The objective of this paper is to describe a methodology on how finite element analysis can be used to model the behaviour of collared joints, and to ultimately develop appropriate SIFs that can be used in pipe stress analyses. 

Stress Intensification Factors or SIFs allows piping to be analysed using beam theory, with a SIF representing local effects of specific piping geometry.  However, the current piping codes do not explicitly provide SIFs for collared type piping joints for use in pipe stress calculations. 

This paper describes a real-life analysis example on collar joints installed on a set of existing fuel transfer lines. The lines, which ranged in size from DN200 to DN350, were concrete lined carbon steel with the collars fillet welded to the carbon steel section of the piping. 

Test coupons made from existing pipe-collar sections were tested in a lab to determine the force required to break the collar welds. Using finite element analysis, the same test coupons were modelled to replicate the  failure tests. 

Multiple iterations were undertaken to determine an appropriate bi-linear stress-strain curve fit for the weld material. The curves of different weld electrode materials were considered. The curve which led to results similar to observed in physical testing was selected. From this, a failure stress across the weld could be determined. This stress, 435 MPa was then used in subsequent models to determine the point at which the weld fails under bending loads. Multiple tests were analysed to allow for possible effects of inclusions and voids. 

Finite element models of the collar geometries were constructed and non-linear analyses were undertaken using the weld strengths determined from the coupon testing data. A simple cantilever type ranagement with a point load at one end was analysed, inducing a bending moment across the collar. 

The point load was increased until the resulting bending moment across the collar at the centre of the cantilevered pipe arrangement experienced stresses in excess of 435 MPa, the weld failure stress. This was done for DN200, 250 and 350 pipe with various wall thicknesses and weld sizes. 

The standard weight DN250 and DN350 piping displayed a linear relationship between weld size and the required force to cause failure in the collar weld. 

The thinner walled DN250 pipe as well as the DN200 pipe analysed tended to buckle before the stress across the collar weld became high enough to induce failure.  In these cases, the collar is not the weakest point in the piping. 

Based on the results, a SIF was developed in accordance with ASME B31.3 for each collar geometry. Hence a pipe stress model of the transfer lines could ultimately be developed using these SIFs to predict the behaviour of the piping. 


Development of Stress Intensification Factors for Collared Type Piping Joints

Division / Sponsors: 
Pressure Vessels & Piping (PVP)


Paper Proceedings: https://asmedigitalcollection.asme.org/PVP/PVP2020/volume/83815

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