10.11 WAVE MOTION ASSESSMENT
10.11.1 Wave Induced Motion Estimates
The motion characteristics of a vessel or floating platform may be reliably predicted by recourse to well-established computer models, or to physical model testing. In either case the results are invariably presented in terms of linear transfer functions. The transfer function contains an amplitude and phase component, and the amplitude is often referred to as a Response Amplitude Operator (RAO). An example response amplitude operator is given in Figure 10.27.
Provided that transfer functions have been derived for all six motion components (roll, pitch, yaw, sway, surge and heave) for a vessel at a defined reference point (often at the centre of gravity, or amidships at the waterline), then RAOs can be readily calculated for any helideck location on the vessel.
Using specialist software this data can be combined with wave climate data (see Section 10.12.1) and limiting motion criteria (see Section 10.12) to derive quantitative helideck downtime estimates (see Section 10.13).
10.12 WAVE CLIMATE
The probability of encountering a given combination of significant wave height and period is defined using a ‘wave scatter table’, which describes the proportion of time when the significant wave height and period lie within specified ranges. Wave scatter tables for open-water sea areas may be obtained from standard reference texts or computer databases (see example in Figure 10.28). Wave scatter tables for specific locations (especially local in-shore conditions) should be obtained from specialist metocean sources.
Wave Scatter Table Worldwide Database , Sea Area 25, Jan - Dec , East
Sig Hgt (m) 28 167 323 278 140 49 13 3 1 Obs 1000
> 14
13 to 14
12 to 13
11 to 12
10 to 11
9 to 10
8 to 9 1
7 to 8 1 1 2
6 to 7 1 1 2 1 5
5 to 6 2 4 4 2 1 14
4 to 5 1 7 12 10 5 2 37
3 to 4 5 22 33 24 10 3 1 98
2 to 3 1 19 66 77 44 15 4 1 226
1 to 2 5 62 140 112 46 12 2 381
0 to 1 21 80 85 38 9 2 235
< 4 4 ~ 5 5 ~ 6 6 ~ 7 7 ~ 8 8 ~ 9 9 ~ 10 10 ~ 11 11 ~ 12 12 ~ 13 > 13
Zero Crossing Period (s)
Figure 10.28 - Example wave scatter table.
Wave scatter tables defined on an ‘all-year’, ‘all-directions’ basis may be adequate for vessels that are to operate at all times of year and whose motions are relatively insensitive to heading (e.g. semi-submersible drilling vessels). Wave scatter tables for directional sectors are needed in cases where vessel motions vary with relative wave heading (e.g. ships), but the manner of analysis will vary depending on whether the vessel heading is fixed or varies with the direction of the weather. The vessel heading relative to waves should be considered in cases where the vessel weathervanes, or operates under heading control (see Section 10.6).
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