ssc-367 - Ship Structure Committee

typically from 0.6 to 0.8 and 1.5 to 2.0, respectively. Very
comprehensive experimental data obtained from full-scale
measurements of the second Christchurch Bay Tower (References5.9,
5.10 and 5.11) validate the coefficients in use today. As
illustrated on Figure 5-3, the Cd and the Cm values applicable for
most cylindrical members near the water surface (Level 3) are 0.66
and 1.8, respectively. Although these values are applicable for
Keulegan-Carpenter (Ke) number in excess of 30, even when Ke is
reduced to 5, the inertiacoefficient,Cm, value reaches 2.0, while
the drag coefficient, Cd, gradually increases to unity at Ke equal
to 10.
These coefficients also decrease with the distance from the water
surface. However,becausethe uncertaintiesinmarine growth (which
directly affects the surface roughness and therefore the drag
coefficient)and the additionaleffortnecessaryto input,check and
justify different coefficients,it is advisableto use one set ofC~
and Cm values.
The use of conventionalMorison’s equation and the wave kinematics
for regular two-dimensionalwaves has proven to be valid for jacket
structures in moderate water depths. Assessment of measureclwave
forcedata (Reference5.12) for extremewave loading associatedwith
directionally spread seas in a hurricaneenvironment in the Gulfof
Mexico compares quite well with those analytically computed.
Morison’s equation is
cylindrical members by
also valid to compute forces on nonapplying
appropriate Cd and Cm values and
equivalent diameters. Suitable values of Cd and Cm for different
cross-sections may be obtained from a Det norske Veritas (DnV)
document (Reference4.16.)
If the extreme loadings are to be computed, an applicable wave
theory, compatiblewith the wave steepness,water depth, etc., must
be used. The applied total load on a structure composed of many
members is then the cumulative sum of loads computed on each member
5-18