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noted that even the mean wind flowing over a changing free surface produces a

fluctuating load due to the variation of the exposed structure with the wave. This

effect is sometimes considered. For a linear wave, this fluctuation may be

determined in a simple straightforward manner if the exposed surface is assumed

to vary sinusoidally.

The accepted steady wind speeds in a design of an offshore structure are

generally taken as the average speed occurring for a period of 1-h duration. The

steady speeds are considered to be the mean speed measured at a reference height,

typically 30 ft (10 m) above the mean still water level. A mean wind speed for a

100-year return period should be used in the design, based on the marginal

distribution of wind speeds at the specific location. The directionality of the wind

may be important in some applications. Wind load on the structure should be

treated as a steady component based on the above mean speed. Additionally, a load
with a time-varying wind component known as the gust should be calculated,

which generates low-frequency motion. The time varying wind is described by a
wind gust spectrum.

Gravity loads include dead loads, operating and equipment weights, live loads
and buoyancy loads. The dead loads include the permanent loads of the structure

and equipment and other fixtures that are not likely to vary during the service life

of the structure. Live loads include the variable loads due to liquid and solid

storage.

A floating structure when at rest in still water will experience hydrostatic

pressures on its submerged part, which act normal to the surface of the structure.

The forces generated from these pressures have a vertical component, which is

equal to the gravitational force acting on the mass of the structure. In other words,

for a freely floating structure, this force is equal to the displacement weight of the

structure. In other directions, the net force is zero.

In the design of offshore structures, current is generally considered time-

invariant represented by its mean value. The current strength, however, may have a
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