Page 63 - Marine Structural Design

40 Part I Structural Design Principles

Ice
Earthquake
Marinegrowth
Some of the above mentioned items are detailed below.
Wind
Wind is a significant design factor. The wind conditions used in a design should be
appropriately determined from collected wind data and should be consistent with other
associated environmental parameters. Two methods are generally used to assess the effects of
wind in design:
Wind forces are treated as constant and calculated based on the 1-minute average velocity.
Fluctuating wind forces are calculated based on a steady component, the 1-hour average
velocity plus a time-varying component calculated from an empirical wind gust spectrum.
The choice of methods depends on the system’s parameters and goals of the analysis. Either
approach may give more severe load than the other, depending on the system‘s mooring and
the wind spectrum used. The design wind speed should refer to an elevation of 10 meters
above the still water level. Rapid changes of wind direction and resulting dynamic loads
should be considered in the design of offshore structures.

Waves
Wind-driven waves are a major component of environmental forces affecting offshore
structures. Such waves are random, varying in wave height/length, and may approach an
offshore structure from more than one direction simultaneously. Due to the random nature, the
sea-state is usually described in terms of a few statistical wave parameters such as significant
wave height, spectral peak period, spectral shape, and directionality, etc.
The calculation of extreme wave loads and their load effects may be based on selected short-
term sea-states. The overall objective of this approach is to estimate loads and load effects
corresponding to a prescribed annual exceedance probability, e.g. lo-’ or IO4, without having
to carry out a full long-term response analysis. This is the so-called design storm concept.
An appropriate formulation of the design storm concept is to use combinations of significant
wave height and peak period along a contour line in the HM and TP plane. Such a contour line
can be established in different ways. The simplest way to establish the contour line at a
probability level of 10” is to first estimate the value of HM along with the conditional
mean value of Tp. The contour line is then estimated from the joint probability model of Hmo
and Tp with constant probability density. An example of such a contour line is shown in Figure
3.1. The estimation of the load effect at the probability level of is then obtained by
determining a proper extreme value for all seastates along the contour line and taking the
maximum of these values.
Current
The most common categories of currents are:
Tidal currents, which are associated with astronomical tides
CircuIation currents, which are associated with oceanic-scale circulation patterns
Storm generated currents