Recommended procedures and methods for design of subsea flanges

Subsea flanges are subject to:

• Internal pressure due to the contained liquid
• External hydrostatic pressure
• Axial loading due to residual lay tension and temperature effects in pipeline
• Bending moments due to thermal effects or misalignment of spool piece
• Thermal reduction in strength of flange and bolting material due to the temperature of the contained liquid
• Thermal stresses due to the temperature gradient between the inside and outside walls of the flange
• Shear across the joint, which can only be resisted by the gasket

It is normal to consider the following conditions as a minimum:

• Bolt tightening at installation
• Hydrotest
• Operational conditions

Our client required a set of recommended procedures and methods to be written for the design of both weld-neck and swivel ring flanges.  An assessment was undertaken of his existing software and methodology.

Standard US and metric flanges were assessed (API B16.5, API 605 API RP6, API 17D, EN 1092 and EN 1759), as well as non-standard designs.

The procedures include the use of standard classified flanges, as well as the design and assessment to PD 5500 or ASME VIII.  Where through-body thermal gradient effects are important, API TR 6AF is used.  Thus, the recommendations start with the simplest selection method through to a full-blown, finite-element design.  A flow-chart helps the designer through the stepwise process of flange design.

Permissible stresses in flanges use either the maximum shear stress or the von Mises equivalent stress approaches.  Care needs to be taken to avoid confusion between these methods and the correct selection of safe stresses.

Choice of appropriate stresses in bolts and gaskets is also included in the recommendations.  Gaskets of different sections and materials are assessed.  These included standard, oval and octagonal, self-energising gaskets of types RX, R and BX.  Self-testing gaskets were also included.