Triaxial Leak Criterion for Generic Premium Connection Under Combined Tension and Pressure Loads

Abstract

Abstract Recent papers introduced a triaxial leak criterion with hydrostatic dependence through the Mean Normal Stress P defined as the average of the three normal stresses. The new criterion with linear dependence on P required two leak constants. Example applications in these papers addressed American Petroleum Institute (API) connections. This paper applies the triaxial criterion to a generic premium connection and determines the leak constants from finite element analysis (FEA) for combined loads with tension. Instead of a linear dependence on P, the FEA results for the generic premium connection correlate better with a quadratic relationship in terms of (P - σn) where σn is the Lubinski neutral stress. The quantity (P - σn) is proportional to effective stress. Quadratic dependence means that three leak constants are needed for premium connections. A plot of the leak limit curve superimposed on the pipe-body yield envelope shows load combinations where leak occurs before the pipe yields. A key conclusion is that internal leak cannot occur without tension for the loads investigated. Indeed, with sufficient tension, both internal and external leak can occur at low pressures for this modified generic premium connection. Actual premium connections should be designed so this triaxial limit curve for leak is outside (or mostly outside) the pipe-body failure envelope.