ssc-367 - Ship Structure Committee

8.
FATIGUE DUETO VORTEX SHEDDING
This section specificallyaddresses fatigue due to vortex shedding.
Fatigue due to vortex-induced vibrations is different from other
forms of fatigue discussed in previous sections only in its loading
characteristics. Generally, relatively small number of slender
members are susceptible to vortex-induced fatigue. However,
response to vortex shedding cannot be predicted using conventional
dynamic analyses techniques because the problem is non-linear. In
compliancewith project objectives, a brief discussion is presented
on vortex sheddingphenomena,analysisand design, damage assessment
and avoidance. A comprehensive discussion, including example
problems, is presented in Appendix D.
VORTEX SHEDDING PHENOMENON
8.1.1
Background
Amember exposedto fluid flow may be subjectedto unsteadydrag and
lift forces caused by sheddingof vortices. While the vortices shed
are most often due to steadywind or current flow, the phenomena can
occur due to combined wave and current action. Depending on the
member’s natural frequency and the velocity of fluid flow, the
member may experience sustained vibrations.
Many structure members may be susceptible to vortex induced
vibrations (VIV). Relatively large diameter cylindrical brace
members of a fixed offshore platform can be designed to avoid VIV.
Component members of a cargo boom on a ship or the flare structure
on production units (FPSO, platform, etc.) are relatively slender
and can not be readily designed to avoid VIV. Then, they need to be
either designed to have adequate fatigue strength to resist the VIV
over the design life of the structure or provided with devices or
spoilers to modify the vortex shedding and/or member natural
frequencies.
8-1