Winds

UFC 4-159-03

3 October 2005

3-8

ENVIRONMENTAL FORCING DESIGN CONSIDERATIONS.

Environmental forces acting on a moored ship(s) can be complex. Winds, currents,

water levels, and waves are especially important for many designs. Specific

environmental design criteria for selected sites of interest can be found in the

Climatalogical Database (WATERS TOOLBOX)

3-8.1

Winds. A change in pressure from one point on the earth to another

causes the wind to blow. Turbulence is carried along with the overall wind flow to

produce wind gusts. If the mean wind speed and direction do not change very rapidly

with time, the winds are referred to as "stationary."

Practical experience has shown that wind gusts with a duration of

approximately 30 seconds or longer have a significant influence on typical moored ships

with displacements of about 1000 tons or larger. Vessels with shorter natural periods

can respond to shorter duration gusts. For the purposes of this UFC, a 30-second wind

duration at a 10-meter (33-foot) elevation is recommended for the design for "stationary"

winds. The relationship of the 30-second wind to other wind durations is shown in

Figure 3-5.

If wind speed and/or direction changes rapidly, such as in a wind gust

front, hurricane or tornado, then winds are "non-stationary". Figure 3-6, for example,

shows a recording from typhoon OMAR in 1992 at Guam. The eye of this storm went

over the recording site. The upper portion of this figure shows the wind speed and the

lower portion of the figure is the wind direction. Time on the chart recorder proceeds

from right to left. This hurricane had rapid changes in wind speed and direction. As the

eye passes there is also a large-scale change in wind speed and direction.

42