Comparison of Methods for Evaluation of Crack Growth at Welds in Offshore Risers
EVENT: ASME Pressure Vessel & Piping
1 Jul 2015
In checking the fitness of fatigue critical welded structure, the stress concentration at the weld due to the weld geometry needs to be considered. Where fatigue is assessed using crack growth methodology, two approaches are commonly used. In the offshore industry in regions where BS7910  is followed, the effect of weld geometry is assessed using the Mk factor approach. The Mk factor directly magnifies the stress intensity. Mk factor solutions are available for T-butt weld joints from the British Standard BS7910.
Alternatively, API579  offers stress intensity solutions that can account for the stress profile through the wall thickness of the pipe. In using this method, the engineer will use an FEA program to find the stress profile for use as an input for the stress intensity factor computation. Since the goal is the assessment of crack growth, the stress profile must represent the cyclic changes in stress. Further, a histogram of such profiles is required.
While the Mk factor approach of BS7910 offers the easier path by supplying factors for pre-solved geometries, the API approach offers an opportunity to refine the solution by conducting relatively simple linear FEA of the un-cracked component. This study compares the two approaches using an example taken from offshore riser fatigue analysis.