An evaluation of <i>p</i>–<i>y</i> curves for fatigue analysis of offshore piles and conductors

The paper evaluates methods for deriving p– y curves for fatigue analysis of slender piles and conductors. Updated ISO standard guidance is compared to a new approach, which is validated by centrifuge tests and based on laboratory measurements of stiffness. This new approach provides a more accurate match to previously-published data on which the ISO guidelines are based. Additionally, a third method is examined—the parallel-Iwan critical state inspired (PICSI) framework—which captures changes in p– y response through cyclic loading and consolidation. Bending moments from a pile–soil Finite Element (FE) analysis using the three methods are compared to data from flexible piles centrifuge testing. Load history effects are evident, with the bending moment profile for a given cyclic amplitude being altered by prior larger cycles, through a changing p– y response. The resulting evolution of the bending moment profile showed migration of the fatigue hotspot along the pile, which improves fatigue life. Whereas the ISO fatigue p– y curves over-predicted the observed stiffness, the alternative simplified p– y method matched the bending moments to ±25% accuracy, and the PICSI framework gave the most accurate bending moment predictions across the full test sequence. These latter two methods are recommended for design use in preference to the ISO approach.