Efficient fatigue analysis of helix elements in umbilicals and flexible risers: Theory and applications

Fatigue analysis of structural components such as helix tensile armors and steel tubes is a critical design issue for dynamic umbilicals and flexible pipes. the basis for assessment of fatigue damage of such elements is the long-term stress cycle distribution at critical locations on the helix elements caused by long-term environmental loading on the system. the long-term stress cycle distribution will hence require global dynamic time domain analysis followed by a detailed cross-sectional analysis in a large number of irregular sea states. an overall computational consistent and efficient fatigue analysis scheme is outlined with due regard of the cross-sectional analysis technique required for fatigue stress calculation with particular attention to the helix elements. the global cross-section is exposed to pure bending,tensile,torsion,and pressure loading. the state of the different cross-section elements is based on the global response. special emphasis is placed on assessment of friction stresses caused by the stick-slip behavior of helix elements in bending that are of special importance for fatigue life assessments. the described cross-sectional analysis techniques are based on an extensive literature survey and are hence considered to represent industry consensus. the performance of the described calculation scheme is illustrated by case studies. copyright © 2012 geir skeie et al.