Application of Actual In-Service Spectrum to SCR Girth Welds

Abstract

Fatigue is normally the limiting design criterion for Steel Catenary Risers (SCRs) and it represents its major engineering challenge. As a consequence, design of this component generally trusts on a very good fatigue resistance. A common practice, widely adopted in oil recovery industry, is to certify that specific welding procedures have proper fatigue strength equal or better than the one adopted in design; such fatigue strength is commonly evaluated under constant amplitude loading. However, SCRs are subjected to complex loading spectrums which are characterized by a wide range of loading amplitudes induced by different sources that include the overall system response of the barge. Therefore, interest arises in verifying the component response to representative loading spectrums of the actual SCR in-service conditions and determining if actual riser components qualification, under constant amplitude loading, presents discrepancies with their resistance under in-service fatigue conditions. This situation has motivated full scale fatigue performance evaluation of SCR girth welds under constant and variable amplitude loading spectrums. The experimental approach was focused on estimating the damage introduced by loading cycles of various magnitudes. Constant and variable amplitude results were compared and the accuracy of Miner’s linear cumulative damage rule has been evaluated. The experimental approach was focused on estimating the damage introduced by loading cycles of various magnitudes. Constant and variable amplitude results have been compared and the accuracy of Miner’s linear cumulative damage rule [1] has been evaluated.