scope:

SUMMARY

This report documents the analysis work done at Stress Engineering Services to establish the load capacity of all threaded flanges given in the October, 1989 Edition of API 6A (Sixteenth Edition) that violate the maximum bore requirement. A total of 12 threaded flanges fit that category when used with line pipe, short or long casing, and non-upset or upset tubing. The capacity of the flange is calculated by a finite element analysis while the joint capacity is evaluated based on strength of material calculations.

In order to calculate the flange capacity, the geometry models of PRAC 86-21 are modified and analysand. The various loads considered were: Bolt Makeup (Paroled), Internal Pressure, Tension, and Bending Moment. The way these loads were applied was also modified since the load is transferred to the flange through the threaded connection. All 12 flanges were analyzed with an axisymmetric finite element model for each of the four load cases. A post-processor program was used to calculate the maximum moment capacity for various levels of pressure and tension, based on linear superposition of results. Three different criteria were used to establish the maximum moment: 1) ASME Section VIII, Division 2 allowable stress categories for the flange with the basic membrane stress allowable established by API, 2) Allowable bolt stresses as established by API, and 3) Loss of preload on the ring joint. The results of this post-processing are presented in plots of pressure vs allowable moment for various tension levels in Appendix A. See Section 2.0 for details of the axisymmetric analysis and Section 3.0 for details of the load capacity calculations. Shrink/fit interference or pipe makeup was not included in the analysis, but it has been considered for one threaded flange. The results of the shrink/fit analysis are provided in Appendix B.

The second task of the study was to determine the effect overboring the flange on the threaded connection. The strength of the threaded pipe which is vital to the connection is established by calculation procedures and methodology similar to API Bulletin 5C3 which are based on ultimate strength. The calculation method for all five different threaded connections is presented in Section 4.0. A comparison of the threaded flange and threaded joint capacities is provided in the same section.