Practical Ship Hydrodynamics

Ship seakeeping 115
height H1/3 for various values of Uc/cp. cp (lower scale) and Uc/cp together
yield the wind velocity Uc that has excited the wind sea characterized by H1/3
and Tp. For swell, we can assume Uc ³ cp. Figure 4.14 shows the relation
between various seaway parameters, the ‘wind force’ and the wind velocity Uc.
20
15
10
5
0
Maturity time in
1
10 day
T P , matured, in s
H 1/3, Matured, in m
Fetch in 100 km
Wind
U 10 = 0.836.BF 1.5
10 20 30 40 50
Uc [m/s]
Figure 4.14 Key wind sea parameters depending on the wind speed Uc (component in wave
propagation direction in 10 m height
Programs to compute the given wind sea spectrum from either Uc, t and x
or H1/3 and Tp are given by Söding (1997).
4.3.4 Wave climate
Predictions of maximum loads, load collectives for fatigue strength analyses
etc. require distributions of the significant seaway properties in individual
ocean areas. The best sources for such statistics are computations of the seaway
based on measured wind fields. ANEP-II (1983) gives such statistical data
extensively for North Atlantic, North Sea, Baltic Sea, Mediterranean Sea and
Black Sea. Based on these data, Germanischer Lloyd derived distributions
for H1/3 and T1 for all of the Atlantic between 50 and 60 longitudinal and
the western Atlantic between 40 and 50 longitudinal (Table 4.2). The table is
basedondataforaperiodof10years.T1 is the period corresponding to the
centre of gravity of the area under the sea spectrum. The modal period is for
this table:
Tp D T1/0.77
The values in the table give 10 6 the time share when T1 was in the given time
interval and H1/3 in the interval denoted by its mean value, at an arbitrary
point in the sea area. FCUM denotes the cumulated share in per cent. Similar
tables can be derived from ANEP-II and other publications for special seaway
directions, seasons and other ocean areas.
Table 4.2 can also be used to approximate other ocean areas by comparing
the wind field in the North Atlantic with the wind field in another ocean area,