Comparison of Sea State Models in Spectral Fatigue Analysis Software Validation with Naval Vessel Sea Trial Data

Ian Thompson

DRDC Atlantic, Dartmouth, NS, Canada, ian.thompson@drdc-rddc.gc.ca

ABSTRACT

Fatigue damage is the governing factor in a naval ship’s design life, can limit the vessel’s operational capability and availability, and can significantly affect through-life maintenance costs. The development of reliable tools to assess a naval ship’s fatigue life will enable better management of aging fleets and help avoid failure-prone areas in new designs. Spectral fatigue analysis of ships involves applying many hydrodynamic loads due to waves to a finite element model. The resulting stress response amplitude operators are combined with wave data from a specified operational profile to determine the stress spectra used along with S-N curves to estimate the fatigue crack initiation life. Data from a naval vessel sea trial are used to validate STRUC_R, spectral fatigue analysis software developed by Cooperative Research Ships (CRS). Hydrodynamic loads are calculated using PRECAL_R, the frequency domain hydrodynamic code also developed within CRS. The calculated stress spectra are compared with measured stresses at several locations on the ship. The effects of using different seaway models of measured conditions are compared, particularly for long-crested and two-dimensional wave spectrum models. As most available wave data are one-dimensional, this will provide a better understanding of the uncertainty the long-crested wave assumption introduces in naval vessel fatigue life calculations.