ssc-367 - Ship Structure Committee

turbulence largely a function of terrain roughness. In an ocean
environment, the wave profi1e makes prediction of wind
characteristics more difficult. As the wind speed increases, the
wave height also increases, thereby increasing the surface
roughness. A surface roughness parameter is used as a measure of the
retarding effect of water surface on the wind speed.
A simple relationship developed by Charnock (ReferenceA.9) is often
used to define the surface roughness parameter and the frictional
velocity in terms of mean wind speed. Further discussion on surface
roughness parameter and drag factor is presented in an ESDU document
(Reference A.10).
Full scale experiments carried out by Bell and Shears (Reference
All) may indicate that although turbulence will decay with the
distance above sea surface, it may be reasonably constant to heights
that are applicable for offshore structures.
Considering that wind flow characteristics are primarily influenced
by energy loss due to surface friction, the mean wind profile for an
ocean environment may be assumed to be similar to that on land and to
follow this power law:
v mz
= Vmzl (z/zL) u
where:
v mz
= mean wind velocity at height z above LAT
v mzl
= mean wind velocity of reference height above
LAT
= height at point under consideration above
IAT
A-24