Page 74 - Marine Structural Design
P. 74
50 Part I Structural Design Principles
Input "Black box"
Input time-history Output time-history
Figure 3.7 The Concept of RAO for a Structure (CMPT, 1998)
Resonant peak
5 10 15 20
Wave period (s)
Figure 3.8 Typical RAO of Barge Roll Motion in Beam Seas (CMPT,
1998)
Another application of the RAO is to calculate loads in irregular waves. Bhattacharyya (1978)
suggests that the total response of a vessel in an irregular seaway is the linear superposition of
the response to the individual components that may be determined using RAO.
In the calculation of H (o,a&), a suitable range of wave frequency, number of frequency
points, and wave headings should be used. The commonly used parameters for an FPSO
analysis are:
Frequency range: 0.20 I o I 1.80 rad/s
Frequency increment: 0.05 rad/s
Wave heading: 0" to 360" with 15" increment
If a finite element method is used, the pressure distribution needs to be mapped from a
hydrodynamic model onto a finite element model with NAXNFXNH loading cases, where:
