Page 69 - Marine Structural Design

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3.3 Environmental Loads and Floating Structure Dynamics

3.3.1 Environmental Loads
According to API Rp 2T (1997), the environmental loads to be considered in the design of
offshore structures include,
wind forces
current forces
wave loads
ice loads
wave impact forces
earthquakes
accidental loads
fire and blast loading
3.3.2 Sea loads on Slender Structures

For slender structures such as jackets, jack-ups, pipelines, risers and mooring lines, viscous
flow phenomena are of importance. Wave loads on slender structures may be predicted using
Morison equation, see Sarpkaya and Isaacson (1981) and Chakrabarti (1987). The Morison
equation assumes the force is the sum of inertia and drag forces.
Vortex-induced vibration (VIV) occurs when the wavelcurrent flow cause resonance with the
natural frequency of the structure. For the design of pipelines and risers, it is necessary to
account for the wave-induced fatigue and VIV induced fatigue @ai, 2001).

3.3.3 Sea loads on Large-Volume Structures
When the size of the structure is comparable to the length of wave, the pressure on the
structure may alter the wave field in the vicinity of the structure. In the calculation of wave
forces, it is then necessary to account for the diffraction of the waves from the surface of the
structure and the radiation of the wave from the structure if it moves (Charkrabarti, 1987).
First Order Potential Forces: Panel methods (also called boundary element methods,
integral equation methods or sink-source methods) are the most common techniques used to
analyze the linear steady state response of large-volume structures in regular waves (Faltinsen,
1990). They are based on potential theory. It is assumed that the oscillation amplitudes of the
fluid and the body are small relative to cross-sectional dimension of the body. The methods
can only predict damping due to radiation of surface waves and added mass. But they do not
cover viscous effects. In linear analysis of response amplitude operator (RAO), forces and
response are proportional to wave amplitude and response eequency are primarily at the wave
frequency.
Second Order Potential Forces: The second order analysis determines additional forces and
responses that are proportional to wave amplitude squared. The second order forces include
steady force, a wide range of low frequency forces (which will excite surge, sway and yaw of
a moored floating system) and high frequency forces (which will excite roll, pitch and heave
springing of a TLP). The most common way to solve non-linear wave-structure problems is to