CA1065626A

CA1065626A - Wave reduction device

Info

Publication number: CA1065626A
Application number: CA283,659A
Authority: CA
Inventors: Denis H. Desty; Roger Duckworth
Original assignee: BP PLC
Current assignee: BP PLC
Priority date: 1976-07-30
Filing date: 1977-07-28
Publication date: 1979-11-06
Also published as: JPS5318232A; NO772660L; GB1580326A; DE2734259A1; FR2359937A1

Abstract

ABSTRACT OF THE DISCLOSURE
Wave reduction device comprising a wholly or partially liquid filled 'mattress' structure. The 'mattress' is rendered buoyant and formed from a number of parallel units, the upper and lower surfaces of the units being connected to yield a puckered structure thereby providing a resistance to liquid flow within the units. The buoyancy is chosen so that the 'mattress' lies awash in the water.

Description

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~ The present inven-tion relates to the reduction of l;.cLuid : wave heights and more particularly :rela-tes -to the reduction of .~ea water wave heights in the vicinity of m~.rine installations ~md the like.
Wave reduction devices may be scaled to protec-t operatiol1s varying frorn oil dril].ing and production platforms, loading .. buoys, ship ~alvage and similar open sea m.qrine operations in exposed ocean locations to fish farms or yichtlDarin~s in estuaries~
It is an object of the present invention to provide a floating wave reduction system which substantially dissipates wave energy whereby the wave height is reduced during its passage to~.rards the installation being protected.
-. Thus according to the present invention there is provided a wave reduction device comprising one or more flexible containers ~.
: which are adapted to be partially or wholly filled with a liquid and means for joining them in a side by side relation~hip and mean~ for rendering the containers buoyant, the upper and lo-iTer faces of each container being drawn towards each other at a number of - pucker points so as to form a puckered surface structure whereby a resistance to liquid motion inside each container is created.
~ The con-tainers are preferably suspended so -that, in still --- water, the upper surface of the container is a~rash or down to a .
maxirnum depth of 1/1Oth of the beam of the device. (The beam is defined as the dimensi.on of the device ~Ihich lies a:l.on~ -the direction of the ;.ncident wave to be reduced in height and the length is the dimension at right angles to the beam alon~ the wave crests). ~:
The containers are suitably constructed from a flexible fabric so that, ~Jhen filled, a pattern of joins or pucker points bet~een the upper and lo-rer sheets of -the container cause the top ;. -, , : - - 2.~

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and bottom surfaces to become puckered. 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 bea~ direction.
The upper and lower surfaces of the container are connected 90 as to provide a resistance to liquid flow within the container and preferably the upper and lower surfaces of the containers are joined 90 that the thickness of the container at the pucker point i9 from 0 to 50~ of the inflated maximum thickness of the container. By inflated 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 pucker point may be of any suitable shape e.g. circular, hexagonal, elliptical or rectangular with curved ends. If asymmetrical shapes are used the largest dimension of said shape is preferably at right .,., ~
angles to the beam of the device. AlternativeIy the upper and lower surfaces may ~imply 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 pucker points comprises from 2 to 30~ of the total surface area of the container. In one embodiment of the invention, the pucker points are preferably arranged in parallel rows and in another preferred embodiment each row is staggered with respect to each other.
Preferably the joins or pucker points between the upper and lower ~u~faces have holes pas~ingthrough them 90 that in use of the device, the liquid whose wave height is to be reduced can - 30 pass through said holes. These holes, preferably, have an area ~o~s~

of 1 to 50~ of the total surface area of said upper and lower surfaces of the device. The presence of the holes help~ to reduce the tendency of the container to dive when subjected to ~ water currents. Also the wave reduction device may h~ve a flexible - flotation collar, e.~. of a closed cell foam, arounds its - periphery to further reduce any dippin~ tendency.
The container is designed to have a beam and length several times greater than its thickness when filled with liquid and preferably each container ha3 a beam from 10 to 30 times the maximum container thickness when filled with liquid. The containers u~ually have a larger beaM than length but containers having a greater length than beam may be used.
Any length of breakwater may be constructed by joining the containersto~ether to obtain the required protection. Preferably the joins should be made along the beam direction.
oont~ners may be moored 90 as to lie close to each other with a uniform or varying gap between them to build up a larger area of effective breakwater.
Buoyancy for the containers may be locali3ed 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 at the puckers. Alternatively the upper and lower sheets of the containers may contain distributed buovancy in th~
form of many air filled pockets.
In R second embodiment of the invent;on, flexible air chambers may be connected to the containers at the join~ between module~. Float3 may be incorporated in some or all of the depressions in the puckered structure of the inflated containers.

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In order to cause wave reduction over the range of condition~
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 thicknes~ 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, one or more discrete length units, is determined bv the area of protected water required. The shape of the complete breakwater installation may be straight or curved. The overall shape is determined by . .~.~ , the shape of the containers and the deployment positions 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.
Typcial 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 spar~ incorporated into the beam edges of the containers. ~owever, in a preferred embodiment mooring may - be achievsd by passing a primary line between two points whichmay 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 ~o 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 . :~
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as a substitute ~or the secondary lines. This arran~ement 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 the containers is preferably greater than 7~% of the total volume and may be increased to give static filling pressures in the bag of up to 3 p3i 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 filled with liquid to - an internal pressure of 0.5 p~i 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 hereinbefore described) i9 deployed with it3 length at right angles to the incident wave direction, and (b) most of the upper surfaces of the device being - awash or below the still liquid level.

2 0 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 pucker points in a 4 - 5 configuration.
Figure 2 shows a plan view of a t~pe B container having pucker points 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.

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Figure 4 ~hows the relationship between wave height reduction, wavelength and liquid fill for a breakwater ~igure 5 illustrates a number of different ways of for~ing the pucker points 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/on warp and 36 kg/cm weft, trouser tear 23 kg.
Distributive buoyancy for type A containers was provided by means of (not shown) 9mm thick ethylene vinylacetate (EVA) - closed cell foam affixed to the top inside face of containers Al and A2 and to the outside lower face of containers A3, A4 and A5. The buoyancy provided by the foam was about 600 kg per container.
Peripheral buoyancy of about 8 kg/metre was provided by 6000 x 150 x 60 mm blocks of EVA foam laced to the edges of ; the containers.
The pucker points ? were formed in a 4 - 5 arrangement around 300 mm diameter holes 3 by cold bonding a fabric reinforced grommet to the top and bottom surfaces of the container (see ~igure 5. 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.

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Figure 2 shows a type B container 7 of similar dimensions to the type ~ container 1. Two similar nylon/butyl rubber/nylon sandwhich materials were used for construction of the container.
(I) The tensile strength was of the order 40 kg/cm (warp) and 36 kg/cm (weft)-and the trouser tear was 22 kg., (II) the tensile strength was 57 kg/cm (warp and weft) and the trouser tear was 34 kg (warp) and 27 kg (weft).
Distributive buoyancy for the type 3 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 ~ containers. Details of construction of the type B containers are shown in the Table 1.
The pucker points 8 were formed in a 5 - 6 arrangement around 300 mm diameter holes 9, The pucker points 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 (A1 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 container 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.

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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 autside 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 brea~later 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. ~-- 9 ~ . : .
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Figure 4 shows results obtained for wave reduction and ~ L/B ratio where L i9 the water wavelength and B is the beam dimension - of tha 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 pucker points consisted of 64 mm diameter welded rings in a 3 - 4 parallel row formatlon.
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. Waveq 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 pucker points, in which in Figure 5 (a), the upper and lower ~urfaces 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.
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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A wave reduction device comprising at least one flexible container, said container being adapted to be filled at least partially with a liquid, said container having means for connecting two or more containers in side by side relationship and having means for rendering the container buoyant, said container having upper and lower faces drawn towards each other at a number of pucker points, and means connecting said upper and lower faces to one another at said pucker points thereby forming a puckered surface structure whereby a resistance to liquid motion inside the container is created.

2. A wave reduction device according to claim 1 in which the thickness of the container at the pucker point is 0 to 50% of the inflated maximum thickness of the container.

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

4. A wave reduction device according to claim 1, 2 or 3 in which the area of the pucker points is from 2 to 30% of the total surface area of the container.

5. A wave reduction device according to claim 1, 2 or 3 in which the shape of the pucker point of the container is circular, hexagonal, elliptical, rectangular, or rectangular with curved ends.

6. A wave reduction device according to claim 1, 2 or 3 in which the shape of the pucker point is asymmetric and the largest dimension of the shape is substantially at right angles to the beam of the device.

7. A wave reduction device according to claim 1 having a plurality of holes passing from the upper to the lower face of the container, the holes passing through the pucker points at which the upper and lower faces of the container are drawn towards each other.

8. A wave reduction device according to claim 7 in which the holes occupy 1 to 50% of the area of the container.

9. A wave reduction device according to claim 1, 2 or 3 in which the upper and lower faces of the container are attached to a grommet.

10. A wave reduction device according to claim 1 in which the pucker points are arranged in parallel rows.

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

12. A wave reduction device according to claim 1 in which the fill of the liquid within the container is 75% or more of the total volume of the container.

13. A wave reduction device according to claim 12 in which the static liquid pressure within the bag is up to 3 psi above atmospheric pressure.

14. A wave reduction device according to claim 1, 2 or 3 in which the liquid within the container is water.

15. A wave reduction device according to claim 1 comprising a plurality of containers joined to each other along the beam direction of the device.

16. A wave reduction device according to claim 1 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 claim 1 in which each container has a beam from 10 to 30 times the maximum container thickness when filled with liquid.

18. A wave reduction device according to claim 1, 2 or 3 in which the thickness of the container is from 0.3 to 10 metres.

19. A wave reduction device according to claim 1, 2 or 3 in which the beam is from 10 to 200 metres.

20. A wave reduction device according to claim 15,16,or 17 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 claim 1 in which the buoyancy means takes the form of gas filled pockets 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 claim 1 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 sheet has a thickness of from 0.5 to 20 mms.

26. A wave reduction device according to claim 23, 24 or 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 claim 1 in which the device is held in position by means of mooring lines.

28. A wave reduction device according to claim 27 in which the mooring means comprises passing a primary line between two mooring points or anchors and taking a number of secondary lines from the primary line to the nearest edge of the device.