 1992. This report
presents the results,
conclusions and recommendations of API PRAC Project 88-51, 89-51 and
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