Analytical Study of Torsional Capacity in API 6BX Type Flange Design and Comparison With FEA

Abstract Flanged connections are critical in high pressure applications such as subsea trees or riser systems. A bolted flange may be subjected to the axial tensile, torsional and bending external loads in service which depends on the orientation of the load path in application. Therefore, the axial tensile, torsional and bending resistance of the bolted flange is vital for the flange strength and reliability. Technical reports API TR 6AF or 6AF2 capacity chart does not include any torsion capacity in the chart. A standard API 6BX flange does not have a feature specifically designed to withstand torsional loading. Torsional loads are predominantly transferred through the mating flange pair via the gasket and raised face. This paper explores the torsion capabilities of API 6BX type flanges based on internal pressure, and external loading, including tension, and bending moment. The objective is to develop an analytical methodology for estimating flange capacities under combined loading without relying on Finite Element Analysis (FEA). The reaction force on the raised face and gasket are calculated based on the required pressure, applied tension, and bending, which in turn predict the torsional capacity. The analytical results are validated through comparison with numerical FEA results for various sizes of the API flanges.