3 October 2005
Wind Gust Fronts. A particularly dangerous wind condition that has
caused a number of mooring accidents is the wind gust front (Mooring Dynamics Due to
Wind Gust Fronts, Seelig and Headland, 1998 and CHESNAVFACENGCOM, FPO-1-
87(1), Failure Analysis of Hawsers on BOBO Class MSC Ships at Tinian on 7
December 1986). This is a sudden change in wind speed that is usually associated with
a change in wind direction (Wind Effects on Structures, Simiu and Scanlan, 1996). The
key problems with this phenomena are: (1) high mooring dynamic loads can be
produced in a wind gust front, (2) there is often little warning, (3) little is known about
wind gust fronts, and (4) no design criteria for these events have been established.
A study of Guam Agana National Air Station (NAS) wind records was
performed to obtain some statistics of wind gust fronts (National Climatic Data Center
(NCDC), Letter Report E/CC31:MJC, 1987). The 4.5 years of records analyzed from
1982 through 1986 showed approximately 500 cases of sudden wind speed change,
which were associated with a shift in wind direction. These wind shifts predominately
occurred in 1 minute or less and never took longer than 2 minutes to reach maximum
wind speed. Figure 3-7 shows sudden changes in wind speed and direction that
occurred over a 2-1/2 day period in October 1982. These wind gust fronts seemed to be
associated with a nearby typhoon.
Table 3-13 gives the joint distribution of wind shifts in terms of the amount
the increase in wind speed and the wind direction change. Approximately 60 percent of
the wind gust fronts from 1982 through 1986 had wind direction changes in the 30-
degree range, as shown in Figure 3-8.
Based on the Guam observations, the initial wind speed in a wind gust
front ranges from 0 to 75 percent of the maximum wind speed, as shown in Figure 3-9.
On the average, the initial wind speed was 48 percent of the maximum in the 4.5-year
sample from Guam (NCDC, 1987).
Simiu and Scanlan (1996) report wind gust front increases in wind speed
ranging from 3 m/sec to 30 m/sec (i.e., 6 to 60 knots). Figure 3-10 shows the
distribution of gust front winds from the 4.5-year sample from 1982 through 1986 on
Guam. This figure shows the probability of exceedence on the x-axis in a logarithmic
format. The square of the wind gust front speed maximums was plotted on the y-axis,
since wind force is proportional to wind speed squared. Figure 3-10 provides a sample
of the maximum wind gust front distribution for a relatively short period at one site.
Those wind gust fronts that occurred when a typhoon was nearby are identified with an
"H." It can be seen that the majority of the higher gust front maximums were associated
with typhoons. Also, the typhoon gust front wind speed maxima seem to follow a
different distribution that the gust front maxima associated with rain and thunderstorms
(see Figure 3-10).
Effects of winds and wind gusts are shown in the examples in Chapter 8 of