GB2617634A

GB2617634A - Improvements to underwater vehicles and methods of use

Info

Publication number: GB2617634A
Application number: GB2211416.9A
Authority: GB
Inventors: Grytten Leiv-Erling; Johan Walther Hellström Carl; Christofer Werenskiold Jens
Original assignee: Beyonc AS
Current assignee: Beyonc AS
Priority date: 2022-08-05
Filing date: 2022-08-05
Publication date: 2023-10-18
Also published as: GB202211416D0; WO2024028279A2

Abstract

The invention provides a method of designing an underwater vehicle for use in an underwater measurement operation. The method comprises using a model of a measurement scenario and calculating a base vector field data set. A vehicle design is incorporated into the model to create a combined model, and a first vector field data set is calculated from the combined model. The first vector field data set is compared with the base vector field data set to assess the suitability of the vehicle design for the measurement operation.

Description

1 IMPROVEMENTS TO UNDERWATER VEHICLES AND METHODS OF USE 3 The present invention relates to a method of designing and/or manufacturing underwater 4 vehicles, underwater vehicles formed by the method, and methods of using the underwater vehicles. Particular embodiments relate to underwater vehicles incorporating electric field 6 and/or magnetic field sensors and methods of use.

8 Background to the invention

Underwater vehicles, including unmanned underwater vehicles such as remotely operated 11 vehicles (ROVs) and autonomous underwater vehicles (AUVs), are used extensively in a 12 range of exploration, surveying, inspection, repair and installation applications. Each ROV 13 or AUV type is applied to tasks within the range of its capabilities, depending on, for 14 example depth and length of operation, manoeuvrability, power and load requirements, and control options. Typically an underwater vehicle will be equipped with tools, sensors 16 and other equipment required for the operation, with minimal modification to the general 17 form of an "off the shelf' base vehicle design.

19 In surveying and/or inspection applications, sensors required for data acquisition may be mounted to tools carried by the vehicle or otherwise incorporated into the vehicle. The 21 sensors may be highly sensitive in order to detect low amplitude signals and/or small 22 signal variations. These sensor types include electric field gradient sensors and magnetic 23 field gradient sensors, as used in applications such as cathodic protection surveys, 24 geological mineral mapping, detection and tracking or pipelines and cables, and shallow water surveying of pipelines and cables. An example of the use of an electric field 26 gradient sensor approach to cathodic protection surveying is described in "Field Gradient 27 Survey of Offshore Pipeline Bundles affected by Trawling", Lauvstad et al., Eurocorr 2016.

28 Modelling methods are described in "Computer modeling of offshore cathodic protection 29 systems: method and experience", RD Strommen, Computer Modeling in Corrosion, ASTM STP 1154, 229-247, 1992.

32 Typically these techniques will be carried out using traditional Work Class ROVs or 33 Inspection Class ROVs, based on an offshore asset or on a large vessel. The ROV itself, 34 equipment on the ROV, or movement of the ROV will influence the detected electric and/or magnetic field readings, restricting how the acquired data can be used for detailed analysis 1 and interpretation. Existing data processing methods to compensate for electric and

2 magnetic field interference are inadequate.

4 W017126975 Al describes a method for surveying a structure by moving at least one electric field gradient sensor along it and measuring the electric field vectors surrounding 6 the structure. The method may involve modelling of the structure, using boundary element 7 methods or finite element methods, to obtain a 2D or 3D vector map of the structure.

9 US2021094660 describes the use of a plurality of electrodes for collecting electric field data from a structure in an underwater environment, and for determining characteristics of 11 this structure or the location of it in surveying of cathodic protection systems.

13 Summary of the invention

According to a first aspect of the invention, there is provided a method of designing an 16 underwater vehicle for use in an underwater measurement operation, the method 17 comprising: 18 a. Using a model of a measurement scenario, calculating a base vector field data set; 19 b. Incorporating a vehicle design into the model to create a combined model, and calculating a first vector field data set from the combined model; 21 c. Comparing the first vector field data set with the base vector field data set to 22 assess the suitability of the vehicle design for the measurement operation.

24 The measurement scenario model and/or the combined model may comprise a Finite Element Model (FEM) or Boundary Element Model (BEM) model. The model may be a 2D 26 or 3D computer model.

28 The vehicle design may be a first vehicle design, and comparing the first vector field data 29 set with the base vector field data set may assess the suitability of the first vehicle design for the measurement operation. The method may comprise repeating step b. for a second 31 and/or further vehicle design to calculate a second and/or further vector field data set. The 32 method may comprise repeating step c. for the second and/or further vector field data set 33 to assess the suitability of the second and/or further vehicle design for the measurement 34 operation. Alternatively, or in addition, the method may comprise comparing the first 1 vector field data set with the second and/or further vector field data set to assess the 2 suitability of the second and/or further vehicle design for the measurement operation 4 The method may comprise optimising the vehicle design for the measurement operation.

Optimising the vehicle design for the measurement operation may comprise minimising a 6 difference between the base vector field data set and the first, second or further vector 7 field data set calculated from a combined model incorporating the vehicle design.

9 Optimising may comprise minimising a difference between calculated vector field data set and the base vector field data set for at least one vehicle design parameter.

12 Alternatively, or in addition, the method may comprise determining a relation between the 13 base vector field data set and the first or further vector field data set calculated from a 14 combined model incorporating the vehicle design. The method may comprise deriving a correction factor or a correction field to be applied to a measured vector field data set.

17 The vehicle may be a remotely operated vehicle (ROV), and may be a mini-ROV. In this 18 context, a mini-ROV may be thought of as an ROV that can be handled manually, for 19 example having a mass of less than 100kg. According to embodiments of the invention, designing a dedicated ROV for the effective incorporation of vector field sensors results in 21 a vehicle that is small in size and weight. The vehicle may therefore be usable in 22 extremely difficult to operate areas such as areas with restricted access and ultra-shallow 23 water. The vehicle may also be manually handled and operated from rope access teams.

Preferably the vehicle has fully omnidirectional flying capabilities. This facilitates 26 placement of the ROV and taking of measurements regardless of anode placement and 27 orientation, without a need for a manipulator arm to place the sensors in the correct 28 position.

The method may comprise creating the model of the measurement scenario using 31 measurement scenario information. Measurement scenario information may comprise 32 information relating to vector field sources, which may be a galvanic anode, a piece of 33 steel, a corroding surface, an electric connector, a subsea structure or a mineral deposit.

1 The method may comprise inputting vehicle design information into the model to create the 2 combined model. The vehicle design information may include any of a range of design 3 factors that could affect the vector fields, including but not limited to: 4 * Shape and dimensions of the vehicle; * Build materials used in the vehicle and/or equipment mounted on the vehicle; 6 * Position and/or orientation of one or more sensors; 7 * Omnidirectional flying features and components; 8 * Electric fields or magnetic fields associated with equipment mounted on or to the 9 vehicle, such as thrusters, other sensor arrays, electric cables, signal generators, magnetic field emitters, and/or electric field emitters.

12 According to a second aspect of the invention, there is provided a method of 13 manufacturing an underwater vehicle comprising the method of designing the underwater 14 vehicle according to the first aspect, and manufacturing the vehicle according to the vehicle design.

17 The second aspect of the invention and its embodiments may comprise one or more 18 essential and/or optional features of the first aspect of the invention or vice versa.

According to a third aspect of the invention, there is provided an underwater vehicle 21 manufactured according to the second aspect of the invention.

23 The third aspect of the invention and its embodiments may comprise one or more essential 24 and/or optional features of the first or second aspects of the invention or vice versa.

26 According to a fourth aspect of the invention, there is provided an underwater vehicle 27 comprising: 28 a frame; 29 one or more propulsion devices; -one or more vector field sensors incorporated into the vehicle; 31 -wherein the vehicle comprises one or more design features selected to modify the 32 disruption of a vector field to be measured by the one or more vector field sensors.

34 In this context, "incorporated into the vehicle" means joined with or forming a part of the vehicle itself, rather than being part of an auxiliary tool or piece of equipment mounted to 1 or carried by a manipulator arm, but does not require that the one or more vector field 2 sensors are fully or partially internal to the vehicle.

4 The one or more vector field sensors may be integrated into the body or frame of the vehicle. The one or more vector field sensors may comprise a field gradient sensor, which 6 may be integrated into the body or frame of the vehicle.

8 In this context, "integrated into the body or frame of the vehicle" means joined with or 9 forming a part of body or frame of the vehicle, rather than being part of an auxiliary tool or piece of equipment mounted to or carried by a manipulator arm, but does not require that 11 the respective sensors are fully or partially internal to the vehicle.

13 The frame may comprise a partially or fully open structure which modifies disruption of the 14 vector field to be measured. The frame may comprise at least one planar member defining a surface of the vehicle. The planar member may comprise one or more 16 apertures to modify disruption of the vector field to be measured.

18 Preferably, disruption of the vector field to be measured is reduced. Alternatively, or in 19 addition, disruption of the vector field to be measured is modified to enable a correction factor or correction field to be applied to measured vector field data.

22 The fourth aspect of the invention and its embodiments may comprise one or more 23 essential and/or optional features of the first to third aspects of the invention or vice versa.

According to a fifth aspect of the invention, there is provided a method of using an 26 underwater vehicle according to the third or fourth aspects of the invention, the method 27 comprising: 28 -locating the underwater vehicle in a measurement location; 29 -acquiring vector field measurement data using the one or more sensors.

31 The method may comprise a method of carrying out a cathodic protection survey of a 32 target. The target may comprise a subsea platform such as a subsea jacket.

34 The target may comprise a pipeline or pipeline system. The target may comprise a subsea production system, such as a subsea manifold, a subsea tree and/or a well frame. The 1 method may comprise a method of detecting and/or tracking a pipeline or cable. The 2 method may comprise a method of surveying shallow water pipelines. The method may 3 comprise a method of tracking subsea electric power cables carrying an AC or DC current.

The method enables tracking of an AC/DC cable as well as detection of defects in the 6 cable causing distortion of the field measured. The method may comprise tracking the 7 burial degree and/or depth of cables, as burying the cable protects the cable from anchor 8 drag and fishing activity (trawling). This may be particularly important in shallow water 9 where seabeds shift, potentially exposing or reducing the depth of burial of cables for windfarms or other offshore infrastructure.

12 The fifth aspect of the invention and its embodiments may comprise one or more essential 13 and/or optional features of the first to fourth aspects of the invention or vice versa.

Brief description of the drawings

17 There will now be described, by way of example only, various embodiments of the 18 invention with reference to the drawings, of which: Figure 1 is a schematic representation of a cathodic protection system and its electric field; 22 Figure 2 is a schematic representation of the system of Figure 1 and the effect on its

23 electric field from a conventional ROV;

Figure 3 is a block diagram showing schematically the steps of a method of designing an 26 underwater vehicle according to an embodiment of the invention; 28 Figure 4A is a schematic representation of a cathodic protection system and the effect on 29 its electric field from an ROV designed according to an embodiment of the invention; 31 Figure 4B is a schematic representation of the cathodic protection system and ROV of 32 Figure 4A from a plan view 34 Figure 5 is a block diagram showing schematically the steps of a method of designing an underwater vehicle according to an alternative embodiment of the invention; 2 Figures 6A to 6D are respectively isometric, bow side, starboard side and plan views of an 3 ROV according to an embodiment of the invention; Figures 7A to 7D are respectively isometric, bow side, starboard side and plan views of an 6 ROV according to an embodiment of the invention; and 8 Figure 8 is a block diagram showing schematically the steps of a method of designing an 9 underwater vehicle according to an embodiment of the invention.

11 Detailed description of preferred embodiments

13 The present invention concerns a method of designing and/or manufacturing underwater 14 vehicles, underwater vehicles formed by the method, and methods of using the underwater vehicles. Such underwater vehicles include, without limitation, remotely operated vehicles 16 (ROVs) used in cathodic protection surveying applications, and accordingly this application 17 is described below to illustrate the principles of the invention. It should be noted that the 18 invention extends to other underwater vehicle types, and is applicable to underwater 19 vehicles in general, with particular application to underwater vehicles which carry or incorporate sensitive electric and/or magnetic field measuring equipment.

22 Referring firstly to Figure 1, there is shown a schematic representation of a cathodic 23 protection system and its electric field, which is a typical measurement scenario for 24 embodiments of the invention. The cathodic protection system 10 is applied to a steel pipeline 12 and comprises an anode 14. Isopotential lines of the electric field generated 26 by the anode 14 are schematically shown at 16. Figure 2 shows the same system 10 27 having a measuring ROV of conventional form in the vicinity of the anode.

29 Figure 2 is a schematic representation of the effect on the electric field from a conventional ROV 20 in the vicinity of the anode 14 to measure the electric field according to known 31 principles of field gradient cathodic protection surveying, using sensor package 22. Lines 32 16 are the isopotential field lines that would be observed in the absence of the ROV 33 according to Figure 1). In practice however, the ROV interferes with the electric field in a 34 complex manner, due to its physical presence in the water, due to electromagnetic fields emanating from the electrical and/or magnetic components of the ROV, and even due to 1 the effect of the ROV on the movement of the surrounding water. The effect on the field is 2 schematically represented by the isopotential field lines 18, which are distorted with 3 respect to the lines 16 of Figure 1. The sensor package 22 is sensitive to the disruption of 4 the field from the ROV, which may render the acquired data insensitive to changes in the field that might be indicative of the performance of the cathodic protection system.

7 To address the described issue, embodiments of the invention use a computer modelling 8 approach to assessing the suitability of an underwater vehicle design for a measurement 9 scenario, as will be described below.

11 Figure 3 is a block diagram showing schematically the steps of a method of designing an 12 underwater vehicle according to an embodiment of the invention. The method, generally 13 shown at 100, utilises a computer model 110 to calculate vector fields 111, 112, which can 14 be compared 114 to assess 116 a vehicle design 104. The method works by establishing or inputting information relating to a measurement scenario 102. Relevant measurement 16 scenario information includes sinks and sources of the electric field and other 17 measurement objects, for example the presence, location and physical properties of 18 measurement objects such as a galvanic anode, a piece of steel, a corroding surface, an 19 electric connector, and/or a subsea structure. The measurement scenario information is input into a computer model, which may be a Finite Element Model (FEM) or Boundary 21 Element Model (BEM) 2D or 3D computer model. The computer model of this 22 embodiment is implemented in software running on a personal computer. An example of 23 commercially available software capable configuring the model is Comsol Multiphysics 24 marketed by Comsol. In alternative embodiments, the computer model may be fully or partially implemented in software running on a remote server and/or cloud-based server, 26 or may be fully or partially implemented in hardware and/or firmware.

28 Having established the model based on the measurement scenario information, a first 29 vector field data set is calculated (111), which corresponds to a base electric field from the measurement objects and field sources (e.g. a galvanic anode). The calculated field data 31 is optionally output to a data storage device (not shown).

33 A vehicle design 104 is then incorporated into the model, to create a combined model of 34 the system of measurement objects and field sources in the presence of the vehicle. The 1 vehicle design may include any of a range of design factors that could affect the vector

2 fields, including but not limited to:

3 * Shape and dimensions of the vehicle; 4 * Build materials used in the vehicle and/or equipment mounted on the vehicle; * Position and/or orientation of one or more sensors; 6 * Omnidirectional flying features and components; 7 * Electric fields or magnetic fields associated with equipment mounted on or to the 8 vehicle, such as thrusters, other sensor arrays, electric cables, signal generators, 9 magnetic field emitters, and/or electric field emitters.

11 The vehicle will generally interfere with the field due to the fact that it has a finite volume, 12 and is made from materials that may interact magnetically or electrically with the fields 13 from the source. Using multi-physics FEM/BEM modelling, the design factors can be 14 accounted for in the model. The updated, combined model, is then used to calculate (112) a second vector field data set, which corresponds to the combined field.

17 The first and second vector field data sets (i.e. the base field and the combined field) are 18 compared (114) and the comparison is assessed to determine if the vehicle design is 19 suitable for the planned measurement operation. More specifically, the differences between the field calculated from the base measurement scenario and the field calculated 21 from the combined model (incorporating the vehicle) are assessed, for example to 22 determine whether the effect on the field is sufficiently small in the location of the sensor 23 package to enable the vehicle built according to the design to be used effectively. If the 24 differences are assessed to be sufficiently small, a vehicle may be manufactured to the design.

27 If there is a deviation between the calculated fields which is too great for effective use of a 28 vehicle manufactured to the design, one option is to derive a relation between the base 29 vector field and the combined vector field. This can be done by solving a matrix equation A*X=B where A is the vector field in the position of the field gradient sensor without the 31 ROV present and B is the vector field with the ROV present. The value of the matrix X is a 32 correction factor which can be applied to the measured vector field. This relation can be 33 used as a correction factor to be applied to the field data acquired in a measurement 34 operation in order to calculate the correct field values. In the simplest form, this correction factor consists of a single vector to be multiplied by the measured vector value to provide 1 the actual vector value without the ROV present. Thus the effect of the vehicle on the field 2 may be compensated for.

4 Figure 4A is a schematic representation of a cathodic protection system and a calculated effect on its electric field from an ROV designed according to an embodiment of the 6 invention. Figure 4B is a schematic representation of the same cathodic protection system 7 and ROV of Figure 4A from a plan view. The cathodic protection system comprises a 8 pipeline 12 and a galvanic anode 14. The ROV 200 comprises a frame formed from an 9 upper planar member 204 and a lower planar member 206 separated by spacer members.

A sensor package 202 is placed with its centre of gravity close to the centre of gravity of 11 the vehicle for optimal ROV flying performance.

13 The ROV design is selected to provide large cutouts 208 in the planar members of the 14 frame to reduce the distortion of the electric field in the area of the sensor package. A comparison of the calculated electric field isopotential lines 18 with the base field, would 16 reveal a relatively small difference in the respective fields (indicated by relatively 17 undistorted isopotential lines in Figure 4A in the position of the sensor package), leading to 18 an assessment that the vehicle is suitable for carrying out cathodic protection surveys.

Although it is described above that the design of the ROV reduces the effect of the 21 difference between the base field and combined field calculations, in other 22 implementations, the design of the vehicle may not reduce the difference between the 23 base field and combined field calculations as such, but may impact them in a way that 24 enables the differences to be compensated for relatively easily, for example by enabling derivation of a relationship and calculation of a correction factor.

27 Referring now to Figure 5, there is shown a block diagram illustrating schematically the 28 steps of a method of designing an underwater vehicle according to an alternative 29 embodiment of the invention. The method, generally shown at 300, is similar to the method 100 and will be understood from Figure 3 and the accompanying description.

31 However, method 300 differs in that it is presented as an iterative method, which repeats 32 the steps of calculating a combined field 112, and comparing with a base field, for a 33 number of design iterations 104. In the event that the assessment step (116) determines 34 that a difference between the respective field calculations is too great, the vehicle design is changed, and the new vehicle design is incorporated into the model 110. A new combined 1 field is calculated and another comparison is made. The process can be repeated until the 2 difference between the field calculations is acceptably low, or is minimised, depending on 3 requirements. Design features that can be adjusted in design iterations include but are not 4 limited to: 6 * Shape and dimensions of the vehicle; 7 * Build materials used in the vehicle and/or equipment mounted on the vehicle; 8 * Position and/or orientation of one or more sensors; 9 * Electric fields or magnetic fields associated with equipment mounted on or to the vehicle, such as thrusters, other sensor arrays, electric cables, signal generators, 11 magnetic field emitters, and/or electric field emitters.

13 Using multi-physics FEM/BEM modelling, the design features can be accounted for in the 14 model, and thus one or more design features may be optimised, and/or may be mitigated against to an acceptable level. When the differences between the field calculations are 16 acceptable, the respective vehicle design may be selected for manufacture of the vehicle.

17 Figures 6A to 6D are respectively isometric, bow side, starboard side and plan views of an 18 ROV according to an embodiment of the invention. The ROV is configured for the 19 incorporation of vector field sensors (including but not limited to electric and/or magnetic field gradient sensors) in a way that the ROV either does not interfere with the field 21 gradient measurements in value or direction, or only interferes in a known way such that 22 the original field gradient value can be reconstructed.

24 Applications of the ROV 600 include acquiring point measurements to calculate the current output of anodes as well as steel current density of a coated or uncoated steel surface.

26 Point measurements are most commonly used to determine the anode activity and steel 27 current density of (without limitation) seawater exposed areas of offshore fixed steel 28 structures (monopiles, jackets, tripods etc.), seawater exposed areas of offshore floating 29 structures (FPSO, Semi-sub, SPAR etc), vessel hulls, and/or mooring lines.

31 Referring to Figures 6A to 6D, the ROV 600 comprises a body 602 comprising an open 32 frame formed from a top plate 604, a bottom plate 606, and support components between 33 the top plate and bottom plate. The support components include support wall members 34 608 partially surrounding a central part of the body 602, and support struts 610 disposed between the top and bottom plates towards the perimeter of the body. Together, the top 1 and bottom plates and support components form a hull or chassis of the ROV, which 2 provides a structure for attachment of functional components of the vehicle. Cables 3 between functional components are contained within the vehicle body. It should be noted 4 that the terms "top" and "bottom", and other position terms used herein are used only to indicate relative positions of the vehicle in a typical orientation, as shown in the drawings.

7 The body 602 is formed primarily from a polymer material, which may conveniently be a 8 suitable density polyethylene. Other materials having suitable strength, density, hardness, 9 resilience, thermal and anti-fouling characteristics may be used in alternative embodiments.

12 The functional components include horizontal thrusters 612 arranged towards respective 13 corners of the body 602, and vertical thrusters 614, spatially separated over the area of the 14 body. Adjacent horizontal thrusters have axes of orientation inclined to one another at 90 degrees, although other configurations are possible. In combination, the thrusters provide 16 the ROV with full omnidirectional flying capability, controllable from the surface via a tether 17 (not shown). The ROV also includes a camera and light package, shown generally at 620 18 at the bow end of the body 602.

A sensor package generally shown at 630 is mounted on the starboard side of the body 21 602. In this embodiment, the sensor package includes a field gradient sensor, for use in 22 acquiring measurements of an electric vector field in, for example, a cathodic protection 23 survey operation. The placement of the field gradient sensor on the body of the ROV is 24 determined from the method described with reference to Figures 3 or 5, in order to mitigate against, or eliminate, the effects of the ROV on the vector field being measured. In this 26 embodiment, the field gradient sensor is located on the centre of gravity of the ROV, and is 27 placed symmetrically between the vertical thrusters 614 on the starboard side. The 28 location of the field gradient sensor is also removed from the flow from the horizontal 29 thrusters to avoid disturbance from rapidly moving water.

31 As well as determining a preferred sensor package placement location using the modelling 32 approach of embodiments of the invention, the ROV body itself has design features 33 determined using the inventive method. In this example, the top plate 604 and the bottom 34 plate 606 are provided with apertures 634a, 634b, 636a, 636b respectively, in the form of cut-outs from the material of the top and bottom plates. On the starboard side, the 1 apertures 634a, 636a are located substantially above and below the position of the sensor 2 package 630, and enable the passage of field lines through the ROV with reduced or 3 minimal disturbance, according to the models of the vector field calculated in the method.

4 On the port side, the apertures 634b, 636b (together with the starboard side apertures) facilitate manual handling of the ROV by functioning as a handle, and also facilitate even 6 weight distribution.

8 Figures 7A to 7D are respectively isometric, bow side, starboard side and plan views of an 9 ROV according to an embodiment of the invention. The ROV, generally shown at 700, is also configured for the incorporation of vector field sensors (including but not limited to 11 electric and/or magnetic field gradient sensors) in a way that the ROV either does not 12 interfere with the field gradient measurements in value or direction, or only interferes in a 13 known way such that the original field gradient value can be reconstructed.

The ROV 700 is designed for use in detection and tracking or pipelines and cables, and 16 shallow water surveying of pipelines and cables. The ROV has similarities with the ROV 17 600 of Figure 6A to 6D, comprising an open frame formed from a top plate 704, a bottom 18 plate 706, and support components between the top plate and bottom plate. Together, the 19 top and bottom plates and support components form a hull or chassis of the ROV, which provides a structure for attachment of functional components of the vehicle. However, the 21 ROV 700 differs in that it comprises frame extensions in the form of wings 707, which 22 laterally extend the top plate 704 on the port and starboard sides, and provide mounting 23 locations for buoyancy modules 709.

The functional components include thrusters 712, 714 which in combination provide the 26 ROV with full omnidirectional flying capability. The ROV also includes a camera and light 27 package, shown generally at 720 at the bow end of the body 702.

29 A sensor package generally shown at 730 is mounted on the starboard side of the body.

In this embodiment, the sensor package includes a field gradient sensor, for use in 31 acquiring measurements of an electric vector field in a pipe or cable tracking operation.

32 The placement of the field gradient sensor on the body of the ROV is determined from the 33 method described with reference to Figures 3 or 5, in order to mitigate against, or 34 eliminate, the effects of the ROV on the vector field being measured. In this embodiment, the field gradient sensor is located on the centre of gravity of the ROV, and is placed 1 symmetrically between the vertical thrusters 714 on the starboard side. The location of the 2 field gradient sensor is also removed from the flow from the horizontal thrusters to avoid 3 disturbance from rapidly moving water.

The ROV 700 also includes an extended frame 711 for mounting pipe or cable tracking 6 sensor heads 713 ahead of the main body 702 in the bow direction.

8 As well as determining a preferred sensor package placement location using the modelling 9 approach of embodiments of the invention, the ROV body itself has design features determined using the inventive method. In this example, the top plate 704 and the bottom 11 plate 706 are provided with apertures 734 and 736, in the form of cut-outs from the 12 material of the top and bottom plates. On the starboard side, the apertures 734, 736 are 13 located substantially above and below the position of the sensor package 730, and enable 14 the passage of field lines through the ROV with reduced or minimal disturbance, according to the models of the vector field calculated in the method.

17 Other features of geometry and flight properties, including but not limited to the geometry 18 of the extended frame 711, lateral frame extensions 707 and apertures/cut outs provided 19 therein, and properties and placement of the buoyancy elements 709 are determined by the modelling approach of the inventive method, so that the ROV either does not interfere 21 with the field gradient measurements in value or direction, or only interferes in a known 22 way such that the original field gradient value can be reconstructed.

24 Figure 8 is a block diagram showing schematically the steps of a method of designing an underwater vehicle according to an embodiment of the invention. The method, generally 26 shown at 400, is similar to the methods 100 and 300, and will be understood from Figures 27 3 and 5 and the accompanying text. Using a model 410, the method calculates fields 412 28 for a number of different design parameters 401. For example, a single design parameter 29 is given multiple values, with a respective field calculation performed for each value. The calculated fields are analysed (414) and compared with a base field calculation 402 to 31 determine which of the design parameter values are within an acceptable range of 32 difference from the base field, or can be corrected for appropriately when acquiring real 33 data. Acceptable levels of difference can be selected 416 for manufacture 420.

1 The described embodiments provide a method of designing an underwater vehicle to 2 incorporate vector field sensors so that the vehicle itself does not interfere, in value nor 3 direction, with vector field measurements. The invention extends to vehicles, including but 4 not limited to ROVs, manufactured according to such designs and/or incorporating novel and inventive design features. Applications of the underwater vehicles include field 6 gradient measurement operations such as those used in determining the anode activity 7 and steel current density of: 8 * Seawater exposed areas of offshore fixed steel structures (monopiles, jackets, 9 tripods etc.) * Seawater exposed areas of offshore floating structures (FPSO, Semi-sub, SPAR 11 etc) 12 * Vessel hulls 13 * Mooring lines The invention extends to other applications in which underwater vehicles use or 16 incorporate vector field sensors. These include detection and tracking or pipelines and 17 cables, and shallow water surveying of pipelines and cables. These include passive 18 tracking, in which the field gradient sensors are used to triangulate the source of the 19 measured field. In this case it is necessary to have full control over the ROV electric and magnetic fields. A custom built ROV where the fields from the ROV have been eliminated 21 will eliminate the need for time consuming calibration which is necessary with existing 22 systems on the market. They also include active tracking, in which an ROV emits an 23 electric signal from a built-in electromagnetic emitter and the field gradient sensor(s) 24 measures the resulting fields and calculates the position of the item disturbing the ideal response. A custom built ROV where the fields from the ROV have been eliminated will 26 eliminate the need for time consuming calibration which is necessary with existing systems 27 on the market.

29 Other applications include geological mineral mapping of the seabed or minerals buried below the seabed. Mineral deposits often have an electric field associated with them due 31 to natural occurrence of anodic and cathodic fields. These can be mapped in up to 3 32 spatial dimensions by using an electric field gradient sensor(s) which measures the electric 33 field in up to 3 spatial dimensions. Mineral deposits often have a magnetic field associated 34 with them due to natural occurrence of magnetic fields in the minerals. These can be mapped in up to 3 spatial dimensions by using a magnetic field gradient sensor(s) which 1 measures the magnetic field in up to 3 spatial dimensions. Mineral deposits with different 2 magnetic permeabilities can be measured as a disturbance of the earth magnetic field and 3 can be mapped with magnetic field gradient sensors.

According to embodiments of the invention, designing a dedicated ROV for the effective 6 incorporation of vector field sensors results in a vehicle that is small in size and weight.

7 The vehicle may therefore be usable in extremely difficult to operate areas such as areas 8 with restricted access and ultra-shallow water. This enables inspection of cathodic 9 protection in areas not possible to inspect today, and also provides more cost effective and environmentally friendly inspection options. Contactless cathodic protection inspection is 11 possible from small ROVs with low CO2, asset-based deployment, and simplified logistics.

12 CP measurement is possible in ultra-shallow water (<20m water depth) and buried 13 pipelines, which are not normally accessible with a vessel or traditional ROV. The method 14 does not compromise the integrity of the coating by penetration, and does not require cleaning of marine growth.

17 The ROV compensates any disturbance of the electrical field by its design and sensor 18 placement.

The invention provides a method of designing an underwater vehicle for use in an 21 underwater measurement operation. The method comprises using a model of a 22 measurement scenario and calculating a base vector field data set. A vehicle design is 23 incorporated into the model to create a combined model, and a first vector field data set is 24 calculated from the combined model. The first vector field data set is compared with the base vector field data set to assess the suitability of the vehicle design for the 26 measurement operation.

28 The invention enables improvements in geometry and/or flight properties. The placement 29 of the sensor packages integrated in the body of a vehicle instead of on a manipulator arm facilitates good flight properties. The modelling approach to the sensor placement and/or 31 vehicle geometry can make the vehicle "invisible" to the field gradient 32 instruments/measurements, or at least reduce the visibility of the vehicle or modify its 33 visibility so that its effects can be compensated for.

1 Various modifications to the above-described embodiments may be made within the scope 2 of the invention, and the invention extends to combinations of features other than those 3 expressly claimed herein.

Claims

1 Claims: 3 1. A method of designing an underwater vehicle for use in an underwater measurement 4 operation, the method comprising: a. Using a model of a measurement scenario, calculating a base vector field data 6 set; 7 b. Incorporating a vehicle design into the model to create a combined model, and 8 calculating a first vector field data set from the combined model; 9 c. Comparing the first vector field data set with the base vector field data set to assess the suitability of the vehicle design for the measurement operation.12

2. The method according to claim 1, wherein the measurement scenario model and/or 13 the combined model comprises a Finite Element Model (FEM) or Boundary Element 14 Model (BEM) model.16

3. The method according to claim 1 or claim 2, wherein vehicle design is a first vehicle 17 design, and comparing the first vector field data set with the base vector field data 18 set assesses the suitability of the first vehicle design for the measurement operation.

4. The method according to any preceding claim, comprising repeating step b. for a 21 second and/or further vehicle design to calculate a second and/or further vector field 22 data set.24

5. The method according to any preceding claim, comprising repeating step c. for the second and/or further vector field data set to assess the suitability of the second 26 and/or further vehicle design for the measurement operation.28

6. The method according to any preceding claim, comprising comparing the first vector 29 field data set with the second and/or further vector field data set to assess the suitability of the second and/or further vehicle design for the measurement operation 32

7. The method according to any preceding claim, comprising optimising the vehicle 33 design for the measurement operation.1

8. The method according to claim 7, wherein optimising the vehicle design for the 2 measurement operation comprises minimising a difference between the base vector 3 field data set and the first, second or further vector field data set calculated from a 4 combined model incorporating the vehicle design.6

9. The method according to claim 7 or claim 8, wherein optimising the vehicle design 7 comprises minimising a difference between calculated vector field data set and the 8 base vector field data set for at least one vehicle design parameter.

10. The method according to any preceding claim, comprising determining a relation 11 between the base vector field data set and the first or further vector field data set 12 calculated from a combined model incorporating the vehicle design.14

11. The method according to any preceding claim, comprising deriving a correction factor or a correction field to be applied to a measured vector field data set.17

12. The method according to any preceding claim, wherein the vehicle is a mini-ROV.19

13. The method according to any preceding claim, wherein the vehicle has fully omnidirectional flying capabilities.22

14. The method according to any preceding claim, comprising creating the model of the 23 measurement scenario using measurement scenario information relating to vector24 field sources.26

15. The method according to claim 14, wherein the vector field sources comprise one or 27 more of a galvanic anode, a piece of steel, a corroding surface, an electric 28 connector, a subsea structure or a mineral deposit.

16. The method according to any preceding claim, comprising inputting vehicle design 31 information into the model to create the combined model.33

17. The method according to claim 16, wherein the vehicle design information includes 34 one or more of: shape and dimensions of the vehicle; build materials used in the vehicle and/or equipment mounted on the vehicle; position and/or orientation of one 1 or more sensors; omnidirectional flying features and components; electric fields or 2 magnetic fields associated with equipment mounted on or to the vehicle.4

18. A method of manufacturing an underwater vehicle comprising the method of designing the underwater vehicle according to any preceding claim, and 6 manufacturing the vehicle according to the vehicle design.8

19. An underwater vehicle manufactured according to the method of claim 18.

20. The underwater vehicle according to claim 19, where a field gradient sensor is 11 integrated into the body or frame of the vehicle.13

21. An underwater vehicle comprising: 14 a frame; -one or more propulsion devices; 16 one or more vector field sensors incorporated into the vehicle; 17 -wherein the vehicle comprises one or more design features selected to modify the 18 disruption of a vector field to be measured by the one or more vector field 19 sensors.21

22. The underwater vehicle according to claim 21, where a field gradient sensor is 22 integrated into the body or frame of the vehicle.24

23. The underwater vehicle according to claim 21 or claim 22, wherein the frame comprises a partially or fully open structure which modifies disruption of the vector26 field to be measured.28

24. The underwater vehicle according to any of claims 21 to 23, wherein the frame 29 comprises at least one planar member defining a surface of the vehicle, and the planar member may comprise one or more apertures to modify disruption of the31 vector field to be measured.33

25. A method of using an underwater vehicle according to any of claims 19 to 20 or 21 to 34 24, wherein the method comprises: -locating the underwater vehicle in a measurement location; 1 -acquiring vector field measurement data using the one or more sensors.3

26. The method according to claim 25 comprising carrying out a cathodic protection 4 survey of a target.6

27. The method according to claim 26, wherein target comprises or more of: a subsea 7 platform such as a subsea jacket; a pipeline or pipeline system; a subsea production 8 system, such as a subsea manifold, a subsea tree and/or a well frame.

28. The method according to claim 25 comprising detecting and/or tracking a pipeline or 11 cable.13

29. The method according to claim 28 comprising surveying shallow water pipelines.

30. The method according to claim 29 comprising tracking subsea electric power cables 16 carrying an AC or DC current.