Premium connection designs are typically evaluated and qualified to broadly adopted industry standards, such as ISO 13679 (2019) and API RP 5C5 (2017) procedures for testing casing and tubing connection in High Pressure and High Temperature (HPHT) wells up to temperatures of 180°C, and ISO/PAS 12835 (2013) for testing casing connection in thermal wells that experience temperatures from 180°C to 350°C. The primary focus of these qualification protocols is to evaluate the sealing capacity and structural integrity of the candidate connection design under loads representative of the conditions that the connection will experience through the well's life cycle.
The test specimens consist of the coupling and the pipe segments on both sides of the coupling. While it may be desirable to evaluate test specimens with lengths equal to that of the field product to capture the temperature, pressure and mechanical loads on the specimen, it is advantageous to limit the length for purposes including handling and controlling the size and cost of the evaluation program. It has been observed that the test results can be affected by the specimen length, so the proper selection of specimen length is a key aspect of these evaluation programs. Current test protocols provide the requirement of a minimum unsupported length for allowing the tests to simulate the strain localization condition. On the other hand, if the unsupported length exceeds a critical value, the test specimens may experience lateral buckling, and preventing buckling adds complexity and cost to the test program. No guidelines have been given in the protocols on the maximum pup length requirement for preventing lateral buckling. Therefore, a better understanding of the impact of specimen length is warranted in order to achieve more reliable and accurate results from the testing program.
This paper presents an investigation of the effect of specimen length on the structural integrity and sealability of premium connections based on Finite Element Analysis (FEA). Parametric FEA was completed to determine the impact of specimen length for several sizes of a generic premium connection design under API RP 5C5 (2017) HPHT well and ISO/PAS 12835 (2013) thermal well conditions. Based on the analysis results, recommendations are made to improve and enhance the guidelines for identifying a suitable specimen length for the test component of an evaluation program.