Results of Performance Tests Performed on the John Watts Casing Connection on 7" Pipe

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Stress Engineering Services (SES) was contracted by Mr. John Watts to test his threaded connection developed for oilfield oil and gas service. This particular test required the application of a variety of loads including axial tension and compression, internal pressure (gas), external pressure (water), bending and both low and elevated temperature. These loads were used to determine the sealing and structural limits of the connection. The connection design tested had tapered threads with 10 threads per inch. A square thread form and a round thread form were tested. The square thread form had a 2{sup o} load flank and 15{sup ... continued below

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Watts, John D. August 1, 1999.

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Stress Engineering Services (SES) was contracted by Mr. John Watts to test his threaded connection developed for oilfield oil and gas service. This particular test required the application of a variety of loads including axial tension and compression, internal pressure (gas), external pressure (water), bending and both low and elevated temperature. These loads were used to determine the sealing and structural limits of the connection. The connection design tested had tapered threads with 10 threads per inch. A square thread form and a round thread form were tested. The square thread form had a 2{sup o} load flank and 15{sup o} stab flank. The round thread had a 0{sup o} load flank and 20{sup o} stab flank. Most of the testing was performed on the round thread form. Both a coupled connection design and an integral connection design were tested. The coupling was a pin by pin (male) thread, with the pipe having a box (female) thread. Both designs have outside and inside diameters that are flush with the pipe body. Both designs also contain a small external shoulder. The test procedure selected for this evaluation was the newly written ISO 13679 procedure for full scale testing of casing and tubing connections. The ISO procedure requires a variety of tests that includes makeup/breakout testing, internal gas sealability/external water sealability testing with axial tension, axial compression, bending, internal gas thermal cycle tests and limit load (failure) tests. This test was performed with four coupled samples and included most of these loads. Two integral samples were also included for limit load testing ISO makeup/breakout tests are divided into three types--initial makeup, IML1, repeated makeup within the same sample, MBL, and repeated makeup using several samples called round robin, RR. IMU and MBL were performed in this project. The ISO sealing and structural procedure is divided into four primary tests and identified as Series A, B, C and Limit Load (failure). Series A and B test to 95% actual yield of the pipe and Series C uses 90% of actual yield. Samples 1 and 3 were tested to Series A and the loads are shown in Figure 1. For these samples, the axial compression was limited to 75% pipe body yield, which was set by Mr. Watts at the beginning of the test. Samples 2 and 4 were tested to Series B with loads shown in Figure 2. This series included 20 degrees per 100 feet bending but no external pressure. Due to premature leaks, no samples were subjected to Series C which included mechanical and thermal cycles. Samples 5 and 6 were tested to failure. The project started with the selection and purchase of a popular size of oilfield pipe, which was 7-inch OD, 32 pound per foot, P-110 casing. While the connections were being threaded, material tensile tests were performed to get the actual strength of the 7-inch pipe. The first samples contained a square thread form. Excessive galling was experienced during the first series of makeup/breakout tests and Mr. Watts decided to change the thread form and remachine the samples. The second samples had a round thread form and performed very well in the makeup/breakout tests. Basically no galling occurred of any consequence. Samples 1 and 3 were to be tested with external water (ISO Series A) while samples 2 and 4 were to be tested with bending (ISO Series B, no external pressure). Testing of all four samples started with tension and internal gas pressure. During this initial pressure testing, samples 1, 3 and 4 developed leaks and the test was stopped before any external pressure or bending was applied. Sample 2 successfully tested to ISO Load Point 5 which included bending before developing a leak. Figure 3 shows the loads at which the samples leaked and the relative pipe body performance capability. Sample 1 and end A of sample 2 held a high pressure while samples 3, 4 and end B of sample 2 leaked at relatively low pressures. All of these leaks were with nitrogen gas pressure. After reviewing the results, it was believed that several conditions may have contributed to the premature leaks: The BOL 2000 thread lubricant contains solid particles that are larger than the solid particles in API 5A2 compound. Possibly the large particles prevented tight thread contact necessary for gas leak tightness. The BOL 2000 lubricant may have caused or at least contributed to the yielding of the pin end during makeup. Pin inward deformation was found in some pins subsequent to testing and breakout. Excessive yielding of the pin or box will contribute to poor thread contact and leakage. The connections were made up to full or near full shoulder contact. This causes the end of the pin to be stressed near the yield strength. This along with the interference caused by the thread compound may cause excessive pin hoop stresses and yielding which in turn gives relatively loose thread contact. This behavior is present when repeated makeup and breakout occurs.

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  • Report No.: NONE
  • Grant Number: FG01-95EE15608
  • DOI: 10.2172/770563 | External Link
  • Office of Scientific & Technical Information Report Number: 770563
  • Archival Resource Key: ark:/67531/metadc719246

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Creation Date

  • August 1, 1999

Added to The UNT Digital Library

  • Sept. 29, 2015, 5:31 a.m.

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  • Dec. 9, 2016, 6:59 p.m.

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Watts, John D. Results of Performance Tests Performed on the John Watts Casing Connection on 7" Pipe, report, August 1, 1999; United States. (digital.library.unt.edu/ark:/67531/metadc719246/: accessed September 21, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.