FLUID-INDUCED STRUCTURAL FORCES
33
Winds and Currents
For a typical fixed-bottom offshore platform, the static drag force due to
wind on the superstructure amounts to about 15 percent of the total force on
the structure (Muga and Wilson, 1970), and often accounts for about 25 percent
of the total overturning moment (Graff, 1981). The wind-induced overturning
moment increases linearly with the height of the structure, and thus, as these
structures are built in deeper and deeper water, the effects of wind drag then
become increasingly significant in design.
A measure of wind velocity is needed to predict both wind loading on the
superstructure and to predict the magnitude of the wind-generated wave forces
on the submerged portion of the structure. A windstorm is often described as
airflow with a mean or steady velocity û(z), with a superimposed fluctuating
velocity. Here z is the height above the still water level. Gould and Abu-Sitta
(1980) point out that the averaging period of one hour has been used in Europe
and Canada in presenting data for â(z) = û(/i), for a reference height of either
z = h= 30 ft or 10 m. Gaythwaite (1981) States that in Great Britain values
of ü(h) chosen for structural design hâve traditionally been averaged over only
one minute. In the United States, however, the concept of the fastest mile of
wind speed is used to define ü(/i). That is, measures are made of wind velocity
during the time it takes for a mile of air to pass a ftxed point, and the annual
extreme condition is used as the reference value. Sachs (1972) and Simiu (1976)
discuss methods of converting such data to a mean velocity. Gaythwaite (1981)
suggests that ail but temporary marine structures should be designed using the
mean, fastest mile of wind speed associated with return periods of 50 to 100
years.
Once ü(/i) is established for a particular offshore site, the mean horizontal
wind velocity at height z above the sea surface is
ü(z)= (j1/nü(h)
(2.27)
The exponent n dépends on many factors. For instance, n = 3 fits data for
rough Coastal areas; n = 7 to 8 for sustained winds over an unobstructed sea;
and n = 12 to 13 for gusts. At heights of 100 ft or more above the surfaces,
the vertical gust-velocity becomes about the same as its horizontal value. Wind
data ü(h) and further discussions of équation (2.27) are given by Muga and
Wilson (1970), Sherlock (1953), Simiu and Scanlan (1978), and Vellozzi and
Cohen (1968).
On a member of the superstructure which is nine or more diameters removed
from neighboring structural éléments, û = û(z) calculated from équation (2.27)
can be used with équation (2.10) to estimate wind drag force, where Cd is based
both on Reynolds number and on the cross-sectional shape of the member.
Values of Cd for common shapes are readily available (Hoerner, 1965, and
Pattison et al., 1977).
For a truss structure in wind, the sum of the drag forces on each individual
member may give a low estimate of the total drag force. This effect, called
33
Winds and Currents
For a typical fixed-bottom offshore platform, the static drag force due to
wind on the superstructure amounts to about 15 percent of the total force on
the structure (Muga and Wilson, 1970), and often accounts for about 25 percent
of the total overturning moment (Graff, 1981). The wind-induced overturning
moment increases linearly with the height of the structure, and thus, as these
structures are built in deeper and deeper water, the effects of wind drag then
become increasingly significant in design.
A measure of wind velocity is needed to predict both wind loading on the
superstructure and to predict the magnitude of the wind-generated wave forces
on the submerged portion of the structure. A windstorm is often described as
airflow with a mean or steady velocity û(z), with a superimposed fluctuating
velocity. Here z is the height above the still water level. Gould and Abu-Sitta
(1980) point out that the averaging period of one hour has been used in Europe
and Canada in presenting data for â(z) = û(/i), for a reference height of either
z = h= 30 ft or 10 m. Gaythwaite (1981) States that in Great Britain values
of ü(h) chosen for structural design hâve traditionally been averaged over only
one minute. In the United States, however, the concept of the fastest mile of
wind speed is used to define ü(/i). That is, measures are made of wind velocity
during the time it takes for a mile of air to pass a ftxed point, and the annual
extreme condition is used as the reference value. Sachs (1972) and Simiu (1976)
discuss methods of converting such data to a mean velocity. Gaythwaite (1981)
suggests that ail but temporary marine structures should be designed using the
mean, fastest mile of wind speed associated with return periods of 50 to 100
years.
Once ü(/i) is established for a particular offshore site, the mean horizontal
wind velocity at height z above the sea surface is
ü(z)= (j1/nü(h)
(2.27)
The exponent n dépends on many factors. For instance, n = 3 fits data for
rough Coastal areas; n = 7 to 8 for sustained winds over an unobstructed sea;
and n = 12 to 13 for gusts. At heights of 100 ft or more above the surfaces,
the vertical gust-velocity becomes about the same as its horizontal value. Wind
data ü(h) and further discussions of équation (2.27) are given by Muga and
Wilson (1970), Sherlock (1953), Simiu and Scanlan (1978), and Vellozzi and
Cohen (1968).
On a member of the superstructure which is nine or more diameters removed
from neighboring structural éléments, û = û(z) calculated from équation (2.27)
can be used with équation (2.10) to estimate wind drag force, where Cd is based
both on Reynolds number and on the cross-sectional shape of the member.
Values of Cd for common shapes are readily available (Hoerner, 1965, and
Pattison et al., 1977).
For a truss structure in wind, the sum of the drag forces on each individual
member may give a low estimate of the total drag force. This effect, called