A method for determining inclinations of land and subsea structures
PT 20190272 (345788)
Key information
Title
A method for determining inclinations of land and subsea structures
Application number
20190272
Case type
National
Status
09.08.2021 Patent meddelt (B1)
Filed
27.02.2019
Effective date
27.02.2019
Publicly available
28.08.2020
Next annual fee due
28.02.2027
Applicant
SUBSEASCANNING AS (NO)
Owner
SUBSEASCANNING AS (NO)
Inventor
Marc Nijdam (DE)
Granted
09.08.2021
Patent number
345788
Expiry date
27.02.2039
Abstract and drawing
Disclaimer: This text has been machine-scanned and may contain errors – please refer to "Publications" for legally binding content.
A method for determining inclinations of land or subsea structures. According to the method a reference plane is created. The reference plane has a known orientation and visual markers. The markers are detected by a camera or scanner and the inclination of the reference plane is compared with markers on the structure and/or along a course of a planned pipeline, thereby obtaining an absolute inclination of said structure and/or along said course. The reference plane may be adjusted about roll and pitch axes until it coincides with the horizontal plane. It may be self-adjusting by gravity or actuators may be used for adjustment.
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PT Varsel om betaling av årsavgift for år 4 + (3352) (PT20190272)
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PT Varsel om betaling av årsavgift for år 4 + (3352) (PT20190272)
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03.01.2022
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PT Varsel om betaling av årsavgift for år 4 + (3352) (PT20190272)
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12.11.2020
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16.03.2020
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20190272
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16.03.2020
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24.09.2019
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PT_report 02:03:48
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Realitet patent
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04.03.2019
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Information to inventor
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27.02.2019
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27.02.2019
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Søknadsskjema Patent
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Description, claims and drawings
Disclaimer: This text has been machine-scanned and may contain errors – please refer to "Publications" for legally binding content.Description
Technical Field[0001] The present invention pertains to a method for measuring inclinations of land or subsea structures, as well as device to be used in connection with measurement of inclinations.Background Art[0002] When subsea structures are to be placed on the seabed, it is often important that the structures are arranged at a predetermined inclination, such as relative to the horizontal plane. Usually, seabed installations have to be arranged horizontally. As an example, when subsea structures are to be connected by piping, it is of vital importance to determine the existing inclination of already installed structures, so that the piping can be tied up correctly with the structure.[0003] To ensure the correct levelling of the structure, levelling instruments, such as inclinometers are used, which measures inclination about two axes, pitch and roll. Existing instruments for long-term monitoring are analogue and will not provide precise easy-to-read values of inclination. Several such instruments are known in the art:[0004] US2015149114 describes a method and system for monitoring the integrity of a subsea pipeline network to transport the production fluid from a subsurface wellhead to a surface facility. The publication fails however to disclose any use of camera or scanner.[0005] US 8806769/US2013104408 describes a levelling instrument in the form of a so-called “bulls-eye” where a sphere that is lighter than a fluid is allowed to roll on the inside of a dome. When the ball is centred in the dome, the instrument is horizontal.[0006] AU2013257432 describes a similar instrument, but here the sphere is heavier than a fluid and rolls on a concave surface. Also, here the instrument is horizontal when the sphere is in the centre of the concave surface.
[0007] The detection of the position of the sphere in these two instruments above is visually done by a camera arranged above the instrument or on an ROV. A serious disadvantage of these instruments is that they require good visibility for the camera and good picture quality. The instrument must be filled with a fluid and has to withstand the high pressures at the installation depth.[0008] Electronic inclinometers for subsea use are also known, such as Inclinometer 42’’ by Oceaneering, RTS Level by Rental & Technology Services AS and OceanSIP-SD by OceanTools.[0009] Existing 2-axis inclinometers that are built for application under water have separate readings for pitch and roll. There is dependence between these two values. This dependence is a source for error and can be demonstrated while levelling the inclinometer-plane through a rotation around the X-axis and the Y-axis during the post-processing. The final, horizontal plane differs for the following two cases: 1) the rotation around the X-axis is performed first and the rotation around the Y-axis is performed second.2) the rotation around the Y-axis is performed first and the rotation around the X-axis is performed second.[0010] The correct process depends on the definition of the axis by the sensor manufacturer. Information about this definition is often not available in the sensor documentation. Consequently, the error is ignored by most users. This leads to unreliable and inaccurate results.[0011] This is an example of inaccuracy: The inclinometer is placed on a subsea structure. Pitch shows 5 deg., roll shows -10 deg. During the data post-processing, the inclinometer plane needs to be rotated in opposite directions in order to receive a levelled plane: -5 deg. in pitch and 10 deg. in roll direction. The order in which the two rotations take place depends on the order the manufacturer has defined. Ignoring it will lead to an error.[0012] The present invention avoids this problem since both angles (pitch and roll), are physically levelled in the field (pitch = roll = 0). The abovedescribed error is zero for this case.
[0013] For air use are a wide variety of digital and analogue inclinometers known.[0014] As indicated above, a particular problem by detecting the readings of inclinometers subsea is the often poor visibility of the water. During installation of subsea structures soil if inevitably whirled up. The particles will take a long time to settle, and in the meantime visual reading of the inclinometer may be difficult. Even when visibility is adequate, interpretation of the numbers shown in the display may be difficult. For example, the number 1.8 may be misinterpreted as 8.1 if the display is seen the wrong way.[0015] The values of pitch and roll may also be switched in the visual reading.Moreover, the correct distinguishing between pitch and roll values is highly depending on a predetermined and correct orientation of the inclinometer when this is placed on the structure. If the inclinometer is oriented in a different direction, misinterpretation of the readings will inevitably occur.[0016] The direction of inclination (+ or -), as well the position of the decimal point, may also be misinterpreted.[0017] Misinterpretations of the values can result in a pipe being produced and/or tied-in at a wrong angle and making it difficult or impossible to connect to the structure. Even if the connection is made, a wrong angle may result in excessive stress on the connection and reducing the lifespan of the connection.[0018] Increasing the size and/or brightness of the display may make it easier to read, but this also means a bulkier device and increased power consumption. This leads to a shorter window for operation.[0019] A suggested solution to the problem of visual reading is to transmit the reading wirelessly to an ROV. However, wireless signals travel poorly in a subsea environment and uses a substantial amount of power. The solution is also inflexible because an ROV has to be equipped with a corresponding receiver to receive the values from the transmitter.
[0020] A particular problem with electronic inclinometers is that they are depending on battery power. Power cables are not desired because of the risk of snagging. The battery has a limited capacity and once depleted, the inclinometer will have to be brought to the surface for recharging of the battery. Experience shows that the expected useable time before depletion is in the magnitude of 24 hours. This creates uncertainty if the battery will last sufficiently long enough for the installation to be completed.[0021] The installation of the subsea structure may often take more than 24 hours, especially if the weather turns bad and the operation has to be interrupted for a while. In such cases, the battery may have depleted, and the recharge will delay the operation even further.[0022] There are inclinometers, such as the above-mentioned Inclinometer 42’’ from Oceaneering, which may be switched off by an ROV operated switch to save power between readings. However, operation of the switch is time consuming and may inadvertently push the inclinometer out of its position.[0023] There are also inclinometers that uses a photocell to control the power consumption by turning off the power when it is dark and on again when light from an ROV is detected. Despite this, there is an uncertainty regarding the battery life, and the solution has proven unreliable.Summary of invention[0024] The is therefore a significant need for a device and a method that requires less power, if any, and where the inclination reading can be visually read despite poor visibility. These and associated objectives are the aim of the present invention.[0025] The objectives are met, according to the invention, by a method where a reference plane is created, said reference plane having a known orientation, said reference plane having visual markers, said markers being detected by a camera or scanner and the inclination of said reference plane being compared with markers on said structure and/or
along a course of a planned pipeline, thereby obtaining an absolute inclination of said structure and/or along said course, as defined in claim 1.[0026] In the preferred embodiment, the reference plane is created and locked in a position close to the structure to be surveyed. The reference plane as well as the structure are equipped with visual markers in the form of 3D points, which can be detected by a 3D scanner or one of more cameras. The scanner or cameras can be carried by a diver or a ROV. The scans or photos from the scanner of cameras will be fed into a per se known photogrammetry software. The digital models created by the software may finally be compared in a 3D capable software, revealing the object’s inclination.[0027] As an alternative to the horizontal plane, a vertical plane may be physically created and locked, which is subsequently tilted by 90 degrees in the appropriate 3D software during postprocessing.[0028] In the preferred embodiment, the device of the invention comprises the following main features:- An electronic inclinometer and a microcontroller that measures pitch and roll values relative to the horizontal plane.- A reference plane.- Visual markers on the reference plane.- An actuator coupled to the inclinometer and the reference plane.[0029] The comparison of the visual markers defining the reference plane and the visual markers on the structure, and hence the determination of the inclination of the structure relative to the reference plane, may be done by methods known to the skilled person, such as 3D-scanning or photogrammetry, in particular close-range photogrammetry.[0030] 3D scanning and photogrammetry techniques are described in a wide range of publications. One publication that gives a very good basic description of these techniques is the pamphlet “Close Range Photogrammetry”, T Luhmann, S Robson, S Kyle and I Harley, 2006, ISBN 1-870325-50-8, which is incorporated herein by reference.
Brief description of drawings[0031]Figure 1 shows an exemplary layout for a projected pipeline between two structures,Figure 2 shows a structure with a reference plane indicator according to the invention arranged on top of the structure,Figure 3 shows another embodiment of the reference plane indicator of the invention,Figure 4 illustrated one principle method of adjusting the reference plane of the reference plane indicator,Figure 5 illustrates an alternative reference plane indicator embodiment, Figure 6 illustrates one alternative embodiment of setting the reference plane, andFigure 7 illustrates another embodiment of setting the reference plane.Detailed description of the invention[0032] The invention will now be described with reference to the accompanying figures.[0033] Figure 1 shows an exemplary layout of a projected pipeline from a first installation A to a second installation B. It is common practice to place frames C with photogrammetry marker along the projected course of the pipeline (the pipeline is not yet installed in figure 1) and around the structures at each end of the projected pipeline. The inclination of these frames will be indicative for the relative inclination of the seabed at the place of the frame. But the angular accuracy over distance using photogrammetry is not always sufficient.[0034] This is the reason why, it is common practice to put photogrammetry markers on the structures A and B, which will be indicative of the inclination of the structure relative to the seabed around the structure.
[0035] To go from relative inclination to absolute inclination of the structures A and B and the seabed along the projected course of the pipeline, the inclination of at least one of the frames and/or of at least one of the structures relative to the horizontal plane must be measured. For this purpose, known inclinometers, as described above, have hitherto been used, with their associated disadvantages.[0036] Figure 2 shows a reference plane indicator 1 according to the invention in a first embodiment. The reference plane indicator has been placed on top of structures (such as structure A in figure 1). The structure A has a terminal 2 for a pipeline (not shown) to be connected.[0037] The reference plane indicator 1 in this embodiment comprises a triangular plate 3 which is supported by one leg 4a, 4b, 4c in each corner 3a, 3b, 3c. On top of the plate 3 are applied fiducial markers (not shown in figure 1).[0038] The legs 4a, 4b, 4c are extendible, so that each corner 3a, 3b, 3c can be adjusted in height.[0039] The reference plane indicator 1 may be placed on a predefined surface 5 on top of the structure A, which has a known orientation relative to the terminal 2. When the reference plane indicator has been safely positioned on the surface 5, the legs 4a, 4b, 4c will be adjusted until the plate 3 has a predetermined orientation relative to the horizontal plane, such as coinciding with the horizontal plane.[0040] Figure 3 shows the reference plane indicator 1 in further detail. Here are three fiducial markers 5 schematically shown on top of the triangular plate 3. The reference plane indicator also has (preferably positioned on the underside of the plate 3) an electronic inclinometer 6 and a processor 7. The inclinometer measures the inclination of the plate 3 about two axes (pitch and roll) relative to the horizontal plane and provides the measured values as input to the processor 7. The processor 7 determines if the legs 4a, 4b, 4c (leg 4c not being visible in figure 3) need to be adjusted in length to level the plate 3 with the horizontal plane.
[0041] The adjustment of the legs 4a, 4b, 4c can be done in several different per se known ways, such as by the legs being telescopic and having a hydraulic or pneumatic actuator, a mechanical screw coupled to an electric motor or a rack and pinion mechanism driven by an electric motor.[0042] The processor 7 will adjust one or two of the legs 4a, 4b, 4c until the pitch and roll values of the inclinometer are coincident with the horizontal plane. In figure 2 the plate 3 has been adjusted 10 degrees in pitch and roll directions relative to the surface of the structure A on which the reference plane indicator 1 has been placed.[0043] To achieve both pitch and roll adjustment, only two of the legs 4a, 4b, 4c need to be adjustable, but having three adjustable legs 4a, 4b, 4c will provide greater versatility.[0044] When the legs 4a, 4b, 4c have been adjusted to bring the plate 3 to coincide with the horizontal plane, there is no more need for any electric power to the reference plane indicator 1. The electric circuits may then be switched off or put into standby mode. As long as the reference plane indicator 1 is left stable at the surface it will show the orientation of the horizontal plane, and the absolute inclination of the terminal 2 and the frames C can be calculated.[0045] It should be noted that the reference plane indicator 1 may also be placed on other surfaces than the structure A, such as on the seabed. Applying a 3D scanner or one or more cameras for photogrammetry, the structure and the reference plane indicator 1 are gathered in a common coordinate system.[0046] Figure 3a shows an alternative variant of the reference plane indicator of figure 3. This comprises fixed legs 4a, 4b, 4c, which are supporting a frame 3’. On top of the frame 3’ is a plate 3 hingedly attached along one edge 3a of the frame 3’. The plate 3 is equipped with visual markers 5. Close to the corner 3b opposite of the edge 3a a lifting mechanism 30 is attached between the flame 3’ and the plate 3. The lifting mechanism comprises an actuator (not shown) that effects the lifting of the plate 3
relative to the corner 3b. This will provide the ability to tilt the plane of the plate 3 about one axis.[0047] A second upper plate (not shown) having visual markers may in turn be hingedly attached to the shown plate 3 about a different edge and a second lifting mechanism may be mounted at a corner in a similar fashion as the shown lifting mechanism to lift the upper plate about a different axis than the plate 3. This way the upper plate can be tilted about two axes to align it with the horizontal plane.[0048] Figure 4 shows an alternative embodiment of a mechanism for adjusting the orientation of the reference plane of the reference plane indicator. Here the reference plane is denoted as 3’. The reference plane 3’ is the top surface of an indicator body 8. The indicator body 8 has a circular curved lower convex surface 9, which is placed against a curved race 10 of a supporting body 11. The curved race 10 has the same curvature as the lower surface 9, albeit being concave. The indicator body 8 is attached to a dog 12, which in turn is coupled to a spindle 13. The spindle is driven by an electric motor 14. By turning the spindle 13, the indicator body 8 will move along the race 10. The inclination of the indicator body 8 and hence the reference plane 3’ depends on the position of the indicator body 8 along the race 10.[0049] To facilitate change of inclination of the reference plane about a transverse axis, a second transverse race can be arranged between the race 10 and the indicator body. This is a well-known structure per se.[0050] If a plate is used as a reference plane, as described in the embodiments above, the plate can also be used to calibrate optical equipment, such as cameras and scanners. To this end, the plate may be equipped with suitable, per se known calibration patterns, such as black and white checkerboard pattern and colour pattern. The pattern may be printed or painted on the plate or made up by engravings or holes in the plate.[0051] Figure 5 shows yet another embodiment of a reference plane indicator 1. In this embodiment the reference plane indicator 1 comprises a
triangular frame 15, which is adapted to rest on the surface on which the reference plane indicator 1 is to be placed. From the frame 15 three posts 16a, 16b, 16c extend. The posts 16a, 16b, 16c are preferably set at a right angle to the plane of the frame15.[0052] The frame 15 and posts 16a, 16b, 16c are hollow tubes forming a common interior chamber 17, which is sealed against the surroundings. The chamber 17 is filled with a suitable liquid, such as oil or alcohol, except for a small portion 18 at the outer end of each post 16a, 16b, 16c. This portion can be filled with a gas, such as air, or a liquid of less density that the liquid in the frame 15 and lower part of the posts. If another liquid is used, this will be incompressible, so that pressure resistant walls against the surroundings is not needed.[0053] A level indicator body 19, which has a density between the two liquids, or between the one liquid and the air or gas, can be used to visually improve the indication of the interface between the two liquids or liquid/gas. The level indicator body 19 will be equipped with fiducial markers.[0054] Due to gravity the level of the densest liquid in the posts will equalize between the post. Consequently, as soon the liquid has settled, the level indicator bodies 19 will be at the same level, i.e. be in the same plane. This plane will coincide with the horizontal plane. Thus, there is no need to have any electrically powered components in this embodiment of the reference plane indicator 1. To facilitate this, the upper ends of the posts may be open, so that the less dense liquid is seawater. The denser liquid may be mercury or another very heavy liquid.[0055] Alternatively, the upper parts of the posts may be interconnected by hoses or tubes 23, as shown, so that the less dense liquid can flow between the posts. The connection can also be made by hoses with a smaller diameter within the posts.
[0056] Moreover, one of the liquids, such as the denser liquid, may be magnetic, such as containing magnetic particles, which enables detection of the interface by a magnetic sensor.[0057] One of the liquids may also be electrically conductive so that the electrical contacts in contact with the liquids above and below the interface may measure the electric conductivity and thereby determine the position of the interface.[0058] Figures 6 and 7 show alternative indicators for determining the liquid level in the posts.[0059] The embodiment of figure 6 shows a post 16 with a piston 20 arranged therein. The piston 20 seals against the walls of the post 16 but will move without significant resistance with the level of the liquid in the lower part of the post 16.[0060] A rod 21 extends from the piston to an indicator body 22. The indicator body 22 is equipped with fiducial markers.[0061] The top of the post 16 may be open to seawater, so that seawater can flow into and out of the upper part of the post 16 in response to the level of the liquid in the lower part of the post, and hence the piston 20 varies. To achieve this, the liquid in the lower part of the post must have a significantly higher density than seawater.[0062] Figure 7 shows yet another alternative embodiment of the indicator.Here the dense liquid in the lower part of the post is in direct contact with the less dense liquid or gas in the upper part of the post, or alternatively in direct contact with seawater. Instead, a reader is arranged on the outside of the post 16, which scans along the post until it finds the interface between the dense liquid and the less dense liquid/gas. When the interface has been detected, the reader maintains its position. The reader is equipped with fiducial markers. If the liquids have different colours it will be easier to detect the interface.[0063] The method of the invention may be implemented as follows:If the reference plane indicator 1 contains electrically powered components the power is switched on. Then the reference plane
indicator 1 is arranged at a stable surface close to or on the structure of which the inclination is to be determined. The reference plane indicator 1 may be carried by an ROV, diver or by a wireline to the location of the stable surface. The stable surface may be the seabed or a nearby structure. The stable surface does not need to be horizontal.[0064] When the reference plane indicator 1 has been placed securely on the stable surface, the device automatically starts to bring the reference plane into a horizontal position. In a preferred embodiment this is done by input from the electronic inclinometer, which is attached to the reference plane, and a processor that gives signals to an actuator to adjust the reference plane. The adjustment of the reference plane continues until the reference plane has the desired position, which in most cases will be a horizontal position, but for certain applications also may be other orientations, such as a vertical plane. A vertical plane may be easier to detect by some types of 3D scanners.[0065] When the horizonal position of the reference plane has been reached, the inclinometer and the actuator may be switched off or enter a standby mode. In this mode, the reference plane will be locked in position so that it is not skewed by water currents or other influences. The ucial markers) on the reference plane will then also be situated in a horizontal plane.[0066] As soon as the reference plane is locked in the horizontal position a or other visual detector may start to compare the markers on the reference plane with the markers on the structure and along the projected course of a pipeline. By using photogrammetry, the angles between the reference plane and any other markers in the vicinity can be determined, both by pitch and roll angle. From these measurements the detailed shape of the pipeline and any need for support along the course of the pipeline can be determined.
Claims
Claims1.A method for determining inclinations of land or subsea structures (A, B), characterised in that a reference plane is created, said reference plane having a known orientation, said reference plane having visual markers (5), said markers (5) being detected by a camera or scanner and the inclination of said reference plane being compared with markers on said structure and/or along a course of a planned pipeline, thereby obtaining an absolute inclination of said structure (A, B) and/or along said course, said reference plane is adjusted about roll and pitch axes until it coincides with the horizontal plane.2.The method of claim 1 , characterised in that the reference plane is selfadjusting by using the gravity to arrange itself with the horizontal.3.The method of claim 1 or 2, characterised in that an electronic inclinometer (6) provides signals to a processor (7) that in turn controls one or more actuators that adjust the reference plane.4.A device for creating a reference plane to perform the method of any of the claims 1 – 3, characterised in that the device comprises a plate arranged on a tiltable structure (3), an electronic inclinometer (6) attached to said plate, a processor (7) coupled to said inclinometer (6), said processor (7) in turn being coupled to at least one actuator, said actuator being adapted to adjust said tiltable structure (3) until said inclinometer (6) gives signals said processor (7) that the plate is coinciding with the horizontal plane.5.The device of claim 4, characterised in that the tiltable structure (3) comprises at least two legs (4a, 4b, 4c) that are adjustable in length.6.The device of claim 5, characterised in that said reference plane is a top surface of an indicator body (8), and that said tiltable structure (3) comprises at least one curved race (10), said top surface being couple to said race (10), and said adjustment of the inclination of the reference plane being performed by moving said indicator body (8) along said race (10).7.The device of claim 4, characterised in that said tiltable structure (3) comprises two curved races set at right angles to one-another.8.A device for creating a reference plane to perform the method of any of the claims 1 – 7, characterised in that the device comprises a plurality of upright tubes (16a, 16b, 16c) which are fluidly connected, said tubes (16a, 16b, 16c) being filled with a liquid with first density, an upper part (18) of said tubes having a second different fluid with a lower density than the first liquid, and that an interface between the first liquid and the second fluid forms said reference plane between the tubes.9.The device of claim 8, characterised in that an indicator body (22) with a density between said first liquid and said second fluid is floating relative to the interface.10.The device of claim 8 or 9, characterised in that said indicator body (22) comprises a piston (20) and a rod (21) extending out of said tube (16) and
visual marker at the opposite end of said rod.11.The device of claim 8, 9 or 10, characterised in that a reader is arranged outside said tube to scan and determine the position of said interface, said reader having a visual marker.12.The device of any of the claims 8 – 11, characterised in that the tube (16) is open to the environments at the top and that if the device is used subsea, the second fluid is seawater.Patentkrav innlevert 16.02.20201. En fremgangsmåte for å bestemme helninger til land- eller undersjøiske strukturer (A, B), karakterisert ved å danne et referanseplan, hvilket referanseplan har en kjent orientering, hvilket referanseplan har visuelle markører (5), hvilke markører (5) blir detektert av et kamera eller en skanner og helningen til nevnte referanseplan blir sammenlignet med markører på nevnte struktur og/eller langs en kurs for en planlagt rørledning, og derved oppnås en absolutt helning av nevnte struktur (A, B) og/eller langs nevnte kurs, hvilket referanseplan blir justert om lengdeaksen eller vertikalaksen inntil det sammenfaller med horisontalplanet.2. Fremgangsmåten i henhold til krav 1, karakterisert ved at referanseplanet er selvjusterende ved bruk av tyngdekraften for å innstille seg med horisontalplanet.3. Fremgangsmåte i henhold til krav 1 eller 2, karakterisert ved at en elektronisk helningsmåler tilveiebringer signaler til en prosessor (7) som i sin tur kontrollerer en eller flere aktuatorer som justerer referanseplanet.4. En anordning for å danne et referanseplan for å utøve fremgangsmåten i henhold til hvilke som helst av kravene 1-3, karakterisert ved at anordningen innbefatter en plate anordnet på en tippbar struktur (3), en elektronisk helningsmåler (6) festet til nevnte plate, en prosessor (7) som er koblet til nevnte helningsmåler (6), hvilken prosessor (7) i sin tur er koblet til minst en aktuator, hvilken aktuator er anpasset til å regulere nevnte tippbare struktur (3) inntil nevnte helningsmåler (6) gir signaler til prosessoren (7) om at platen er sammenfallende med horisontalplanet.5. Anordning i henhold til krav 4, karakterisert ved at den tippbare strukturen (3) innbefatter minst to ben (4a, 4b, 4c) som har justerbar lengde.6. Anordning i henhold til krav 5, karakterisert ved at nevnte referanseplan er en øvre overflate av et indikatorlegeme (8), og at nevnte tippbare struktur (3) innbefatter minst en buet bane (10), og nevnte regulering av helningen til
referanseplanet blir utført ved å bevege nevnte indikatorlegeme (8) langs nevnte bane (10).7. Anordning i henhold til krav 4, karakterisert ved at nevnte tippbare struktur (3) innbefatter to buede baner anordnet i rette vinkler på hverandre.8. En anordning for å danne et referanseplan for å utøve fremgangsmåten i henhold til hvilke som helst av kravene 1-7, karakterisert ved at anordningen innbefatter et flertall av oppadstående rør (16a, 16b, 16c) som er fluidforbundet, hvilke rør (16a, 16b, 16c) er fylt med en væske med en første tetthet, der en øvre del (18) av nevnte rør har et andre forskjellig fluid med en lavere tetthet enn den første væsken, og at en grenseflate mellom den første væsken og det andre fluidet danner nevnte referanseplan mellom rørene.9. Anordning i henhold til krav 8, karakterisert ved at et indikatorlegeme (22) med en tetthet mellom den første væsken og den andre fluidet flyter i forhold til grenseflaten.10. Anordning i henhold til krav 8 eller 9, karakterisert ved at nevnte indikatorlegeme (22) innbefatter et stempel (20) og en stang (21) som strekker seg ut av nevnte rør (16) og visuell markør ved den motsatte enden av nevnte stang.11. Anordning i henhold til krav 8, 9 elle 10, karakterisert ved at en leser er anordnet på utsiden av nevnte rør for å skanne og bestemme posisjonen til nevnte grenseflate, hvilken leser har en visuell markør.12. Anordning i henhold til hvilke som helst av kravene 8-11, karakterisert ved at røret (16) er åpent mot omgivelsene ved toppen og at dersom anordningen brukes undersjøisk, er det andre fluidet sjøvann.