Page 25 - 6634

atmospheric pressure and tidal effects. But current is also present in the subsurface

and seafloor region.

In the early years of offshore development, the common belief was that

currents are confined to the upper waters of the ocean and practically no current

exists below a water depth of about 1000 m. In recent days, however, it is

recognised that a number of classes of currents exist in the deep waters, and some

are known to extend to large depths. Improved definition and knowledge of these

currents and their sources will lead to an improved design criteria for offshore

structures extending in ultra-deep water. Examples of these classes of currents are

tropical cyclones such as hurricanes, extratropical cyclones, and cold air outbreaks

and currents arising from major surface circulation features. The most common

categories of current are:

wind-generated currents,
tidal currents (associated with astronomical tides).

circulational currents (associated with oceanic circulation patterns),
loop and eddy currents, and

soliton currents.
In most cases current is turbulent, but is generally approximated by the

corresponding mean flow. In the design of offshore structures, it is customary to

consider current as time invariant. For the design value, a 100 yr current is often

chosen. The environmental conditions in design are obtained from the site-specific

data. Therefore, it is difficult to be too specific in terms of the magnitude of current

in offshore locations. However, some order of magnitudes of different types of

current prevalent in the open oceans may be cited.

A surface current speed with a 10-year return period should be used in the

offshore structure design, based on the marginal distribution of current speeds at

the location. In certain geographical areas, current loads can be the governing

design loads. In areas where the current speed is high, and the sea states are

represented with small wave heights, e.g. West Africa, an environmental condition
25