WO2007037696A1

WO2007037696A1 - Sheltering device for radar type liquid level measuring apparatus

Info

Publication number: WO2007037696A1
Application number: PCT/NO2006/000134
Authority: WO
Inventors: Øivind JENSSEN; Øivind TØMMERÅS; Oddbjørn MALMO
Original assignee: Kongsberg Maritime As
Priority date: 2005-09-27
Filing date: 2006-04-11
Publication date: 2007-04-05
Also published as: KR20080050367A; JP2011141292A; KR20080098477A; KR100952536B1; KR100952528B1

Abstract

In one aspect, the invention comprises a sheltering device for liquid level radar measurements, comprising a wave screen in fluid communication with the liquid. In another aspect, the invention comprises an apparatus for determining liquid level, comprising a radar measuring device with an antenna for sending signals towards the liquid's surface and a waveguide. The apparatus is characterized in that it comprises a sheltering device adapted for placement under the still pipe and over the tank bottom, where the sheltering device is in fluid communication with the liquid.

Description

SHELTERING DEVICE FOR RADAR TYPE LIQUID LEVEL MEASURING APPARATUS

The invention relates to radar type instruments employed to measure liquid volume in a storage container, in particular radar instruments disposed on top of a waveguide (stilling well or still pipe) that extends vertically down the container. Said waveguide might for various reasons be terminated at a short distance above the base of the container, thus leaving a small gap between the waveguide and the base. Liquid surface below the waveguide lacks the protection provided by the waveguide, and is, thus, fully exposed to wave phenomenon on the surface.

Although the invention will be described in relation to storage tanks onboard marine vessels it is possible to implement it in any type of container as e.g. land based LNG storage tanks, petroleum product tanks, chemical tanks and liquid nutrition tanks.

With particular reference to liquid at low level in storage tanks onboard marine vessels, e.g. vessels carrying liquefied natural gas, it is well known that the radar echo produced by the liquid surface is subject to disturbance by capillary waves and turbulence, at one side, and disturbance by echoes produced at the base of the tank, on the other side. It is generally recognized that both types of disturbance should be minimized in order to allow a radar level gauge instrument to measure liquid level with the accuracy required for custody transfer metering onboard marine vessels. In radar level measurements (as e.g. described in US 6,184,818), still pipes are used to avoid interference with equipment inside the tank, and to prevent surface turbulence in the liquid from causing fluctuations in the reflected signal. Still pipes are pipes which extend from the upper part of the tank up to a certain distance above the tank's bottom. An antenna is located in or immediately over the pipe to direct radiation downwards with the pipe acting as a waveguide. This arrangement will not operate satisfactorily when the liquid level is low and the liquid's surface lies under the still pipe's opening because no still zone on the liquid surface will be provided by the pipe. Further, a high degree of turbulence is present at the lower zone of the tank because the pumping devices have their openings in this zone. Besides, according to the prior art, to avoid interference due to bottom reflection signals a deflection plate or a device with a similar function, e.g. an attenuator, is set directly underneath the antenna. The application of these types of technology for level measurements in LNG tanks was described already in 1996 (committee drafts for ISO International Standard 13689:2001). WO 01/29523 describes an apparatus for determining the liquid level in a tank by means of a radar level measuring device where an absorber for microwaves is placed at the base of the tank below the opening of the pipe to absorb microwave energy to prevent echo from the base of the tank. Such a device will not be suitable for use in its main application field, membrane tanks, where for some designs all devices must be situated at a distance from the base of the tank and not directly on it. Further this device provides no protection against surface turbulence.

This invention is aimed at obtaining controlled measurement conditions close to the tank bottom in situations where the still pipe for any reason cannot extend all the way to the bottom of the tank. The expression controlled measurement conditions refers in the context of the present application to measurement conditions where noise due to among other things equipment in the. tank, surface turbulence and bottom reflections is reduced. For vessel tanks sources for such disturbances are wave motions in the tanks set up by movements of the vessel by wind generated waves and swell, or changes in the trim/list angle of the vessel by ballasting or cargo loading/unloading. Vibrations from machinery may also propagate through tank walls and other mechanical structures inside the tanks and set up small waves/capillary waves inside the tanks. Inside the tanks the cargo pumps will generate both vibrations and wave motions. Specifically in LNG tanks additional sources of disturbance are the spray pumps/stripping pumps and cargo drop lines, and for all types of cryogenic cargo, boiling will create disturbances at the surface.

The need for controlled measurement conditions near the tank bottom arises e.g. in LNG membrane tanks, where there for certain types of membranes (i.e. Invar metal sheet membranes) shall always be a safety distance between the lower end of the still pipe, and the thin membrane tank bottom. A related limitation may be that it is not possible to put or support any structural parts directly on the membrane tank bottom, nor is it possible to make a well in the tank floor to put such devices or to measure the liquid level by measuring upwards in the tank. In addition the pipe and the support structure of the pipe (a tripod or similar structure) may be subject to thermal contraction relative to the tank walls at cryogenic conditions so that at least at low temperatures there will be a substantial free beam space between the lower end of the pipe and the tank bottom where the measuring accuracy is degraded by disturbances of the liquid surface and interference with the bottom reflection. The invention has thus as an object to provide a device by means of which high accuracy measurements can be achieved near the bottom of the tank.

It is an object of the invention to provide a device which is easy to produce and install, i.e. with a minimum of separate parts and a low weight so that brackets or other support mechanisms can be made slender. The requirements to the relative positioning of the device below the pipe must not bee more critical than satisfied by normal construction methods for such tanks (i.e. by welding of brackets).

It is another object of the invention to provide a device with a design which prevents damage to the tank membrane, and avoids any possibility of parts being broken off during operation, as these parts can get soaked into and damage the cargo pumps.

In one aspect the invention comprises a sheltering device for radar measurement of low liquid levels in a container, characterized in that it comprises a wave screen situated near the bottom of the container and in fluid communication with the liquid.

In another aspect the invention comprises an apparatus for determining low liquid level in a container, comprising a radar measuring device with an antenna for sending signals towards the liquid's surface and a waveguide. The apparatus is characterized in that it comprises a sheltering device situated near the bottom of the container, where the sheltering device is in fluid communication with the liquid.

Thus, one aspect of the invention provides a solution to shelter a sufficiently large portion of the liquid surface to provide the best possible conditions to produce a good quality radar echo at said liquid surface. In a second aspect the invention comprises further a device adapted to minimize disturbance produced by radar signals echoed from the base of the tank. This can be achieved by means of various stealth techniques as e.g. absorbers, deflection panels and diffractive geometries, or, as might be perceived, combinations of these techniques. Both aspects of the invention aim to facilitate enhanced signal fidelity respecting the echo produced by a liquid surface close to the base of the container, thus also allowing the radar instrument to perform accurate measurements of a liquid at very low level in the storage container.

In an embodiment of the invention the device is made up of a single type of material such as a fiber mat or woven material containing fibers of carbon or a similar material, and thus inherently satisfies the requirements for calming down the surface and be sufficiently soft to constitute no risk to the membrane. This embodiment will not be appropriate in the cases where the high microwave absorption efficiency of e.g. a resonant attenuator structure is necessary, or where the risk that fibers can fall off and enter into the cargo pumps is not acceptable.

Different designs of the device are therefore perceived for different applications and requirements.

In another embodiment the device is at least partly made of and/or covered by a radar absorbing material. In another embodiment, the device comprises fastening means for fastening to the tripod guidance structure. In a special variant of this embodiment the fastening means comprises at least an articulation and/or is made of a thermal insulation material, and/or comprises an extendable part.

In another embodiment of the invention the sheltering device is adapted to minimize disturbances produced by radar signals echoed from the base of the tank. In another embodiment the sheltering device comprises stealth devices as e.g. attenuators, deflection panels and diffractive geometries. In a special variant of the last mentioned embodiment said stealth devices are provided on the wave screen and/or on the bottom part.

In another variant the wave screen is situated within the radar beam's main lobe. This specific embodiment of the invention is adapted to tanks where there are size limitations, e.g. Samsung 1442 (130x230x180 mm inner measurements). In another variant the device is sized large in relation to the radar beam's radiation lobe, in this manner interaction with the wave screen will be negligible.

In another embodiment the wave screen is adapted to surround a still pipe to provide a telescopic arrangement.

In one embodiment of the invention, the wave screen comprises perforations. In another embodiment the device comprises a bottom part adapted to minimize disturbance produced by radar signals echoed from the base of the tank.

In another variant the bottom part is rectangular or square-shaped. In another embodiment the bottom part comprises a resonant absorber to attenuate the major part of the incident radar signal in such way that a small radar echo will be detected at a location displaced downwards to increase the virtual depth of liquid.

In another embodiment the bottom part is equipped with a grating pattern slab for transforming the input impedance to match the impedance of LNG. In one variant of this embodiment the bottom part comprises a layer of attenuating material and preferably screws for fastening the attenuating material layer to the backing metal. In another variant the layer of attenuating material is situated under the grating pattern. In another variant the metal backing comprises at least two zones which are situated at different distances from the grating pattern (quarter wave step). The attenuator material has as a function to make reflections from the bottom part as weak as possible. The metal backing which provides a quarter wave step causes the reflections from the two zones to be cancelled in the direction back towards the pipe. . The attenuator being resonant, that is the attenuator's thickness is adapted to the wavelength of the radar signal so that the radar energy is trapped and dissipated within the attenuator. Preferably the limit between the two zones corresponds to the axis of symmetry of the radar beam's lobe. In another embodiment the sheltering device according to the invention is shaped as a container, where the wave screen comprises the container's wall and the bottom part comprises the container's base.

In one embodiment of the invention, the sheltering device is equipped with an attenuator in its lower part.

The aim of the container is to provide a still zone which extends from the bottom of the tank to at least some few centimeters above the lower end of the still pipe, in total rarely exceeding 25 centimeters, though, thus avoiding surface waves and bottom reflection to corrupt the measuring signal. In one embodiment of the invention the sheltering device's walls are at least partially perforated to establish fluid communication with the liquid outside. Said perforations will be adapted to provide communication with a low time delay so that the surface inside the sheltering device follows the average surface outside the device with a maximum deviation which is (significantly) less than the specified overall measuring accuracy, at the highest specified pumping rate for that installation. For present LNG applications the delay must typically be less than 1-2 mm.

Another effect of the perforations of the walls is however that disturbances from the outside will enter into the sheltered area. For this reason the perforated area is in one embodiment of the invention minimized and located at the bottom periphery of the sheltering device to suppress capillary waves from entering until the tank level reaches the very bottom of the shelter, and arranged in an irregular pattern so that constructive interference between the various openings is minimized. On the other hand, the perforations also have the effect that standing waves within the shelter are damped, and for this reason the perforated area in one embodiment of the invention is large and spread all over the sidewalls of the sheltering device in an irregular pattern, and/or it may consist of vertical or oblique slices. To obtain the wanted hydrodynamic filtering effect so that rapidly changing disturbances are prevented from entering, while the less rapid change in average level is maintained inside the shelter, the perforations in one embodiment of the invention are made up from a large number of small holes, rather than a few big holes. The edges of each hole are preferably sharp (90 deg) to increase hydrodynamic losses, and thereby obtain sufficient damping of rapid level fluctuations. From the above elaborations one can understand that the design of the sheltering device according to the invention can be optimized for each type of installation in a compromise between the three partly contradicting considerations; sufficiently rapid communication of level, prevention of waves from entering the shelter, damping of waves inside the shelter.

As mentioned above the wave screen will in one embodiment comprise perforations. These will permit to get the surface of the liquid sufficiently calm to obtain stable measurements, nevertheless providing swift communication with the surrounding liquid to obtain sufficiently low time constants and delays in the readings during the most rapid pumping operations in the bottom zone. This is especially important in the embodiment of the invention where the wave screen is closed. A sheltering device according to the invention comprises a preferably perforated wave screen, preferably comprising a radar transparent material or covered on the inside with a radar absorbing material and with physical dimensions and shape so that a minimum of the incident radar signal is reflected back to the radar receiver (stealth function). This achieves also an effective avoidance and suppression of resonant standing liquid surface waves including capillary waves within the device.

The wave screen in one embodiment of the invention is narrow so that it encompasses the still pipe and is allowed to slide as a telescope relative to the still pipe at contraction. IN another embodiment of the invention for use when there is a risk that the telescope can be arrested by thermal effects or any other misalignment of the moving parts of the construction the diameter of the wave screen is increased so that in any case there is a clearance between the pipe and the sheltering device. The wave screen in one embodiment of the invention has a tubular form while in another embodiment it does not form a closed geometry and comprises a curved wall. In one embodiment of the invention the sheltering device comprises a bottom part. This is designed so that it will not damage the membrane during mounting or if any damage of the system should occur during operation (double error). The sheltering device has a minimum of sharp edges at the bottom side, and in one embodiment it is equipped with distance holding pieces in non-metallic material, e.g. Teflon to minimize any risk of fretting or wearing of the tank membrane. The minimum distance above the tank membrane is decided in the concrete application as a compromise between the safety distance applied by the tank designer and the shipyard, and the requirements to minimum gauging level specified by the shipowner. Typically the safety distance is 3-5 mm for Invar sheet membrane tanks. The bottom part of the sheltering device can comprise an absorbing material, which in essence works as an attenuator, and which in the preferred design covers the full area of said bottom. The design may, however, take different forms in that the absorbing material may be distributed at different heights throughout the bottom plane of the sheltering device, thus constituting a grating (or diffracting) structure well suited to divert residual echoes produced at the bottom part outside the detection reach of the radar.

To minimize the probability of boiling inside the sheltering device, in one embodiment the sheltering device is at least partially made in a material with a low specific heat capacity, so that the temperature inside the shelter at any instant is equal to the surrounding temperature.

The material satisfies also other environmental requirement such as e.g. applicability to temperature range, aggressive chemicals or corrosive liquids, and the device in one embodiment is substantially thermally isolated from the supporting structure by use of an insulating part in the supporting bracket, so that heat transferred from the top of the tank via the tripod structure and the still pipe is prevented from heating up the liquid inside the shelter. Both the sheltering device and the isolating part of the bracket can for LNG Tanks be made for instance in plastic materials, of which Teflon may be most commonly used for low temperature applications.

To minimize the probability of generating capillary waves within the sheltering device, the support of the device includes in one embodiment of the invention means for vibration damping which mechanically isolate the device from vibrations induced by the pumps or other parts of the vessel through the tank structure.

The vibration dampers are for instance made of metallic springs or rubber blocks, but at cryogenic temperatures, most materials become brittle or loose their flexibility, and in such case only a few materials are used such as polyester films and similar plastics. In some applications the device is allowed to be mounted directly to the tank bottom by welding, screwing or by use of a clamping bracket. In one embodiment of the invention it comprises a thin Teflon sheet or similar which is between the device and the tank membrane to allow a free relative sliding in case the two are not made of materials with equal thermal constants. In embodiments adapted for cases where the device cannot be mounted directly on the tank bottom, the device comprises fastening means in form of e.g. an arm or a bracket connected to a support structure which rests on the bottom or in any other way is mounted at a fixed vertical distance relative to the bottom of the tank. In a preferred embodiment this support structure is the guiding structure for the tripod, which is also used to support the stripping pump used for the final emptying of the tank. Thereby there will always be a fixed relative and selectable vertical distance between the lowest measured level and the lowest liquid level reached by pumping.

In one embodiment of the invention, the sheltering device comprises fastening means adapted for mounting on constructional parts which contract relative to the vertical position of the tank bottom. This is achieved by means of an arm connected to the device and the constructional part of interest which arm automatically adjusts for the effects of the contraction for instance by using details of bi-metals in the construction or by active control of the vertical position of the device. In one embodiment the sheltering device is at least partly made of and/or covered by a radar absorbing material, which is suitably woven or fabricated to provide the desired effect of damping standing hydrodynamic waves in the shelter. Suitable materials are for instance fibers of Kevlar or other carbon materials. As mentioned before in one embodiment the sheltering device can be substantially made up from a fiber mat or woven material containing fibers of carbon material or similar material . This embodiment combines the desired effects of perforations of the walls to allow liquid communication through the wall, damping of hydrodynamic waves from the outside and at the inside, and attenuation of microwaves by absorption by the carbon and scattering from the irregular surface. Such a soft device will inherently also satisfy the requirement to avoid damage to the thin tank membrane below. To improve the device's rigidity, in one embodiment of the invention the mat is sprayed with adhesive or plastic material to moisten and gather the fibers. It is also possible to equip the device with a support ring. In one embodiment of the invention the internal wave pattern inside the sheltering device is broken up by making the shelter irregular and non-symmetrical in the horizontal plane, using wedges or corrugations of the sidewalls. Preferably simple, regular geometries are avoided, because of the strong half wave resonances. The preferred overall shape is an odd-numbered polygon, preferably also with unequal side lengths.

As one can see, disturbance produced by capillary waves and turbulence can be reduced to an acceptable level by employing a sheltering device according to the invention (comprising a wave screen) to shelter a sufficiently large portion of the liquid surface to provide a fairly even surface for producing a radar echo of good quality. Said wave screen can take various shapes and designs to provide an effective damping force on capillary waves and turbulence without corrupting the quality of the radar echo from the liquid. As for the latter feature various stealth techniques may be employed, e.g. absorbers, deflection panels and diffractive geometries, or, as might be perceived, combinations of these techniques. On the other hand, disturbance produced by radar signals echoed from the base of the tank can be minimized in many ways. One option is to employ a material for the device or at least for parts of it (e.g. the bottom part) that soaks up radar energy to such a degree that only a tolerable fraction of the energy is cast back as a detectable echo from the base of the container. Another option is to employ an oblique metal plate that deflects the major part of the radar signal aside before reaching the base of the container, thus also rendering the echo signal from the base of the container barely detectable for the radar. A third method involves a grid, or array, of geometric shapes and patterns designed to diffract the major part of the echo energy outside the detection reach of the radar. Those skilled in the art will easily conceive that solutions combining the conceptual ideas of radar signal diffraction, deflection and absorption might form a considerable class of devices well suited to minimize disturbances caused by echo signals from the base of the container.

Although various features of the invention have been described as different embodiments of the invention, it will b e obvious for the skilled man that these can be combined to provide a device adapted for a specific use.

An embodiment of the invention will now be described by means of the drawings, where:

Figure 1 shows a device according to the invention in a first embodiment. Figure 2 shows a second embodiment of the device according to the invention. Figure 3 shows diffractive geometries and different wave shield geometries.

Figure 4 shows a detail of the bottom structure in an embodiment of the device according to the invention.

Figure 5 shows a third embodiment of the invention. Figure 6 illustrates different methods for mounting the device to the tank.

Figure 7 shows a sheltering device according to the invention and mounting of the same.

Figure 8 shows a sheltering device according to the invention, comprising a grating pattern. Figures 8 and 9 show tests and analysis of these tests.

Figure 1 shows a sheltering device 1 for liquid level radar measurements, comprising a wave screen 2 and a bottom part 3, where the wave screen 2 is in fluid communication with the liquid 4 (means for fluid communications not shown) The figure shows also a still pipe 5 and a support arm or fastening means 6. In this embodiment the sheltering device 1 is shaped as a container, where the wave screen 2 comprises the container's wall and the bottom part 3 the container's bottom. The tank's bottom 10 is shown in the figure as well as a bottom structure 11.

Figure 2 shows one embodiment of the invention where wave screen 2 only extends on part of the periphery of bottom part 3. This figure shows also perforations 12 in the wave screen 2.

Figures 3 A and 3B show diffractive geometries on the bottom part 3. These diffractive geometries can also be situated in the wave screen 2, and combinations of diffractive geometries in the wave screen 2 and in the bottom part 3 are also possible. The figures show different portions of the bottom part 3: 20, 21, 24, 25, 26 which lie at different levels (distances dl and d2). This, preferably combined with an overlapping of the edges of said portions (e.g edges 22 and 23) permits creation of a diffraction pattern which reduces bottom noise. Distances d2 can be alike for . all portions or they can vary, that is there can be one distance between portion 24 and portion 27 and another distance between portion 24 and portion 25. Those skilled in the art will understand that distances dl and d2 are closely linked to both the refractive index of the liquid medium and the operating frequency of the radar. However, deliberate alterations of said distances may well provide diffractive backscatter that adapt neatly to the geometric shape of the wave screen, thus also minimizing the amount of energy (albeit residual) echoed back to the radar instrument. As an example, one might assume an operating frequency of 10 GHz, and a fluid of liquefied natural gas (LNG), suggesting that said distances might range from 3 to 10 millimeters (a lower frequency suggests larger distances, and vice versa).

Figure 3 shows different embodiments of the device according to the invention. The figures show a bottom part 34 which in a preferred embodiment of the invention is an attenuator, rings 300, 301 which fasten of the bottom part and provide stealth effect, and wave shield 2. Figures 3C, 3D and 3E show different geometries for the bottom part 3 (cylindrical, odd-numbered polygon and irregular shape with few or no parallel sides respectively).

Figure 3F shows an embodiment of bottom part 3 where it contains a woven structure and a support ring 100 for rigidity. Figure 3 G shows an embodiment of the invention where the wave screen 2 has a corrugated form. This embodiment has the advantage that internal standing waves within the shelter are broken up and dissipated more rapidly by the corner structure.

In figures 3H-3K the wave screen comprises thin sheet metal or plastic materials, i.e. stainless steel, aluminium, Teflon or similar. In the embodiments shown in figures 3L-3N it comprises woven material. The various shapes, types of wall structure and microwave attenuator design and geometry described above will in some embodiments of the invention be combined to provide optimized calming devices for various applications and requirements.

Figure 30 shows an embodiment of the device according to the invention comprising a plate 501 which is adapted for placement (fits) between corrugations in the tank bottom. Plate 501 provides a relatively large area where the sheltering device can be placed, the sheltering device can be moved on the plate to achieve optimal placing of the device taking into consideration the radar pipe/radar footprint. Plate 501 comprises walls which provide wave screen 2. The device comprises also a perforated screen or wave absorber 500. The figure shows also stilling chambers 502.

Figure 3P shows an alternative embodiment of the invention where a cylindrical sheltering device comprising wave screen 2, perforated screen 500 and attenuator plate 34 is situated on a plate member 501.

Figure 4 shows details as to the basic structural features of the bottom part 3 which in one embodiment of the invention comprises an attenuating device. From bottom up the structure comprises a metal support 31 that in the preferred design is placed as close as possible to the base 10 of the storage container (tank) without touching the base. The latter design feature calls for a support that for its lower surface 32 extends fairly parallel to the base of the container (tank), whilst, however, the upper surface 33 of the support or bottom part may well include panels and facets set at an oblique angle relative to the base of the container. The second structural feature of the attenuating device 3 is an absorbing material 34 attached to upper surface of the support to soak up a good portion of the energy that has penetrated the liquid surface 4 on its way down from the still pipe 5. Those skilled in the art will easily recognize that both the upper and the lower surface of the absorbing material 34 might pose a mismatch as to radar signal wave propagation, and that both surfaces are thus likely to produce a radar echo. This double-echo feature is utilized commercially in products generally referenced as resonant absorbers (Dallenbach layers), which are well suited to be employed as the absorbing material 34 in the attenuating device 3. As a further structural feature of the attenuating device 3 an additional layer of plastic material 35 is included. Said layer aims primarily to provide protection of the absorbing material, if so required, whilst also providing some design freedom as to optimizing the radar signal performance of the attenuator 3 in regard of its intended application. In order to exemplify this design freedom in one embodiment of the invention the device can comprise an absorbing material 34 with high attenuation factor or, as an alternative, far too thick to provide resonance according to the Dallenbach design, thus leaving only the upper surface of the absorbing material to produce a detectable echo. This echo can, however, be reduced considerably by providing an embodiment of the invention comprising a layer of plastic material 35 with suitable thickness and refractive index, thus also providing a solution that render a viable alternative to the Dallenbach design. Further to this, careful examination of the Dallenbach design has led the inventors to envisage a novel feature and inventive idea as to the measurable location of the echo produced by the bottom attenuating device 3, which renders a beneficial aspect with respect to the intended use of said device. The resonant nature of the Dallenbach layer will either eliminate backscatter, or it may be designed to produce a certain backscatter. The latter feature will inevitably displace the detectable location of the echo produced by the attenuating device 3. Said displacement can by careful choice of absorbing material be forced to provide a detectable location below the support 31 , thus also providing a virtual depth of liquid larger than the real depth. This beneficial feature renders an increased margin for software based algorithms that might be employed to resolve the liquid echo from an echo produced by the attenuating device 3. This appreciable property of the attenuating device 3 applies equally well in case a layer of plastic material 35 with suitable thickness and refractive index is applied to provide the above referenced alternative to the Dallenbach design. Figure 5 shows a third embodiment of the invention, where the sheltering device comprises only a wave screen 2 and where said wave screen 2 does not form a complete pipe geometry but is partly open. .

Figure 6 illustrates different methods for mounting the device 1 to a tank. In figure 6a the sheltering device is fastened by a bracket to a tripod guidance 701 or similar structure. This figure shows the device mounted at the tank bottom 10 but at a distance from it (no contact). Figure 6b shows the device mounted on the tank bottom 10 with separate metal backing. In this case the device can be fastened by bolting, welding or gluing and is in contact with the tank bottom. Figure 6c shows the device mounted on the tank bottom, using the metal membrane as backing. The device can be fastened by bolting, welding or gluing and the figure shows also a clamping ring 700.

Figure 7a illustrates positioning of an embodiment of the invention. In this embodiment the bottom part 3 of the sheltering device 1 comprises a microwave attenuator, and is besides equipped with draining holes 600. The figure shows in full scale the minimum required gauging height of 26mm, the distance between the lower surface of the bottom part 3 and the tank's bottom, which in this case is 4mm and the distance between the upper surface of the bottom part 3 and the tank's bottom, which is 8mm. The figure shows also a plate joint 610 which in the illustrated case (Invar tanks) has a height of 15mm, and the tank membrane 10. Figure 7b shows an embodiment of the sheltering device 1 according to the invention equipped with fastening means 800 whereby the device can be fixed to a tripod guidance structure or any other support resting on the tank bottom 10.

Figure 7c shows one alternative positioning of the sheltering device 1 according to the invention where the bracket of the device is fixed to another bracket protruding from the said tripod guidance structure where also the cargo stripping pump is fixed.

Figure 7d shows another possibility regarding positioning of the sheltering device 1. Figure 7e shows mounting details with dimensions.

Figure 8a illustrates a sheltering device with a grating pattern. Sheltering device 1 comprises a wave screen 2 and a bottom part 3. According to this embodiment of the invention the bottom part 3 comprises a grating pattern layer 902. The lower surface of the grating pattern, and correspondingly the bottom layer 900 has zones with different thickness and provides a quarter wave step 901. An attenuating material layer 34 is situated between the grating pattern layer 902 and the bottom layer 900. A test object was produced corresponding to figure 8a for test purposes. This test object comprises a plastic material where the impedance is equal to the impedance of kerosene which causes the slab to be invisible both submerged in LNG and kerosene. On the other hand the slab is visible in air. The quarter wave step 901 and the attenuating material 34 are in this embodiment of the invention adapted to remove the influence of the reflection of the back side of the slab material. As mentioned before in one embodiment of the invention the sheltering device is provided with fastening means for fastening the slab to the attenuating material. In a special variant of the embodiment the fastening devices are screws which press the slab tightly down to the attenuating material, the bottom part is rectangular and the screws are mounted symmetrically close to each short wall. Figure 8b shows a sheltering device formed as a container and comprising a grating pattern.

Figures 8c-8g show results of tests of the device submerged in kerosene. Kerosene has a dielectric constant close to LNG' s. In the measurements the distance to the slab is 147mm (72 mm in the introductory tests). The device used is mainly as illustrated in figure 8b but provided with fastening screws and where the holes in the walls and the leaky corners are taped with duct tape to avoid leakage of kerosene.

Figure 8c shows an initial measurement with air above slab. Gamma=1.98. The result is an almost perfect pulse (gamma=2.00 equals perfect Hanning pulse), and easy to detect (45 dB) (Note that the pulse at 0 ullage stems from the test pipe and can be neglected).

Figure 8d shows an initial measurement without liquid in the sheltering device, that is the measurement condition includes a plain metal reflector (the device bottom) and air above. Gamma = 2.12. Figure 8e shows the results of a test with 20 mm kerosene above slab. Very fine pulse with gamma= 2.08. Figure 8f shows results with 60mm kerosene above slab. Gamma= 2.15. Figure 8g shows results with 140mm kerosene above slab, gamma =2.07. The centre of gravity of these measurements and similar ones at different levels of kerosene are compared to ruler results in Figure 9a.

The results shown above prove that the sheltering device is effective, with no interfering echoes, and the focus can be moved to accuracy. To examine this part, the reference (ruler) reading was compared to the radar reading. The deviation between the radar measurement and the nominal kerosene level measured by a ruler can be seen in figure 9a. Figure 9a shows that the standard deviation for this set of data is 4,2mm, which is within the requested accuracy of +-5mm.

Figure 9b shows a comparison of level measurements with radar versus ruler. The standard deviation in the measurements is 4.2mm, i.e. the accuracy is within +- 5mm. The estimated accuracy of the ruler measurement (reference) is +-2mm as indicated by the error bars.

Figure 9c shows kerosene amplitude as a function of level, and the results in this figure are consistent. The above mentioned measurements show that the rectangular shaped shielding device does not produce significant unwanted reflections.

The experiments also confirm that an accuracy higher than +- 5mm down to a level of 10-15 mm above the slab can be obtained.

Although different features of the invention have been described as belonging to different embodiments, it will be possible within the scope of the invention to combine some or all of said features in a single embodiment. Thus, in one embodiment of the invention the device comprises stealth devices on the bottom part and perforations on the wave screen. In another embodiment the device comprises perforations in the bottom part and in the wave screen and is adapted to surround a still pipe. In yet another embodiment the device comprises only a wave screen which is substantially made up from a fiber mat.

Claims

I . Sheltering device for radar measurement of low liquid levels in a container, cha racterised in that it comprises a wave screen situated near the bottom of the container and in fluid communication with the liquid. 2. Sheltering device according to claim 1, characterised in that it is substantially made up from a fiber mat or woven material containing fibers of carbon or similar material.

3. Sheltering device according to claim 1 , characterised in that it is at least partly made of and/or covered by a radar absorbing material.

4. Sheltering device according to claim 1, characterised in that it comprises fastening means for fastening to the tripod guidance structure.

5. Sheltering device according to claim 4, characterised in that the fastening means comprises at least an articulation and/or is made of a thermal insulation material, and/or comprises an extendable part.

6. Sheltering device according to claim 1, characterised in that it is adapted to minimize disturbance produced by radar signals echoed from the base of the tank.

7. Sheltering device according to claim 6, characterised in that it comprises stealth devices as e.g. attenuators, deflection panels and diffractive geometries.

8. Sheltering device according to claim 1, characterised in that the wave screen is situated within the radar beam's main lobe.

9. Sheltering device according to claim 1, cha racterised in that the wave screen is adapted to surround a still pipe to provide a telescopic arrangement. 10. Sheltering device according to claim 1, characterised in that the wave screen comprises perforations.

I 1. Sheltering device according to claim 1 , cha racterised in that it comprises a bottom part adapted to minimize disturbance produced by radar signals echoed from the base of the tank.

12. Sheltering device according to claim 11, characterised in that the bottom part is rectangular or square-shaped.

13. Sheltering device according to claim 11, characterized in that the bottom part comprises a resonant absorber to attenuate the major part of the incident radar signal in such way that a small radar echo will be detected at a location displaced downwards to increase the virtual depth of liquid.

14. Sheltering device according to claim 6, characterised in that the bottom part is equipped with a grating pattern slab. 15. Sheltering device according to claim 14, cha racterised in that it comprises a layer of attenuating material.

16. Sheltering device according to claim 15, characterised in that it comprises screws for fastening the slab to the attenuating material layer. 17. Sheltering device according to claim 15, characterised in that the layer of attenuating material is situated under the grating pattern.

18. Sheltering device according to claim 15, characterised in that the attenuating material layer comprises at least two zones which are situated at different distances from the grating pattern (quarter wave step).

19. Sheltering device according to claim 18, characterised in that the limit between the two zones corresponds to the axis of symmetry of the radar beam's lobe. 20. Sheltering device according to claim 11, cha racterised in that it is shaped as a container, where the wave screen comprises the container's wall and the bottom part comprises the container's base.

21. Sheltering device according to claim 20, characterised in that said stealth devices are provided on the wave screen and/or on the bottom part.

22. Apparatus for determining low liquid level in a container, comprising a radar measuring device with an antenna for sending signals towards the liquid surface and a waveguide, characterised in that it comprises a sheltering device according to one of the preceding claims.