WO1980000262A1

WO1980000262A1 - Method and devices for protecting fixed undersea structures

Info

Publication number: WO1980000262A1
Application number: PCT/GB1979/000123
Authority: WO
Inventors: T Aldridge
Original assignee: Moat Ltd; T Aldridge
Priority date: 1978-07-24
Filing date: 1979-07-20
Publication date: 1980-02-21
Also published as: AU4915479A; EP0016080A1; DK106680A; NO792426L

Abstract

To inhibit fouling by marine life of an undersea structure, the part of the structure to be protect (32) is provided with a plurality of elongate fronds (20) of flexible plastics material, such that they can move against each other under the influence of water currents to create a constantly moving surface. Preferably the device is backed by a foam layer (50) which inhibits water movement against the surface of the structure thus reducing corrosion.

Description

METHOD AND DEVICES FOR PROTECTING FIXED UNDERSEA STRUCTURES

This invention relates to a method of protecting a fixed undersea structure , such as an oil platform for example , and to devices for use in the method. The invention relates particularly to inhibiting fouling of the structure by marine life, and to reducing corrosion.

It is well known that structures of this type , which are typically formed of concrete or steel , will accumulate on their surfaces considerable quantities of marine life . This marine life is of many types. Usually mussels cause the greatest problem, but other life of different sorts , such as barnacles for example but also including seaweed, will also grow on them. This causes several problems , including (i) giving increased weight which the structure must support, (ii) causing more drag in the water, requiring the structure to be stronger than would otherwise be necessary , (iii) blocking holes such as Jalan holes which are provided in the structure for engineering reasons , (iv) encouraging corrosion , and (v) making inspection of the condition of the structure very difficult.

With a view to reducing this problem the conventional solution has been to paint the structure with an anti-fouling paint. The main problem with this is that the paint has only a limited effe ctive life , and after that has expired the problem is as great as ever. Accordingly , it has hitherto been necessary to undertake frequent cleaning programmes , which require a diver to clean the structure by physical means . This .is dangerous , and very expensive , and in certain areas , for example in the North Sea , can only be undertaken in about three months of the year.

Proposals have been made to use more sophisticated devices which generate electric currents or ultrasound. These devices have limited effectiveness and are relatively expensive. Maintenance is difficult, and in the case of devices producing electric currents can be dangerous.

Thus none of the prior approaches to the problem have been very satisfactory.

It is known that mussels and many other forms of life will not grow on sand, or where there is an abundance of seaweed. We have appreciated that this provides the key to an improved anti-fouling system.

The present invention is defined in the appended claims , to which reference should now be made. Thus to inhibit fouling we deliberately introduce a moving surface , which can be of carefully controlled dimensions and, naturally, will not grow, so that the growth of mussels and like creatures on the structure is at least substantially reduced. The constant varying movement of the fronds caused by water currents makes the surface unattractive for them.

The fronds can be in the form of any suitable strips or lengths of material.

Various examples of the use of the invention will now be given with reference to the drawings , in which : Fig 1 is a sectional view through a hole in a breakwater provided with an anti-fouling device in accordance with the invention; Fig 2 is a perspective broken-away view of the device ; Fig 3 is a perspective view of the end of the device , with the flange removed;

Fig 4 is a detail showing the flange ;

Fig 5 and 6 are respectively a sectional and a perspective broken-away view of an alternative device ; Fig 7 and 8 are views similar to Figs 5 and 6 of a further device ;

Fig 9 and lO are views similar to Figs 5 and 6 of the device of Fig 1 , and are included for purposes of comparison ; Fig 11 is a perspective view similar to Fig 3 showing an improved form of fixing for the device ; Fig 12 is a section through the device of Fig 11; Fig 13 shows the device bf Fig 11 in the locked position;

Fig 14 illustrates an anti-fouling device applied to the leg of an oil platform; Fig 15 is a diagrammatic sectional view of the device of Fig 14 ; Fig 16 is a sectional view through a device embodying the invention; Fig 17 is a sectional view through an alternative device ; Fig 18 is a sectional view through the leg of an oil platform to which the device of Fig 17 is about to be applied; and

Fig 19 is a detail illustrating how the parts of the device of

Fig 17 are joined together. The various figures of the drawings are only diagrammatic sketches but serve to illustrate the construction and use of several embodiments of the invention.

Tie anti-fouling device shown in Figs 1 to 4 , 9 and 10 is designed to fit in the Jalan holes of an underwater breakwater. One hole 10 in a breakwater 12 is shown in Figs 1 , 2 and 9 , a further adjacent hole 10 can be seen in Fig 10. The device consists essentially of a sleeve portion 14 which is slit , as shown in Fig 3 , so that it can be rolled xsp sufficiently to fit through the hole , where it can expand to bear against the inner surface 16 of the hole. Each end of the device carries a flange 18 shown in Figs 2 and 4 , which, when the sleeve is expanded in the hole , together restrain the device against further movements in the axial direction, these are not shown in Figs 3 , 9 and 10.

At each end of the device adjacent the rim or mouth of the sleeve there is a ring of flexible fronds 20 of plastics material. Each frond is fixed at one end to the sleeve while the other end is free . Preferably the fronds taper towards the free ends .

The fronds are sufficiently flexible to move with the water currents in the manner shown in Figs 1, 9 and 10, and are of such length and number that they sweep across the whole of the inner surface 22 of the sleeve 14. The fronds can be formed of a so-called artifical weed which is made commercially for anti-scouring purposes , that is , to stop sand shifting. In such use it can for example be dropped to either side of an undersea pipeline where this passes over a sandy area, and will sink into the sand giving it resistance against scouring by the sea. In such use it is not placed where it moves over the pipeline surface , and instead is submerged in the sand to either side of the pipeline.

The continuous movement of the fronds 20 within the hole 10 inhibits the build-up of marine life within the hole by preventing settlement and interferring with their feeding mechanisms , and thus reduces substantially the danger of the hole becoming blocked. Block Jalan holes could lead to the breakwater being knocked down by a strong flow of water. The necessity for frequent cleaning of the holes is therefore reduced by the device.

In Figs 5 and 6 an alternative device is illustrated, in which the fronds 20A are fixed at both their ends , at respective ends of the sleeve 14A. Again, the flanges 18 are omitted for clarity. The device shown in Figs 7 and 8 is preferred, in which fronds 20B extend throughout the length of the inner surface of the device 14B.

The particular shape of the fronds is not of great importance providing they have the necessary flexibility. They can conveniently be of differing lengths. One of the virtues of the system is its simplicity, in initial construction , in fitting, and in operation. Nevertheless , with the use of the devices described it is anticipated that the requirements for cleaning will be significantly reduced.

If the fronds are being fitted in a concrete structure as it is being made, they can simply be embedded in the concrete , in which case the sleeve becomes unnecessary.

A modification of the sleeve is shown in Figs 11 to 13. The sleeve has ridges or teeth on the facing overlapping surfaces of the sleeve which form a ratchet, so that once the sleeve is opened up in the hole it will not collapse again.

Figs 14 and 15 illustrate the use of a method embodying the invention on the steel leg of an oil platform. Hence the fronds are woven into a mat 30 which is secured around the leg 32 of the platform. The fronds 20 again can be arranged in many different ways but are preferably distributed over the whole surface to be protected. They then move freely in the water as indicated diagram matically in Fig 15. The size of the fronds can vary as required by the particular circumstances. In one example of the device of Figs 1 to 4 , the thickness of the fronds at their fixed ends is 5 or 6 mm by 1 to 2 mm, and they may conveniently be in the range 15 to 30 cm long. In one example of the device of Figs 7 and 8, the fronds 20B are at a density of tip to about 20 per sq cm.

Two examples of the cons truction of an anti-fouling device for use on the leg of an oil platform will now be given. In the first example , illustrated in the sectional view of Fig 16 , a three-layer mat is formed. The base layer 40 is a non-woven mixed polymix fibre sold under the Trade Mark "Terram" of Imperial

Chemical Industries Limited (ICI) . There is then a layer of a water- impermeable material 42 such as the polythene sheeting sold by ICI under the Trade Mark "Visqueen" . The outer layer 44 may consist of a teylene-reinforced alkathene such as sold by ICI under the Trade Mark "Paraweb" , to which are attached fronds 46 of a toughened nylon such as that sold by ICI under the Trade Mark "Melinex" .

The impermeable sheet 42 is intended to minimise corrosion beneath the device by reducing the movement of water over the surface of the structure. The base layer 40 protects the device from abrasion during fitting and from puncturing caused by irregularities on the metal surface.

In this instance the fronds 46 are of strip form being about 1 cm wide , 75 Aim thick and up to 10 cm long. However fronds of lengths up to about 50 cm could be useful. They are threaded into tiie mat 44 and secured (if necessary) with a pressure-sensitive adhesive . Although the use of adhesives with polythene is generally unsatisfactory this adhesive will secure the nylon fronds to the polythene sufficiently well to make installation of the device possible ; once the device is in position and tightened up , the adhesive is no longer important. Attachment the device should be as simple as possible to minimise underwater work in fitting it. Blocks of gunmetal can be mounted on stainless steel rods which are themselves threaded through the mat 44. "Xylan" fluoropolymer-coated mild steel T-bolts can be used to secure opposite ends of the mat and can be snapped into place before tightening.

In the second example, the device comprises a base layer of polyethylene foam 50 of about 5 cm thidcness when unstressed and an outer sheet 54 of flexible polypropylene or similar material of about

1 mm thickness, as shown in Fig 17. The fronds 56 are attached to the outer sheet as in the previous example. The impermeable middle sheet could optionally be included.

The device is formed in three sectors 60 as shown in Fig 18 to surround a circular leg 62. Adjacent sectors are attached as indicated in Fig 19 by means of nylon webbing 64 which passes through slots 66 near the edges of the sectors. The webbing is tightened until the foam layer is about half its original thickness.

The use of the device of Figs 17 to 19 enhances the corrosion resisting properties of the method.

Such corrosion is of two types, a. electrolytic corrosion, the result of placing a metal (steel) in an electrolyte (seawater) and b. bacterially-induced corrosion arising from the metabolic products of certain microorganisms, notably organic acids and hydrogen sulphide. Both these forms of corrosion are enhanced by the continual movement of seawater over the metal surface which replenishes electrolyte and provides substrate for bacterial growth. By preventing movement of seawater close to the metal surface, the device reduces corrosion from these two sources. In an alternative arrangement, fronds of hollow section are used. The overall density of the fronds can be more suitable and provide more surface area for the weight.

The devices described do not necessarily stop all fouling of the structure to which they are attached. They do, however, produce a useful reduction in the fouling and last for a sufficient period of time to make their use of considerable value.

Claims

1. A method of protecting a fixed undersea structure , which method comprises providing the part of the structure which is to be protected with a plurality of elongate fronds of flexible plastics material such that they can move against each other under the influence of water currents.

2. A method according to claim 1, wherein the fronds are of strip form.

3. A method according to claim 1 , wherein the fronds are of hollow section.

4. A method according to claim 1 , wherein the fronds are mounted on a plastics foam material which is compressed against the structure.

5. A device for protecting a fixed undersea structure , the device comprising a plurality of elongate fronds of flexible plastics material capable of moving against each other under the influence of water currents , and means for fixing the fronds on an undersea structure.

6. A method of protecting a fixed undersea structure , the method comprising providing an artificial device having elements capable of continuous movement in seawater, and attaching the device to a fixed undersea structure.

7. A method of protecting a fixed undersea structure , comprising surrounding a portion of the structure with a plastics foam material having an outer plastics layer thereon.