Geoscience Reference
In-Depth Information
the ship-sea system re
ecting cycles and frequencies in the impacts of ship and waves.
As a result, the prognosis of the ship course depends on the relative spatial distribution of
the ship course and rough sea as well as on different restrictions for the ship moving
(other ships, waterway etc.). Adaptive regulator of the ship course provided with this
model gives in real time recommendations for the ship course change minimizing the
risk of emergency conditions (strike with barrier, beaching etc.).
Figure
4.34
explains the interrelation between physical factors in the ship-sea
system. According to this scheme, period and frequency of collision between ship
and waves is calculated using the following formulas:
fl
T
e
¼ L
x
=
½V
x
Vcos
v; x
l
¼ 2
p
½V
x
Vcos
v=
L
x
;
ð
4
:
52
Þ
where T
e
is the period of the ship collision with wave components,
ˉ
l
is the angle
frequency of collision (=2
ˀ
/T
l
), L
ˉ
is the wavelength, V is the ship speed, V
ˉ
is the wave
velocity,
is the corner between the ship speed vector and the direction of waves.
From Fig.
4.34
and Eq. (
4.52
) it is easy to see that if sea wave is moving stern of
ship (90
ˇ
°
<
ˇ
< 270
°
, cos
ˇ
< 0), the frequency of collisions is more than in other
cases (0
° ≤ ˇ ≤
90
°
, 270
° ≤ ˇ ≤
360
°
). Ship moving before the wave can be
associated with small values of V
ˉ
−
and, therefore, frequency of collisions
will be also low in contrast to other situations. Hence, physical factors of the ship-
sea system dynamics are essentially diverse into two zones of the angle
Vcos
ˇ
changes.
The knowledge of these factors gives the possibility to control a ship management.
In the case of an astern wave, according to (
4.52
) there are three situations:
ˇ
If V
ˉ
> Vcos
there is an overtaking wave. In this case, waves overhaul the ship
and the frequency of collisions between the ship and waves is low.
ˇ
ˇ
If V
ˉ
= Vcos
there is a semi-static situation. In this case, the frequency of
collisions between the ship and waves is near zero and if at the time t = 0 the
ship is situated in stable position regarding to waves, this state is retained at any
time t=t
1
.
Fig. 4.34 Diagram for the interaction between physical factors in the sea-ship system
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