FLUID-INDUCED STRUCTURAL FORCES
35
design of floating and fixed structures. Offshore tidal currents, or the horizontal
water flow due to the vertical rise and fall of tides, often attain a maximum
velocity of 1 to 2 knots and may even reach 10 knots in sonie locations. Tidal
currents are higher in the spring than in any other time of the year. Data on
tidal currents for many offshore locations on the coasts of the United States and
Asia are published annually by the National Oceanic and Atmospheric Administration, U.S. Department of Commerce. Data for the tidal current ut(z) as a
function of water depth z generally follows a power law similar to that for wind,
équation (2.27). That is
ut(z)= 11 +-)
ut(0)
(2.30)
\
a/
where d is the total water depth, z is the coordinate of the tidal velocity (a
négative number measured downward from the sea surface), ut(0) is the tidal
velocity at the surface z = 0, and ut(—d) — 0 at the seafloor.
The other important current is uw(z), the current generated by a sustained
wind blowing over the sea surface. The velocity profile of this wind-stress current
is approximated as linear with depth, with a maximum value uw(0) at the sea
surface, where uw( —d) = 0 at the seafloor. That is
uw(z) = (1 + 3) Uw(0)
(2-31)
\
dJ
The magnitude of uw(z) is generally about 1 to 5 percent of the sustained wind
speed.
In the absence of vortex shedding, the steady-state drag force per unit length
at the depth location z of a stationary, submerged, tubular member can be
calculated from équation (2.10) in which the effective fluid velocity u is the sum
of the two current velocities given by the last two équations, and the horizontal
wave particle velocity, uwave, is discussed in Chapter 3. That is,
u = Ç1 + -)
uj(0) + ^1 + -J um(0) • uwave
(2.32)
With the design values for Ut(0), uw(0), and uwaVe, and with a knowledge of the
geometry for ail the structure’s tubular members whose longitudinal axes are
perpendicular to u, the total horizontal drag force from those tubular members
of the structure is obtained by using équation (2.32) with équation (2.10) and
integrating the resuit over those tubular members.
Currents do affect structural integrity in other ways. Current-induced scouring, for instance, can undermine pile-supported jacket template platforms and
gravity platforms by erroding surrounding sand and soil. Currents carry ice that
can impact and damage structures. High currents accelerate the corrosion rate
of submerged métal structures. Currents also modify waves and wave loading
of structures (Tung, 1974). Except for the impact of ice, which is discussed
briefly in the next section, these environmental hazards to offshore structures
are subjects that are beyond the scope of this book.
35
design of floating and fixed structures. Offshore tidal currents, or the horizontal
water flow due to the vertical rise and fall of tides, often attain a maximum
velocity of 1 to 2 knots and may even reach 10 knots in sonie locations. Tidal
currents are higher in the spring than in any other time of the year. Data on
tidal currents for many offshore locations on the coasts of the United States and
Asia are published annually by the National Oceanic and Atmospheric Administration, U.S. Department of Commerce. Data for the tidal current ut(z) as a
function of water depth z generally follows a power law similar to that for wind,
équation (2.27). That is
ut(z)= 11 +-)
ut(0)
(2.30)
\
a/
where d is the total water depth, z is the coordinate of the tidal velocity (a
négative number measured downward from the sea surface), ut(0) is the tidal
velocity at the surface z = 0, and ut(—d) — 0 at the seafloor.
The other important current is uw(z), the current generated by a sustained
wind blowing over the sea surface. The velocity profile of this wind-stress current
is approximated as linear with depth, with a maximum value uw(0) at the sea
surface, where uw( —d) = 0 at the seafloor. That is
uw(z) = (1 + 3) Uw(0)
(2-31)
\
dJ
The magnitude of uw(z) is generally about 1 to 5 percent of the sustained wind
speed.
In the absence of vortex shedding, the steady-state drag force per unit length
at the depth location z of a stationary, submerged, tubular member can be
calculated from équation (2.10) in which the effective fluid velocity u is the sum
of the two current velocities given by the last two équations, and the horizontal
wave particle velocity, uwave, is discussed in Chapter 3. That is,
u = Ç1 + -)
uj(0) + ^1 + -J um(0) • uwave
(2.32)
With the design values for Ut(0), uw(0), and uwaVe, and with a knowledge of the
geometry for ail the structure’s tubular members whose longitudinal axes are
perpendicular to u, the total horizontal drag force from those tubular members
of the structure is obtained by using équation (2.32) with équation (2.10) and
integrating the resuit over those tubular members.
Currents do affect structural integrity in other ways. Current-induced scouring, for instance, can undermine pile-supported jacket template platforms and
gravity platforms by erroding surrounding sand and soil. Currents carry ice that
can impact and damage structures. High currents accelerate the corrosion rate
of submerged métal structures. Currents also modify waves and wave loading
of structures (Tung, 1974). Except for the impact of ice, which is discussed
briefly in the next section, these environmental hazards to offshore structures
are subjects that are beyond the scope of this book.