Pile Fatigue Failures. II: Mechanism Studies

Abstract

An analytical model of a vertical, singly standing pile is constructed to study the mechanism by which fatigue damage is produced in girth butt welds near the mudline. Top restraints are known to have been present on the terminal piles for a portion of the construction period. These are ignored to form an upper bound estimate of fatigue damage prior to the December 4–6, 1974 storm. Motion response and fatigue damage calculations with representative sea state spectra hindcast for the construction period suggest that failure of the first pile would not be expected prior to the December storm. Failure shortly after the storm can be demonstrated as a result of synchronization between the pile natural frequency and the vortex shedding frequency in waves at the storm peak. Physical observations reported from the field support the analytical findings. For a simple design tool, the potential synchronization region can be located on a scatter diagram plot of significant wave height versus spectral peak period.