ssc-367 - Ship Structure Committee

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of cycles. Thus, the reduced velocity necessary for the onset of
cross-flow vibrations in steady current should be avoided.
0.3.3 Critical Flow Velocities
The criteria for determining the critical flow velocities for the
onset of VW can be expressed in terms of the reduced velocity
(Section D.2):
v cr = (Vr)cr (fn* d)
where:
(Vr) Cr = 1.2 for in-line oscillations in water
= 1.7 for in-line oscillations in air
= 3.9 for cross-flow oscillations in water
=
4.7 for cross-flow oscillations in air
0.4. AMPLITUDES OF VIBRATION
Amplitudes of vibrations can be determined by several methods. A DnV
proposed procedure (Reference 0.8) is simple to apply and allows
determination of member natural frequencies, critical velocities and
maximum amplitudes of vortex-shedding induced oscillations. The
procedure yields consistent results, comparable to the results
obtained by other methods, except for oscillation amplitudes. The
DnV calculation of oscillation amplitudes is based on a dynamic load
factor of a resonant, damped, single-degree-of-freedomsystem. this
approach is not valid unless the nonlinear relationship between the
response and damping ratio is known and accounted for. Consequently,
in-line and cross-flow vortex shedding amplitudes are assessed
separately.
D-9