WO2012007743A4

WO2012007743A4 - Underwater marker

Info

Publication number: WO2012007743A4
Application number: PCT/GB2011/051299
Authority: WO
Inventors: Carl Peter Tiltman; Andrew Malcolm Tulloch
Original assignee: Subsea Asset Location Technologies Limited
Priority date: 2010-07-16
Filing date: 2011-07-11
Publication date: 2012-10-18
Also published as: GB2494830B; DE112011102375T5; WO2012007743A2; GB2494830A; GB201300349D0; WO2012007743A3

Abstract

A marker to mark underwater objects characterised in that it undergoes whole body resonance when exposed to an incident acoustic wave. In one arrangement the marker comprises a shell (12) surrounding a water core. In other arrangements the marker is toroidal or cylindrical or tubular in shape. Holes (20) are provided in the shell to allow water freely to enter and leave the core when the marker is immersed in water. In the case of cylindrical or tubular shells, the cylinders or tubes can be open ended to allow water freely to flow into the interior of the shell. Best results are obtained when the marker is a sphere of aluminium, aluminium alloy or brass between 200mm and 400mm inclusive in diameter with a shell wall between 6mm and 15 mm inclusive thick, operating with an interrogating sonar or other acoustic transducer at 30 to 80KHz inclusive.

Description

UNDERWATER MARKER

This invention relates to passive acoustic reflectors and markers for marking objects and other items of interest underwater. In this specification the word reflector is used generally to cover such devices.

Recently a number of acoustic reflectors have been proposed for use under water to mark and act as guidance to underwater facilities such as oil wells, stanchions, bridge piers, cables and the like. In general these acoustic reflectors comprise a core surrounded by a shell. Acoustic waves falling on the shell are partially transmitted into the core to be reflected from the shell wall opposite the entry, and partially around the shell. The reflected wave and the waves transmitted around the wall of the shell combine constructively to reflect back from the reflector towards the source of the original acoustic waves a strong acoustic wave. An example of such a device is found in

WO

WO 2006/075167

A THE SECRETARY OF STATE FOR DEFENCE 20060720

. In WO2006/075167A a passive acoustic reflector for use underwater and comprises a shell surrounding a core, said shell having one or a plurality of acoustic windows at more or more frequencies and through which acoustic waves at said frequencies may pass, at least in part, through the shell into the core to be focussed and reflected back from the interior of shell wall opposite said window(s), part of the wave also passes around the inside the shell and combines constructively with the reflected wave to provide a very strong output. Such reflectors, although simple in concept, are quite sophisticated in their application.

A new simple acoustic reflector is proposed herein particularly for use where large numbers of low cost devices are needed for straight forward applications, for example marking fishing nets. Although a comparatively simple device, the reflector proposed herein is not limited in its application to simple tasks.

According to the present invention an acoustic reflector for use underwater is characterised in that it comprises a shell having at least one circular cross section surrounding a core said core being of water when the reflector is submerged in water.

In a preferred embodiment the shell has one or a plurality of holes therein by which water may freely enter and leave the interior of the shell when the reflector is in water.

Preferably the reflector is characterised in that the shell is spherical, cylindrical (including tubular), toroidal, an ovoid or conical hollow body with one or a plurality of holes therein by which water may freely enter and leave the interior of the body when the body is in water.

Best results are obtained when the body is metal, although non-metal bodies, such a glass fibre reinforced polyphthalamide are also effective.

Best results are obtained if the shell wall is between 6 and 30mm thick, the circular cross section is between 200 and 400 mm in diameter and reflector is used in combination with an interrogating acoustic transmitter operating at 4 to 80 KHz, with peak performance at about 62KHz. Aluminium and its alloys work well for reflectors; ideally the circular section of the shell should be relatively small – no more than 400mm, larger balls taking up too much room. For ease of deployment and minimising cost the shell wall should be no more than 15mm thick, metal shells less than 6mm thick are insufficiently rigid to be used in this application.

A degree of optimisation of the interrogating frequency may be required in order to achieve best results for a given shell size and wall thickness.

In order that the invention may be more fully understood, reflector s according to the invention are described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a cross section a reflector according to the invention with an aluminium alloy shell; and

Figure 2 is a cross section the reflector of figure 1.

Figure 3 illustrates the use of a bar mounting with the present invention;

Figure 4 shows the use of tubular marker of the present invention marking a pipeline;

Figures 5A and 5B shows a toroidal reflector according to the present invention; figure 5A being a section on the line A-A’ of figure 5B and

Figure 6 shows the use of a toroidal reflector to monitor scouring.

In Figures 1 and 2 two

hemispheres

13 and 14 comprise the shell 12 of a spherical acoustic reflector 10 according to the invention. The spherical shape of the shell 12 naturally provides the necessary circular cross section required for this invention. The hemispheres are made of aluminium alloy 6061T6. The shell is hollow. A number of holes 20 are provided in the

hemispheres

13 and 14 communicating with the hollow interior 16 of the shell. The diameter of holes 20 is not critical -1mm to 2mm is typical; the diameter should be sufficient to allow air to escape freely from the hollow interior 16 of the shell 12 and for water to enter from outside of shell 12.

A tongue 22 is provided around the rim 24 of one of the hemispheres 13. A groove 26 is provided on the rim 28 of the other hemisphere 14 to receive tongue 22 when the

hemispheres

13 and 14 are assembled together. The tongue 22 is glued in place within the groove 26

As an alternative to aluminium or aluminium alloy, a shell comprising 25% glass reinforced polyphthalamide sold under the trade name Zytel® HTN51G25HSL by E.I. du Pont de Nemours has been used. The components are identical to those of Figures 1 and 2 save for the different shell materials, and the provision of a circumferential raised portion or latch around one face of the tongue 22 and a corresponding detent on a face of the wall of the groove 26 to receive the latch, such that when the two

hemispheres

13 and 14 are assembled together around the core, the circumferential latch engages in detent as can be seen. 25% glass fibre reinforced polyphthalamide is sold under the trade name Zytel® HTN51G25HSL by E.I. du Pont de Nemours and Company. A similar glass fibre reinforced polyphthalamide is marketed under the trade mark Amadel by Solvey SA. Polyphthalamides with higher glass fibre content are also obtainable and provide harder shells, but as the glass fibre content increases so does the brittleness of the shell.

Other suitable non-metals to form the shell include epoxy impregnated carbon fibre, Kevlar® (aramid) fibre, Zylon® [poly(p-phenylene-2,6-benzobisoxazole) or PBO] fibre impregnated with epoxy resin, nylon 6, and epoxy impregnated polythene fibre (e.g. Dyneema®).

With a non-metal shell damage has occurred to the shell when it is stored on-shore or on the deck of a vessel. This damage can be reduced by coating the shell with polyurethane, which is closely matched acoustically to sea water, although care must be taken to avoid blocking the holes.

In figure 3, the two

hemispherical halves

13 and 14 of a reflector spherical shell 12 of a reflector in accordance with the invention are as described in figures 1 and 2. Other identical features are not described in detail but can be identified with reference to figures 1 and 2. One hemispherical half 13 of the shell 12 is provided with an internally treaded hole 34. The internal threads 36 of the hole co-operate with the external threads 42 of one end of bar 40. The other externally screw threaded end of the bar 40 may be screwed into a suitable internally threaded socket to mount the reflector in place. Ideally the bar 40 is made of the same material as the shell 12 of the reflector.

The arrangements in figure 3 enable the reflector of the invention to be mounted above ground, or against an object to be marked.

Toroidal shaped reflectors are made in a similar way, from two halves of the toroid each having half circular cross section being joined together to form the final toroidal shape.

Cylindrical or tubular reflectors made of aluminium or its alloys can simply be extruded. If a reflector is made of a non-extrudable material such as glass reinforced polyphthalamide, the manufacturing process is similar to that described in paragraph [0018] is used with two elongate halves of the cylinder or tube being brought together. For long tubes, this latter may not be entirely practicable, and a shell of an extrudable material such as aluminium or its alloys would be preferred for use in such cases.

Both toroidal and cylindrical shells have the necessary circular cross section required for this invention.

Examples parameters and results are:

Example 1 Water in steel spherical shell - outside shell diameter 300mm
- Shell Thickness 10.25mm
- Frequency 38.0KHZ
- Target Echo Strength = -10.1dB
Example 2 Water in steel spherical shell - outside shell diameter 400mm
- Shell Thickness 14.0 mm
- Frequency 27.9KHZ
- Target echo strength = -7.6dB
Example 3 Water in 25% glass reinforced polyphthalamide spherical shell - outside shell diameter 300 mm
- Shell Thickness 13.5 mm
- Frequency 12.0KHZ
- Target echo strength = -14.0dB
Example 4 Water in 25% glass reinforced polyphthalamide spherical shell - outside shell diameter 400 mm
- Shell Thickness 15.5mm
- Frequency 9.0KHZ
- Target echo strength = -11.5dB
Example 5 Water in aluminium alloy 6061T6 spherical shell - outside shell diameter 200 mm
- Shell Thickness 12.75mm
- Frequency 37.4KHZ
- Target echo strength = -16.7dB
Example 6 Water in aluminium alloy 6061T6 spherical shell - outside shell diameter 300 mm
- Shell Thickness 18.75mm
- Frequency 25.2KHZ
- Target echo strength = -13.11dB
Example 7 Water in aluminium alloy 6061T6 spherical shell - outside shell diameter 400 mm
- Shell Thickness 24.5mm
- Frequency 19.1KHZ
- Target echo strength = -10.6dB
A similar response to that of example 7was obtained from an aluminium alloy 6061T6 sphere with a shell thickness 19mm thick broadly over in the frequency range 28 to 35 KHz
Example 8 Water in aluminium spherical shell - outside shell diameter 300 mm
- Shell thickness 14mm
- Frequency 38 to 80 KHz
- Target Echo Strength better than -15db across frequency range
Example 9 Water in aluminium alloy 6061T6 spherical shell - outside shell diameter 400 mm
- Shell thickness 11mm
- Frequency 38 to 80 KHz
- Target Echo Strength better than -15db across frequency range
Example 10 Water in aluminium alloy 6061T6 spherical shell - outside shell diameter 200 mm
- Shell thickness 13mm
- Frequency 38 to 80 KHz
- Target Echo Strength better than -15db across frequency range

The best results were obtained when the shell wall is between 6 and 30mm thick, the sphere is between 200 and 400 mm in diameter and reflector is used in combination, in the case of aluminium or its alloys, with an interrogating acoustic transmitter operating at around 60 KHz in the range 35 to 80 KHz. Aluminium and its alloys work well for smaller diameter spheres, clearly for the application envisaged the ball should be relatively small, larger spheres taking up too much room. Although steel shell reflectors performed well, steel is not the preferred material for underwater application in this invention because of its propensity to corrode, even if treated it is likely to deteriorate much more quickly than the alternatives.

Although the results above are quoted for spheres, similar results are obtained from toroids, cylinders and tubes when the acoustic source directs acoustic waves normally to an external surface of the shell around the circular cross section.

In contrast to the reflector described in WO2006/075167A, there is little or no focussing of the acoustic waves within the shell of this invention, as the reflected acoustic wave appears largely to be generated by reflection from the front surface of the reflector and acoustic resonance of the shell itself. As a result little information is conveyed to a detector concerning the configuration of the reflector. Thus acoustic reflectors according to this invention are appropriate for low cost applications, where it is required to mark an object such as a fishing net, but may be less appropriate for more sophisticated underwater applications, such as marking oil or gas installations where information about the reflector itself it often needed, for guidance purposes. In particular a reflector in the form of a toroid can be placed around the outside of an underwater stanchion or other under water object to mark it.

In each of the examples the target echo strength given was the best achieved for the given shell material and diameter. In the case of steel and the 25% glass filled polyphthalamide shell the response with a 200mm outside diameter shell was inadequate for practical use. It will be seen from the results that the response tends to be stronger as the cross section increases, but at the same time the best interrogating frequency decreases.

Other shells made of epoxy impregnated fibres, such as carbon fibre, Kevlar® (aramid) fibre, Zylon® [poly(p-phenylene-2,6-benzobisoxazole) or PBO] , or polythene fibre (e.g. Dyneema®) will display acoustic resonance behave in a resonance I a similar way to glass reinforced polyphthalamide, although optimum interrogating frequencies and shell thicknesses will vary.

Brass can substitute for aluminium or its alloy and overall at the frequencies at which this invention operates best (4 to 80 KHz) metal shells perform better than non-metal shells.

Figure 4 shows a schematic diagram of a pipe section 50 fitted with a number of aluminium tubular reflector s 52 each according to the invention. Each reflector has a hollow interior 54, with open ends 56 which, under water, allow water freely to enter and leave the interior 54.The tubes are isolated from the pipe sections 50 by conventional electrically insulating lugs 58.The pipe section 50 has conventional end flanges 60 with holes therein allowing it to be bolted to another pipe section. The pipe section with the reflectors can be prefabricated on land and joined by means of the bolt holes in the flanges 60 to another like fitted pipe. In this way, a pipe line fitted with acoustic makers can be assembled as part of the normal process for laying an underwater pipeline.

In operation the reflectors of Figure 4 work in exactly the same way as the spherical reflectors shown in figure 1 and 2. Acoustic waves at about 60 KHz transmitted from an acoustic source, are incident on the tubular reflector 52. The tube 52 undergoes whole body vibration at this frequency which is rerated as an acoustic wave and is detectable at the source of original acoustic transmission

Although the tubular reflectors in figure 4 have been described in relation to a pipe section, the reflectors can be applied to other objects, such as oil rig platforms, accommodation platforms for workers at sea, and other objects to be placed under water

Another embodiment of the invention is shown in figures 5A and 5B. A hollow toroidal reflector 70 constructed following the principals previously described. The reflector has a shell 74 comprising two semi-circular cross sectioned

halves

72 and 73, joined by a glued tongue 77 and groove 78 joint. The shell 74 has a plurality of holes 75, which when the reflector is immersed in water, permits water freely to flow from the outside of its shell 74 the shell to the hollow interior 76. The shell in this example is aluminium or aluminium alloy, although any of the alternatives mentioned in relation to the earlier examples may be used.

Acoustic waves at about 60KHz transmitted from an acoustic source are incident on the external surface of the reflector 70 which undergoes whole body resonance, the resonance generates acoustic waves which can be detected, thus effectively acoustically marking any object around which the reflector 70 is placed.

In figure 6 the lower portion 90 of a stanchion such as a bridge pier is shown extending below surface 91 of the sea into the sea bed 92. A series of toroidal acoustic reflector s 94A, 94B, 94C and 94D as described with reference to figures 5A and 5B is mounted below the sea surface around the pier 90. Those reflector s 94A and 94B above the sea bed can be used to mark the stanchion 90. Currents will scour the sea bed preferentially around the stanchion 90, eventually lowering the sea bed level to 96 exposing the reflector 94C which was initially below the sea bed. Detection of this reflector 94C as a result of resonance on interrogation by a low frequency sonar signal will provide an early warning of scouring, and the need for possible attention.

As scouring continues and the sea bed drops further as indicated by line 98, a further reflector 94D is exposed, which may indicate that a potentially dangerous situation has developed and the underwater mounting of stanchion 90 may need urgent attention.

Claims

An acoustic reflector for use underwater characterised in that it comprises a shell having a least one circular cross section surrounding a core, said core being of water when the reflector is submerged in water.
An acoustic reflector according to claim 1 characterised in that the shell has one or a plurality of holes therein by which water may freely enter and leave the interior of the body when the body is in water.
An acoustic reflector according to claim 1 or 2 characterised in that the shell is spherical, cylindrical, toroidal, an ovoid or conical hollow body
An acoustic reflector according to any claim 1, 2 or 3 characterised in that the shell material is selected from the group comprising a glass reinforced polyphthalamide, a resin impregnated fibre, or metal.
An acoustic reflector according to claim 4 characterised in that the shell comprises a metal selected from the group comprising aluminium, aluminium alloy and brass.
An acoustic reflector according to anyone of claims 2 to 5 characterised in that the shell wall is between 6mm and 30mm inclusive thick.
An acoustic reflector according to claim 6 characterised in that the shell wall is spherical and between 200mm and 400mm inclusive in diameter.
An acoustic reflector according to any preceding claim characterised in that it is in combination with an acoustic source operating at between 4 and 80 KHz inclusive.
An acoustic reflector according to claim 8 characterised in that the reflector is in combination with an acoustic source operating at about 62KHz.
An acoustic reflector according to any preceding claim characterised in that it is cylindrical and attached to a pipe.