API REPORT 88/89/91-51
Investigation of Pipe Thread Compounds
|Publication Date:||1 November 1992|
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 requirements.
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 objectives.
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 available.
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 specified range.
CAT. I-M - Compounds with midrange particle size.
CAT. I-S - Compounds with particle sizes at the small end of the specified range.
CATEGORY II - Compounds containing no lead or zinc. CATEGORY III - Compounds, other than API Modified, that contain lead and/or zinc.
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 CAT.III.
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 psi.
3. Full-scale make-up and break-out tests on both the 4½" connections and 2-7/8" N-80 E.U.E. tubing.
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™/graphi
The conclusions from the first two test phases can be summarized as follows:
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 characteristics.
2. There are alternate compounds, including non-lead and non-zinc that demonstrate sealing 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 connection dependent.
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, mainly:
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 LP-C&T connections.
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 this phase.
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 the program:
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 compounds.
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 91-51.