smart technologies for safety engineering

Adaptive Impact Absorption 197
Pressure in the front cavity [Pa]
[x10 6 ]
0.80
0.60
0.40
0.20
0.00
0.00 0.20 0.40 0.60 0.80 1.00 1.20
Time [s]
(a)
Final ship velocity [m/s]
0.00
−2.00
−4.00
−6.00
0.00 0.20 0.40 0.60
Time [s]
(b)
0.80 1.00 1.20
Figure 5.40 Active mitigation of the ship rebound: (a) pressure of the gas in the cavity, (b) corresponding
final velocity of the ship
used. The optimal flow resistance coefficient and pressure change during impact are the same
as in the case of semi-active acceleration mitigation (cf. Figure 5.38(a)). In the considered
example, the pressure at the moment when the ship is stopped is not completely reduced, but
the chamber volume is minimal and work that can be done by gas is relatively small. Therefore
the semi-active system is very efficient and allows almost 96 % of the initial ship energy to be
dissipated.
In the active strategy, the valve opening remains constant while the ship is approaching
the tower, as in a semi-active approach. When the ship velocity decreases to zero (at time
0.54 s), the valves are fully opened to release a surplus of pressure (cf. Figure 5.40(a), dark
line). Then the valve is closed again and backward deformation of the chambers is reduced
due to arising underpressure. As a result of this strategy, the final ship velocity is diminished
to 1.05 m/s (cf. Figure 5.40(b), dark line). Nonzero final rebound velocity is the consequence
of ship interaction with strongly deformed AIS walls, which repel the ship.
The influence of inflatable structure walls can be further reduced by minimizing their deformation
and thereby the amount of strain energy accumulated. In the present control strategy
the valves remain closed during the whole compression stage of impact in order to achieve the
highest possible stiffness of the front chambers of the inflatable torus. The ship is stopped after
crushing only a part of the pneumatic structure at time 0.42 s and then an immediate pressure
release is executed (cf. Figure 5.40(a), bright line). Finally, the rebound velocity is reduced to
0.82 m/s, which means that more than 98 % of the initial ship energy is dissipated.
5.5.2.3 Minimization of Stresses in the Tower Wall
Another purpose of applying the inflatable structure is mitigation of tower response to impact.
In particular, the pneumatic structure reduces local stresses in the front tower wall by preventing
direct contact of the ship and the tower. The front tower wall is subjected to bending caused by
pressure loading and the level of stress depends approximately on the actual value of pressure.
Therefore, minimization of stresses is equivalent to minimization of front chamber pressure.