Page 137 - Marine Structural Design

Chapter 6 mshore Smtchral Analysis 113

For shius: A global extreme sea-state is used and imposed on the structure. Inertia loads are
calculated based on a conservatively assumed motion of the vessel.
For offshore structures: A different analysis is carried out and a sufficient number of periods
should be analyzed for the following reasons:
To adequately cover the site-specific wave conditions
0 To satisfactorily describe transfer functions at and around the wave cancellation and
amplification periods
To satisfactorily describe transfer functions at and around the heave resonance period of
the structure
Global wave frwuency: Structural responses should be established by an appropriate
methodology, for example:
A regular wave analysis
A design wave analysis
A stochastic wave analysis
Once the extreme waves are selected for a design, wave induced loads may be computed by
commercial programs, such as AQUA, WAMIT, etc. The phase angles of waves should be
represented properly. The structural members are therefore designed to withstand the
maximum stresses resulting from various phases of waves.
Wind Loads
Wind loads are usually considered static loads and are calculated based on the actual area and
wind pressure by simply using the following formula:
F~nd = 'knd . (6.20)
Pwind = v2 .c, .c, (6.21)

where,
V = Wind velocity
C,, = Height coefficient
C, = Shape coefficient

A,, = Projected area perpendicular to the wind direction
The height and shape coefficients are specified in classification rules. The quasi-static wind
pressure Eq.(6.21) was derived in accordance with Bernoulli's theorem for an idea fluid
striking an object which states that the dynamic pressure may be expressed as:

(6.22)
where, p denotes the mass density of air. Wind loads may be applied as surface loads if the
projected areas are modeled. In most cases, they are applied as horizontal concentrated loads
at appropriate elevations.