Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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FIELD OF THE INVENTION
This invention relates to floating breakwaters.
BACKGROUND TO THE INVENTION
A floating breakwater generally consists of a floating structure intended
by adaptation in one way or another to dissipate or substantially
attenuate water wave energy to provide an area oF relatively still water
in the lee of the breakwater. Breakwaters oF the above type are well known;
few have, however, actually been built but various alternative proposals
consisting oF variances of those mentioned below hava been ~ut forward.
1. Rigid floating breakwaters such as pontoons or floating platforms and
floating sloping barriers;
2. Flexible structures such as floating membranes or mattresses, water or
air-filled bags;
3. Maze-like structures ~hich are at least partially submerged and rely on
jet diffusion; and
4. Pneumatic and hydraulic breakwaters which among others have been tested
mainly under laboratory conditions.
Many and in fact most of the known proposals present enormous practical
constructional difficulties and no single method of wave attenuation by means
of a floating breakwater stands out as the obvious answer to the problems
which have been encountered and none has achieved an outstanding
breakthrough in terms of combined simplicity, economy, performance and
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durability. Quite apart from economy of construction and operation,
the problems associated with known floating breakwaters are principally
undue oscillation of the breakwater, insuf-ficient reduction o-f the
incident wave, s-tructural failure of the floating structures, undue
stressing of the mooring and general maintenance.
OBJECT OF THE INVENTION
It is the object of this invention to provide a floating breakwater which
has greater properties of wave attenuation than known breakwaters and
which is of relatively simple constructional design.
SUM~ARY OF THE INVENTION ~
A floating breakwater according to the invention consists of a structure
which defines a plurality of compartments which in use are open top and
bottom in a direction normal to the surface of the water with the
- breakwater being characterised in that the structure includes elongated
members which are tubular in cross section and adapted in use to be
located below the surface of the water, upwardly directed elements which are
; 15 attached to the tubular members and extend over their length substantially
to enclose the compartments horizontally at the surface of the water with
the sides of the compartments across the structure being out of parallel
with each other over a substantial portion of their leng-ths and buoyancy
control chanbers at water level attached to the structure to ensure
positive buoyancy of the structure.
In a preferred form of the invention the compartments are substantially
triangular in plan with adjacent compartments over the length of the structure
being of opposite aspect.
BRIEF DESCRIPTION OF THE DRA~INGS
An embodiment of the invention will now be described by way o~ example only
with reference to -the drawings in which:
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Figure 1 is a plan view of the breakwater of the invention,
Figure ~ is a fragmentary perspective view of a portion of the breakwater
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of Figure 1, and
Figure 3 is a sectional end elevation of one of the structural members
of the breakwater.
DETAILED DESCRIPTION OF THE DRA~IINGS
The breal<water of the invention is shown in Figure 1 of the drawings to
consist of a structure indicated generally at 10 which deFines a plurality
oF substantially triangular compartments 12 and 1~ which are open top and
bottom and are respectively of opposite aspect.
As is more clearly seen in Figure 2 the structure is composed principally
of tubular members 1~, 18 and 20. The members 16 and 18 carry upwardly
directed fins or wave spoilers 22 and the member 20 carries a plurality of
downwardly inclined beach members 24 which are located on the members by
spaced brackets 26. Although the spoilers 22 are shown in the drawing as
being continuous elements they could be vertically separated at intervals
to minimise torsional forces on the tubular members, the spoilers
themselves and the nodes of the structure in use.
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The tubular members 16 and 1~ are connected to a first series of air tight
floatation chambers 28 and a second series of vertically divided floatation
chambers 30 are located between the tubular members 18 in the apices of the
compartments 14.
The struc-ture is constructed in the form of modules, such as that enclosed
~ by chain lines in Figure 1, with each module including a compartment 12 and
`~ two half compartments 14. Each half compartmen-t 14 includes half of a
floatation chamber 30. The ends of the tubular members 16 on the breakline
of the modules could carry radial flanges for joining one module to
another to form the breakwater.
In this embodiment of the invention the modules are rigidly coupled but
in another embodiment could be joined by means adapted to permit limited
movement between modules.
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The tubular members 16 could consist of short sections of tube which are
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coupled by flanges and one or more of the sections could be removed to
shorten the lee wall of the structure should it be desired that the
breakwa-ter have a convexly curved configuration in plan. In this event
suitable spacing members, not shown, wou1d have to be provided between
and joined to the halves of selected floatation compartmen-ts 30.
The tubular members of the structure are divided over their lengths by
bulkheads ;nto a plurality of watertight buoyancy compartments with each
compartment including a hatch through which ballast may be introduced into
and removed From the compartments to vary the buoyancy of the structure.
All of the compartments of the structure in this embodiment of the
invention are made from reinforced concrete but could be made from suitably
coated structural steel or even ~rom a synthetic rubber or like material.
The dimensions of the structure will depend principally on the period and
amplitude of the waves to be attenuated at a particular site. It has, for
example, been determined from extensive flume testing of a 1:100 model of
the breakwater of the invention that a breakwater beam of about 10 metres
(from the sea to lee wall of the structure) will provide adequa-te
pro-tection on inland ial~es with a fe~ch of about 30 km and on which wind
generated waves can a-ttain periods of about three to five seconds and
heights of up to 2,5 metres. On open seas with an unrestricted fetch, waves
generally have periods of between five and twenty seconds and can reach
heights exceeding 20 metres. In such seas the required beam dimension may
be between as much as 80 and 120 metres depending on the degree o~
attenuation required. In a breakwater having an 80 metre beam dimensio~ the
tube members 16 and 20 require an external diameter of about 12 metres. The
diagonal members 18 are of slightly lesser diameter.
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In practice, the modules of the breakwater are coupled at a convenient place
and towed to the site of use of the breakwater. Alternatively, the modules
could be towed singly-to the site and there assembled.
At the site of use the assembled breakwater is suitably anchored in position
with the wall of the structure including the beach members 24 arranged to
be normal to the predominant wave direction ~t the site, as indicated by the
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arrow in Figure 1.
The buoyancy compar-tments in the tubular mernbers are then partially filled
with a sùitable ba11ast which may be water, a particulate material or
pelle-ts of a heavy material. Preferably, however, the ballast should be
less fluent than a liquid to minimise mass distribution problems and
consequent instability of the structure due to movement of the ballast
during operation of the breakwater. The structure is loaded with ballast
until the e-Ffective density of the tubular portion of the structure
approximates unity and so becomes substantially neutrally buoyant. The
10 sealed floatation chambers 28 and 30, in this condition of the structure,
must be of such dimension and capacity as to impart sufficient positive
buoyancy to the structure to overcome the inherent instabili~y of a
neutrally buoyant body and to hold the tubular portion of the structure
stably submerged at a depth proportional to the position illustrated in
Fiyure 3 with the spoiler fins 22 projecting above mean water level. With
the breakwater floated in this manner a substantial proportion o-f the mass
OT ihe struc-ture is located below the turbulent surface zone of the water
and is substantially less influenced by the predominant sur-Face wave energy
than it would have been had the tubular portion of the structure been
located at water level.
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The spoiler fins 22, in st.ll water, enclose the compartments 12 and 14 at
and below the sur-face of the water. The beach members 2~ are partially
submerged.
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It was observed from scale model studies of the breakwater o, the invention
that incident waves normal.to the beach members induced a pulsàting ef-fec-t
in adjacent compartments of the structure in the sense that the water level .
in compartments of one aspect were out of phase with the water levels in
the compartments of opposite aspect, the water level differential being.a
function o-f both wave height and period.
The complete mechanism of energy interception of the breakwater is not yet
fully understood It is thought that because of the confiyuration of the
structure the respecti~/e masses of water enclosed within adjacent
~- compartments are excited by orbital motion of water beneath them and an out
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of phase damping of wave energy results from the pulsatory movement of
the horizontally confined masses of water. Part of the wave energy is
conver-ted to heat by turbulence in the compartments and as with any
- floating structure part of the incident energy is reflected.
~ Model tests of the breakwater indicate that the wave attenuation
performance o-f the breakwater is substantially improved by submerging the
bulk of the mass of the structure to reduce the sensitivity of response
rate of the structure to surface wave action.
~leasurements taken during testing of breakwater models showed that wave
attenuation was a direct function of the ratio of wave length to beam width
of the structure of the invention and that the breakwater of the invention
performed well when compared with other floating structures of similar
dimensions.
The invention is not limited to the precise constructional details as herein
described and the tubular structural members could, for example, be of any
suitable cross sectional shape. Additionally, the buoyancy compartments 28
and 30 need not necessarily be located at the nodes of the structure and
could Le located at any other ~uitable position on tha s'ructure
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