smart technologies for safety engineering

196 Smart Technologies for Safety Engineering
Inverse of flow resistance coefficient
Pressure in the front cavity [Pa]
[x10 6 ]
10.00
8.00
6.00
4.00
2.00
0.00
[x10 6 ]
0.60
0.50
0.40
0.30
0.20
0.10
0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60
Time [s]
0.00
0.00 0.20 0.40
Time [s]
0.60 0.80
Mass flow rate [kg/s]
Ship acceleration [m/s2]
7.00
6.00
5.00
4.00
3.00
2.00
1.00
0.00
20.00
16.00
12.00
8.00
4.00
0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60
Time [s]
0.00
0.00 0.20 0.40
Time [s]
0.60 0.80
Figure 5.39 Two strategies of active control: with pressure release (dark line), additional inflation at
the beginning of the impact (bright line)
In a passive system with no pressure release, the main part of the ship kinetic energy (E ship )
is converted into internal gas energy (U) and the small part is converted to AIS walls strain
energy (E AIS ). The kinetic and potential energy of the tower (E tower ) can be neglected due
to small tower displacement and velocity. During the rebound stage of impact the gas expands,
its internal energy decreases and it is changed back into kinetic energy of the ship. Thus, in
a passive system, the absolute value of the final ship velocity is close to the initial one. In
contrast to a passive system, in a semi-active or active system with pressure release, internal
gas energy is not accumulated but dissipated by letting the air out of the cavities (H). Hence,
the value of gas exergy (its ability to perform useful work) at the moment when the ship is
stopped is decreased and the ship rebound is mitigated.
Semi-active adjustment of the initial pressure does not affect the final ship velocity since
it does not cause energy dissipation. However, the ship rebound is significantly decreased by
using inflatable structure comprising a valve with a constant opening. The minimal final ship
velocity of 1.25 m/s is obtained in the case when the whole stroke of an inflatable structure is