Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Title: ~ , Underwater shock protection device
The invention relates to an object to be set up or transported in or on
water, provided with a structure for protecting the object against underwater
shock, which structure comprises resilient members provided over an outside
skin of the object that is operatively situated in the water.
Such an object is known as a marine vessel modified according to US
patent 4,193,367. In that patent specification, the outside skin of the vessel
is
covered with pre-stressed membranes, such as fiber glass plates, destined to
prevent or at Least curb transmission of a shock wave ,in the water, resulting
from an underwater explosion, to the heart of a vessel.
The membranes have as a disadvantage that the construction is
rather vulnerable in use, that it is complicated and can easily spring a leak
as
a result of which a water mass can penetrate between the membranes and the
vessel, which adversely affects the boating properties of the vessel.
Moreover,
due to the leakage, the protection against shock may be reduced or be undone.
In addition, adjacent the edges of the membranes, a relatively large surface
will remain that continues to be vulnerable to impact of the shock wave. At
this location, the shock will indeed be partially transmitted to the vessel.
The object of the invention is to provide an improvement of such a
structure, which is relatively simple and inexpensive in manufacture and
maintenance, and which does not adversely affect the boating properties.
This object is achieved by providing an object with a structure as
mentioned in the opening paragraph, while the members have been
manufactured from a layer of elastic material in which, at least in a side
remote from the object, gas-filled spaces are present, such that the members
are compressible over a distance of at least one water displacement amplitude
resulting from an underwater shock.
Due to such a structure, a protective layer is formed around the
vessel, so that a pressure wave resulting from an underwater explosion cannot
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reach 'the vessel. As the protective layer ha's a very low acoustic impedance,
the
pressure wave is reflected back into the water as a reflection wave. The
energy
of the shock wave is then absorbed in the water, in a manner similar to that
near the water surface, in that, as a result of this reflection, bulk-
eavitation
occurs. Consequently, there is hardly any transmission of the energy of the
underwater shock wave, so that in the vessel hardly any additional shock
resisting measures need be taken as long as the path of displacement of the
pressure wave does not exceed the maximum displacement distance of the
compressible material. This has as an advantage that the inside construction
of the vessel needs to be far less shock resistant than in conventional
designs
for protection against underwater shock. Further, the invention can be
utilized
without making use of complex mechanical constructions, which are expensive
in maintenance and can easily become defective.
This reflection phenomenon, for that matter, is of a totally different
nature than curbing a collision between a vessel and water waves slamming
against the vessel. Constructions axe known, as, for instance, from US
patent 3,960,100, wherein the impact energy is stored as resilient,energy in
gas chambers, and is transmitted to the vessel in a retarded manner, i.e. with
lower peak forces. As the maximum water displacement amplitude resulting
from underwater shock is relatively small (as a r ule, this is approximately
6 - 10 cm), relatively limited thickness dimensions of the material will
suffice,
so that a very robust and easily applicable structure is obtained.
Tn a preferred embodiment, the spaces are connected to gas pressure
regulators. In particular in submarines, which operate at greater depths, due
to the prevailing water pressure, the volume of the chambers would become too
small if not adjusted, so that the pressure wave has a path of displacement
which will be greater than the chamber diameter or the chamber height.
Regulating the amount (=mass) and the pressure of the gas in the chamber can
prevent the occurrence of "bottoming", i.e. resilient material colliding
internally, whereby the shock wave can no longer reflect but slams into the
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vessel: Further, the regulators have as,an advantage that small leakages; due
to, for instance, local leakages or diffusion of the gas from the chambers,
can be
eliminated.
In a further preferred embodiment, the spaces are connected to a gas
buffer member. Such a member prevents gas pressures from rising too high in
the chambers, in that, during compression, a portion of the gas can flow away
into a buffer. As a result, the chambers can be designed to be relatively
small.
Fuxther, it is advantageous when the specific gravity of the
compressible material has a gradient having, at an outside remote from the
object, a relatively low specific gravity, and at the inside, a relatively
higher
specific gravity. Due to a low specific gravity at the outside, the impedance
difference between the 'surrounding water and the shock protection structure
is increased, as a result of which a better reflection of the shock wave
occurs.
Due to an increased specific gravity towards the inside, the structure can be
of
robust design, while shock absorbing properties of the compressible material
itself can be optimally utilized.
In order to impxove the boating properties and the mechanical
strength, a relatively thin, rigid plate can be provided on an outside of the
compressible material. This plate can be made of steel, aluminum, fiber glass
or a different, relatively light material which, as long as its thickness is
not too
great, remains relatively transparent to the shock wave.
In a preferred embodiment, the gas-filled compressible material is
manufactured from a substantially homogenous, impermeable foam of a
thickness of approximately IO - 20 cm.
An alternative preferred embodiment consists of the gas-filled
compressible material comprising flexible tubes welded together over a
longitudinal side, which tubes have been brought to a predetermined gas
pressure, which tubes have a diameter of a thickness of approximately
10-20 cm. Preferably, the tubes are disposed continuously next to and along
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each other, thereby covering the surface of the vessel to be protected,
preferably completely.
The invention also relates to a structure for protecting an object
against impact of an underwater shock according to one of the above-
mentioned aspects.
The invention will be further elucidated with reference to the
Figure. In the Figure:
Fig. 1 shows a schematic representation in cross section of a marine
vessel, such as, for instance, a frigate, provided with a protective structure
according to the invention;
Fig. 2 shows a schematic representation in cross section of a
submarine, provided with a protective structure according to the invention;
Fig. 3 shows a detail of a protective structure according to a first
embodiment;
Fig. 4 shows a detail of a protective structure according to a second
embodiment; and
Fig. 5 shows a schematic representation of a protective structure
according to a third embodiment.
In the Figure, the same or corresponding parts are indicated with
the same reference numexals.
Tn Fig. 1, a schematic representation is shown of a marine vessel 1
which is exposed to an underwater explosion 2. Due to the explosion 2, a shock
wave 3 (pressure wave) is generated moving radially outwards as a spherical
front at the speed of sound imwate.r:. When.~he shQCk wave arrives at the
water
surface 4, the shock is negatively reflected and returns as a reflection wave
5.
"Bulk cavitation" occurs, as a result of which cavitation bubbles 6 can form.
The shock wave 3 is characterized by a step-wise displacement of the
water over a distance s. The size of the displacement depends on the intensity
of the explosion and the distance to the explosion and is in the order of, at
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most, 6 cm. During the occurrence of this shock displacement, mbrizentaneous
.,
peak pressures can occur in the order of 100 bars or more.
When the shock wave arrives at the marine vessel 1, without the use
of the protective measures according to the invention, a part of the shock
wave
5 3 is introduced into the structure of the ship. Shock transmission takes
place
and the ship is shock-loaded, which leads to extremely large local
accelerations. As' a rule, the displacement s in the ship is then in the same
order as the water displacement and is therefore also at most 6 cm. As a
result
of the shock, equipment on board of the vessel can become defective.
Furthermore, the outside skin of the vessel may become damaged. Especially
with less ductile materials, this is an actual threat (fiber glass reinforced
polyester mine combating vessels, aluminum vessels).
According to the invention, on the outside skin of the marine vessel
a resilient, gas-~.lled plastic or rubber-like material 7 has been pxovided
which
is compressible over a distance of at least twice a water displacement
amplitude resulting from underwater shock. What is achieved by using gas-
filled plastic or rubber-like material is that the compressible properties are
improved due to the' presence of gas in the material, so that a displacement
of
a magnitude of twice the shock displacement can easily be performed, without
entailing the risk of "bottoming". In this last case, the material impacts
internally, giving rise to a high internal pressure in the gas and material.
On the other hand, as the structure has a relatively low density, the
acoustic impedance of the surrounding water towards the plastic or rubber-like
material exhibits a sharp discontinuity. The fact is that the acoustic
impedance of this gas damper, defined as the product of the density p and the
speed of sound c of the gas (nitrogen or air etc.) is considerably less
(pc=1.25*330=412.5) than that of water (pc=1000*1500=1,500,00).
As a result, the damper behaves as air, so that at the buffer, bulk
cavitation occurs. At the location of the damper, the shock is negatively
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reflected and returns into the water as a reflection wave, so that the shock
is
(virtually) not introduced into the vessel.
A displacement shock of water only occurs over approximately
6 - 12 cm. This displacement is absorbed without problems by the 'shock
damper (foam or air bed).
Through the use of a shock damper with sufficient resilience path
(more than 12 cm), and a sufficiently low rigidity, the force transmitted by
the
damper to the ship's skin is very strongly reduced.
In Fig. 2, a schematic representation of a submarine ~ is shown.
Around the submarine, air chambers 9 have been provided, the amount of gas
(or mass of gas) of which in the chamber can be regulated. By adding or
blowing off gas during change of the diving depth, the height of the air
chamber can remain virtually constant, while the pressure of the chambers
remains virtually equal to the actual ambient pressure of the water at the
respective diving depth. The height of the chambers 9 remains almost
constant, so that also at a greater depth under water, the shock resistance
remains at the proper level and the buoyancy is not adversely affected.
By means of a duct system 11, a gas-buffer member 10 is connected
to the air chambers 9. The gas-buffer member is connected to a compressor
andlor a gas bottle with gas at high pressure I2, for regulating the gas
pressure and/or the gas volume of the chamber. In addition thereto or as an
alternative, use can be made of a blow-off valve, which opens at a maximally
acceptable pressure load.
In Fig. 3 it is represented how, by means of flexible plastic hoses or
an air mattress 13, a vessel skin 14 can be covered. The hoses have been
welded together over a longitudinal side, and have been brought at a
predetermined gas-pressure. The hoses can have a diameter of approximately
12 - 20 cm. The hoses 13 are closed at one end, and, at another end, can also
be closed or connected to a compressorlgas bottle (not shown).
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In Fig. 4 it is schematically represented how in a preferred
embodiment the structure for protection against underwater shock 'has been
built up. The gas-filled compressible material is manufactured from a
substantially homogeneous, impermeable foam 15 with a thickness of
approximately 10 - 20 cm. On the foam 15, for protection or for a lower
boating
resistance, on an outer side of the compressible material, a relatively thin,
rigid plate 16 has been provided from, for instance, steel, aluminum, plastic.
The thickness of the protective layer must be as thin as possible; preferably
less than 10 mm.
The specific gravity of the compressible ma~erial has a gradient with
a relatively very low specific gravity on. the outside 17 and a relatively
higher
specific gravity on the inside 18. The whole is attached to a steel vessel
skin
14, for instance by means of gluing or vulcanization.
Try Fig. 5, schematically, it is represented how a vessel skin 14, such
as for instance a skin of a frigate or a submarine, can be covered with a
structure for protection against underwater shock according to the invention.
The structure consists of laminated elements 19 which can have a standard
size of, for instance,'1 by 1 meter. The elements are provided close together
on
a vessel skin 14, by means of bolt connections, gluinglwelding and/or magnetic
coupling (not shown). This last variant offers the possibility of temporarily
protecting a vessel against underwater explosions, by magnetically fixing the
elements on the vessel skin.
The elements consist of a layer of foam rubber 15 with a thickness of
approximately 25 cm, which is covered on both sides with relatively thin,
rigid
plates 16 of steel, aluminum and/or fiber glass.
The invention is not limited to the preferred embodiment
represented in the drawing but may contain all sorts of variations thereon.
For
instance, in a protective structure, use can be made of a combination of
inflatable elements with foam rubber elements, allowing the impact resistance
of a vessel to be temporarily increased. Also, in the protecting elements,
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reinforced sections can be present for increasing the 'self supporting
capacity of
the elements. Such variants are all understood to fall within the scope of the
invention as defined in the following claims.
