GB1580326A

GB1580326A - Wave reduction device

Info

Publication number: GB1580326A
Application number: GB31828/76A
Authority: GB
Original assignee: BP PLC
Current assignee: BP PLC
Priority date: 1976-07-30
Filing date: 1977-03-04
Publication date: 1980-12-03
Also published as: FR2359937A1; JPS5318232A; CA1065626A; DE2734259A1; NO772660L

Description

(54) WAVE REDUCTION DEVICE (71) We, THE BRITISH PETRO LEUM COMPANY LIMITED, of Britannic House, Moor Lane, London, EC2Y 9BU, a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to a device for the reduction of liquid wave heights and more particularly relates to a device for the reduction of sea water wave heights in the vicinity of marine installations and the like.

Wave reduction devices may be scaled to protect operations varying from oil drilling and production platforms, loading buoys, ship salvage and similar open sea marine operations in exposed ocean locations to fish farms or yacht marinas in estuaries.

It is an object of the present invention to provide a floating wave reduction device which substantially dissipates wave energy whereby the wave height is reduced during its passage towards the installation being protected.

Thus, according to the present invention there is provided a wave reduction device comprising one or more flexible containers adapted to be partially or wholly filled with a liquid and means rendering the or each container buoyant, the device also having constricting means provided so that when the or each container is distended by internal liquid pressure, its upper and lower surfaces are drawn towards each other by the constricting means at a plurality of spaced apart locations to form constrictions causing the surfaces to become indented. The term "constriction" is also intended to comprise the case in which the upper and lower surfaces are in contact.

The containers are preferably deployed so that, in still water, the upper surface of the container is awash.

The containers are suitably constructed from a flexible fabric so that, when filled, a pattern of constrictions are formed by the constricting means between the upper and lower sheets of the container and the top and bottom surfaces become indented. Their internal structure provides a resistance to the free movement of liquid within the container but most preferably no bulkhead should be incorporated which completely prevents liquid flow from front to back, i.e. in the beam direction.

The upper and lower surfaces of the containers are preferably connected so as to provide a resistance to liquid flow within the container and preferably the upper and lower surfaces of the containers are joined so that the thickness of the container when distended with liquid at the constrictions is from 0 to 50% of the distended maximum thickness of the container. By distended maximum thickness is meant the thickness of the container when at its total volume (as hereinafter defined). This may be achieved, for example, by joining the opposite faces by use of a ring weld. Each constriction may be of any suitable shape, e.g. circular, hexagonal, elliptical, rectangular, or rectangular with curved ends and preferably has a smooth or non-wrinkled contour when the container is distended.If asymmetrical shapes are used the largest dimension of said shape is preferably at right angles to the beam of the device. Alternatively the upper and lower surfaces may simply be drawn closer together by e.g. a suitable form of clip, moulding or grommet to provide the resistance to liquid flow in the container without the opposite surfaces touching. The area of the constrictions comprises from 2 to 30% of the total surface area of the container. In one embodiment of the invention, the constrictions are preferably arranged in parallel rows and in another preferred embodiment each row is staggered with respect to each other.

Preferably the constrictions between the upper and lower surfaces have holes passing through them so that in use of the device, the liquid whose wave height is to be reduced can pass through said holes. These holes, preferably, have an area of 1 to 50% of the total surface area of said upper and lower surfaces of the device. The presence of the holes helps to reduce the tendency of the container to dive when subjected to water currents. Also the wave reduction device may have a flexible flotation collar, e.g. of a closed cell foam, around its periphery to further reduce any dipping tendency.

The container or containers are preferably designed to have a beam and length several times greater than its thickness when filled with liquid and preferably each container has a beam from 10 to 30 times the maximum container thickness when filled with liquid.

The containers usually have a larger beam than length but containers having a greater length than beam may be used. The beam is defined as the dimension of the device which lies along the direction of the incident wave to be reduced in height and the length is the direction at right angles to the beam parallel to the wave crests.

Any length of breakwater may be constructed by joining the containers together to obtain the required protection. Preferably the joins should be made along the beam direction.

Containers may be moored so as to lie close to each other preferably in a side by side relationship. They may have a uniform or varying gap between them to build up a larger area of effective breakwater.

Buoyancy for the containers may be localised or distributed. In one embodiment the buoyancy takes the form of a closed cell foam which is distributed over the surfaces of the containers. Preferably the foam is distributed evenly and over either the upper surface or the lower surface of the containers. The foam may be cut away in the region of the constrictions. Alternatively the upper and lower sheets of the containers may contain buoyancy in the form of many gas filled pockets distributed over the surfaces of the containers.

In a second embodiment of the invention, flexible air chambers may be connected to the containers at the joins between the containers.

Floats may be incorporated in some or all of the depressions in the indented structure of the distended containers.

In order to cause wave reduction over the range of conditions encountered at a chosen location the device is desirably dimensioned to produce the required performance for the longest wave length and greatest wave height (crest to trough) specified for reduction at the location.

The maximum thickness of the container is preferably 0.3 to 10 metres. The beam dimension should be, preferably, 10 to 200 metres.

The overall length of the breakwater installation made up from one or more containers, is determined by the area of protected water required. The shape of the complete break water installation may be straight or curved.

The overall shape is determined by the shape of the containers and the deployment posi tions of the mooring lines.

The containers are preferably made from a polymer coated synthetic fabric in which the polymer may be in the form of a closed cell foam. Preferred fabrics are polyester and nylons and the most preferred material is nylon reinforced rubber sheet. Typical thicknesses used are from 0.5 to 20 mm. Alternatively non-reinforced elastic polymer sheets e.g. a rubber, may be used which when filled have the characteristics of a balloon or bladder.

Again the polymer may be in the form of a closed cell foam.

The mooring lines may be connected to flexible load distributing spars incorporated into the beam edges of the containers. However, in a preferred embodiment mooring may be achieved by passing a primary line between two points which may be anchors or vessels and taking a number of secondary lines from the primary line to the nearest edge of the breakwater. These secondary lines are adjusted in length so as to pull the primary line into a shallow curve and the edge of the breakwater is held in the desired shape. Netting may be used as a substitute for the secondary lines. This arrangement may be duplicated to moor other edges of the breakwater if necessary. In an alternative embodiment, the edge of the breakwater may be fabricated in a curve so as to eliminate need for secondary lines.

The fill of the liquid within a container is preferably greater than 75% of the total volume and may be increased to completely fill the container to give static filling pressures in the bag of up to 3 psi above atmospheric pressure at the top surface of the container liquid. By total volume of the container is meant the volume of the container when floating on the liquid surface and completely filled with liquid to an internal pressure of 0.5 psi above atmospheric pressure at the top surface of the container liquid.

The invention also includes a method of liquid wave reduction whereby: (a) a wave reduction device (as herein before described) is deployed with its length at right angles to the incident wave direction, and (b) the containers of the device are de ployed so that the upper surfaces of the containers are awash.

The invention will now be described by way of example only with reference to Figures 1 to 6 of the accompanying drawings.

Figure 1 shows a plan view of a type A container having constrictions in a 4-5 configuration.

Figure 2 shows a plan view of a type B container having constrictions in a 5-6 configuration.

Figure 3 shows a plan view of a deployed breakwater comprising 5 type A and 5 type B containers with associated buoys and mooring lines, the arrow representing the direction of the incident waves.

Figure 4 shows the relationship between wave height reduction, wavelength and liquid fill for a breakwater, Figure 5 illustrates a number of different ways of forming the constrictions of the containers and Figure 6 shows a diagrammatic representation of a four container breakwater when deployed on open water. Figure 1 shows a type A container 1 measuring about 5.8 x 12 metres. The containers 1 were made from a nylon fabric coated with a 656 gram/m2 polychloroprene/ natural rubber blend. The tensile strength of the fabric was of the order 63 kg/cm warp and 36 kg/cm weft, trouser tear 23 kg.

Distributive buoyancy for type A containers was provided by means of (not shown) 9 mm thick ethylene vinylacetate (EVA) closed cell foam affixed to the top inside face of containers Al and A2 (figure 3) and to the outside lower face of containers A3 A4 and AS. The buoyancy provided by the foam was about 600 kg per container.

Peripheral buoyancy of about 8 kg/metre was provided by 6000 x 150X60 mm blocks of EVA foam laced to the edges of the con tainnrs.

The constrictions 2 were formed in a 4-5 arraligement around 300 mm diameter holes 3 by cold bonding a fabric reinforced grommet to the top and bottom surfaces of the container (see Figure 5). The parallel space apart relationship of the surfaces in figure 5 is prior to the container being distended by liauid. The distance between the edges of the holes 3 was 850 mm and gave a maximum inflated thickness of the container of 540 mm.

Valves 4, 5 were incorporated for filling and monitoring internal pressure and zippers 6 were used for rapid emptying of the container.

The details of construction of the type A containers are shown in the Table 1.

Figure 2 shows a type B container 7 of similar dimensions to the type A container 1.

Two similar nylon/butyl rubber/nylon sandwich materials were used for construction of the container. The first material (I) had a tensile strength of the order 40 kg/cm (warp) and 36 kg/cm (weft) and the trouser tear was 22 kg. The second material (II) had a tensile strength of 57 kg/cm (warp and weft) and the trouser tear was 34 kg (warp) and 27 kg (weft).

Distributive buoyancy for the type B containers was provided by means of 6 mm thick EVA foam with a buoyancy of 400 kg/container affixed to the external lower face of the container 7.

Peripheral buoyancy, filling and pressure relief valves were similar to the type A containers. Details of construction of the type B containers are shown in the Table 1.

The constrictions 8 were formed in a 5-6 arrangement around 300 mm diameter holes 9. The constrictions 8 were stitched circular doublers (Figure 5) cold bonded in position with the circular holes 9 cut out after the bonding.

Figure 3 shows a breakwater 10 comprising ten flexible water filled containers 11 joined in parallel with small gaps between. Five of the containers are of type A (Al to 5) and the other five are of type B (B6 to 10). The total array of containers has dimensions of about 63 x 11 metres. The buoyancy of the containers was such that the upper surfaces were awash.

The mooring system for the breakwater was made up as follows. Four primary ropes 12 of 20 mm diameter polypropylene were held in parabolic form by moorings and buoys 13.

The edges of the containers 11 were attached to the primary ropes 12 by 6 mm diameter nylon lines 14 in a zig-zag configuration. This arrangement keeps the breakwater in its configuration when subjected to wave and tidal forces.

When the breakwater 10 had been deployed and moored, the individual containers 11 were then filled with water. Ten sealed foam filled 25 litre plastic buckets each containing a 35 ampere-hour, 12 volt accumulator on the inside and a 12V, 6800 litre/hour bilge pump on the outside were used to fill the breakwater.

The initial filling took about 3 hours.

The performance of the breakwater was assessed by wave rider buoys 15 placed in front of and behind the breakwater and to one side where a reference buoy was not under any influence from the breakwater. Examples of the results obtained for a sea trial are indicated in the following Table 2.

TABLE 1 Data on Type A and Type B Breakwaters

Buoyancy Estimated wt Maximum of each 12mx Construction Constriction Inflated Calculated Position of 6m section of Spacing Thickness Capacity Distributed Edge Distributed Number kg container mm mm litres kg kg Buoyancy A1 A2 264 Hot bonded 850 540 21,000 600 300 Inside top A3 A4 A5 264 Hot bonded 850 540 21,000 600 300 Outside bottom B6 B7 B8 166 Material II 700 445 16,000 400 300 Outside bottom Cold bonded B9 166 Material I 700 445 16,000 400 300 Outside top Cold bonded B10 166 Material I 700 445 16,000 400 300 Inside top Cold bonded TABLE 2 Sea Trial - Wave Reduction

Incident Wave Transmitted Wave Wave Reduction Height (m) Height (m) HTS Zero Crossing Zero Crossing (Significant) (Significant) #1- # 100 Period Incident Period Transmitted (HIS) (HIS) HIS (secs) (secs) 0.22 0.08 64% 1.63 1.83 0.27 0.10 63% 1.84 1.94 0.42 0.13 69% 1.79 2.22 0.31 0.16 48% 2.03 1.99 0.41 0.16 61% 1.92 2.24 Figure 4 shows results obtained for wave reduction and L/B ratio where L is the water wavelength and B is the beam dimension of the breakwater using a wave tank at different water fills. The breakwater comprised three containers. Each container was formed from two 2078 x 1219 mm sheets of nylon reinforced polyvinyl chloride welded along their edges. The constrictions consisted of 64 mm diameter welded rings in a 3--4 parallel row formation. Buoyancy was provided by strips of 2-3 mm closed cell plastic foam bonded to the upper or lower surfaces of the containers.

The wave tank used had a length of 15.2 metres, a width of 3.6 metres and a water depth of 900 mm. Waves were generated at one end by an hydraulically driven cam and at the other end a beach minimised wave reflection.

The results show an increase of wave height absorbed with degree of water fill up to a fill of 22 gallons. At greater fill, the wave height reduction diminishes. The total fill volume of the container was considered to be 28 gallons.

Figure 5 (a) and (b) shows two ways of fabricating the constrictions, in which in Figure 5 (a), the upper and lower surfaces of fabric are cold bonded to a partly reinforced rubber grommet, and in Figure 5 (b), the upper and lower surfaces of the fabric are cold bonded to a circular stitched doubler.

WHAT WE CLAIM IS:- 1. A wave reduction device comprising one or more flexible containers adapted to be partially or wholly filled with a liquid and means rendering the or each container buoyant, the device also having constricting means provided so that when the or each container is distended by internal liquid pressure, its upper and lower surfaces are drawn towards each other by the constricting means at a plurality of spaced apart locations to form constrictions causing the surfaces to become indented.

2. A wave reduction device according to claim 1 in which the thickness of the or each container at the constriction is 0 to 50% of the distended maximum thickness of the container when filled with liquid.

3. A wave reduction device according to claim 2 in which the upper and lower faces of the or each container at the constriction are joined by bonding or stitching.

4. A wave reduction device according to claims 1 to 3 in which the area of the constrictions is from 2 to 30% of the total surface area of the container.

5. A wave reduction device according to claims 1 to 4 in which the shape of the constrictions of the container is circular, hexagonal, elliptical, rectangular, or rectangular with curved ends.

6. A wave reduction device according to claims 1 to 4 in which the shape of the con strictions is asymmetric and the largest dimen sion of the shape is substantially at right angles to the beam of the device.

7. A wave reduction device according to any of the preceding claims having a plurality of holes passing from the upper to the lower face of the or each container, the holes passing through the constrictions.

8. A wave reduction device according to claim 7 in which the holes occupy 1 to 50% of the total surface area of the or each of the containers.

9. A wave reduction device according to any of the preceding claims in which the upper and lower faces of the or each of the containers are attached to a grommet.

10. A wave reduction device according to any of the preceding claims in which the constrictions are arranged in parallel rows.

11. A wave reduction device according to claim 10 in which the constrictions of the parallel rows are staggered with respect to each other.

12. A wave reduction device according to any of the preceding claims in which the fill of the liquid within the container is 75% or more of the total volume (as hereinbefore defined) of the container.

13. A wave reduction device according to claim 112 in which the static liquid pressure within the bag is up to 3 psi above atmospheric pressure.

14. A wave reduction device according to any of the preceding claims in which the liquid within the or each of the containers is water.

15. A wave reduction device according to any of the preceding claims comprising a plurality of containers joined to each other along the beam direction of the device.

16. A wave reduction device according to any of claims 1 to 15 in which a plurality of containers are spaced apart and joined to each other in a substantially parallel configuration.

17. A wave reduction device according to any of the preceding claims in which the or each of the containers has a beam from 10 to 30 times the maximum container thickness when filled with liquid.

18. A wave reduction device according to any of the preceding claims in which the thickness of the or each of the containers is from 0.3 to 10 metres.

19. A wave reduction device according to any of the preceding claims in which the beam is from 10 to 200 metres.

20. A wave reduction device according to claims 15 to 19 in which at least part of buoyancy means is provided by flexible air chambers connected to the containers and lying at the joins between the containers.

21. A wave reduction device according to any of claims 1 to 19 in which the buoyancy means takes the form of gas filled pockets

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. Figure 4 shows results obtained for wave reduction and L/B ratio where L is the water wavelength and B is the beam dimension of the breakwater using a wave tank at different water fills. The breakwater comprised three containers. Each container was formed from two 2078 x 1219 mm sheets of nylon reinforced polyvinyl chloride welded along their edges. The constrictions consisted of 64 mm diameter welded rings in a 3--4 parallel row formation. Buoyancy was provided by strips of 2-3 mm closed cell plastic foam bonded to the upper or lower surfaces of the containers. The wave tank used had a length of 15.2 metres, a width of 3.6 metres and a water depth of 900 mm. Waves were generated at one end by an hydraulically driven cam and at the other end a beach minimised wave reflection. The results show an increase of wave height absorbed with degree of water fill up to a fill of 22 gallons. At greater fill, the wave height reduction diminishes. The total fill volume of the container was considered to be 28 gallons. Figure 5 (a) and (b) shows two ways of fabricating the constrictions, in which in Figure 5 (a), the upper and lower surfaces of fabric are cold bonded to a partly reinforced rubber grommet, and in Figure 5 (b), the upper and lower surfaces of the fabric are cold bonded to a circular stitched doubler. WHAT WE CLAIM IS:-

1. A wave reduction device comprising one or more flexible containers adapted to be partially or wholly filled with a liquid and means rendering the or each container buoyant, the device also having constricting means provided so that when the or each container is distended by internal liquid pressure, its upper and lower surfaces are drawn towards each other by the constricting means at a plurality of spaced apart locations to form constrictions causing the surfaces to become indented.

distributed over the surfaces of the containers.

22. A wave reduction device according to claim 21 in which the gas filled pockets are in a closed cell foam.

23. A wave reduction device according to any of the preceding claims in which the container is made from a synthetic fabric.

24. A wave reduction device according to claim 23 in which the synthetic fabric is a polyester, a nylon, a rubber or a nylon reinforced rubber sheet, or a laminate of one or more polymers.

25. A wave reduction device according to claim 23 in which the fabric has a thickness of from 0.5 to 20 mms.

26. A wave reduction device according to claims 23 to 25 in which the fabric is coated with a polymer in the form of a closed cell foam.

27. A wave reduction device according to any of the preceding claims in which the device is held in position by means of mooring lines.

28. A wave reduction device according to claim 27 having the mooring means comprising a primary line between two mooring points or anchors, there being a number of secondary lines from the primary line to the nearest edge of the device.

29. Wave reduction devices as hereinbefore described with reference to Figures 1 to 6 of the accompanying drawings.

30. A method of liquid wave reduction whereby (a) a wave reduction device according to any of claims 1 to 29 is deployed with its length at right angles to the incident wave direction, and (b) the containers of the devices are deployed so that, in still liquid, the upper surfaces of the containers are awash.