TRI-AXIAL TENSION STRENGTH OF THICK-WALLED CASING STRINGS BASED ON UNIFIED STRENGTH THEORY

In order to study mechanical properties of thick walled casing strings in oil and gas wells, a computational formula for the tri-axial tension strength of thick walled casing strings is deduced according to unified strength theory, considering the impacts of the intermediate principal stress effect and tension/compression ratio of materials. In addition, the influences of various factors on the tri-axial tension strength of casing strings under different load conditions are analyzed, including the internal pressure/external pressure ratio, tension/compression ratio, intermediate principal stress, and diameter/thickness ratio. The results show that the strength criterion has a significant influence on the tri-axial tension strength of casing strings. The strength disparity (SD) effect of materials also has a conspicuous impact on tri-axial tension strength. The tri-axial tension strength increases almost linearly with the growth of tension/compression ratios if the internal pressure/external pressure ratio is less than or equal to 1, while the influence of the SD effect of materials is related to loads when the internal pressure/external pressure ratio is more than 1. By considering the intermediate principal stress effect, full use can be made of the self-bearing capacity and strength potential of materials, so as to improve the tri-axial tension strength of casing strings and to bring certain benefits. The internal pressure/external pressure ratio and diameter/thickness ratio also have certain influences on the tri-axial tension strength of casing strings. The applicability of the proposed computational formula is broader and has unified the various expressions for the tri-axial tension strength of casing strings by using different yield criteria. Therefore, it is suitable for all kinds of materials which have both SD and intermediate principal stress effects, and it will assist greatly in the design of thick walled casing strings in oil and gas wells.