3 October 2005
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)
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
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.