There are several models of wind force that are or were commonly used.
There is a need for determining not only the mean or steady wind forces, but also the dynamic wind forces, particularly as these forces may act at natural frequencies of the system. The dynamic wind forces arise from the turbulence in the wind field.
All sorts of complicated physical mechanisms are involved in affecting the 3-dimensional wind field over some region of water at any given time (high atmosphere winds, barometric pressure, density stratification, air-sea interactions, proximity to land, proximity to other structures, etc.), for engineering purposes we define certain design scenarios (hurricane winds, squalls, gusts) and describe these with standard models with parameters that can be set based on local historical measurements.
There is a need for determining not only the mean or steady wind forces, but also the dynamic wind forces, particularly as these forces may act at natural frequencies of the system. The dynamic wind forces arise from the turbulence in the wind field.
All sorts of complicated physical mechanisms are involved in affecting the 3-dimensional wind field over some region of water at any given time (high atmosphere winds, barometric pressure, density stratification, air-sea interactions, proximity to land, proximity to other structures, etc.), for engineering purposes we define certain design scenarios (hurricane winds, squalls, gusts) and describe these with standard models with parameters that can be set based on local historical measurements.
API Wind Model:
Assume wind speed is horizontally uniform but varies with height about the sea surface according to a power law profile,
Where z is the elevation about mean water level and by convention the reference elevation Zref=10m, and the mean speed at that reference elevation is Uref. Over bar denotes a time-average over one-hour. Common value for a=0.125.
Forristall (1988) analyzed and proposed that the wind spectrum should be modeled as
Forristall (1988) analyzed and proposed that the wind spectrum should be modeled as
Norwegian Petroleum Directorate (NPD)
One-hour mean wind profile is given by
And the spectrum is
Where n=0.468 and
The NPD model, compared to API spectrum, shifts even more energy to the low frequency end, a degree of conservatism that seems to make designers more comfortable.
The following figure is drawn from NOAA model product for South US coasts.
The following figure is drawn from NOAA model product for South US coasts.