THREAD-COMPOUND TEST PROCEDURES BEING DEVELOPED

API is planning to issue a new bulletin that will outline standardized test procedures and set minimum performance for thread compounds used on OCTG (oil country tubular goods) connections with API thread forms.

These performance standards will replace the recommended compound described in API Bulletin 5A2.

The proposed bulletin will detail test equipment, procedures, and performance requirements for thread compounds. API will no longer recommend a specific compound formulation.

THREAD COMPOUNDS

Specifications for the original compound, made with a silicone grease, were published in 1951.1 Because of the high cost of silicone grease, a supplementary study in 1955 2 modified the compound to include a petroleum soap-based grease that became known to the industry as API modified high-pressure thread compound. The upper pressure limit of the modified compound was determined to be 8,000 psi.

The program to develop the new procedures was prompted by concerns about the apparent difference in performance properties of API compounds made by different thread-compound manufacturers and even between different lots from the same manufacturer. There were also environmental concerns because of the heavy metal content of API modified.

In 1982 a task group study of commercially available API compounds concluded3 that 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.

A technical advisory committee (TAC) was formed in 1984 to supervise a test program on thread-compound performance. This program (PRAC 84-51 and PRAC 85-51)4 5 included two test phases. The conclusions were as follows:

Variation in component particle size within the specifications of Bulletin 5A2 has a significant influence on the sealability of API modified and, to a lesser amount, also affects the makeup characteristics.
There are alternate compounds, including nonlead and nonzinc, that demonstrate sealing performance equal to or better than API modified.
To effectively seal the buttress thread form at high pressure (7,000 psi) or elevated temperature (280 F.) requires large, easily deformed particles such as lead or polytetrafluoroethylene (PTFE, Teflon).
Thread form variables (pitch, lead, taper) can cause sealing and makeup performance to be connection dependent.
Frictional/galling tests currently in use do not adequately model OCTG connections and are not useful for performance evaluation.

In 1988, API Committee 5 approved the TAC recommendations to develop a set of standardized tests on a laboratory scale. These tests would be used to define three primary requirements of thread-compound performance: sealability, friction/makeup characteristics, and galling resistance.

The tests were designed to eliminate or minimize thread form influence and to measure relative thread-compound performance. The minimum standard baseline was the performance of average API modified thread compound.

TEST EQUIPMENT

The first fixture designed (PRAC 88-51)6 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.

Separate grooved plates model the "worst case" leak path cross sectional area of eight round and also buttress thread forms (Fig. 1). The leak path or groove length of each plate is 200 in.

The grooved plate is coated with the test compound and pressed against the lower seal plate with an hydraulic ram (Fig. 2). The fixture is pressured in 500 psi increments until a leak occurs or maximum test pressure is attained.

The fixture can be heated with thermalelectric platens to elevated temperature. Preliminary test results with the fixtures have been promising.

API Committee 7 developed a bench-top apparatus to determine frictional or makeup characteristics (Fig. 3). This tester uses a torque transducer and a rotary encoder to obtain torque vs. turn data for a small test specimen (1-1 1/2 diameter).

The tester's dc motor is controlled by a silicon-controlled rectifier (SCR).

PROPOSED WORK

The current test phase (PRAC 89-51) will use a specimen to model the thread interference of an OCTG connection. Test specimens that will provide consistent torque curves are being evaluated.

Also being evaluated are various galling test proposals. These tests will model the high bearing stress, slow rotational speed, flat surface contact of a tapered threaded connection.

The PRAC 89-51 phase will include reproducibility testing to determine the statistical validity of the proposed test. Extensive fullscale testing for correlation and verification purposes will be required.

The goal is to present the new bulletin at the July 1991 API Annual Standardization Conference. To meet this deadline will require additional funding and donations of both goods and services from API members and other industry organizations.

Additional information on the new bulletin can be obtained from H.B. McDonald of Bestolife Corp., Dallas, chairman of API Committee 5, TAC on thread compound performance.

The new bulletin will allow manufacturers the flexibility to meet customer requirements and provide the industry with standardized, clearly defined test procedures that will yield useful results at a reasonable cost.

REFERENCES

Mayberry, M.G., "Development of an API Thread Compound," API 31st Annual Meeting, Chicago, November 1951.
Ruppel, T.D., and Willingham, C.B., "Final Report on Supplementary Tests of an Experimental Thread Compound Having a Soap-base Petroleum Grease Vehicle," Pittsburgh, September 1955.
Minutes of API Committee 5, Task group on thread compounds, Jan. 7, 1982.
Report on API PRAC Project 8451, "Investigation of Pipe Thread Compounds," December 1985.
Report on API PRAC Project 8651, "Investigation of Pipe Thread Compounds," May 1987.
Report on API PRAC Project 8851, "Investigation of Pipe Thread Compounds," June 1989.