Assessment of Fatigue Damage from Variable Amplitude Loads in Risers

The in-service fatigue loading of risers may include contributions from first and second order vessel motions, wave loadings, vortex induced vibration (VIV), vortex induced motion (VIM), pressure variation and slugging. All of these loadings are variable amplitude random loadings where the sequence of cycles can change the fatigue outcome. The loading sequence effects can be addressed in analysis through cycle counting methods and through use of the most appropriate methods of damage accumulation. High loads can be responsible for either fatigue crack growth retardation or acceleration depending up loading sequence. The Rainflow Counting method takes into account the whole loading algorithm and assumes the structure memory sustains all through the loading. The Simple Range Count method counts cycles by every single reversals and does not take into account the load sequence or material memory. Fatigue damage accumulation can be accomplished using a linear model like Palmgren-Miner or a nonlinear model. Fatigue life can be estimated using crack growth methods, S-N or strain life methods. For each method some tools are available that can account for sequence effects. The fatigue analysis of the riser system is illustrated step by step. This study particularly looks at how different cycle counting methods affect the fatigue damage estimate obtained with the S-N approach, strain-life approach and fatigue crack growth approach. In the study Rainflow and Simple Range cycle counting methods are applied to variable amplitude loads typical of deepwater riser systems.