The effect of higher harmonic forces on fatigue life of marine risers

Higher harmonic responses measured in flexible riser model tests conducted within the Norwegian Deepwater Programme (NDP) are studied to evaluate the effect of higher harmonics of force on the fatigue life of risers. A large third-harmonic contribution and sometimes a fifth-harmonic are observed for the majority of test cases. Since existing methods of fatigue life estimation account for the effect of hydrodynamic forces only at the frequency of vortex shedding, a new methodology is introduced which takes into account the influence of the higher harmonic components: First, a new response reconstruction technique is used to reconstruct the entire span-wise riser motion in both the cross-flow and in-line directions based on measurements at specific points along the length. Then, a force database obtained from forced in-line and cross-flow motions of rigid cylinders is used to estimate the higher harmonic components of the force along the entire riser length. A significant decrease in fatigue life is obtained when the higher harmonic components of the fluid forces are considered.