OA12234A - Apparatus for deploying a load to an underwater target position with enhanced accuracy and a method to control such apparatus. - Google Patents

Description

012234 1

APPARATUS FOR DEPLOYING A LOAD TO AN UNDERWATER TARGET POSITION WITHENHANCED ACCURACY AND A METHOD TO CONTROL SUCH APPARATUS

The présent invention relates to an apparatus for deploying an object to anunderwater target position, the apparatus being provided with a beacon to transmitacoustic rays and a plurality of thrusters to control positioning of the apparatus withrespect to the underwater target position.

Such an apparatus is known ffom WO 99/61307.

The prior art apparatus is used for deploying and/or recovering loads up to 1000tons or more on the seabed at great depths, for instance, up to 3,000 meter or more. Duringdeployment, the apparatus is controlled by controlling equipment on board of a vesselfloating on the sea surface. The controlling equipment needs to know the exact location ofthe apparatus as accurate as possible. To that end, the beacon on board of the apparatustransmits acoustic rays through the sea water to the vessel. An appropriate acousticreceiver receives these acoustic rays and couverts them into electrical signais used tocalculate the position of the apparatus with respect to the vessel.

However, it is found that with increasing depth of the apparatus below the seawater the accuracy of the location measurement decreases due to bending of the acousticrays in the sea water.

The object of the invention is therefore to further enhance the accuracy of thelocation measurement of such an apparatus during use in sea water or any other fluid.Moreover, such location measurement is needed on-line (real-time).

To obtain this object, the apparatus as defined at the outset is characterized in that itis provided with a sound velocity meter to measure velocity of sound in a fluidsurrounding said apparatus. Thus, the velocity of sound at a certain location in the fluidcan be continuously measured and used to update a sound velocity profile, i.e., data as tothe sound velocity as a function of depth in the fluid. From these data, local bending of theacoustic rays can be determined on-line (real-time). So far, such on-line détermination hasnot been possible. This allows corrections of location measurements in real-time.

In a preferred embodiment, the thrusters comprise a first set of thrusters arranged to provide a torque control function and a second set of thrusters arranged to provide at least a translation function, each thruster of the second set of thrusters being provided with a rotary actuator. 012234

This is a very advantageous embodiment Only two thrusters are necessary toprevent any undesired rotation of the apparatus attached to the load during deploymentthus avoiding ail problems related to twisting and tuming of hoist wires carrying the load,as already explained in WO 99/61307. Moreover, only two rotatable thrusters are neededto control positioning of the apparatus with its load attached to it to the desired horizontalcoordinates. Thus, prior to lowering the load with the apparatus the apparatus can movethe load to the desired horizontal coordinates and when these coordinates hâve beenreached the hoist wire(s) can lower the load to its desired location on the seabed while thethrusters keep the load on the desired coordinates and prevent any undesired rotation ofthe load. Only when the desired target position on the seabed is reached a possible rotationof the load to a desired orientation need be carried out by the thrusters dedicated to thetorque control.

It is observed that rotatable thrusters on an underwater apparatus for deployingloads to a desired position are known from US-A-5,898,746.

The apparatus is preferably provided with load cells to measure weight of the loadattached to the apparatus. When the load is put on the seabed by this weight suddenlydecreases. Thus, a signal indicating that the weight of the load suddenly decreases can beused to détermine when the apparatus may be detached from the load.

The invention also relates to a processing arrangement arranged to drive an appa-ratus for deploying an object to an underwater target position, the apparatus being pro-vided with a beacon to transmit acoustic rays, a plurality of thrusters to control posi-tioning of the apparatus with respect to the underwater target position, and a soundvelocity meter to measure velocity of sound in a fluid surrounding the apparatus, the Proc-essing arrangement being provided with an acoustic receiver to receive the acoustic rays,the processing arrangement is arranged to use data derived from the acoustic rays in acalculation to détermine the position of the apparatus characterized in that the processingarrangement is arranged to receive online sound velocity meter data from the soundvelocity meter to détermine a sound velocity profile in the fluid and to calculate from thesound velocity profile bending of the acoustic rays transmitted by the apparatus throughthe fluid and to use this in the calculation to détermine the position of the apparatus inreal-time.

Such a processing arrangement is able to control driving of said apparatus to adesired location in a desired orientation with very high accuracy, even at great depth under 012234 3 water. While the apparatus with its load is lowered, the processing arrangement constantlyreceives Sound velocity data and détermines a sound velocity profile comprising soundvelocity data from the water surface to the depth of the apparatus. The processingarrangement uses these data to détermine acoustic ray bending as a fonction of the depthin the water and thus to correct any position calculation of the apparatus.

Such a processing arrangement may be on board of a vessel floating on the watersurface. However, it is to be understood that part of the fonctionality of determining thesound velocity profile and calculating the acoustic ray bending may be carried out by oneor more processors iocated elsewhere, even on board of the apparatus itself.

Preferably, a forther sound velocity meter is provided just below the water surfaceto provide actual data regarding any ray bending in the water surface layers and thus toforther correct any position calculation of the apparatus. Réception of the acoustic rays transmitted by the apparatus is preferably performedby an acoustic array attached to the hull of the vessel.

In a very preferred embodiment, the vessel, the acoustic array and the apparatus areail provided with a distinct gyrocompass measuring respective heaves, rolls and pitches.Output data from these gyrocompasses are used to forther increase accuracy of theposition measurement of the apparatus.

The invention also relates to a System comprising such a vessel and an apparatustogether.

The invention also relates to a method of driving an apparatus for deploying anobject to an underwater target position, the apparatus being provided with a beacon totransmit acoustic rays, a plurality of thrusters to control positioning of the apparatuswith respect to the underwater target position, and a sound velocity meter to measurevelocity of sound in a fluid surrounding the apparatus, the method comprising the steps of: • receiving the acoustic rays, • using data derived from the acoustic rays in a calculation to détermine the position ofthe apparatus characterized by the steps of: • receiving sound velocity meter data from the sound velocity meter and determining aSound velocity profile in the fluid, and 012234 • calculating from the Sound velocity profile bending of the acoustic rays transmittedby the apparatus through the fluid and to use this in the calculation to détermine theposition of the apparatus.

This method may be entirely controlled by a suitable computer program after beingloaded by the Processing arrangement. Therefore, the invention also relates to a computerprogram product comprising data and instructions that after being loaded by a processingarrangement provides said arrangement with the capacity to cany out a method as definedabove.

Also a data carrier provided with such a computer program product is claimed.

Below, the invention will be explained in detail with reference being made to thedrawings. The drawings are only intended to illustrate the invention and not to limit itsscope which is only defined by the appended daims.

Figure 1 shows a schematic overview of a FPSO (floating, production, storage andoffloading System) dedicated to offshore petrochemical recoveries.

Figure 2 shows a crâne vessel according to the prior art and displaying a loadrigged to the crâne block with relatively long wire ropes whereby it is possible to see thatthe control of the load is virtually impossible at great depth.

Figure 3 shows a crâne vessel and an underwater System for deploying and/orrecovering a load to and/or from the seabed according to the prior art.

Figure 4 shows a detailed overview of a possible embodiment of the underwaterSystem.

Figure 4a shows a detailed overview of one of the rotatable thrusters.

Figure 5 shows the underwater System viewed from above.

Figures 6a and 6b schematically show the underside of the main module with somedetectors.

Figure 7a shows a schematic block diagram of the electronic equipment on boardof the vessel.

Figure 7b shows a schematic block diagram of the electronic equipment related to an acoustic array and related to the underwater System.

Figure 8 shows the définition of three different coordinate Systems used during driving the underwater System to its target position.

Description of the preferred embodiment 01 2234 5

With référencé to figure 1, the layout présents a FPSO 1 with swivel productionstack 11 from which risers 2 départ, said risers connecting to their riser bases 3 at theseabed 4. During production lifetime, it is paramount for the FPSO 1 to remain within anallowable dynamic excursion range and therefor the FPSO 1 is moored to the seabed 4 bymeans of mooring legs 5 which are held by anchors 6, or altematively by piles.

Exploitation of oil or gas by means of a production vessel 1, requires that severalrelatively heavy objects be positioned at the seabed 4 with a high accuracy.

To secure an appropriate and safe anchoring by means of the mooring legs 5, it isrequired that these mooring legs 5 hâve approximately the same length. In practice for thisapplication anchors can be used with a weight of 50 ton and more, which are placed at theseabed 4 with an accuracy to within several meters. Moreover not only is the anchor 6itself very heavy, but the mooring leg attached to the anchor 6 has a weight that equalsseveral times the weight of the anchor 6 itself.

Also for other objects like the "templates", "gravity riser bases", "productionmanifolds" etceteras applies that these objects hâve to be put on the seabed 4 withrelatively high accuracy.

The objects that are shown in figure 1 that are required for exploiting the oil andgas at sea and that hâve to be put on a seabed, are not only very heavy, but very expensiveas well.

Figure 2 shows a vessel 20, according to the prior art, having hoisting meansthereon, like a crâne 21. The crâne 21 is provided with a hoisting wire 22, by means ofwhich an object or a load 4 can be put on the seabed 5. In order to position the load 23 it isnecessary to move the surface support together with the crâne 21.

The resuit will be that, at one given time, the load 23 inertia will be overcome butdue to the load 23 accélération, an uncontrollable situation will occur, whereby the targetarea will be overshot. Because of the fact that the hoisting wire 22 and the load 4 aresusceptible to influences like the sea current, the load 23 will not move straightdownward, when the hoisting wire 22 is being lowered. Also the heave, roll and pitch ofthe vessel 20 will hâve a négative influence on the accuracy that can be achieved.

Figure 3 shows a crâne vessel 40 provided with an underwater apparatus or System 50 for deploying a load 43 on the seabed 4. The vessel 40 comprises first hoist means, for example a winch 41, provided with a first hoist wire 42. By means of this hoist wire 42 the load 43, for instance a template can be deployed and placed at the bottom of the sea. 012234 6

As mentioned above, the exploitation of oil and gas fields using a floatingproduction platform requires that several heavy objects must be placed at the seabed 4,moreover, these objects hâve to be placed on the seabed 4 with a very high accuracy.Because of the fact that nowadays the exploitation has to be done at increasing depths upto 3000 m and more, achieving the required accuracy is getting harder. E.g., one of theproblems to be solved is the possible rotation of the load 43 carried by hoist wire 42.

In order to control the position of the load 43 when deploying it and in order to beable to position the load 43 on the seabed 4 within the required accuracy, the apparatus orSystem 50 has been secured to the lifting wire 42. A preferred embodiment of the System50 will be described with reference to figures 4, 5, 6a and 6b.

The System 50 may engage the end of the lifting wire 42. Altematively, the System50 may directly engage the load 43 itself. The System 50 comprises a first or main-module51, provided with drive means such as thrusters 56(i), i = 1, 2, 3, ...1,1 being an integer(figures 4 and 5). The System further comprises a second or counter module 52. Thiscounter-module 52 is also provided with thrusters 56(i). In use the thrusters of the main-module 51 and of the counter-module 52 will be positioned at opposite sides of the liftingwire 42.

The System 50 is coupled to the vessel 40 by means of a second lifting wire 45,which can be operated using second hoist means, for instance a second winch 44. Thesecond hoist wire 45 is, for instance, set overboard by means of an A-ftame 49. Thesecond winch 44 and the second hoist wire 45 will be normally lighter than the first hoistmeans 48 and the primary hoist wire 42, respectively. The system 50 is further connectedto the vessel 40 by means of an umbilical 46. This umbilical 46 can be attached to thehoist wire 45 or can be lowered from a tertiaiy winch 47 separately. The electricity wiringfor providing power to the system 50, as well as electrical wiring or optical fibers are forinstance accommodated in the umbilical. In the system 50 usually means are provided toconvert the electrical power into hydraulic power. The hydraulic power consequently willbe used for controlling i.a. the thrusters 56(i) and auxiliaiy tooling amenities.

Since lately the work is being done at an increasing depths, twisting and tuming of the loads 43 and long hoist wires 42 is becoming a bigger problem still. Since heavy loads 43 are attached at the underside of the hoist wire 42, such twisting and tuming can impel a relatively large wear on the hoist wires, so severe damage can occur at the hoist wires.

This wear can be so severe that a hoist wire 42 will break and the load 43 will be lost. 01 2234 7

Another problem is that because of enormous twists in the wires, the wires at the vesselcan run out of the sheaves.

Because of the fact that the thrusters 56(i) of the main-module 51 and of thecounter-module 52, respectively, are positioned at opposite sides of the lifting wire 42, acounter-torque can be exerted at the hoist wire 42 in both directions. In this way by meansof the System an anti-twist device is formed. In order to improve the abilities of this anti-twist device, preferably, the distance between the main-module 51 and the counter-module52 can be altered.

Figure 4 shows a detailed overview of a possible embodiment of the System 50 fordeploying a load 43 on the seabed 4. Figure 5 shows the System according to figure 4,from above.

The System 50 comprises the main-module 51, the counter-module 52 and an arm53. The arm 53 can be detached from the main-module 51. That means that the main-module 51 can also be used separately, as a modular System. The arm 53 is provided witha recess 54. On opposite sides of this recess 54 two jacks 57, 58 are provided, at least oneof which can be moved relative to the other. In between the end surfaces of these jacks 57,58 an object, such as a crane-block of load 43, can be clamped. In order to improve thecontact between the jacks 57, 58 and the object, the respective ends of the jacks areaccommodated with clamping shoes lined with a friction element, from a high frictionmaterial such as dedicated rubber.

In use, the thrusters 56(i) can be used to position the System 50 relative to a targetarea on the seabed 4. The thrusters 56(i) can be actuated from a fîrst position mainly insidethe System 50, to a position in which the thrusters projects out of the System 50. The twoupper thrusters 56(2), 56(3) are rotatable with respect to the underwater System 50. Theyare, for instance, installed on respective rotary actuators 65(1), 65(2). The purpose thereofwill be explained later. Thruster 56(2) has been shown on an enlarged scale in figure 4a.

In figure 5 it is shown that there are two positions 61, 62 on top of the main-module 51 to connect the main module to the second lifting wire 45 and/or to theumbilical 46. When the main-module 51 is used separately position 61 can be used. Themain-module 61 will be balanced when the module 61 is deployed, both in the air andunderwater.

When the System 50 is used, the connection between the vessel 40 and the System50 will be fixed in position 62 in order to keep the System in balance, both in the air and 8 012234 underwater. Το improve the balance of the System, an auxiiiary counterweight 55 can besecured to the system 50.

In use, the apparatus 50 will not hâve any buoyancy. In order to improve themovability of the System under water, the arm 53 is provided with holes 59, in order toavoid structural damage due to an increasing pressure while being lowered and to ensurequick drainage during the recovery phase.

As mentioned above, it is advantageous when the counter-module 52 can be movedrelative to the main-module 51. This can be accomplished by using jacks 64a.

The module 51 comprises an outer frame and an inner frame (both not shown). Theinner frame preferably is cylinder-shaped. By connecting the outer frame to the innerframe, a very strong construction can be accomplished. The strength of the construction isnecessary in order to avoid prématuré fatigue in the system.

The module 51 is, for instance, partly made of high-tensile Steel and therebydesigned to be used as intégral part of either the first 42 or second hoist wire 45. Thismeans that the top side of the module 51 will be connected to a first part of the hoist wire45, and that the underside of the module 51 will be connected to a second part of the hoistwire 45, or the underside of the module 51 will be attached directly to the load. in this waythe load on the hoist wire will be transferred through the module 51.

As mentioned before, the module 51 is provided with a thruster drive 270 forconverting electrical power, delivered through the umbilical 46, into hydraulic power.This thruster drive 270 may comprise motors, a pump, a manifold and a hydraulicréservoir. Such converting means are known to persons skilled in the art and need nofurther explanation here. In order to communicate relevant data as to its position, bothabsolute and relative to other objects, to the contrai system and/or an operator on board ofthe vessel 40, the module 51 further comprises sensor means and control means that willbe explained in detail below. The module 51 is equipped with a sensor junction box.Moreover, the module 51 comprises light-sources 87, a gyrocompass 256 including heave,roll and pitch sensors, a pan and tilt color caméra 97, a USBL responder 255 including adigiquartz depth sensor 253, a sound velocity meter 258, and a sonardyne mini Rovnav264. At the underside of the module 51 are mounted on several platforms light sources 94,a pan and S.I.T. caméra 93, an altimeter 262, a Doppler log unit 266, and a dual headscanning sonar 260. They are installed there to hâve only clear sea water below them, inuse. They are schematically shown in figures 6a and 6b. It is to be understood that they 01 22 34 9 may be located elsewhere, e.g., at the underside of module 52. Moreover, load cells 268are part of the System 51. AU these components are schematically indicated in figure 7b.

As mentioned above, the use of high resolution sonar equipment 260 together witha distance log, measured by Doppler log unit 266, is important to achieve the requiredaccuracy, once the load has reached its intended depth. The sonar equipment 260 will beused to détermine the position with respect to at least one object positioned at the seabed.Using the distance log, it will then be possible to dissociate the positioning activities fromthe surface support, as weü as from any other acoustic transponder devices such as LBL(Long Base Line) arrays (or other, e.g., USBL), while accuracy in the order of centimeterswill be achieved within a large radius.

Figure 7a shows the electronic equipment 200 installed on the vessel 40, whereasfigure 7b shows déployable acoustic array 250 with velocity meter 248 and a gyrocompass 252. Figure 7b also shows underwater electronic equipment 249 installed onthe underwater System 50.

The equipment shown in figure 7a comprises four processors: a navigation proc-essor 202, acoustic processor 224, a sonar control processor 236, and a thruster controlprocessor 240. The navigation processor 202 is interfaced to the other three processors224,236,240 for mutual communications and complementarity.

The navigation processor 202 is also interfaced to a surface positioning equip-ment DGPS (Differential Global Positioning System) 204, a vessel gyrocompass 206,four display units 208, 210, 212, 214, a printer unit 218, a keyboard 220, a mouse 222,and a fiber optic (de)multiplexer unit 244. If necessary, a video splitter 216 may beprovided to transmit one SVGA signal output of the navigation processor 202 to two ormore display units. In figure 7a, display units 212, 214 are connected to the navigationprocessor 202 via video splitter 216.

The fiber optic (de)multiplexer unit 244 is also connected to the acoustic proces-sor 224, the sonar control processor 236, and the thruster control processor 240.

The acoustic processor 224 is connected to a command and control unit 226which is connected to a keyboard 230, a mouse 232 and a display unit 228, ail togetherforming a USBL surface unit 234.

The acoustic processor 224 is connected to déployable acoustic array 250 with motion sensor unit 252 and velocity meter 248. In use, the acoustic array 250 is, pref- erably, mounted 2.5 meters below the keel of vessel 40. 012234 10

The fiber optic (de)multiplexer unit 244 is connected to a further fiber optic(de)multiplexer 246 installed on the underwater System 50. An optical fiber intercon-necting both fiber optic (de)multiplexers 244, 246 is preferably accommodated in um-bilical 46 (figure 3).

The sonar control processor 236 is connected to a display unit 238. The thrustercontrol processor 240 is connected to a display unit 242.

The underwater equipment 249 is shown in figure 7b in the form of a block dia-gram. The USBL responder 255 with digiquartz depth sensor 253, a gyrocompass withmotion sensors 256, (removable) Sound velocity meter 258, a dual head scanning sonar260, altimeter 262, sonardyne mini Rovnav 264, Doppler log 266, load cells 268, andthruster drive control 270 are ail connected to the fiber optic (de)multiplexer 246.

Moreover, figure 7b shows two beacons 272,274 that can be installed on the sea-bed or on the load to be deployed (or on other structures already on the seabed). Thesebeacons 272, 274 can, e.g., be interrogated by means of the sonardyne mini Rovnav264 (or équivalent equipment) to transmit acoustic signais back to the System 50 thatcan be used by the system 50 itself to détermine and measure distances and orientationsrelative to these beacons. Such an acoustic telemetry link results in very high précisionrelative position measurements. The number of such beacons is not limited to the twoshown in figure 7b.

Functionality

The functions of the components shown in figures 7a and 7b are the foliowing.

The navigation processor 202 is collecting the surface positioning equipment data(DGPS receivers, DGPS corrections, vessel's gyrocompass and vessel's motion sensors204 and 206), in order to calculate and display the vessel's attitude and its fixed offsets.

Via the fiber optic (de)multiplexers 244 and 246, the navigation processor 202sends different settings to the navigation instruments of the System 50, i.e., Doppler log266, altimeter 262, and gyrocompass and motion sensors 256. After setting up, itreceives the data from those instruments, as well as, via the acoustic processor 224, therange/bearing and depth data of the system 50 to calculate and to display the attitudesand absolute coordinates of the system 50.

An integrated software in the navigation processor 202 has been developed, in- cludîng a dynamic positioning controller software able to work in manual or automode to décidé the intended heading of the system 50 and to select between many way points 01 2234 11 and to carry out the intended positioning. Moreover, the operator on board of the vesselcan input offsets to the selected way point, the offsets being input with XY coordinatesrelative to the heading of the system 50. There is another possibility to select severalother types of sub-sea positioning devices via an arrangement of specifically designedWindows on the screens (electronic pages) of the display units 208-214, to stabilize andfilter the position. To ensure that the operator has as many tools as possible to get theoptimal resuit, there is an other part in the software showing different status of the sub-sea instruments in use for the calculation of the position of the system 50 on-line (real-time).

Embarked gyrocompass 256 including heave, roll and pitch sensors 88 on boardof the system 50 provides data as to the exact attitudes of both the system 50 and theload 43 to be installed on the sea bed. At the surface of the sea, in a control van,operators are able to check those attitudes on-line (real-time), during descent but alsoonce the load 43 is laying on the sea bed for final vérification.

The vessel gyrocompass 206, as well as the gyrocompass with motion sensors252 installed on the acoustic array 250 that could be used for the same functions, istransmitting the vessel's heading to the navigation processor 202. The navigation proc-essor 202 will use this vessel's heading to calculate different offsets.

The display units 208, 210, 212, and 214, respectively, are arranged to displaynavigation settings, a view of the sea bed, a view of the surface, in the control van forthe operators and another one on the vessel bridge for the marine department operators.

The USBL command and control unit 226 consists of a personal computer pro-viding control and configuration of the system and displaying the man-machine-inter-face for operator control.

The acoustic processor 224, preferably, consists of one VME rack which per-forais corrélation process on received signais, corrections to bathy-celerimetry and ves-sel's attitude. Moreover, it calculâtes coordinates of any beacon used. The acousticprocessor 224 is linked to the navigation processor 202 through Etemet.

The acoustic array 250 includes means for transmission and réception. The acoustic array 250 can be used as a transducer to acoustically communicate with one or more beacons. Such a transducer mode is advantageous when the umbilical 46 fails and is unable to transmit interrogation signais down to the system 50. Then, acoustic inter- rogation signais can be transmitted down by lhe transducer directly through the sea 012234 12 water. In ail other cases, the acoustic array 250 will be used in a réception mode.Réception is done with two orthogonal réception bases which measure distances andbearing angles of beacons relative to the acoustic array 250. Each réception base in-cludes two transducers. Each received signal is amplified, fîltered and transferred to theacoustic processor 224 for digital signal processing.

The sound velocity meter 248 installed on the acoustic array 250 is updating inreal-time the critical and unsettled sound velocity profile situated just undemeath thevessel 40, This is of great importance since turbulences of the sea water appear to bevery heavy in these layers just undemeath the vessel 40.

The gyrocompass 252 is preferably used as motion sensor unit transmitting theacoustic array attitude to the acoustic processor 224 in order to rectify data as to theposition of the System 50 sub-sea.

In a preferred embodiment, the beacon 254 is working in a responder mode andhas the following characteristics: - the triggering interrogation signal generated by the acoustic processor 224 is notacoustic but electrical and is transmitted to the beacon 254 through the cable linkbetween the vessel 40 and the System 50; - interrogation frequencies are remotely controlled by an operator through the man-machine-interface.

As indicated above, the beacon 254 can also be used in a transponder mode.

Then, the beacon 254 is triggered by a surface acoustic signal transmitted by the acous-tic array 250 and then delivers acoustic reply signais to the acoustic array 250 through acoded acoustic signal.

The digiquartz depth sensor 253 included in the beacon 254 allows transmittingvery accurate depth data of the System 50 to the acoustic processor 224. The acousticprocessor 224 uses these data to improve the calculation of the sub-sea positioning ofthe System 50 and its load 43.

The sound velocity meter 258, mounted on the underwater System 50, is trans- mitting data as to the velocity of sound in sea water at the depth of the underwater Sys- tem 50 to the acoustic processor 224 during descent and recovery. The sound velocity data is used to update calculated sound velocity profiles in the sea water as a function of depth in real-time and to calculate acoustic ray bending from these profiles as func- 012234 13 tion of depth in the sea water and thus to correct calculations of the sub-sea position ofthe System 50.

The dual head scanning sonar 260 is used to measure ranges and bearings of theSystem 50 to any man-made or natural target on the seabed and to output correspondingdata as digital values to the navigation processor 202. The positions of such man-madeor natural targets can either be predefmed or the navigation System can allocate coordi-nates to each of the selected objects. After the objects hâve been given coordinates,they can be used as navigation references in a local coordinate System. This results inan accuracy of 0.1 meter in relative coordinates.

The altimeter 262 mounted on the system 50 is measuring the vertical distance ofthe underwater system 50 to the seabed and transmits output measuring data to theacoustic processor 224.

The Doppler log unit 266.provides data as to the value and direction of the seawater current at the depth of the underwater System 50. These data are used in twoways.

First of ail, the data received from the Doppler log unit 266 and the gyrocompasswith motion sensor 256 is used by the acoustic processor 224 to smooth on-line (real-time) the random noise related to using USBL. To obtain such a smoothing a fîlter isused, e.g., a Kalman fîlter, a Salomonsen fîlter, a Salomonsen light fîlter, or any othersuitable fîlter in the main processor unit 224. Such filters are known to persons skilledin the art. A brief summary can be found in appendix A.

Secondly, the output data of the Doppler log unit 266 regarding current strength,current direction, together wit data regarding présent and intended heading of theunderwater system 50 are transmitted to the thruster control processor 240 via the navi-gation processor 202. Based on the intended direction the thruster drive control 270will be automatically controlled. Manual control may also be provided for.

In a very advantageous embodiment the Doppler log unit 266 (or any other suit-able sensor) is used to measure température and/or salinity of the sea water surroundingthe system 50. Data as to local température and/or salinity is transmitted to the naviga-tion processor 202 that calculâtes and updates température and/or salinity profiles as afunction of depth in the sea water. These data are also used to détermine acoustic raybending through the sea water and, thus, to correct calculations of the position of thesystem 50. 012234 14

The sonardyne mini Rovnav 264 is optional and may be used to provide relativeposition of the System 50 to local beacons on the seabed as explained above. For in-stance, a Long Base Line (LBL) array may already be installed on the seabed and usedfor that purpose. 5 The load cells 268 are used to measure the weight of the load 43 as engaged by the underwater System 50. When this weight decreases this is an indication that the loadis now placed on the seabed (or other target position) and that the System 50 may bedetached from the load 43. Output data from the load cells is transmitted to the naviga-tion processor 202 through the (de)multiplexers 244,246. 10 The thruster drive control 270 is used to drive the thrusters 56(i) in order to drive the underwater System 50 to the desired position as will be explained in detail below.

In figure 7a, four different processors 202, 224, 236 and 240 are shown to carry outthe fiinctionality of the System according to the invention. However, it is to be understoodthat the functionality of the System can, altematively, be carried out by any other suitable 15 number of cooperating processors, including one main frame computer, either in parallelor master slave arrangement. Even remotely located processors may be used. There maybe provided a processor on board of the underwater System 50 for performing some of thefunctions.

The processors may hâve not shown memory components including hard disks, 20 Read Only Memory's (ROM), Electrically Erasable Programmable Read Only Memory's(EEPROM) and Random Access Memory's (RAM), etc. Not ail of these memory typesneed necessarily be provided.

Instead of or in addition to the keyboards 220, 230 and the mice 222, 232 otherinput means known to persons skilled in the art, like touch screens, may be provided too. 25 Any communication within the entire arrangement shown may be wireless.

In figure 5, the situation is shown that the two upper thrusters 56(2) and 56(3) are directed in an other direction than the thrusters 56(1) and 56(4). The thrusters 56(2),56(3) are mounted on rotary actuators 65(1), 65(2), which allow the thrusters 56(2),56(3) to be vectored by tuming them up to 360°. Preferably, the thrusters 56(2), 56(3) 30 can be independently controlled such that they may be directed each to a different direction.

To allow the thruster control processor 240 to accurately position the underwater

System 50, a common coordinate System must be established between the navigation 01 22 34 15 processor 202 and the thruster control processor 240. First of ail, there is a standardcoordinate System used by the navigation processor 202. However, two other coordi-nate reference Systems are preferably established for the underwater System 50.

Figure 8 shows the three different coordinate Systems. The coordinate Systemrelated to the navigation processor 202 is indicated with "navigation grid". This coordi-nate System uses this “navigation grid” direction and its normal.

The thrusters 56(2), 56(3) are controlled to provide a driving force in a directiontermed "thruster mean direction". This direction together with its normal defines thesecond coordinate System.

The third coordinate System is defmed relative to the "system direction" which isdefined as the direction perpendicular to. a line interconnecting the thrusters 56(1),56(4).

Now, an error in the path followed by the underwater System 50 can be defined internis of an error vector that can be split into one component parallel to the thrustermean direction termed the "mean error" and a component normal to the thruster meandirection termed "normal mean error". Appropriais sensors on the underwater System50 will provide the navigation processor 202 with the thruster mean direction and Sys-tem direction. From these data the navigation processor 202 will create a grid as shownin figure 8.

The error is defined as the desired position DP minus the system position TP suchthat a vector R-Φεν is generated relative to the navigation grid reference, i.e.: DP - TP = ROen

Moreover: Φτν is the system orientation minus the navigation grid orientation, Φμτ is the mean thruster orientation minus the system orientation.

Then: DP - TP = R&em, Φεμ ~ Φεν - (Φτν + Φμτ)

Now R®em is known, the mean and the normal to the mean errors can be caiculated.

The two thrusters 56(1) and 56(4) are used to counteract the twisting forces applied by the lifting cable 42, equipment drag and the rotational moment induced by the vectoring of positioning control. A control loop for the orientation requires that the navigation processor 202 is provided with the actual System orientation and the desired system orientation. The actual system orientation is measured by the gyrocompass 256. 01 2234 16

The desired orientation is manually input by an operator. From these two orientationsthe control loop in the navigation processor 202 computes an angular distance betweenthe required orientation and the actual orientation as well as the direction of rotationrequired to move the System 50 accordingly. A simple control loop controlled by thethruster control processor 240 then adjusts the power to the thrusters 56(1) and 56(4) torotate the System 50 appropriately.

On power up of the System 50, both thrusters 56(2) and 56(3) will be, preferably,orientated such that the thruster mean direction is directed parallel to the system direc-tion. Then, the thrusters 56(2), 56(3) will be given a small vector angle déviation fromthe system direction to assist in positioning the system 50 in two planes. The size ofthis vector is, preferably, manually adjustable and may be needed to be configured foreach different job in dependence on actual sea conditions. Once the thrusters 56(2) and56(3) hâve been centered and vectored, a positioning loop can take over control of thesystem 50.

The positioning loop comprises two more phases.

In the first next phase, which is executed while the system 50 is still near the seasurface, the sea current direction will be measured by the Doppler log unit 266. The seacurrent direction will be transmitted to the navigation processor 202. Using this direc-tion, the thruster control processor 240 receiving proper commands from the navigationprocessor 202 will drive the rotary actuators 65(1), 65(2) such that the thruster meandirection substantially opposes the sea current direction. During this rotation of therotary actuators 65(1), 65(2) none of thrusters 56(i) is powered. The system directionwill be measured by the fiber optic gyrocompass 256. The depth is constantly measuredby the digiquartz depth sensor 254 and the altitude by the altimeter 262. The mean andnormal to the mean errors as calculated in accordance with the équations above willthen be used by the positioning loop to apply power to the thrusters 56(2) and 56(3) todrive the System 50 to the desired location.

During driving the system 50 with load 43 to the desired coordinates by means ofthrusters 56(2), 56(3) the thrusters 56(1), 56(4) are used to counteract any rotation ofthe system 50 with its load 43. This provides for better control since, especially forheavy loads, rotation movements may resuit in other undesired movements of the load,which may be difficult to control. When the system 50 with its load is on the desiredcoordinates the load together with the system 50 is lowered by means of the hoisting 17 01 2234 wire 42. During descending the load 43, the load 43 is constantly controlled by System 50 to keep it on the desired location without any rotation.

In a next phase, the System 50 is for instance approximately 200 m or less fromthe seabed 4. Then, the Doppler log unit 266 goes into a bottom track mode. Thischanges the operation into a more accurate and fast responding mode for the finalapproach of the target location on the seabed 4. Now, the Doppler log unit 266 and thegyrocompass with motion sensors 256 are used to filter the random noise of the USBL.Once fîltered, a good read out of the navigation data including an accurate velocity ofthe System 50 will make the position control loop both extremely rapid and stable. Avery fine tuned control loop results in which control up to some centimeters movementis achieved. Now, the sonar unit 260 and the Doppler log unit 266 are used to provideinformation regarding the surroundings of the target point such that the load 43 can bepositioned on the right coordinates and in the right orientation. Then, a rotation, ifnecessary, may be applied to the load 43 by thrusters 56(1), 56(4) as controlled bythruster control processor 240.

Two control loops are provided for the thrusters 56(2), 56(3): a mean error con-trol loop and a further control loop to reduce the normal mean error.

The mean error control loop will adjust the power equally to both thrusters 56(2),56(3) so as to reduce the mean error. As the System 50 reaches the target coordinatesthe driving power to the thrusters 56(2), 56(3) will be reduced to such a level that theSystem 50 is able to maintain its position in the sea current. In other words, initially, thedriving power was set at a level that was proportional to the mean error. However, asthe system 50 moves doser to the target coordinates the control loop will slowly reducethe driving power applied to the thrusters 56(2), 56(3). As the system 50 reaches thetarget coordinates an equilibrium will be reached where the driving power to the thrust-ers 56(2), 56(3) counteracts the strength of the sea current. The mean error control loopprovides equal power with equal sign to both thrusters 56(2), 56(3). A further control loop is applied to reduce the normal mean error. This furthercontrol loop adjusts the individual power applied to the thrusters 56(2), 56(3) such thata movement perpendicular to the sea current is generated. The further control loopapplies equal power of opposite sign to both thrusters 56(2), 56(3) to this effect. Thepower applied to the thrusters 56(2), 56(3) in order to reduce the normal mean error,preferably, reduces linearly to zéro as the system 50 moves to the target coordinates. At 18 012234 the point where the normal to the mean error reaches zéro and assuming that the seacurrent direction has not changed, the System 50 will exactly be located above the tar-get position on the sea bed 4 and the thrusters 56(2), 56(3) are powered to keep theSystem 50 on the correct coordinates and to correct for the sea current.

If the sea current direction changes the control loops referred to above will berequired to adjust the power applied to the thrusters and ultimately to change the Sys-tem direction. As the new current direction acts upon the System 50, the normal meanerror will start to increase as the System 50 is moved from the target coordinates. Toovercome this effect, the size of the normal mean error will again be controlled toreduce to zéro. The system direction is changed such that the sea current or natural driftof the system 50 is counteracted.

The direction of rotation of the rotary actuators 65(1), 65(2) will be defined bythe sign of the normal mean error. To reduce the time required to slew the rotaryactuators 65(1), 65(2) to the required position, an algorithm will be used by the thrustercontrol processor 240 to détermine the shortest route to the required orientation.

It is envisaged that manual control by means of, for instance, a joystick (notshown) connected to the navigation processor 202 is also arranged.

During the positioning of the system 50 a velocity control is also, preferably,applied. Preferably, the doser is the system 50 to the coordinates of the target, theslower will be the velocity of the system 50. For instance, when the distance betweenthe system 50 and the target is more than a predetermined first threshold value, thethrusters are controlled to provide the system 50 with a maximum velocity. Betweenthis first threshold value and a second threshold value of the distance to the target coor-dinates, the second threshold value being lower than the first threshold value, a linearlydecreasing velocity profile is used. Within a distance smaller than the second thresholdvalue the system is kept on a velocity of substantially zéro. USBL measurement

The USBL measurement principle is based on an accurate phase measurementbetween two transducers. In one embodiment, a combination of short base line (SBL)and ultra short base line (USBL) is used which enables to use a large distance betweentransducers without any phase ambiguity. For an USBL, the accuracy dépends on thesignal to noise ratio and the distance between the transducers (like in an interferometry 19 01 22 34 method). Then, the trade-off is for ffequency which is limited by the range and hydro-dynamïc part in terms of dimensions.

Ambiguity is calculated by using an SBL measurement combined with corréla-tion data processing. The signal-to-noise ratio is improved by use of such corrélationProcessing. The following expression defines the general accuracy for a USBL: LJ—— cos £ V noise where: σθ : Angular standard déviation L : transducer distance λ : wavelength Θ : bearing angle

The expression given above indicates that the accuracy is improved by increasingthe transducer distance L, i.e., by increasing the array. Moreover, a higher ffequencyresults in a better accuracy. Hydrodynamic aspects and phase ambiguity reduce theseparameters. Signal-to-noise ratio is increased by using corrélation data processing.

To optimize range and accuracy, a frequency of 16 kHz is preferably used forphase meter measurements. A corrélation process enables to increase the distance rangewhile keeping a narrow puise length for multipath discrimination.

For ambiguity phase measurements, the System opérâtes in SBL to détermine arange sector and in USBL within the sector to achieve the best accuracy.

The range may be increased beyond 8000 m by using a rather low ffequency.

Appendix A 20 01 2234

Kalman filter

The Kalman filter is probably the most well-known technique in the offshore in-dustry. It gives a fast filtering method based on comparison towards predicted values,which are calculated on basis of the latest history. We will not go into details aboutKalman filtering, but refer to, e.g., "Kalman Filtering - Theory and Practice", by M.S.Grewal and A.P. Andrews Prentice Hall (ISBN 0-13-211335-X).

The position track can be combined with the velocity data (Doppler log), eachpoint will be improved on basis of the neighboring points, the distance in time and theactual speed. The weight between Kalman value and the velocity improved is decidedby the Doppler efficiency coefficient·, higher values will take speed more into consid-ération.

Advantage: Disadvantage:

It's fairly fast Rather ’un-smooth' resuit

Can be improved with speed Not the best combination of speed and position

Simple filter

The Simple filter runs through ail positions, and calculâtes a smooth curve givinga minimum squared error, i.e. a kind of Least Square Fit line

Advantage: Disadvantage:

It's fast No Doppler-log data is used

The resuit is smooth Does not iike curved tracks

Salomonsen filter

The Salomonsen filter, which is named after the Danish mathematician HansAnton Salomonsen, Professor and phD at University of Aarhus, is a highly integratedfilter. It takes advantage of the short-term stability of the Doppler track and combines itwith the long-term robustness of the position track.

Description 21 01 22 34

The filter is used in a situation where we hâve time tacked position data along atrack as well as Doppler data. The Doppler Data are usually very précisé but do notgive any information about the absolute positions. On the other hand the position dataare absolute positions but they are usually not very précisé.

The filter combines the two sets of data to produce a précisé track with absolute posi-tions. This is done as follows. 1. The Doppler data are used to construct the shape of the track, i.e. a track formed asa cubic spine. .2. Beginning at the origin (0, 0) and with velocities as defined by the Doppler data. 3. Then the position data are used to position the track correctly. The track is trans-lated, rotated, and stretched/compressed linearly to fît the position data aswell aspossible using least squares techniques. 4. It will mainly be a translation. However, the other modifications serve to correct forpossible systematic errors in the Doppler data.

The fact that the position data are used only to make the modifications in 2 meansthat the position data are subject to considérable averaging. This reduces the uncer-tainty of the position measurements. Thus, if there are many position data the absoluteposition of the track should be expected to be much more précisé than each single posi-tion measurement. H.A. Salomonsen

Mathematical description

The algorithm is divided into five steps:

Step 1:

Calculate accélérations for each point 1/2 hk+l(Xl"+Xk+l ")=Xk+l'-Xk'

Where hk = tk-tk-l tk = timestamp for speed measurement

Xk - speed measurement at tk

Xk"= calculated accélération at tk •κ··' 22 012234

Step 2:

Calculate next position based on accélération and speed, and previous calculated posi-tion (based on previous speed measurements and accélérations)

Xk+1 = Sqr(hk+l)/6(2Xk"+Xk+T)+hk+l Xk'+XkWhere xk = calculated position at tk (speed timestamp)

Step 3:

Calculate the positions at actual timestamps (using position of first speed measure-ments) X(t)=l/2hk+l {((hk+1 )A2 (t-tk)+l/3(tk+l-t)A3-l/3(hk+l)A3)Xk"+l/3(t-tk)A3 Xk+1"}Where X(t) = position at time t

Step 4:

Add position of first speed measurements to calculated positions

Step 5:

Move, rotate, stretch of compress calculated positions to best fit of real position line

Advantage: Disadvantage:

It combines the best of Doppler and positions. It is slow due to complex matrixTakes ail data in considération Dépendent on good Doppler-log

The resuit is smooth

Salomonsen Light

The light version of Salomonsen filter, which was first introduced in the NaviBatOn-line program, was invented to hâve a faster solution combining the better of twomethods.

Due to its on-line nature, it only uses history in deciding to filter a point. Hence the resuit will be rougher at the start of line and getting better as it moves along.

Basic operation. 012234

The filter is started with a reset call to initialize the filter. The reset is made usingthe first velocity measurement. The filter uses both velocity and position data. A cubicspine curve is created using the velocity records and fitting the positions as good aspossible to this curve.

Then the filter is reading a position record it is stored for later processing.

When a velocity record is read a 'knot' is created. Any positions read between theprevious and the présent velocity records (in time) are adjusted to fit the curve.

History

The filter gain parameter, value 0 to 1, Controls the influence of Doppler-log dataand history on the current point.

For the value 1 the Doppler-log data and history in the line hâve the greaterweight. Smaller values are only when there are more position records than valid,velocity records.

Useful values will be in the range 0.9 to 1, e.g. 0.99.

Error correction

The position and velocity records may be compared with predicted values usingprevious data. Limits may be set when to reject data.

Resetting

If there are many erroneous data points there is a risk that the filter looses track.The operator may reset the filter manually, i.e. kill its history (attempts are made todesign an auto-reset).

Advantage: Disadvantage:

It combines the best of Doppler and positions ’un-smooth' at the start of line

It is fast

The overall resuit is smooth

Can handle noisy Doppler data

Claims (24)

1. 24 daims 01 2234

   1. Apparatus (50) for deploying an object (43) to an underwater target position, theapparatus being provided with a beacon to transmit acoustic rays and a plurality ofthrusters (56(i), i = 1, 2, .. .1,1 being an integer) to control positioning of said apparatuswith respect to said underwater target position characterized in that the apparatus is pro-vided with a sound velocity meter (258) to continuously measure velocity of sound in afluid surrounding said apparatus and to transmit sound velocity data in real-time.
2. 2. Apparatus according to claim 1, wherein said thrusters comprise a fîrst set ofthrusters (56(1), 56(4)) arranged to provide a torque control fonction and a second setof thrusters (56(2), 56(3) arranged to provide at least a translation fonction, eachthruster of said second set of thrusters (56(2), 56(3)) being provided with a rotaryactuator (65(1), 65(2)).
3. 3. Apparatus according to any of the daims 1 or 2, wherein said apparatus is providedwith a gyrocompass with motion sensors (256) to sense roll and pitch of the apparatus inuse.
4. 4. Apparatus according to any of the preceding daims, wherein the apparatus is pro-vided with a sonar unit (260) to détermine the position of said apparatus with respect to atleast one object extemal to said apparatus.
5. 5. Apparatus according to daim 4, wherein the apparatus is provided with a Dopplerlog unit (266) to measure current strength of said fluid.
6. 6. Apparatus according to any of the preceding daims, comprising load cells (268) tomeasure weight of a load (43) engaged by the apparatus.
7. 7. Apparatus according to any of the preceding daims, wherein the apparatus is pro-vided with a température sensor (266) to measure température in said fluid and to transmittempérature data in real-time. 25 012234
8. 8. Apparatus according to any of the preceding daims, wherein the apparatus is pro-vided with a salinity meter (266) to measure salinity of said fluid and to transmit salinitydata in real-time.
9. 9. A processing arrangement arranged to drive an apparatus (50) for deploying anobject (43) to an underwater target position, the apparatus being provided with a bea-con to transmit acoustic rays, a plurality of thrusters (56(i), i = 1, 2, ...I, I being an in-teger) to control positioning of said apparatus with respect to said underwater targetposition, and a sound velocity meter (258) to continuously measure velocity of sound in afluid surrounding said apparatus and to transmit sound velocity data in real-time, theProcessing arrangement being provided with an acoustic receiver (250) to receive saidacoustic rays, the processing arrangement is arranged to use data derived from saidacoustic rays in a calculation to détermine the position of the apparatus, the processingarrangement being arranged to receive on-line sound velocity meter data from said soundvelocity meter (258) to continuously détermine a sound velocity profile in said fluid and tocalculàte from said sound velocity profile bending of said acoustic rays transmitted bythe apparatus through the fluid and to use this in the calculation to détermine theposition of said apparatus in real-time.
10. 10. Processor arrangement according to claim 9, wherein said thrusters of the appa-ratus comprise a fïrst set of thrusters (56(1), 56(4)) arranged to provide a torque controlfunction and a second set of thrusters (56(2), 56(3) arranged to provide at least a trans-lation function, each thruster of said second set of thrusters (56(2), 56(3)) being pro-vided with a rotary actuator (65(1), 65(2)), and said processing arrangement is arrangedto perform the following fonctions in use: • to control application of driving power to said thrusters of said fïrst set of thrusters(56(1), 56(4)) to keep said apparatus in a desired orientation in a fïrst plane defmed bydriving forces generated by said thrusters (56(1), 56(4)) of said fïrst set; • to control application of driving power to said thrusters of said second set of thrusters(56(2), 56(3)) and to said rotary actuators (65(1), 65(2)) to move said apparatus in amean direction and a direction perpendicular to said mean direction to a desired loca-tion, said mean direction and said direction perpendicular to said mean direction being 26 012234 in a second plane defined by driving forces generated by said thrusters (56(1),56(4)) of said second set.
11. 11. A processing arrangement according to claim 10, in said apparatus said first andsecond plane not being coïncident, the processing arrangement being arranged to receivefirst sense signais ffom a gyrocompass with motion sensors (256) on the apparatus (50)regarding roll and pitch of the apparatus in use.
12. 12. A processing arrangement according to claim 11, wherein the first sense signaisfrom the gyrocompass with motion sensors (256) are used in the calculation to déterminethe attitude of the apparatus.
13. 13. A processing arrangement according to any of the daims 9-12, the apparatus in-cluding a température sensor (266), wherein the processing arrangement is arranged toreceive température data from said température sensor, to update a température profile insaid fluid and to assist a correction of determining the position of said apparatus in real-time.
14. 14. A processing arrangement according to any of the daims 9-13, the apparatus in-cluding a salinity meter (266), wherein the processing arrangement is arranged to receivesalinity data from said salinity meter, to update a salinity profile in said fluid and to correctdetermining the position of said apparatus in real-time.
15. 15. A vessel provided with a processing arrangement according to any of the daims 9through 14.
16. 16. A vessel according to claim 15, wherein the vessel is provided with an acousticarray (250) attached to a hull of the vessel and an ultra short base line surface unit (234)on board of the vessel arranged to communicate with said acoustic array (250), theacoustic array (250) being arranged to receive acoustic signais from at least said appa-ratus (50) and to provide acoustic array output data to said processing arrangement,which is arranged to perform, in real-time, the calculation of the position of at least said 27 01 2234 apparatus (50) relative to said acoustic array (250) based on said acoustic arrayoutput data.
17. 17. A vessel according to claim 16, wherein the acoustic array (250) comprises asound velocity meter (248) to measure velocity of sound in fluid layers just below thevessel and to provide sound velocity meter output data to said processing arrangement,said processing arrangement being arranged to correct said calculation of said positionof said apparatus (50) based on said sound velocity meter output data in real-time.
18. 18. A vessel according to claim 16 or 17, wherein the acoustic array (250) comprisesan acoustic array gyrocompass (252) to measure heave, roll and pitch of the acousticarray (250) and to provide acoustic array gyrocompass output data to said processingarrangement, the processing arrangement being arranged to correct said calculation ofsaid position of said apparatus (50) based on said acoustic array gyrocompass outputdata in real-time.
19. 19. A vessel according to any of the daims 15 through 18, wherein the vessel com-prises a vessel gyrocompass (206) to measure heave, roll and pitch of the vessel and toprovide vessel gyrocompass output data to said processing arrangement, the processingarrangement being arranged to correct said calculation of said position of said appara-tus (50) based on said vessel gyrocompass output data in real-time.
20. 20. A System comprising a vessel according to any of the daims 15 through 19 andan apparatus according to any of the daims 1 through 8, the apparatus and the proc-essing arrangement being arranged to communicate with one another.
21. 21. A System according to claim 20, wherein the apparatus and the processingarrangement are coupled via fiber optic (de)multiplexers (244, 246) interconnected byan optical fiber.
22. 22. A method of driving an apparatus (50) for deploying an object (43) to an under-water target position, the apparatus being provided with a beacon to transmit acousticrays, a plurality of thrusters (56(i), i = 1,2, ...I, I being an integer) to control position- 01 22 34 ing of said apparatus with respect to said underwater target position, and a soundvelocity meter (258) to continuously measure velocity of sound in a fluid surrounding saidapparatus and to transmit sound velocity data in real-time, the method comprising thesteps of: 5 · receiving said acoustic rays, • using data derived from said acoustic rays in a calculation to détermine the position ofthe apparatus • receiving sound velocity meter data from said sound velocity meter (258) andcontinuously determining a sound velocity profile in said fluid, and 10 · caîculating from said sound velocity profile bending of said acoustic rays trans- mitted by the apparatus through the fluid and to use this in the calculation todétermine the position of said apparatus.
23. 23. A computer program product comprising data and instructions that after being 15 loaded by a processing arrangement provides said arrangement with the capacity to carry out a method according to claim 22.
24. 24. A data carrier provided with a computer program product according to claim 23.