Development of BOP API Flange Capacity Using Simulation Approach

Abstract Flanges commonly selected for use in offshore equipment are based on the designs found in American Petroleum Institute (API) standards and depend on the predicted load during planned operations and capacity of the flange. The flange capacity is derived from standard analysis techniques provided by API Technical Reports 6AF, 6AF1, and 6AF2. It is however possible to conduct analysis to determine the flange capacities using more sophisticated methods to improve the accuracy of the analysis which can potentially lead to deriving higher capacity results that are less conservative. This paper presents a detailed analysis of an API 18-3/4" 15 ksi BX-164 flange capacity at ambient and an elevated temperature of 375°F. It also clarifies stress and leakage criteria and details the discrepancies in analysis methodology and capacity results between API 6AF2 and ourapproach. To calculate the capacity of an API 18-3/4" 15 ksi BX-164 flange, a 180-degree 3D model is created to calculate the detailed load and stress distribution under different combinations of loadsincluding a flange, hub, and gasket. Both flange and hub are modeled with linear elastic material properties while the gasket is modeled with elastic-plastic material properties. For the flange capacity at elevated temperatures, a steady state decoupled thermal analysis is performed first to obtain the temperature gradient and subsequent thermal stress. The material properties are derated due to elevated temperature per API 6MET. An elastic-plastic approach is then employed to calculate the capacity under a series of load combinations of pressure, tension and bending, using the design factors defined in ASME BPVC, Section VIII, Division 2. The analysis result is based on both stress and leakagecriteria to determine the limiting capacity under various load cases. This work presents a capacity chart that shows the relationship between pressure and bending under various tensions at both ambient and elevated temperatures and further clarifies the stress and leakage criteria. Results show that the overall capacity at ambient temperature based on this analysis is significantly higher than published API reports. Additionally, the capacity of the flanged connection at 375°F is reduced when compared to the capacity at ambient temperature. The capacity reduction from elevated temperature is more pronounced at lower pressures. The flange capacity at both ambient and elevated temperatures is governed by bolt stress criteria. As publicly available capacity charts for API 18-3/4" 15 ksi BX-164 flanges calculated using modern finite element approaches are rare, the analysis method presented in this paper defines a process to calculate the flange capacity at elevated temperatures. This approach can potentially lead to deriving higher capacity results that are less conservative which aids in operational and well design decisions.