ssc-367 - Ship Structure Committee

vortex shedding will occur, i.e., Vmin = ‘r(req’d) x ‘n x ‘-
e.
Check the applied velocity profile to see if Vmax is greater
than Vmin. If Vmax is less than Vmin, then no vortex
oscillations can occur.
f.
For Vmax greater than Vmin, vortex oscillations can occur. The
displacement amplitude is based on stability parameter Ks, and
is determined from Figure O-2 for in-line vibration. A
conservative approach is used to determine Y/d vs Ks. For Ks <
0.6 the first instability region curve is used. For Ks > 0.6
the second instability region curve is used. This
conservatively represents an envelope of maximum values of Y/d
vs Ks from Figure O-2. Displacement amplitude is normalized to
Y/d.
9*
Given (Y/d), calculate the bending stress, fb.
h.
Multiply bending stress fb by an SCF of 1.S to produce hOt SpOt
stress fH. A larger
SCF will be used where necessary.
i.
From the maximum hot spot stress, the hot spot stress range is
calculated as 2fH.
j.
Allowable number of cycles to failure (N) should be calculated
using an applicable S-N curve (based on weld type and
environment).
k.
Assume conditions conducive to resonant vortex shedding occur
for a total time of T (seconds) per annum (based on current or
wind data relevant to applicable loading condition).
1.
Hence, in time T, number of cycles n = fnT and the cumulative
damage D1 = n/N = fnT/N in one year.
Step 2:
D-15