The present invention relates to a method of designing and/or manufacturing underwater vehicles, underwater vehicles formed by the method, and methods of using the underwater vehicles. Particular embodiments relate to underwater vehicles incorporating electric field and/or magnetic field sensors and methods of use.

Background to the invention

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

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 sensors may be highly sensitive in order to detect low amplitude signals and/or small signal variations. These sensor types include electric field gradient sensors and magnetic field gradient sensors, as used in applications such as cathodic protection surveys, 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 gradient sensor approach to cathodic protection surveying is described in “Field Gradient Survey of Offshore Pipeline Bundles affected by Trawling”, Lauvstad et al., Eurocorr 2016. Modelling methods are described in “Computer modeling of offshore cathodic protection systems: method and experience”, RD Strommen, Computer Modeling in Corrosion, ASTM STP 1154, 229-247, 1992.

Typically these techniques will be carried out using traditional Work Class ROVs or Inspection Class ROVs, based on an offshore asset or on a large vessel. The ROV itself, 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 and interpretation. Existing data processing methods to compensate for electric and magnetic field interference are inadequate.

WO17126975 A1 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 the structure. The method may involve modelling of the structure, using boundary element methods or finite element methods, to obtain a 2D or 3D vector map of the structure.

LIS2021094660 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 this structure or the location of it in surveying of cathodic protection systems.

Summary of the invention

According to a first aspect of the invention, there is provided a method of designing an underwater vehicle for use in an underwater measurement operation, the method comprising: a. Using a model of a measurement scenario, calculating a base vector field data set; 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; 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.

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 or 3D computer model.

The vehicle design may be a first vehicle design, and comparing the first vector field data 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 and/or further vehicle design to calculate a second and/or further vector field data set. The method may comprise repeating step c. for 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. Alternatively, or in addition, the method may comprise comparing the first vector 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.

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

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.

Alternatively, or in addition, the method may comprise determining a relation between the base vector field data set and the first or further vector field data set calculated from a 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.

The vehicle may be a remotely operated vehicle (ROV), and may be a mini-ROV. In this context, a mini-ROV may be thought of as an ROV that can be handled manually, for 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 a vehicle that is small in size and weight. The vehicle may therefore be usable in extremely difficult to operate areas such as areas with restricted access and ultra-shallow water. The vehicle may also be manually handled and operated from rope access teams.

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

The method may comprise creating the model of the measurement scenario using measurement scenario information. Measurement scenario information may comprise information relating to vector field sources, which may be a galvanic anode, a piece of steel, a corroding surface, an electric connector, a subsea structure or a mineral deposit. The method may comprise inputting vehicle design information into the model to create the combined model. The vehicle design information may include any of a range of design factors that could affect the vector fields, including but not limited to:

• 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 or more sensors;

• Omnidirectional flying features and components;

• Electric fields or magnetic fields associated with equipment mounted on or to the vehicle, such as thrusters, other sensor arrays, electric cables, signal generators, magnetic field emitters, and/or electric field emitters.

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

The second aspect of the invention and its embodiments may comprise one or more 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 manufactured according to the second aspect of the invention.

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

According to a fourth aspect of the invention, there is provided an underwater vehicle comprising: a frame; one or more propulsion devices; one or more vector field sensors incorporated into the vehicle;

- wherein the vehicle comprises one or more design features selected to modify the disruption of a vector field to be measured by the one or more vector field sensors.

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 or carried by a manipulator arm, but does not require that the one or more vector field sensors are fully or partially internal to the vehicle.

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 may be integrated into the body or frame of the vehicle.

In this context, “integrated into the body or frame of the vehicle” means joined with or 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 the respective sensors are fully or partially internal to the vehicle.

The frame may comprise a partially or fully open structure which modifies disruption of the 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 apertures to modify disruption of the vector field to be measured.

Preferably, disruption of the vector field to be measured is reduced. Alternatively, or in 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.

The fourth aspect of the invention and its embodiments may comprise one or more 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 underwater vehicle according to the third or fourth aspects of the invention, the method comprising: locating the underwater vehicle in a measurement location; acquiring vector field measurement data using the one or more sensors.

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

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 method may comprise a method of detecting and/or tracking a pipeline or cable. The method may comprise a method of surveying shallow water pipelines. The method may 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 cable causing distortion of the field measured. The method may comprise tracking the burial degree and/or depth of cables, as burying the cable protects the cable from anchor drag and fishing activity (trawling). This may be particularly important in shallow water where seabeds shift, potentially exposing or reducing the depth of burial of cables for windfarms or other offshore infrastructure.

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

Brief description of the drawings

There will now be described, by way of example only, various embodiments of the invention with reference to the drawings, of which:

Figure 1 is a schematic representation of a cathodic protection system and its electric field;

Figure 2 is a schematic representation of the system of Figure 1 and the effect on its electric field from a conventional ROV;

Figure 3 is a block diagram showing schematically the steps of a method of designing an underwater vehicle according to an embodiment of the invention;

Figure 4A is a schematic representation of a cathodic protection system and the effect on its electric field from an ROV designed according to an embodiment of the invention;

Figure 4B is a schematic representation of the cathodic protection system and ROV of Figure 4A from a plan view;

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; Figures 6A to 6D are respectively isometric, bow side, starboard side and plan views of an ROV according to an embodiment of the invention;

Figures 7A to 7D are respectively isometric, bow side, starboard side and plan views of an ROV according to an embodiment of the invention; and

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.

Detailed description of preferred embodiments

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

Referring firstly to Figure 1 , there is shown a schematic representation of a cathodic protection system and its electric field, which is a typical measurement scenario for 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 by the anode 14 are schematically shown at 16. Figure 2 shows the same system 10 having a measuring ROV of conventional form in the vicinity of the anode.

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 principles of field gradient cathodic protection surveying, using sensor package 22. Lines 16 are the isopotential field lines that would be observed in the absence of the ROV (i.e. according to Figure 1). In practice however, the ROV interferes with the electric field in a 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 the effect of the ROV on the movement of the surrounding water. The effect on the field is schematically represented by the isopotential field lines 18, which are distorted with respect to the lines 16 of Figure 1. The sensor package 22 is sensitive to the disruption of 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.

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

Figure 3 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 shown at 100, utilises a computer model 110 to calculate vector fields 111 , 112, which can 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 scenario information includes sinks and sources of the electric field and other measurement objects, for example the presence, location and physical properties of measurement objects such as a galvanic anode, a piece of steel, a corroding surface, an 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 Element Model (BEM) 2D or 3D computer model. The computer model of this embodiment is implemented in software running on a personal computer. An example of commercially available software capable configuring the model is Comsol Multiphysics marketed by Comsol. Alternatively, or in addition, the computer model may be fully or partially implemented in software running on any suitable computer or an equivalent processing device. In embodiments of the invention, the computer model may be fully or partially implemented in software running on a remote server and/or cloud-based server, and in further alternatives may be fully or partially implemented in hardware and/or firmware. The computer model may also be implemented in combinations of hardware, firmware and software.

Having established the model based on the measurement scenario information, a first 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 is optionally output to a data storage device (not shown). A vehicle design 104 is then incorporated into the model, to create a combined model of the system of measurement objects and field sources in the presence of the vehicle. The vehicle design may include any of a range of design factors that could affect the vector fields, including but not limited to:

• 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 or more sensors;

• Omnidirectional flying features and components;

• Electric fields or magnetic fields associated with equipment mounted on or to the vehicle, such as thrusters, other sensor arrays, electric cables, signal generators, magnetic field emitters, and/or electric field emitters.

The vehicle will generally interfere with the field due to the fact that it has a finite volume, and is made from materials that may interact magnetically or electrically with the fields from the source. Using multi-physics FEM/BEM modelling, the design factors can be 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.

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

If there is a deviation between the calculated fields which is too great for effective use of a vehicle manufactured to the design, one option is to derive a relation between the base 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 ROV present and B is the vector field with the ROV present. The value of the matrix X is a correction factor which can be applied to the measured vector field. This relation can be used as a correction factor to be applied to the field data acquired in a measurement 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 the actual vector value without the ROV present. Thus the effect of the vehicle on the field may be compensated for.

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 invention. Figure 4B is a schematic representation of the same cathodic protection system and ROV of Figure 4A from a plan view. The cathodic protection system comprises a pipeline 12 and a galvanic anode 14. The ROV 200 comprises a frame formed from an 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 the vehicle for optimal ROV flying performance.

The ROV design is selected to provide large cutouts 208 in the planar members of the 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 reveal a relatively small difference in the respective fields (indicated by relatively undistorted isopotential lines in Figure 4A in the position of the sensor package), leading to 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 difference between the base field and combined field calculations, in other implementations, the design of the vehicle may not reduce the difference between the base field and combined field calculations as such, but may impact them in a way that enables the differences to be compensated for relatively easily, for example by enabling derivation of a relationship and calculation of a correction factor.

Referring now to Figure 5, there is shown a block diagram illustrating schematically the steps of a method of designing an underwater vehicle according to an alternative 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. However, method 300 differs in that it is presented as an iterative method, which repeats the steps of calculating a combined field 112, and comparing with a base field, for a number of design iterations 104. In the event that the assessment step (116) determines 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 field is calculated and another comparison is made. The process can be repeated until the difference between the field calculations is acceptably low, or is minimised, depending on requirements. Design features that can be adjusted in design iterations include but are not limited to:

• 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 or more sensors;

• Electric fields or magnetic fields associated with equipment mounted on or to the vehicle, such as thrusters, other sensor arrays, electric cables, signal generators, magnetic field emitters, and/or electric field emitters.

Using multi-physics FEM/BEM modelling, the design features can be accounted for in the 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 acceptable, the respective vehicle design may be selected for manufacture of the vehicle. Figures 6A to 6D are respectively isometric, bow side, starboard side and plan views of an ROV according to an embodiment of the invention. The ROV is configured for the 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 gradient measurements in value or direction, or only interferes in a known way such that the original field gradient value can be reconstructed.

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

Referring to Figures 6A to 6D, the ROV 600 comprises a body 602 comprising an open frame formed from a top plate 604, a bottom plate 606, and support components between the top plate and bottom plate. The support components include support wall members 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 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. Cables between functional components are contained within the vehicle body. It should be noted 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.

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

The functional components include horizontal thrusters 612 arranged towards respective corners of the body 602, and vertical thrusters 614, spatially separated over the area of the 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 the ROV with full omnidirectional flying capability, controllable from the surface via a tether (not shown). The ROV also includes a camera and light package, shown generally at 620 at the bow end of the body 602.

A sensor package generally shown at 630 is mounted on the starboard side of the body 602. In this embodiment, the sensor package includes a field gradient sensor, for use in acquiring measurements of an electric vector field in, for example, a cathodic protection survey operation. The placement of the field gradient sensor on the body of the ROV is 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 embodiment, the field gradient sensor is located on the centre of gravity of the ROV, and is placed symmetrically between the vertical thrusters 614 on the starboard side. The location of the field gradient sensor is also removed from the flow from the horizontal thrusters to avoid disturbance from rapidly moving water.

As well as determining a preferred sensor package placement location using the modelling approach of embodiments of the invention, the ROV body itself has design features determined using the inventive method. In this example, the top plate 604 and the bottom 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 apertures 634a, 636a are located substantially above and below the position of the sensor package 630, and enable 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. 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 weight distribution.

Figures 7A to 7D are respectively isometric, bow side, starboard side and plan views of an 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 electric and/or magnetic field gradient sensors) in a way that the ROV either does not interfere with the field gradient measurements in value or direction, or only interferes in a 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 shallow water surveying of pipelines and cables. The ROV has similarities with the ROV 600 of Figure 6A to 6D, comprising an open frame formed from a top plate 704, a bottom plate 706, and support components between the top plate and bottom plate. Together, the 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 ROV 700 differs in that it comprises frame extensions in the form of wings 707, which laterally extend the top plate 704 on the port and starboard sides, and provide mounting locations for buoyancy modules 709.

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

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 acquiring measurements of an electric vector field in a pipe or cable tracking operation. The placement of the field gradient sensor on the body of the ROV is 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 embodiment, the field gradient sensor is located on the centre of gravity of the ROV, and is placed symmetrically between the vertical thrusters 714 on the starboard side. The location of the field gradient sensor is also removed from the flow from the horizontal thrusters to avoid disturbance from rapidly moving water.

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

As well as determining a preferred sensor package placement location using the modelling 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 plate 706 are provided with apertures 734 and 736, in the form of cut-outs from the material of the top and bottom plates. On the starboard side, the apertures 734, 736 are located substantially above and below the position of the sensor package 730, and enable 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.

Other features of geometry and flight properties, including but not limited to the geometry of the extended frame 711 , lateral frame extensions 707 and apertures/cut outs provided 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 with the field gradient measurements in value or direction, or only interferes in a known way such that the original field gradient value can be reconstructed.

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 shown at 400, is similar to the methods 100 and 300, and will be understood from Figures 3 and 5 and the accompanying text. Using a model 410, the method calculates fields 412 for a number of different design parameters 401. For example, a single design parameter 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 determine which of the design parameter values are within an acceptable range of difference from the base field, or can be corrected for appropriately when acquiring real data. Acceptable levels of difference can be selected 416 for manufacture 420.

The described embodiments provide a method of designing an underwater vehicle to incorporate vector field sensors so that the vehicle itself does not interfere, in value nor direction, with vector field measurements. The invention extends to vehicles, including but not limited to ROVs, manufactured according to such designs and/or incorporating novel and inventive design features. Applications of the underwater vehicles include field gradient measurement operations such as those used in determining the anode activity and steel current density of:

• Seawater exposed areas of offshore fixed steel structures (monopiles, jackets, tripods etc.)

• Seawater exposed areas of offshore floating structures (FPSO, Semi-sub, SPAR etc)

• Vessel hulls

• Mooring lines

The invention extends to other applications in which underwater vehicles use or incorporate vector field sensors. These include detection and tracking or pipelines and cables, and shallow water surveying of pipelines and cables. These include passive tracking, in which the field gradient sensors are used to triangulate the source of the 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 will eliminate the need for time consuming calibration which is necessary with existing systems on the market. They also include active tracking, in which an ROV emits an electric signal from a built-in electromagnetic emitter and the field gradient sensor(s) 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 eliminate the need for time consuming calibration which is necessary with existing systems on the market.

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 to natural occurrence of anodic and cathodic fields. These can be mapped in up to 3 spatial dimensions by using an electric field gradient sensor(s) which measures the electric field in up to 3 spatial dimensions. Mineral deposits often have a magnetic field associated 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 measures the magnetic field in up to 3 spatial dimensions. Mineral deposits with different magnetic permeabilities can be measured as a disturbance of the earth magnetic field and can be mapped with magnetic field gradient sensors.

According to embodiments of the invention, designing a dedicated ROV for the effective incorporation of vector field sensors results in a vehicle that is small in size and weight. The vehicle may therefore be usable in extremely difficult to operate areas such as areas with restricted access and ultra-shallow water. This enables inspection of cathodic protection in areas not possible to inspect today, and also provides more cost effective and environmentally friendly inspection options. Contactless cathodic protection inspection is possible from small ROVs with low CO2, asset-based deployment, and simplified logistics. CP measurement is possible in ultra-shallow water (<20m water depth) and buried pipelines, which are not normally accessible with a vessel or traditional ROV. The method does not compromise the integrity of the coating by penetration, and does not require cleaning of marine growth.

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

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.

The invention enables improvements in geometry and/or flight properties. The placement 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 vehicle geometry can make the vehicle "invisible" to the field gradient instruments/measurements, or at least reduce the visibility of the vehicle or modify its visibility so that its effects can be compensated for. Various modifications to the above-described embodiments may be made within the scope of the invention, and the invention extends to combinations of features other than those expressly claimed herein.