IE49105B1 - Floating breakwater - Google Patents

Description

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 have been put 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 which 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 durability. Quite apart from economy of construction and operation, the problems associated with known floating breakwaters are principally undue oscillation of the breakwater, insufficient reduction of the incident wave, structural failure of the floating structure, undue stressing of the mooring and general maintenance.

A floating breakwater according to the invention consists of a structure which defines a plurality of substantially rigid 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 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 lengths and buoyancy control chambers at water level attached to the structure to ensure positive buoyancy of the structure.

It is believed possible by means 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.

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.

An embodiment of the invention will now be described by way of example only with reference to the drawings in which: IOS Figure 1 is a plan view of the breakwater of the invention, Figure 2 is a fragmentary perspective view of a portion of the breakwater of Figure 1, and Figure 3 is a sectional end elevation of one of the structural members of the breakwater.

DETAILED DESCRIPTION OF THE DRAWINGS The breakwater 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 14 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 15, 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.

The tubular members 16 and 18 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 structure 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 compartment 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.

The tubular members 16 could consist of short sections of tube which are 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 breakwater have a convexly curved configuration in plan. In this event suitable spacing members, not shown, would have to be provided between and joined to the halves of selected floatation compartments 30.

The tubular members of the structure are divided over their lengths by bulkheads into 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 from 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 adequate protection on inland lakes with a fetch of about 30 km and on which wind generated waves can attain 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 of attenuation required. In a breakwater having an 80 metre beam dimension the tube members 16 and 20 require an external diameter of about 12 metres. The diagonal members 18 are of slightly lesser diameter.

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 at the site, as indicated by the arrow in Figure 1.

The buoyancy compartments in the tubular members are then partially filled with a suitable ballast which may be water, a particulate material or pellets 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 effective density of the tubular portion of the structure approximates unity and so becomes substantially neutrally buoyant. The 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 instability 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 Figure 3 with the spoiler fins 22 projecting above mean water level. With the breakwater floated in this manner a substantial proportion of the mass of the structure is located below the turbulent surface zone of the water and is substantially less influenced by the predominant surface wave energy than it would have been had the tubular portion of the structure been located at water level.

The spoiler fins 22, in still water, enclose the compartments 12 and 14 at and below the surface of the water. The beach members 24 are partially submerged.

It was observed from scale model studies of the breakwater of the invention that incident waves normal, to the beach members induced a pulsating effect 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 of 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 configuration of the structure the respective masses of water enclosed within adjacent compartments are excited by orbital motion of water beneath them and an out 48105 of phase damping of wave energy results from the pulsatory movement of the horizontally confined masses of water. Part of the wave energy is converted 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 of 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.

Measurements 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 peed not necessarily be located at the nodes of the structure and could be located at any other suitable position on the structure.

Claims (8)

1. A floating breakwater consisting of a structure which defines a plurality of substantially rigid compartments which in use are open top and bottom in a direction substantially normal to the surface of 5 the water, wherein 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 attached to the tubular members and extend over their length substantially to enclose the compartments horizontally at the surface 10 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 lengths and buoyancy control chambers at water level attached to the structure to ensure positive buoyancy of the structure,

2. A breakwater as claimed in Claim 1 wherein the compartments 15 are substantially triangular in plan.

3. A breakwater as claimed in Claim 2 wherein the triangular compartments are arranged in the structure in opposite aspect.

4. A breakwater as claimed in any of Claims 1 to 3 wherein the structural members are arranged in the structure to define 20 substantially parallel lee and sea walls which are joined by transverse members to form the compartments.

5. A breakwater as claimed in Claim 4 wherein the upwardly directed elements on the lee wall and transverse structural members are vertical fins which are substantially narrower than the 25 structural members and the elements on the sea wall are elements which are downwardly inclined into the water to provide a sloping beach over and beneath which waves may enter compartments of the structure.

6. A breakwater as claimed in any one of the above claims wherein the tubular structural members include buoyancy compartments for ballast for adjusting the buoyancy of the structure.

7. A breakwater as claimed in any one of Claims 4 to 6 wherein 5 the buoyancy control chambers are fixed to the structure at the corners of the compartments and extend upwardly from the tubular members.

8. A floating breakwater substantially as herein described with reference to, and as illustrated in the accompanying drawings.