Understanding of material properties for tubular design under HPHT conditions goes well beyond the basics of the classic methods routinely employed in the industry. The objective of this paper is to highlight several important characteristics of the stress-strain response of high strength tubular materials which differ at elevated temperatures relative to room temperature, and to demonstrate the significant impact these differences can have on the pipe body strength and connection mechanical/sealability performance under HPHT conditions.
Coupon test results of high strength tubulars commonly used in HPHT wells are presented to demonstrate the key differences in the stress-strain response at elevated temperatures versus room temperature. The tensile tests were also performed in a manner to emphasize the importance of strain rate effect on the material response. Analytical equations are used to demonstrate the strain rate effect on pipe body ductile rupture and collapse strengths. In addition, advanced non- linear Finite Element Analysis (FEA) results are presented to further illustrate the substantial impact that the temperature and rate-dependent material properties have on pipe body and connection performance in HPHT applications.
The coupon test results show a significant change in the tubular material response at elevated temperatures, such as reduction of key mechanical properties (e.g. Young’s modulus, yield and ultimate tensile strengths, etc.) and substantial strain rate effects at temperatures representative of HPHT conditions. Calculations using analytical equations provide a quantitative estimation of critical load thresholds, beyond which the structural integrity of tubulars may be subjected to increased failure risk when strain rate effect is not accounted for. Advanced FEA simulations provide a further understanding of the pipe body and connection response under HPHT load conditions. Based on the observations from this study, recommendations are made on a few special considerations in well tubular design for elevated temperature applications. In addition, the findings also provide the basis for a critical discussion of the applicable ASTM/API test standards and the need to understand the effect of different strain/loading rates that may be employed in material characterization and full-scale testing of tubular products at elevated temperatures.
Collectively the information and results presented in the paper are expected to be very useful to the new generation of engineers charged with the tubular design for challenging well applications involving elevated temperature and severe load conditions.
The paper raises an awareness of the importance of considering strain rate effects for tubular design at elevated temperatures. The outcome of this study will also be used to support the recommendation of using a yield strength determination closer to or at the material proportional limit rather than a constant strain value (per API 5CT (2018)) for specifying the load envelope to be employed when qualifying tubular connections for elevated temperature service.