American petroleum institute welding neck flange design for subsea pipelines

Flange design has been a subject of intensive research and discussion over many decades as engineers strive to maximize the capacity of flanges in application while reducing costs and providing effective seals. Calculation frameworks such as those contained within American Society of Mechanical Engineers (ASME) VIII have been used for a considerable number of years to provide reasonably accurate models for analysing the various factors contributing to flange selection. However, they do not cover all situations and this is especially significant within the oil and gas industry where American Petroleum Institute standards can 'confuse' the design and selection process for flanges. Therefore, in this article the authors present a revised approach to the design of welding neck flanges used in subsea applications that is based on the existing design approaches contained within ASME VIII, Kellogg, Kuppan and PD8010. This revised approach to welding neck flange design also examines the key influences of gasket geometry and loading, as well as the matching of a particular flange to its pipe capacity. Finally, results of the authors' calculations are compared against existing finite element-derived data contained within API 6AF.