Method of controlling an autonomous underwater vehicle

Abstract

A method of controlling an autonomous underwater vehicle, AUV, when the AUV is cruising along a subsea surface is disclosed. The method comprises: monitoring a position of the AUV (100) by directing a laser beam from a laser interferometry sensing device (136) to the subsea surface (2); determining a visibility below the AUV (100); and controlling a cruising height o the AUV (100) relative to the subsea surface (2) based on the visibility.

Description

[0001] METHOD OF CONTROLLING AN AUTONOMOUS UNDERWATER VEHICLE

[0002] The present invention relates to a method of controlling an autonomous underwater vehicle (AUV), and an AUV capable of performing such a method. Particularly, the method pertains to controlling an AUV when the AUV is cruising along a subsea surface.

[0003] Autonomous underwater vehicles (AUVs) are used for various purposes, and can take many forms and sizes to fulfil their particular function.

[0004] Some AUVs are adapted for surveying purposes, e.g. mapping and monitoring of subsea structures. AUVs for this purpose typically have an elongate cylindrical shape with a propeller at the rear end to propel the AUV through the water. Many commercially available AUVs with tail propellers can be classified as ‘cruising’ AUVs, or ‘hovering’ AUVs.

[0005] Cruising AUVs comprise a number of control surfaces, such as fins or rudders, provided along their surface for steering of the AUV. Cruising AUVs require continuous motion in the forward direction in order for their control surfaces to steer the AUV. Cruising AUVs are also generally bottom-heavy, so as to be ballasted in a preferred orientation under the influence of gravity and to counteract any undesired roll torque induced by the propeller. However, this may limit the pitch or diving angle attainable by the AUV. Each of these factors may limit the manoeuvrability of cruising AUVs.

[0006] Hovering AUVs, on the other hand, do not require constant forward motion to be able to steer, and can hold their position in water. Hovering AUVs employ a number of additional thrusters or propellers oriented in different directions and distributed along the body of the AUV, so as to stabilise and move the AUV in water. Sizes of hovering AUVs vary from small and light, for example with weights of a few kilograms, to very large and heavy, for example on the order of 2,000kg. However, hovering-type AUVs are typically larger than other designs of AUV for a given sensor payload. It can be impractical to use large hovering AUV systems in some circumstances due to the difficulties of transporting and manipulating the vehicle.

[0007] Reliable and accurate control of AUVs is particularly important for AUVs adapted for surveying purposes, since the AUV should be suitably positioned during subsea operations to gather data associated with surveying the seabed.WO 2015 / 161892 A1 discloses a method of navigation for an AUV, using laser interferometric techniques.

[0008] It is desired to provide an AUV with improved control when the AUV is cruising along a subsea surface during subsea operations, e.g. when surveying subsea surfaces.

[0009] Viewed from a first aspect of the present invention, there is provided a method of controlling an autonomous underwater vehicle, AUV, when the AUV is cruising along a subsea surface. The method comprises: monitoring a position of the AUV by directing a laser beam from a laser interferometry sensing device to the subsea surface; determining a visibility below the AUV; and controlling a cruising height of the AUV relative to the subsea surface based on the visibility.

[0010] Laser interferometric techniques are more accurate than conventional, acoustic techniques used to monitor the position of the AUV when navigating subsea. However, laser interferometric techniques for determining the position of the AUV are more constrained than acoustic techniques as to when they can be appropriately used. Due to the rapid attenuation of light underwater, laser interferometric techniques are reliable when the AUV is cruising close to the subsea surface against which the laser beam from the laser interferometry sensing device is directed.

[0011] This operational range for laser interferometric sensing devices can also be limited further due to varying visibility at the subsea surface, e.g. due to disturbed sediment or turbid conditions more likely to be experienced at a subsea boundary. As such there are tight constraints on the cruising height of the AUV relative to the subsea surface, when using laser interferometric techniques when monitoring the position of the AUV. This can result in a trade-off between the subsea surveying capability of the AUV (which will have a greater field of view the further the AUV is from the subsea surface it is surveying), and the accuracy of laser interferometric techniques used to monitor the position of the AUV when cruising along a subsea surface.

[0012] By determining a visibility below the AUV and controlling a cruising height of the AUV relative to the subsea surface based on the visibility, the AUV can be controlled in a manner which balances the need for accurate laser interferometric sensing against the need for more efficient subsea surveying by maintaining a distance acceptable for both operations, when cruising along the subsea surface. This dynamic form of control can improve the accuracy of the positional monitoringcapabilities of the laser interferometric by accounting for varying visibility during subsea operations for the AUV, whilst also enabling the AUV to cruise at higher distances above the subsea surface when the visibility below the AUV allows for it.

[0013] The cruising height of the AUV is generally controlled such that monitoring of the position of the AUV relative to the seabed remains viable using the laser interferometry sensing device. Thus, the method may comprise determining a new cruising height based on a change in visibility, such that a position of the AUV relative to the seabed can be monitored using the laser interferometry sensing device.

[0014] The cruising height may be continuously varied based on the visibility. Alternatively, the cruising height may be controlled discontinuously based on the visibility.

[0015] If the visibility is below a first predetermined threshold, the cruising height may be decreased; and if the visibility is above a second predetermined threshold that is greater than the first predetermined threshold, the cruising height may be increased.

[0016] In some embodiments, where the visibility is below the first threshold, the cruising height is controlled to be less than or equal to 5 metres. Where the visibility is above the first threshold and below the second threshold, the cruising height may be controlled to be greater than 5 metres and less than or equal to 10 metres. Where the visibility is above the second threshold, the cruising height may be controlled to be greater than 10 metres.

[0017] The cruising height may be controlled to remain above a minimum cruising height. This may ensure the AUV does not collide with the subsea surface. The minimum cruising height may be at least 1 metre, at least 1.5 metres, or at least 2 metres. The minimum cruising height may be less than 3 metres, or less than 2.5 metres, or less than 2 metres.

[0018] A maximum cruising height of the AUV relative to the subsea surface may be less than 20 metres above the subsea surface. In other embodiments, the maximum cruising height may be less than 15 metres above the subsea surface.

[0019] Monitoring a position of the AUV may further comprise: receiving reflected light from the subsea surface; determining a velocity of the AUV relative to the subsea surface based on an interference pattern between the reflected light and a reference beam; and determining a change in position of the AUV relative to the subsea surface based on the velocity.The laser interferometry sensing device may be used to obtain a measurement relating to the visibility below the AUV.

[0020] The laser interferometry sensing device may be arranged to determine the measurement relating to the visibility below the AUV based on an intensity of reflected light received from the subsea surface.

[0021] In other embodiments, the AUV may comprise a visibility sensing device used to obtain a measurement relating to the visibility below the AUV. This visibility sensing device can be separate to, i.e. independent of, the laser interferometry sensing device.

[0022] The visibility sensing device may be configured to determine the visibility using light scattering.

[0023] The visibility sensing device may comprise a plurality of LEDs arranged to emit light into the water, and a photodiode arranged to receive scattered light. The visibility sensing device may determine a measurement relating to the visibility below the AUV based on the amount of scattered light.

[0024] The visibility sensing device may emit visible light and / or infrared light. The visibility sensing device may be configured to determine the visibility using laser scattering.

[0025] Alternatively, the visibility sensing device may be configured to determine the visibility using acoustic scattering.

[0026] The visibility sensing device may be configured to determine the visibility based on a measurement of suspended particulate concentration below the underwater vehicle. The visibility sensing device may be an acoustic doppler current profiler configured to determine the suspended particulate concentration below the underwater vehicle using acoustic backscattering.

[0027] The AUV may comprise a hull having a nose end and a tail end; a first laser interferometry sensing device located towards the nose end, and a second laser interferometry sensing device located towards the tail end. The method may comprise: receiving a first measurement relating to a velocity of the AUV relative to the subsea surface from the first laser interferometry sensing device; receiving a second measurement relating to a velocity of the AUV relative to the subsea surface from the second laser interferometry sensing device; determining a change of incline of the subsea surface relative to the AUV based on the first measurement and the second measurement; and based on the change of incline, controlling a pitch of the AUV.The first laser interferometry sensing device and / or the second laser interferometry sensing device can also be used as the laser interferometry sensing device monitoring a position of the AUV. Thus, one or more of the laser interferometry sensing devices of the AUV may have multiple functions when the AUV is cruising along a subsea surface.

[0028] Viewed from a second aspect of the present invention, there is provided a method of controlling an autonomous underwater vehicle, AUV, when the AUV is cruising along a subsea surface. The AUV comprises: a hull having a nose end and a tail end, a first laser interferometry sensing device located towards the nose end, and a second laser interferometry sensing device located towards the tail end. The method comprises: receiving a first measurement relating to a velocity of the AUV relative to the subsea surface from the first laser interferometry sensing device; receiving a second measurement relating to a velocity of the AUV relative to the subsea surface from the second laser interferometry sensing device; determining a change of incline of the subsea surface relative to the AUV based on the first measurement and the second measurement; and based on the change of incline, controlling a pitch of the AUV.

[0029] When navigating subsea, and particularly when cruising along or traversing over a subsea surface at a close distance to the subsea surface, the AUV may be at risk of colliding with projections or inclined surfaces of the subsea surface.

[0030] By controlling a pitch of the AUV based on a change of incline of the subsea surface relative to the AUV determined based on the first measurement of relating to velocity obtained by the first laser interferometry sensing device and the second measurement of relating to velocity obtained by the second laser interferometry sensing device, possible collisions with oncoming changes of incline and / or projections from the seabed can be anticipated, and avoided.

[0031] It will be appreciated that, through the method according to the second aspect, the pitch of the AUV can be controlled such that the AUV does not collide with the subsea surface.

[0032] A beam emitted from the first laser interferometry sensing device may be emitted in a direction extending out in front of the nose end of the AUV.

[0033] By emitting its beam out in front of the nose end of the AUV, sudden changes in incline of the subsea surface may be better anticipated. Additionally, obstacles projecting directly upwards from the subsea surface that are not anticipated by any prior change in incline may be better detected.The method may comprise: obtaining a first estimated velocity based on the first measurement and a second estimated velocity based on the second measurement; and determining a change of incline of the subsea surface relative to the AUV based on a difference between the first estimated velocity and the second expected velocity.

[0034] If the difference between the first estimated velocity and the second expected velocity is above a predetermined threshold, the method may comprise controlling a pitch of the AUV such that the AUV is ascending from the subsea surface in a substantially vertical orientation. This manoeuvre may result in the AUV sharply turning to avoid in possible collisions, responsive to the detected change in incline indicating the onset of an obstacle or otherwise dramatic projection from the subsea surface.

[0035] The method may comprise, using the first laser interferometry sensing device: emitting a first laser beam from the first laser interferometry sensing device to the subsea surface; receiving reflected light from the subsea surface; and determining the first measurement based on an interference pattern between the reflected light and a first reference beam.

[0036] The method may comprise, using the second laser interferometry sensing device: emitting a second laser beam from the second laser interferometry sensing device to the subsea surface; receiving reflected light from the subsea surface; and determining the second measurement based on an interference pattern between the reflected light and a second reference beam.

[0037] The pitch of the AUV may be controlled such that the AUV remains substantially parallel to the subsea surface.

[0038] The method may comprise monitoring a position of the AUV by directing a laser beam from a laser interferometry sensing device to the subsea surface. The laser interferometry sensing device may be the first laser interferometry sensing device or the second laser interferometry sensing device.

[0039] Monitoring a position of the AUV may further comprise, using a / one of the laser interferometry sensing devices: receiving reflected light from the subsea surface; determining a velocity of the AUV relative to the subsea surface based on an interference pattern between the reflected light and a reference beam; and determining a change in position of the AUV relative to the subsea surface based on the velocity.The change in position of the AUV relative to the subsea surface may be based on the first measurement and / or the second measurement.

[0040] The method may comprise: determining a visibility below the AUV; and controlling a cruising height of the AUV relative to the subsea surface based on the visibility. This step is particularly performed when monitoring a position of the AUV by directing a laser beam from a laser interferometry sensing device to the subsea surface.

[0041] The methods of the first aspect and the second aspect of the present invention are combinable with one another. Thus, the above-description of the method of the second aspect, including but not limited to all technical advantages and alternative embodiments, may be equally applicable to the method of the first aspect. Similarly, the above-description of the method of the first aspect, including but not limited to all technical advantages and alternative embodiments, may be equally applicable to the method of the second aspect.

[0042] As mentioned above, when navigating subsea, and particularly when cruising along or traversing over a subsea surface at a close distance to the subsea surface as would be the case when monitoring a position of the AUV using laser interferometric techniques, the AUV may be more likely to collide with projections or inclined surfaces of the subsea surface. Thus, using the methods of the first aspect and the second aspect in combination may result in the AUV being able to monitor the position of the AUV more accurately using laser interferometric techniques, with a lower risk of collision with the subsea surface above which the AUV is cruising.

[0043] The following features may be features of the method of the first aspect and / or of the method of the second aspect, of the present invention.

[0044] A longitudinal axis of the hull generally extends between the nose end and the tail end, i.e. in a forward-aft direction of the AUV. Generally, in use, the nose end defines a forward-facing end of the AUV during forward propulsion of the AUV. The AUV may roll about the longitudinal axis. That is, the roll axis of the AUV may and the longitudinal axis of the AUV are generally identical.

[0045] The AUV may also comprise a port side and a starboard side. The hull may comprise a transverse axis extending between the port side and the starboard side, i.e. in a left-right direction of the AUV. The AUV may pitch about the transverse axis. That is, the pitch axis of the AUV and the transverse axis of the AUV are generally identical.The AUV may also comprise a top side and a bottom side. The hull may comprise a vertical axis extending between the top side and the bottom side, i.e. in an up-down direction of the AUV. The AUV may yaw about the vertical axis. That is, the yaw axis of the AUV and the vertical axis are generally identical.

[0046] The AUV may comprise at least two or four laser interferometry sensing devices.

[0047] The AUV may comprise a / the first laser interferometry sensing device located towards the nose end, and a / the second laser interferometry sensing device located towards the tail end.

[0048] The position of the AUV may be monitored based on an average of the measurements relating to velocity obtained by the first and the second laser interferometry sensing devices.

[0049] By monitoring the position of the AUV based on the average of the measurements relating to velocity obtained by the first and the second laser interferometry sensing devices, unwanted components introduced due to a tilt of the AUV relative to the subsea surface may be eliminated.

[0050] The AUV may comprise a third laser interferometry sensing device located towards the port side, and a fourth laser interferometry sensing device located towards the starboard side.

[0051] The position of the AUV may be monitored based on an average of the measurements relating to velocity obtained by the third and the fourth laser interferometry sensing devices.

[0052] By monitoring the position of the AUV based on the average of the measurements relating to velocity obtained by the third and the fourth laser interferometry sensing devices, unwanted components introduced due to a roll of the AUV relative to the subsea surface may be eliminated.

[0053] The AUV may comprise a propulsion mechanism configured to propel the AUV, wherein the propulsion mechanism comprises a pair of counter-rotating propellers. The rotational speed of each propeller is independently controllable. The method may comprise controlling a roll of the AUV by independently controlling the rotational speed of each propeller of the pair of counter- rotating propellers.

[0054] By using a pair of counter-rotating propellers, roll torques generated by each propeller may cancel out when the propellers are operated at the same rotational speed. Accordingly, the use of a pair of counter-rotating propellers can improve the roll control of the AUV by mitigating against the generation of undesirable roll torqueduring propulsion of the AUV. The use of a pair of counter-rotating propellers may also negate the need for additional control surfaces associated with mitigating undesirable roll torques induced by a propeller. Since no control surfaces are required to counteract roll torques introduced by the propellers, the AUV need not experience drag forces associated with the presence of said control surfaces.

[0055] Further, where it is desirable to adjust a rotational orientation of the AUV about its roll axis, the rotational speeds can be asymmetrically controlled since their respective speeds are independently controllable, providing further roll control for the AUV.

[0056] The AUV may comprise a steering mechanism configured to rotate the propulsion mechanism in at least one of a pitch and a yaw direction, relative to the hull. The method may comprise adjusting a position of the AUV using the steering mechanism.

[0057] Using the steering mechanism to provide pitch and yaw control to the propulsion mechanism may also provide greater and more responsive control for the AUV. For example, actuating the steering mechanism such that the propulsion mechanism is greatly offset to the longitudinal axis of the hull provides a tighter turning circle for the AUV in water, thereby improving its manoeuvrability.

[0058] Since the AUV does not require control surfaces to induce steering due to the presence of the steering mechanism, the AUV need not experience drag forces associated with the presence of said control surfaces. The AUV may therefore have an improved hydrodynamic profile, reducing drag it may experience and further improving the manoeuvrability of the AUV as a result.

[0059] The AUV may comprise an internal ballast located in the hull, and an internal ballast positioning mechanism configured to controllably move the internal ballast relative to the hull. The method may comprise controlling a / the roll of the AUV using the internal ballast positioning mechanism.

[0060] The internal ballast positioning mechanism may (also) provide roll control for the AUV, such that the AUV can also be oriented in any desired orientation about its roll axis. The weight of the internal ballast will align the AUV with respect to the direction of gravity, and as such moving the internal ballast relative to the hull can result in the hull effectively moving about the roll axis.

[0061] In some embodiments, the combined and selective use of both the pair of counter-rotating propellers and the internal ballast positioning mechanism may provide improved roll control for the AUV across the breadth of its subseaoperations. For example, the combined use of the independently controllable propellers and the internal ballast positioning mechanism can cause a swifter and more reactive roll control if required, providing the AUV with greater manoeuvrability. Further, in situations where one form of roll control is less suited for use the other may be able to compensate such that the AUV maintains full roll control capabilities at all times during subsea operations. For example, roll control through use of the pair of counter-rotating propellers may be more suited in situations where the AUV is ascending and / or descending, whilst roll control through use of the internal ballast positioning mechanism may be more suited in situations where the AUV is travelling in a generally horizontal, or not far-from horizontal, orientation.

[0062] Accordingly, the provision of the internal ballast positioning mechanism and the pair of counter-rotating propellers in combination may provide improved roll control for the AUV.

[0063] Through the use of any or all of the propulsion mechanism, the steering mechanism, and the internal ballast positioning mechanism, the AUV may be suitably controlled to cruise along the subsea surface. This is because the AUV according to the present invention may have improved manoeuvring and positioning capabilities, through the use of the aforementioned mechanisms.

[0064] Particularly, reliable and effective roll control through the use of the propulsion mechanism and / or the internal ballast positioning mechanism, and / or reliable and effective pitch and / or yaw control through the use of the steering mechanism, may enable the AUV to remain substantially parallel to the subsea surface. As a result, the introduction of unwanted components into the measurements obtained by the laser interferometry sensing devices of the AUV may be avoided by minimising tilt and roll of the AUV relative to the subsea surface.

[0065] Further, reliable and effective roll control through the use of the propulsion mechanism and / or the internal ballast positioning mechanism, and / or reliable and effective pitch and / or yaw control through the use of the steering mechanism, may enable the AUV to closely follow the contours of the subsea surface, such that the AUV remains within a suitable distance of the subsea surface for accurate monitoring of the position of the AUV using laser interferometric techniques.

[0066] The subsea surface may be a seabed.

[0067] The method of the first aspect and / or the second aspect may comprise the use of the AUV of the third aspect and / or the fourth aspect, as will now bedescribed below. Thus, the methods of the first and the second aspects may have one or more or all of the features (including optional features) of the third and fourth aspects now described.

[0068] The ALIVs according to the third and the fourth aspects now described below also each comprise a controller. The controller may comprise a processor and a memory, and may be in wired or wireless communication with one or more components of the AUV. The controller, such as via the memory thereof, may store computer-readable instructions which, when executed by the processor, causes the AUV to form one or more operations. These operations may include one or more or all of the steps of the methods of the first and / or the second aspects. Thus, the AUVs according to the third and the fourth aspects, as will now be described below, may have one or more or all of the features (including optional features) of the methods of the first and second aspects described above.

[0069] Viewed from a third aspect of the present invention, there is provided an autonomous underwater vehicle, AUV, for performing subsea operations. The AUV comprises: at least one laser interferometry sensing device configured to obtain a measurement relating to a position of the AUV using a laser beam directed at the subsea surface; a visibility sensing device configured to obtain a measurement relating to visibility below the AUV; and a controller in communication with the at least one laser interferometry sensing device and the visibility sensing device.

[0070] When the AUV is cruising along the subsea surface, the controller is configured to: monitor a position of the AUV using the laser interferometry sensing device; determine a visibility below the AUV using the visibility sensing device; and control a cruising height of the AUV relative to the subsea surface based on the visibility.

[0071] Laser interferometric techniques are more accurate than conventional, acoustic techniques used to monitor the position of the AUV when navigating subsea. However, laser interferometric techniques for determining the position of the AUV are more constrained than acoustic techniques as to when they can be appropriately used. Due to the rapid attenuation of light underwater, laser interferometric techniques are reliable when the AUV is cruising close to the subsea surface against which the laser beam from the laser interferometry sensing device is directed.

[0072] This operational range for laser interferometric sensing devices can also be limited further due to varying visibility at the subsea surface, e.g. due to disturbed sediment or turbid conditions more likely to be experienced at a subsea boundary.As such there are tight constraints on the cruising height of the AUV relative to the subsea surface, when using laser interferometric techniques when monitoring the position of the AUV. This can result in a trade-off between the subsea surveying capability of the AUV (which will have a greater field of view the further the AUV is from the subsea surface it is surveying), and the accuracy of laser interferometric techniques used to monitor the position of the AUV when cruising along a subsea surface.

[0073] By providing a controller configured to determine a visibility below the AUV and configured to control a cruising height of the AUV relative to the subsea surface based on the visibility, the AUV can be controlled in a manner which balances the need for accurate laser interferometric sensing against the need for more efficient subsea surveying by maintaining a distance acceptable for both operations, when cruising along the subsea surface. This dynamic form of control can improve the accuracy of the positional monitoring capabilities of the laser interferometric by accounting for varying visibility during subsea operations for the AUV, whilst also enabling the AUV to cruise at higher distances above the subsea surface when the visibility below the AUV allows for it.

[0074] The controller may be configured to continuously vary the cruising height based on the visibility.

[0075] Alternatively, the controller may be configured to control the cruising height discontinuously based on the visibility.

[0076] The controller may be configured to: decrease the cruising height if the visibility is below a first predetermined threshold; and increase the cruising height if the visibility is above a second predetermined threshold that is greater than the first predetermined threshold.

[0077] In some embodiments, the controller is configured to: control the cruising height to be less than or equal to 5 metres where the visibility is below the first threshold; control the cruising height to be greater than 5 metres and less than or equal to 10 metres where the visibility is above the first threshold and below the second threshold; and control the cruising height to be greater than 10 metres where the visibility is above the second threshold.

[0078] The controller may be configured to control the cruising height to remain above a minimum cruising height. This may ensure the AUV does not collide with the subsea surface. The minimum cruising height may be at least 1 metre, at least1.5 metres, or at least 2 metres. The minimum cruising height may be less than 3 metres, or less than 2.5 metres, or less than 2 metres.

[0079] The controller may be configured to control the cruising height to remain below a maximum cruising height. The maximum cruising height may be less than 20 metres above the subsea surface. In other embodiments, the maximum cruising height may be less than 15 metres above the subsea surface.

[0080] The laser interferometry sensing device may be configured to: direct a leaser beam at the subsea surface; receive reflected light from the subsea surface; and determine a measurement relating to a velocity of the AUV relative to the subsea surface. The measurement relating to a velocity of the AUV is a measurement relating to a position of the AUV, and can be used by the laser interferometry sensing device and / or the controller to monitor a position of the AUV relative to the seabed.

[0081] The laser interferometry sensing device may be used as the visibility sensing device.

[0082] The laser interferometry sensing device may be arranged to determine the measurement relating to the visibility below the AUV based on an intensity of reflected light received from the subsea surface.

[0083] In other embodiments, the visibility sensing device can be separate to, i.e. independent of, the laser interferometry sensing device.

[0084] The visibility sensing device may be configured to determine the visibility using light scattering.

[0085] The visibility sensing device may comprise a plurality of LEDs arranged to emit light into the water, and a photodiode arranged to receive scattered light. The visibility sensing device may determine a measurement relating to the visibility below the AUV based on the amount of scattered light.

[0086] The visibility sensing device may emit visible light and / or infrared light. The visibility sensing device may be configured to determine the visibility using laser scattering.

[0087] Alternatively, the visibility sensing device may be configured to determine the visibility using acoustic scattering.

[0088] The visibility sensing device may be configured to determine the visibility based on a measurement of suspended particulate concentration below the underwater vehicle. The visibility sensing device may be an acoustic dopplercurrent profiler configured to determine the suspended particulate concentration below the underwater vehicle using acoustic backscattering.

[0089] The AUV may comprise a hull having a nose end and a tail end; a first laser interferometry sensing device located towards the nose end, and a second laser interferometry sensing device located towards the tail end; wherein the controller is in communication with the first laser interferometry sensing device and the second laser interferometry sensing device. When the AUV is cruising along the subsea surface, the controller is configured to: receive a first measurement relating to a velocity of the AUV relative to the subsea surface from the first laser interferometry sensing device; receive a second measurement relating to a velocity of the AUV relative to the subsea surface from the second laser interferometry sensing device; determine a change of incline of the subsea surface relative to the AUV based on the first measurement and the second measurement; and based on the change of incline, control a pitch of the AUV.

[0090] The first laser interferometry sensing device and / or the second laser interferometry sensing device can also be used as the laser interferometry sensing device configured to obtain a measurement relating to a position of the AUV using a laser beam directed at the subsea surface. Thus, one or more of the laser interferometry sensing devices of the AUV may have multiple functions when the AUV is cruising along a subsea surface.

[0091] Viewed from a fourth aspect of the present invention, there is provided an autonomous underwater vehicle, AUV, for performing subsea operations. The AUV comprises: a hull having a nose end and a tail end; a first laser interferometry sensing device located towards the nose end; a second laser interferometry sensing device located towards the tail end; and a controller in communication with the first laser interferometry sensing device and the second laser interferometry sensing device. When the AUV is cruising along the subsea surface, the controller is configured to: receive a first measurement relating to a velocity of the AUV relative to the subsea surface from the first laser interferometry sensing device; receive a second measurement relating to a velocity of the AUV relative to the subsea surface from the second laser interferometry sensing device; determine a change of incline of the subsea surface relative to the AUV based on the first measurement and the second measurement; and based on the change of incline, control a pitch of the AUV.When navigating subsea, and particularly when cruising along or traversing over a subsea surface at a close distance to the subsea surface, the AUV may be at risk of colliding with projections or inclined surfaces of the subsea surface.

[0092] By providing a controller configured to control a pitch of the AUV based on a change of incline of the subsea surface relative to the AUV determined based on the first measurement of relating to velocity obtained by the first laser interferometry sensing device and the second measurement of relating to velocity obtained by the second laser interferometry sensing device, possible collisions with oncoming changes of incline and / or projections from the seabed can be anticipated, and avoided.

[0093] The first laser interferometry sensing device may be configured to emit a beam in a direction extending out in front of the nose end of the AUV.

[0094] By emitting its beam out in front of the nose end of the AUV, sudden changes in incline of the subsea surface may be better anticipated. Additionally, obstacles projecting directly upwards from the subsea surface that are not anticipated by any prior change in incline may be better detected.

[0095] The controller may be configured to: obtain a first estimated velocity based on the first measurement and a second estimated velocity based on the second measurement; and determine a change of incline of the subsea surface relative to the AUV based on a difference between the first estimated velocity and the second expected velocity.

[0096] If the difference between the first estimated velocity and the second expected velocity is above a predetermined threshold, the controller may be configured to control a pitch of the AUV such that the AUV is ascending from the subsea surface in a substantially vertical orientation. This manoeuvre may result in the AUV sharply turning to avoid in possible collisions, responsive to the detected change in incline indicating the onset of an obstacle or otherwise dramatic projection from the subsea surface.

[0097] The first laser interferometry sensing device may be configured to: emit a first laser beam from the first laser interferometry sensing device to the subsea surface; receive reflected light from the subsea surface; and determine the first measurement based on an interference pattern between the reflected light and a first reference beam.

[0098] The second laser interferometry sensing device may be configured to: emit a second laser beam from the second laser interferometry sensing device to thesubsea surface; receive reflected light from the subsea surface; and determine the second measurement based on an interference pattern between the reflected light and a second reference beam.

[0099] The controller may be configured to control the pitch of the AUV such that the AUV remains substantially parallel to the subsea surface.

[0100] The AUV may comprise at least one laser interferometry sensing device configured to obtain a measurement relating to a position of the AUV using a laser beam directed at the subsea surface. When the AUV is cruising along the subsea surface, the controller is configured to: monitor a position of the AUV using the at least one laser interferometry sensing device. The at least one laser interferometry sensing device may be the first laser interferometry sensing device or the second laser interferometry sensing device.

[0101] The at least one laser interferometry sensing device may be configured to: direct a laser beam at the subsea surface; receive reflected light from the subsea surface; and determine a measurement relating to a velocity of the AUV relative to the subsea surface based on an interference pattern between the reflected light and a reference beam.

[0102] The AUV may comprise a visibility sensing device configured to obtain a measurement relating to visibility below the AUV; wherein the controller is in communication with the at least one laser interferometry sensing device and the visibility sensing device. When the AUV is cruising along the subsea surface, the controller is configured to: determine a visibility below the AUV using the visibility sensing device; and control a cruising height of the AUV relative to the subsea surface based on the visibility. The controller may be particularly configured to perform these operations when monitoring a position of the AUV using the at least one laser interferometry sensing device.

[0103] The change in position of the AUV relative to the subsea surface may be based on the first measurement and / or the second measurement.

[0104] One or more or all features of the AUV of the third aspect and one or more or all features of the AUV of the fourth aspect may be combinable with one another. Thus, the above description of the AUV of the third aspect, including but not limited to all technical advantages and alternative embodiments, may be equally applicable to the AUV of the fourth aspect. Similarly, the above description of the AUV of the fourth aspect, including but not limited to all technical advantages and alternative embodiments, may be equally applicable to the AUV of the third aspect.As mentioned above, when navigating subsea, and particularly when cruising along or traversing over a subsea surface at a close distance to the subsea surface as would be the case when monitoring a position of the AUV using laser interferometric techniques, the AUV may be more likely to collide with projections or inclined surfaces of the subsea surface. Thus, using the features of the AUV of the third aspect and the features of the AUV of the fourth aspect in combination may result in the AUV being able to monitor its position more accurately using laser interferometric techniques, with a lower risk of collision with the subsea surface above which the AUV is cruising.

[0105] The following features may be features of the AUV of the first aspect and / or the AUV of the second aspect, of the present invention.

[0106] A longitudinal axis of the hull generally extends between the nose end and the tail end, i.e. in a forward-aft direction of the AUV. Generally, in use, the nose end defines a forward-facing end of the AUV during forward propulsion of the AUV. The AUV may roll about the longitudinal axis. That is, the roll axis of the AUV may and the longitudinal axis of the AUV are generally identical.

[0107] The AUV may also comprise a port side and a starboard side. The hull may comprise a transverse axis extending between the port side and the starboard side, i.e. in a left-right direction of the AUV. The AUV may pitch about the transverse axis. That is, the pitch axis of the AUV and the transverse axis of the AUV are generally identical.

[0108] The AUV may also comprise a top side and a bottom side. The hull may comprise a vertical axis extending between the top side and the bottom side, i.e. in an up-down direction of the AUV. The AUV may yaw about the vertical axis. That is, the yaw axis of the AUV and the vertical axis are generally identical.

[0109] The AUV may comprise at least two or four laser interferometry sensing devices.

[0110] The AUV may comprise a / the first laser interferometry sensing device located towards the nose end, and a / the second laser interferometry sensing device located towards the tail end. The controller may be in communication with the first laser interferometry sensing device and the second laser interferometry sensing device.

[0111] The controller may be configured to monitor the position of the AUV based on an average of the measurements relating to velocity obtained by the first and the second laser interferometry sensing devices.By monitoring the position of the AUV based on the average of the measurements relating to velocity obtained by the first and the second laser interferometry sensing devices, unwanted components introduced due to a tilt of the AUV relative to the subsea surface may be eliminated.

[0112] The AUV may comprise a third laser interferometry sensing device located towards the port side, and a fourth laser interferometry sensing device located towards the starboard side. The controller may be in communication with the third laser interferometry sensing device and the fourth laser interferometry sensing device.

[0113] The controller may be configured to monitor the position of the AUV based on an average of the measurements relating to velocity obtained by the third and the fourth laser interferometry sensing devices.

[0114] By monitoring the position of the AUV based on the average of the measurements relating to velocity obtained by the third and the fourth laser interferometry sensing devices, unwanted components introduced due to a roll of the AUV relative to the subsea surface may be eliminated.

[0115] The AUV may comprise a propulsion mechanism configured to propel the AUV, wherein the propulsion mechanism comprises a pair of counter-rotating propellers. The rotational speed of each propeller is independently controllable.

[0116] The controller may be in communication with the propulsion mechanism. The controller may be configured to control a roll of the AUV by independently controlling the rotational speed of each propeller of the pair of counter-rotating propellers.

[0117] By using a pair of counter-rotating propellers, roll torques generated by each propeller may cancel out when the propellers are operated at the same rotational speed. Accordingly, the use of a pair of counter-rotating propellers can improve the roll control of the AUV by mitigating against the generation of undesirable roll torque during propulsion of the AUV. The use of a pair of counter-rotating propellers may also negate the need for additional control surfaces associated with mitigating undesirable roll torques induced by a propeller. Since no control surfaces are required to counteract roll torques introduced by the propellers, the AUV need not experience drag forces associated with the presence of said control surfaces. Further, where it is desirable to adjust a rotational orientation of the AUV about its roll axis, the rotational speeds can be asymmetrically controlled since theirrespective speeds are independently controllable, providing further roll control for the AUV.

[0118] The propulsion mechanism may comprise two or more pairs of counterrotating propellers, the rotational speed of each propeller being independently controllable.

[0119] The propulsion mechanism may be located at the tail end of the hull.

[0120] The AUV may comprise a steering mechanism configured to rotate the propulsion mechanism in at least one of a pitch and a yaw direction, relative to the hull.

[0121] The controller may be in communication with the steering mechanism. The controller may be configured to adjust a position of the AUV using the steering mechanism.

[0122] The steering mechanism may connect the propulsion mechanism to the hull. The steering mechanism may comprise an active joint configured to rotate the propulsion mechanism. The active joint may be any one of: a cardan joint; a motorised ball joint; a pneumatic actuator; an electric actuator; or a hydraulic actuator.

[0123] Using the steering mechanism to provide pitch and yaw control to the propulsion mechanism may also provide greater and more responsive control for the AUV. For example, actuating the steering mechanism such that the propulsion mechanism is greatly offset to the longitudinal axis of the hull provides a tighter turning circle for the AUV in water, thereby improving its manoeuvrability.

[0124] Since the AUV does not require control surfaces to induce steering due to the presence of the steering mechanism, the AUV need not experience drag forces associated with the presence of said control surfaces. The AUV may therefore have an improved hydrodynamic profile, reducing drag it may experience and further improving the manoeuvrability of the AUV as a result.

[0125] The AUV may comprise an internal ballast located in the hull, and an internal ballast positioning mechanism configured to controllably move the internal ballast relative to the hull.

[0126] The controller may be in communication with the internal ballast positioning mechanism. The controller may be configured to control a / the roll of the AUV using the internal ballast positioning mechanism.

[0127] The internal ballast positioning mechanism may (also) provide roll control for the AUV, such that the AUV can also be oriented in any desired orientation aboutits roll axis. The weight of the internal ballast will align the AUV with respect to the direction of gravity, and as such moving the internal ballast relative to the hull can result in the hull effectively moving about the roll axis.

[0128] The internal ballast positioning mechanism may be configured to rotate the internal ballast relative to the hull, e.g. about the longitudinal axis or roll axis of the hull. The internal ballast positioning mechanism may comprise a motor configured to rotate the internal ballast relative to the hull.

[0129] The internal ballast and the internal ballast positioning mechanism may be located towards the tail end of the hull. The internal ballast and the internal ballast positioning mechanism may be located adjacent to the propulsion mechanism.

[0130] The controller may be configured to control the roll of the AUV using at least one of the pair of counter- rotating propellers and the internal ballast positioning mechanism, so as to maintain the AUV substantially parallel to the subsea surface when using the laser interferometry sensing device(s).

[0131] The controller may be configured to control the pitch of the AUV using at least one of the pair of counter-rotating propellers and the internal ballast positioning mechanism, so as to maintain the AUV substantially parallel to the subsea surface when using the laser interferometry sensing device(s).

[0132] The combined and selective use of both the pair of counter- rotating propellers and the internal ballast positioning mechanism may provide improved roll control for the AUV across the breadth of its subsea operations. For example, the combined use of the independently controllable propellers and the internal ballast positioning mechanism can cause a swifter and more reactive roll control if required, providing the AUV with greater manoeuvrability. Further, in situations where one form of roll control is less suited for use the other may be able to compensate such that the AUV maintains full roll control capabilities at all times during subsea operations. For example, roll control through use of the pair of counter-rotating propellers may be more suited in situations where the AUV is ascending and / or descending, whilst roll control through use of the internal ballast positioning mechanism may be more suited in situations where the AUV is travelling in a generally horizontal, or notfar-from horizontal, orientation.

[0133] Accordingly, the provision of the internal ballast positioning mechanism and the pair of counter-rotating propellers in combination may provide improved roll control for the AUV.Through the use of any or all of the propulsion mechanism, the steering mechanism, and the internal ballast positioning mechanism, the AUV may be suitably controlled to cruise along the subsea surface. This is because the AUV according to the present invention may have improved manoeuvring and positioning capabilities, through the use of the aforementioned mechanisms.

[0134] Particularly, reliable and effective roll control through the use of the propulsion mechanism and / or the internal ballast positioning mechanism, and / or reliable and effective pitch and / or yaw control through the use of the steering mechanism, may enable the AUV to remain substantially parallel to the subsea surface. As a result, the introduction of unwanted components into the measurements obtained by the laser interferometry sensing devices of the AUV may be avoided by minimising tilt and roll of the AUV relative to the subsea surface.

[0135] Further, reliable and effective roll control through the use of the propulsion mechanism and / or the internal ballast positioning mechanism, and / or reliable and effective pitch and / or yaw control through the use of the steering mechanism, may enable the AUV to closely follow the contours of the subsea surface, such that the AUV remains within a suitable distance of the subsea surface for accurate monitoring of the position of the AUV using laser interferometric techniques.

[0136] The AUV may comprise a pair of subsea surveying sensors located on opposing sides of the hull and configured to scan underwater surfaces or structures.

[0137] The pair of subsea surveying sensors may be diametrically opposed.

[0138] Alternatively, the pair of subsea surveying sensors may be offset by less than 180 degrees. The pair of subsea surveying sensors may be arranged symmetrically about the hull, i.e. such that each subsea surveying sensor of the pair is equally offset from a vertical axis of the AUV.

[0139] The pair of subsea surveying sensors may be a pair of sidescan sonar transducers located on opposing sides of the hull and each configured to emit a sonar beam. Each sidescan sonar transducer may extend in a forward-aft direction of the AUV. The sonar beam may be a conical or fan-shaped beam.

[0140] The pair of subsea surveying sensors may be a pair of multibeam echosounders or a pair of cameras. The AUV may comprise multiple pairs of subsea surveying sensors.

[0141] The controller may be in communication with the pair of subsea surveying sensors, the controller being configured to control the roll of the AUV, using at leastone of the pair of counter- rotating propellers and the internal ballast positioning mechanism, so as to scan an underwater surface or structure at least partially higher or lower than the AUV using the pair of subsea surveying sensors.

[0142] The AUV may comprise an acoustic sensor configured to monitor the velocity of the AUV, such as a Doppler Velocity Logger (DVL). The AUV may comprise an inertial navigation system (INS) configured to monitor the acceleration and / or the angular velocity of the AUV.

[0143] The acoustic sensor and / or the INS may be in communication with the controller. The controller may be configured to monitor a position of the AUV relative to the seabed using the acoustic sensor and / or the INS.

[0144] Monitoring of the position of the AUV via the INS and / or the acoustic sensor can be used to supplement monitoring of the position of the AUV via the at least one laser interferometry sensing device. Particularly, the position of the AUV may be monitored via the INS and / or the acoustic sensor when the AUV is above the maximum cruising height.

[0145] The subsea surface may be a seabed.

[0146] Certain preferred embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings in which:

[0147] Figure 1 shows an autonomous underwater vehicle, AUV, from a side view; Figure 2 shows the AUV of figure 1 in a perspective view;

[0148] Figure 3 shows a modified form of the AUV shown in figure 1;

[0149] Figure 4 shows the AUV of figure 3 in a perspective view;

[0150] Figures 5 and 6 show the AUV at different positions during subsea operations;

[0151] Figure 7 illustrates an AUV navigating during subsea operations;

[0152] Figure 8 shows an AUV cruising along a subsea surface; and

[0153] Figure 9 shows an AUV approaching an incline when cruising along a subsea surface.

[0154] Figure 1 shows the autonomous underwater vehicle, AUV, 100 from a side view, and figure 2 shows the same AUV 100 from a perspective view, in closer detail.

[0155] The AUV 100 comprises a hull 101 formed of multiple sections and modules, extending from a nose end of the AUV 100 to a tail end of the AUV 100. Generally, in use, the nose end defines a forward-facing end of the AUV 100 duringforward propulsion of the AUV 100. A longitudinal axis of the AUV 100 extends from the nose end to the tail end, i.e. in a forward-aft direction of the AUV 100.

[0156] The hull 101 is a rigid hull 101 insofar as it maintains a stiff and inflexible housing for the various components located therein. However, the rigid hull 101 can be provided with one or more articulatable, flexible sections as desired for performing various subsea operations. The hull 101 is generally cylindrical.

[0157] The hull 101 comprises a nose 102, a navigation section 103, a battery module 104 and a propulsion section 105. The nose 102 is located at the nose end of the AUV 100 and can house a payload. The navigation section 103 includes various controllers and sensors for controlling the operations and navigation of the AUV 100. The battery module 104 is configured to provide a source of electrical power for the AUV 100 which, in the present embodiment, is a 1.7 kWh battery, (batteries of differing capacity are contemplated in other embodiments). The propulsion module 105 is configured to generate a motive force to propel the AUV 100 subsea, and to control the positioning of the AUV 100.

[0158] The navigation section 103 comprises a sidescan sonar transducer 131 on each side of the hull 101 of the AUV 100. The sidescan sonar transducers 131 are for scanning structures or landscapes during subsea operations. Each sidescan sonar transducer 131 extends in a forward-aft direction of the AUV 100 and is configured to emit a conical or fan-shaped beam in a direction perpendicular to the forward-aft direction of the AUV 100. The sidescan sonar transducers 131 are diametrically opposed about the hull 101 in the present embodiment, although in other embodiments they can be positioned at different circumferential positions as desired. Other subsea surveying sensors, such as multibeam echosounders (MBESs) and cameras, may also be fitted.

[0159] The navigation section 103 also comprises a communications module 132 comprising a WiFi transceiver for communications with any nearby vessels. A strobe light for signalling is also provided as part of the communications module 132.

[0160] Additionally, the navigation section 103 comprises equipment configured to determine a position of the AUV 100. For example, the navigation section 103 may comprise a GPS sensor for monitoring a position of the AUV 100, which may form part of the communications module 132. Additionally, or alternatively, the navigation section 103 comprises a Doppler Velocity Log, DVL, 133 for estimating a velocity of the AUV 100, and hence a position of the AUV 100 during subseaoperations, relative to the seabed. The navigation section 103 may also comprise a series of transponders 134 forming a high precision acoustic positioning system for additionally monitoring the position of the AUV 100 during subsea operations. The DVL 133 and the transponders 134 are located on an underside of the hull 101. A positioning transponder similar to or the same as 134 may also be fitted at the top side of the AUV 100.

[0161] The AUV 100 also includes, as part of the navigation section 103 or otherwise, a laser interferometry sensing device 136 for determining a position of the AUV 100 during subsea operations. The laser interferometry sensing device 136 is located on the underside of the hull 101. The AUV 100 can comprise one or multiple laser interferometry sensing devices 136. Each laser interferometry sensing device 136 can operate according to the principles described in

[0162] WO 2015 / 161892 A1.

[0163] The laser interferometry sensing device 136 determines a position of the AUV 100 as follows. Firstly, the laser interferometry sensing device 136 emits a laser beam from underneath the AUV 100 to the seabed 2. The laser beam reflects off the seabed 2, and the laser interferometry sensing device 136 receives the reflected light. An interferometer of the laser interferometry sensing device 136 is used to combine the reflected light with a reference beam, from which a velocity of the underwater vehicle relative to the seabed 2 is calculated. The velocity of the AUV 100 relative to the seabed 2 is calculated using laser Doppler interferometric techniques.

[0164] Pitch (i.e. tilt) and roll of the AUV 100, relative to the subsea surface, can introduce unwanted components into measurement relating to a position of the AUV 100, obtained by the laser interferometry sensing device 136.

[0165] Figures 3 and 4 illustrate the AUV 100 shown in figures 1 and 2 respectively, but now provided with two laser interferometry sensing devices 136A, 136B. A first laser interferometry sensing device 136A is located towards the nose end of the AUV 100, whilst a second laser interferometry sensing device 136B is located towards the tail end of the AUV 100.

[0166] The two laser interferometry sensing devices 136A, 136B essentially form a pair of sensors which can be used to account for unwanted components introduced into the velocity measurements obtained by the laser interferometry sensing devices 136A, 136B. In particular, by providing the pair of sensors 136A, 136B about a pitch axis of the AUV 100 (i.e. by providing one towards the nose end andthe other towards the tail end of the AUV 100) unwanted components introduced due to the pitch of the AUV 100 relative to the seabed 2 can be eliminated or at least reduced. This may be done by taking the average of the measurements obtained by the laser interferometry sensing devices 136A, 136B.

[0167] Whilst not illustrated, the AUV 100 can also have third and fourth laser interferometry sensing devices disposed about the roll axis of the AUV 100, used to eliminate or at least reduce unwanted components introduced into the velocity measurements by a roll of the AUV 100 relative to the seabed 2. To this end, the third laser interferometry sensing device is located towards a port side of the AUV 100, and the fourth laser interferometry sensing device is located towards a starboard side of the AUV 100. Again, unwanted components may be eliminated by taking an average of the measurements obtained by each of the laser interferometry sensing devices being used to monitor the position of the AUV 100.

[0168] Thus, embodiments of the invention contemplate the AUV 100 having one, two, or four laser interferometry sensing devices used to monitor a position of the AUV 100 relative to the seabed 2.

[0169] The navigation section 103 also comprises an inertial measurement unit (IMU; not shown) located within the hull 101 of the AUV 100. The inertial measurement unit measures, and provides information on, the orientation of the AUV 100 to further aid in determining a position of the AUV 100 when performing subsea operations.

[0170] The navigation section 103 also comprises an interchangeable data storage module 135. The data storage module 135 is interchangeable in the field, and can therefore be changed rapidly when the AUV 100 is surfaced, which allows the AUV 100 to be rapidly redeployed, rather than recovering the AUV 100 and downloading data from the data storage module 135 by a wired connection or the like.

[0171] The AUV 100 comprises a controller comprising a processor and a memory. The controller is in communication, e.g. wired or wireless communication, with the payload located in or towards the nose 102, the various sensors and controllers of the navigation section 103, the battery of the battery section 104, and also the various components of the propulsion section 105. The controller is located in the navigation section 103 in the present embodiment, but can be provided in an alternative section or module as appropriate in other embodiments. The memory of the controller stores computer-readable instructions which, when executed by the processor of the controller, causes the AUV 100 to perform various operations.The propulsion section 105 comprises a propulsion mechanism 152. The propulsion mechanism 152 is located at the tail end of the hull 101. In other embodiments, however, the propulsion mechanism 152 can be located at a nose end of the hull 101.

[0172] The propulsion section 105 comprises an internal ballast module 151 and a propulsion mechanism 152. The propulsion mechanism 152 is located at the tail end of the hull 101. In the present embodiment the internal ballast module 151 is located adjacent to the propeller mechanism 152 at the tail end of the hull 101. In other embodiments, however, the propulsion mechanism 152 and the internal ballast mechanism 151 can be located elsewhere along the hull 101, and elsewhere relative to one another.

[0173] The internal ballast module 151 comprises an internal ballast. The internal ballast is configured to self-right the AUV 100, or bias the hull 101 of the AUV 100 to be in a particular rotational orientation, such that the roll of the AUV 100, i.e. the rotational orientation of the AUV 100 about a roll axis coincident with the longitudinal axis of the AUV 100, can be controlled. The weight of the internal ballast will align the AUV 100 with respect to the direction of gravity, thereby orienting the AUV 100 about the roll axis accordingly. In use, the internal ballast can therefore be used to orient the AUV 100 in any desired orientation about the roll axis.

[0174] The internal ballast module 151 further comprises an internal ballast positioning mechanism configured to position the internal ballast relative to the hull 101 of the AUV 100. The internal ballast will always cause the AUV 100 to attempt to orient itself in a particularly orientation under the influence of gravity.

[0175] Accordingly, controlling a position the internal ballast relative to the hull 101 can influence the rotational position of the hull 101. The internal ballast module 151 thus provides roll control to the AUV 100, since it is configured to position the AUV 100 in one of a plurality of rotational orientations about its roll axis.

[0176] In the present embodiment the internal ballast positioning mechanism is configured to rotate the internal ballast about a central axis of the hull 101 of the AUV 100, and can therefore be considered to be an internal ballast rotation mechanism. The internal ballast rotation mechanism is a motor configured to rotate the internal ballast about the central axis of the hull 101.

[0177] The propulsion mechanism 152 is configured to propel the AUV 100 during subsea operations. The propulsion mechanism 152 comprises a pair of counter-rotating propellers 153 and a plurality of fins 154. The counter- rotating propellers 153 each generate a motive force for propelling the AUV 100 through water, and the fins 154 improve the efficiency of the propellers 153 by stabilising the motion of the AUV 100. The rotational speed of each propeller in the pair of counter-rotating propellers 153 is independently controllable. This can provide roll control to the AUV 100 as follows.

[0178] In prior art systems where a single propeller blade was used, the rotational motion of the propeller would cause the AUV to roll undesirably. This would be counteracted by various control surfaces, together with the use of a fixed ballast at the bottom of the AUV, to maintain a normal orientation of the AUV. In the present embodiment, the use of a pair of counter-rotating propellers 153 results in the torques induced by each propeller cancelling out. Therefore, where the pair of counter-rotating propellers 153 are operated at the same rotational speed, no roll torque is undesirably induced. The use of a pair of counter-rotating propellers 153 can therefore improve the roll control of the AUV 100 by mitigating against the generation of undesirable roll torque during propulsion of the AUV 100.

[0179] Further, where it is desirable to induce a roll torque, e.g. to adjust a rotational orientation of the AUV 100 about its roll axis, the rotational speeds of the propellers 153 can be asymmetrically controlled. When the rotational speeds are imbalanced, the roll torque components induced by each propeller do not cancel out and accordingly the desired roll torque can be induced.

[0180] Whilst the internal ballast module 151 and the propulsion module 152 each provide independent roll control to the AUV 100, the combined and selective use of both modules 151, 152 provides improved roll control for the AUV 100 across the breadth of its subsea operations.

[0181] For example, the combined use of the independently controllable propellers and the internal ballast positioning mechanism can cause a swifter and more reactive roll control if required, providing the AUV 100 with greater manoeuvrability. Further, in situations where one form of roll control is less suited for use the other may be able to compensate such that the AUV 100 maintains full roll control capabilities at all times during subsea operations.

[0182] Figures 5 and 6 illustrate the rotational orientation of the AUV 100 being controlled about its roll axis during subsea operations. The roll of the AUV 100 is controlled using the internal ballast positioning mechanism and the pair of counterrotating propellers 153. In the example illustrated, the AUV 100 is performingsidescan operations and therefore emits sonar beams from each of its sidescan sonar transducers 131, located on opposing sides of the AUV 100. The direction of the emitted beams is dependent on the orientation of the AUV 100. Accordingly, by controlling the position of the AUV 100 about its roll axis the direction of the beams emitted by the sidescan sonar transducers 131 can also be controlled, e.g. to aim at surfaces or structures higher or lower than the AUV 100 (as well as level with the AUV 100). Controlling the direction of the beams emitted by the sidescan sonar transducers 131 has particular utility when surveying inclined surfaces or structures of or on the seabed, since the beams can be directed straight at said surfaces or structures.

[0183] The AUV 100 also comprises a steering mechanism 155. The propulsion mechanism 152 is mounted to the hull 101 via the steering mechanism 155. The steering mechanism 155 is configured to provide directional control to the propulsion mechanism 152 relative to the hull 101, and hence to the pair of counterrotating propellers 153. Accordingly, the steering mechanism 155 can provide pitch and yaw control for the AUV 100 by directing the motive force generated by the propulsion mechanism 152 relative to hull 101.

[0184] The steering mechanism 155 comprises a motorised joint, such as a motorised cardan joint, that is configured to provide pitch and yaw control to the propulsion mechanism 152 relative to the axis of the hull 101 of the AU V 100. The motorised joint is protected by an oil-filled bellows defining a part of the exterior surface of the AUV 100. In other embodiments, the motorised joint can instead be a motorised ball joint, or can be replaced with any other suitable active joint such as a pneumatic, electric or hydraulic actuator.

[0185] Compared to prior art AUVs that use control surfaces to provide steering, using the steering mechanism 155 to provide pitch and yaw control directly to the propulsion mechanism 152 can provide greater and more responsive control for the AUV 100. For example, actuating the steering mechanism 155 such that the propulsion mechanism 152 is greatly offset to the longitudinal axis of the hull 101 provides a tighter turning circle for the AUV 100 in water, thereby improving its manoeuvrability. Also, since the AUV 100 does not require any control surfaces to induce steering due to the presence of the steering mechanism 155, the hull 101 of the AUV 100 does not experience drag forces associated with the presence of control surfaces, thus improving its hydrodynamic profile and reducing drag.Further, the use of control surfaces as in the prior art requires the AUV to have forward motion to induce a directional thrust. For the AUV 100 of the present embodiment, however, the AUV 100 can make a directional turn from a standstill since the steering mechanism 155 directly controls the direction of the propulsion mechanism 152 and hence the motive force it generates.

[0186] Figure 7 illustrates a process of controlling the position of the AUV 100 through direct control of the propulsion mechanism 152 using the steering mechanism 155. The AUV 100 is shown traversing the seabed 2, which comprises a flat portion 2A and a contoured hill portion 2B. The AUV 100 traverses from left to right in figure 7, such that the AUV 100 progresses from position (i) through to position (v). The position of the AUV 100 about its roll axis is controlled throughout the process (e.g. through controlling the speed of the pair of counter-rotating propellers 153, and / or through control of the internal ballast mechanism 151), such that undesirable roll of the AUV 100 does not inadvertently disturb the directional control provided by the steering mechanism 155.

[0187] The AUV 100 follows a path generally parallel to the contours of the seabed 1. At position (i), the AUV 100 travels generally parallel to the flat portion 2A of the seabed 2, with zero pitch control induced by the steering mechanism 155.

[0188] As the AUV 100 approaches the hill portion 2B of the seabed 2 at position (ii), the steering mechanism 155 is actuated to induce a pitch up motion of the AUV 100. The AUV 100 turns in response, such that the AUV 100 proceeds along a path parallel to the contour of the hill portion 2B at position (iii). The steering mechanism 155 can be controlled again to induce no change in the pitch of the AUV 100.

[0189] At position (iv), the hill portion 2B flattens out. The steering mechanism 155 is controlled to induce a pitch down motion of the AUV 100. The AUV 100 turns accordingly.

[0190] At position (v), the hill portion 2B descends and the AUV 100 is steered to be parallel to the angle of descent. The steering mechanism 155 is controlled to induce no change in the pitch of the AUV 100.

[0191] The steering mechanism 155 is therefore controllable such that the AUV 100 follows the contours of the seabed 1 in a substantially parallel manner. That is, the tighter turning circle provided by the steering mechanism 155 in the pitch and yaw directions provides the AUV 100 with the required manoeuvrability to stay close to the surface of the seabed 1 during subsea operations. The speed of thepair of counter-rotating propellers 153 can also be varied such that the AUV 100 can follow different turning circles and radius, where necessary.

[0192] Navigation of the AUV 100 when performing subsea operations will now be discussed in further detail.

[0193] When navigating subsea, and particularly when cruising along or traversing over a subsea surface, the AUV 100 is unable to reliably communicate with positioning systems or reference points located at the surface. This is due to the opacity of water to wavelengths of light particularly outside of the visible spectrum, which might otherwise be used for submarine communications.

[0194] The AUV 100 therefore makes use of sensors including the DVL 133 and the IMU. By knowing the orientation of the AUV 100 and the speed with which the AUV 100 is travelling, a position of the AUV 100 can be determined. The AUV 100 is able to autonomously perform subsea operations at least partially on the basis of this information.

[0195] To further increase the accuracy with which the velocity of the AUV 100 is determined, the laser interferometry sensing device 136 is also used. Compared to acoustic sensing, as is used by the DVL 133, laser interferometric techniques can be orders of magnitude more accurate in determining the velocity of the AUV 100.

[0196] Due to the rapid attenuation of light underwater, laser interferometric techniques used to determine the velocity of the AUV 100 are only reliable when the AUV 100 is cruising within a limited range above a surface from which light emitted by the laser interferometry sensing device 136 might reflect. This range can also be further limited due to varying visibility at the subsea surface, e.g. due to disturbed sediment or turbid conditions more likely to be experienced at a subsea boundary. As such, interferometric techniques may only be acceptably reliable in a range of 0 to 10 metres above the seabed, for example. However, this can result in a trade-off between the subsea surveying capability of the AUV 100 (which has a greater field of view the further the AUV 100 is from the subsea surface it is surveying), and the accuracy of the velocity measurements taken by the laser interferometry sensing device 136.

[0197] Thus, in accordance with embodiments of the present invention, the AUV 100 is configured to determine a visibility below the AUV 100, and is configured to control a cruising height of the AUV 100 based on the determined visibility. As such, the AUV 100 can be controlled in a manner which balances the need for accurate laser interferometric sensing against the need for more efficient subseasurveying by maintaining a distance acceptable for both operations, when cruising along the subsea surface.

[0198] Figure 8 shows an AUV 100 cruising along a subsea surface. In the illustrated embodiment, the subsea surface is the seabed 2. The AUV 100 cruises through the sea 1, above the seabed 2, in an orientation generally parallel to the seabed 2. In the illustrated embodiment, the AUV 100 navigates from left to right, and hence travels through steps A to E in order.

[0199] At position A, the AUV 100 is at a distance, or height, hi from the seabed 2. The AUV 100 is travelling substantially parallel to the seabed 2 at this height hi.

[0200] The AUV 100 maintains its cruising height hi parallel to the surface of the seabed 2 also when changes in inclination of the seabed 2 are encountered. At position B, the AUV 100 traverses parallel to an inclined plane of the seabed 2, maintaining its existing cruising height hi.

[0201] At both positions A and B, the AUV 100 is travelling through an area of the sea 1 which has a visibility V1.

[0202] A measurement of turbidity of the sea 1 is taken to be the visibility, and is measured in either Formazin Nephelometric Units (FNU) as defined by ISO 7027 or Nephelometric Turbidity Units (NTU) as defined by USEPA Method 180.1. Any other suitable measurement of visibility in water is also contemplated.

[0203] For the avoidance of doubt, a ‘better’, ‘greater’ or similar visibility indicates less attenuation of light in a region, i.e. a further distance for which something may be seen. A ‘worse’, ‘lower’ or similar visibility indicates greater attenuation of light in a region, i.e. a shorter distance for which something may be seen. A region of greater visibility will have a lower turbidity. A region of poorer visibility will have a higher turbidity.

[0204] At position C, the AUV 100 continues to cruise above the seabed 2, substantially parallel to the seabed 2. However, the AUV 100 now cruises at a height h2 above the seabed 2. Height h2 is greater than height hi. The AUV 100 cruises at a greater height h2 above the seabed 2 because the AUV 100 is travelling through a region of water 1 with a better visibility V2 than the visibility V1 at positions A and B.

[0205] At positions A and B, the controller determines that the visibility below the AUV 100 is the visibility V1 , and thus controls the AUV 100 to cruise at the height hi. The height hi is suitable for using the laser interferometry sensing device 136 to reliably determine a position of the AUV 100 relative to the seabed 2, at a regionhaving a visibility V1. Upon reaching position C, the controller then determines that the visibility below the AUV 100 is the visibility V2, and thus controls the AUV 100 to cruise at the height h2 because the visibility V2 has improved. The height h2 is suitable for using the laser interferometry sensing device 136 to reliably determine a position of the AUV 100 relative to the seabed 2, at a region having a visibility V2.

[0206] Since the visibility V2 at position C is better than the visibility V1 at positions A and B, the AUV 100 can be controlled to cruise at a greater height h2 from the seabed 2. This is because the light emitted from the laser interferometry sensing device 136 will not be as strongly attenuated by the seawater 1 of visibility V2 at position C, as compared to the visibility V1 at positions A and B. As such, laser interferometric sensing techniques can still be used whilst maximising a field of view of the subsea surveying sensors 131 of the AUV 100.

[0207] At position D, the AUV 100 continues to cruise above the seabed 2, substantially parallel to the seabed 2. However, the AUV 100 now cruises at a height h3 above the seabed 2. Height h3 is lower than height hi and height h2. The AUV 100 cruises at a lower height h3 above the seabed 2 because the AUV 100 is travelling through a region of water 1 with a worse visibility V3 than the visibility V1 at positions A and B and the visibility V2 at position C.

[0208] Upon reaching position D, the controller of the AUV 100 determines that the visibility below the AUV 100 is the visibility V3, and thus controls the AUV 100 to cruise at the height h3 because the visibility V3 has worsened. The height h3 is suitable for using the laser interferometry sensing device 136 to reliably determine a position of the AUV 100 relative to the seabed 2, at a region having a visibility V3.

[0209] Whilst the field of view of the sidescan sensors 131 will be smaller at the lower cruising height h3, the laser interferometer sensing device 136 can still be used to reliably determine the velocity of, and hence the position of, the AUV 100. As such, the information gathered through the subsea surveying process can still be accurately mapped to a subsea position or coordinate, and / or navigation of the AUV 100 can remain accurate during subsea operations.

[0210] In the present embodiment, the AUV 100 comprises a separate visibility sensing device (not shown). The visibility sensing device comprises a plurality of light emitting diodes (LEDs) which emit light into the water 1, and a photodiode arranged to receive any light scattered by particulate or the like suspended in the water 1. The visibility sensing device determines a measure of the visibility basedon the amount of scattered light. The controller is configured to receive this measurement.

[0211] In other embodiments, the visibility sensing device can be an acoustic doppler current profiler configured to determine the suspended particulate concentration below the underwater vehicle using acoustic backscattering. The visibility can be determined based on this measurement.

[0212] In other embodiments, the laser interferometry sensing device 136 can be arranged to measure the visibility of the water 1, in place of or in addition to the separate visibility sensing device. The visibility can be determined, using the laser interferometry sensing device 136, based on the intensity of the reflected light received from the seabed 2.

[0213] In the present embodiment, the cruising height of the AUV 100 is continuously controlled based on the visibility underneath the AUV 100.

[0214] In other embodiments, the cruising height of the AUV 100 is discontinuously controlled based on the visibility underneath the AUV 100.

[0215] Where the visibility is below a first threshold, the cruising height is controlled to be less than or equal to 5 metres. Where the visibility is above the first threshold and below a second threshold greater than the first threshold, the cruising height is controlled to be between 5 and 10 metres. Where the visibility is above the second threshold, the cruising height is controlled to be between 10 and 15 metres.

[0216] The visibility below the AUV 100 can also vary with the distance of the AUV 100 from the seabed 2. Thus, the cruising height of the AUV 100 can be controlled accordingly.

[0217] In some embodiments, where the visibility is below a first threshold, the cruising height is decreased. The cruising height remains above a minimum threshold, e.g. 2 metres, to ensure the AUV 100 does not collide with the seabed. Where the visibility is above the first threshold and below a second threshold greater than the first threshold, the cruising height can remain unchanged. If the visibility is above the second threshold, the cruising height of the AUV 100 is increased. The cruising height remains below a maximum threshold, e.g. 20 metres, to ensure that laser interferometric sensing techniques remain suitable for use.

[0218] Whilst navigating subsea, the minimum cruising height of the AUV 100 relative to a subsea surface can be set to approximately 2 metres. Setting a minimum cruising height ensures that the AUV 100 does not collide with the subseasurface and maintains enough space for adjusting the pitch of the AUV 100 as required, e.g. when approaching an incline.

[0219] A maximum cruising height of the AUV 100 relative to the subsea surface can also be set to approximately 15 metres. Above the maximum cruising height determinations of the position of the AUV 100 via interferometric techniques may not be reliable regardless of the regardless of the visibility underneath the AUV 100, e.g. due to attenuation of light signals in water. In other embodiments, the maximum cruising height can be set to less than 10 metres or less than 20 metres, or anywhere between 10 to 20 metres.

[0220] Whilst the embodiment discussed above and as illustrated in figure 8 shows the AUV 100 with a substantially constant roll orientation, the roll of the AUV 100 can also be controlled when navigating subsea should the incline of the subsea surface change relative to the roll axis of the AUV 100 (as discussed above in relation to figures 5 and 6, for example). This can occur where the AUV 100 moves along a subsea hillside, rather than up or down a subsea hillside, for example.

[0221] Particularly, the roll of the AUV 100 is generally controlled such that the underside of the AUV 100 remains substantially parallel to the subsea surface 2 above which the AUV 100 cruises. The roll is controlled through the use of the pair of counter-rotating propeller blades 153, and / or the use of the internal ballast module 151.

[0222] Reliable control of the roll of the AUV 100 can be of particular benefit when navigating subsea using laser interferometric sensing techniques. When passing a hillside, having the underside of the AUV 100 be parallel to the subsea surface can avoid the introduction of unwanted components into the velocity measurements obtained by the laser interferometric sensing devices 136 (as described above). As such, more or all laser interferometry sensing devices 136 present on the underside of the AUV 100 can be reliably used when determining the position of the AUV 100 relative to the subsea surface.

[0223] Figure 9 illustrates the AUV 100 approaching an incline when approaching a subsea surface which, in the present embodiment, is a seabed 2. The AUV 100 has a first laser interferometry sensing device 136A located towards the nose end of the AUV 100, and a second laser interferometry sensing device 136B located towards the tail end of the AUV 100.

[0224] When navigating subsea, and particularly when cruising along or traversing over a subsea surface at a close distance to the subsea surface, the AUV 100 maybe more likely to collide with projections or inclined surfaces of the subsea surface. The AUV 100 therefore makes use of its laser interferometry sensing devices 136A, 136B to determine any changes of incline of the subsea surface relative to the AUV 100, such that the pitch of the AUV 100 can be controlled to avoid possible collisions.

[0225] As the AUV 100 cruises along the seabed 2, the first laser interferometry sensing device 136A emits a first beam and the second laser interferometry sensing device 136B emits a second beam. Both the first beam and the second beam are directed towards the seabed 2. When the seabed 2 is substantially flat, or parallel to the longitudinal axis of the AUV 100, the velocity measurements obtained by the first and second laser interferometry sensing devices 136A, 136B generally corroborate one other (i.e. there are no unwanted components introduced into the velocity measurements due to a relative tilt between the AUV 100 and the seabed 2).

[0226] Figure 9 illustrates a scenario where the AUV 100 approaches an inclined surface of the seabed 2. Light emitted from the first laser interferometry sensing device 136A is reflected at position P1 on the seabed 2, and light emitted from the second laser interferometry sensing device 136B is reflected at position P2 on the seabed 2. Position P1 is a distance d1 higher than position P2. The light reflected at position P1 is used to obtain a first measurement relating to a velocity of the AUV 100 relative to the seabed 2, and the light reflected at position P2 is used to obtain a second measurement relating to a velocity of the AUV 100 relative to the seabed 2.

[0227] Owing to the distance d1 between positions P1 and P2, the first measurement and the second measurement do not generally corroborate one another. Unwanted components will be introduced into at least the first measurement due to the relative tilt between the AUV 100 and the incline of the seabed 2 at position P1. As such, the velocities determined by the first interferometry sensing device 136A and the second interferometry sensing device 136B will differ.

[0228] Based on the first and second measurements, the AUV 100 determines a change of incline of the subsea surface relative to the AUV 100. The change of incline can be based on a difference between a first velocity based on the first measurement a second velocity based on the second measurement.In the example scenario illustrated in figure 9, the AUV 100 determines that there is a positive change of incline. To avoid a collision with the seabed 2, a pitch of the AUV 100 is controlled based on the change of incline. The pitch of the AUV 100 is increased, such that the nose end of the AUV 100 pitches up.

[0229] In the illustrated embodiment, the first laser interferometry sensing device 136A is arranged to emit the first beam out in front of the AUV 100, or at least to emit the first beam in a direction extending out in front of the AUV 100.

[0230] Emitting the first beam out in front of the AUV 100 can improve the reaction time of the AUV 100 for detecting the change of incline, by having the first beam strike position P1 at an earlier time than if the first beam was emitted directly below the AUV 100. That is, the onset of any difference between the velocities calculated based on the first measurement and the second measurement respectively will be sooner, when the first beam is emitted out in front of the AUV 100.

[0231] Whilst the scenario illustrated in figure 9 and illustrated above is described in regard to avoiding a collision with an inclined subsea surface, the same control techniques can be applied when detecting a descending subsea surface. Thus, if the seabed 2 has a negative change of incline, it can be detected that the seabed 2 is descending. Based on the change of incline, the pitch of the AUV 100 is then controlled such that the AUV 100 pitches down. In this manner, the pitch of the AUV 100 is controlled such that the AUV 100 follows the contouring of a subsea surface along which it cruises.

Claims

- 37 -CLAIMS1. A method of controlling an autonomous underwater vehicle, AUV, when the AUV is cruising along a subsea surface, the method comprising:monitoring a position of the AUV by directing a laser beam from a laser interferometry sensing device to the subsea surface;determining a visibility below the AUV; andcontrolling a cruising height of the AUV relative to the subsea surface based on the visibility.

2. A method as claimed in claim 1, wherein the AUV comprises a hull having a nose end and a tail end, a first laser interferometry sensing device located towards the nose end, and a second laser interferometry sensing device located towards the tail end;the method comprising:receiving a first measurement relating to a velocity of the AUV relative to the subsea surface from the first laser interferometry sensing device;receiving a second measurement relating to a velocity of the AUV relative to the subsea surface from the second laser interferometry sensing device;determining a change of incline of the subsea surface relative to the AUV based on the first measurement and the second measurement; andbased on the change of incline, controlling a pitch of the AUV.

3. A method of controlling an autonomous underwater vehicle, AUV, when the AUV is cruising along a subsea surface;wherein the AUV comprises a hull having a nose end and a tail end, a first laser interferometry sensing device located towards the nose end, and a second laser interferometry sensing device located towards the tail end;the method comprising:receiving a first measurement relating to a velocity of the AUV relative to the subsea surface from the first laser interferometry sensing device;receiving a second measurement relating to a velocity of the AUV relative to the subsea surface from the second laser interferometry sensing device;determining a change of incline of the subsea surface relative to the AUV based on the first measurement and the second measurement; and- 38 -based on the change of incline, controlling a pitch of the AUV.

4. A method as claimed in claim 3, comprising:monitoring a position of the AUV by directing a laser beam from a laser interferometry sensing device to the subsea surface;determining a visibility below the AUV; andcontrolling a cruising height of the AUV relative to the subsea surface based on the visibility.

5. A method as claimed in claim 1, 2 or 4, wherein the cruising height is continuously varied based on the visibility.

6. A method as claimed in claim 1, 2 or 4, wherein if the visibility is below a first predetermined threshold, the cruising height is decreased; andwherein if the visibility is above a second predetermined threshold that is greater than the first predetermined threshold, the cruising height is increased.

7. A method as claimed in claim 1, 2, 4, 5 or 6, wherein a maximum cruising height of the AUV relative to the subsea surface is less than 20 metres above the subsea surface;preferably wherein the maximum cruising height is less than 15 metres above the subsea surface.

8. A method as claimed in any of claims 1 , 2, or 4 to 7, wherein monitoring a position of the AUV further comprises:receiving reflected light from the subsea surface;determining a velocity of the AUV relative to the subsea surface based on an interference pattern between the reflected light and a reference beam; and determining a change in position of the AUV relative to the subsea surface based on the velocity.

9. A method as claimed in any of claims 1 , 2 or 4 to 8, wherein the laser interferometry sensing device is used to obtain a measurement relating to the visibility below the AUV.

10. A method as claimed in claim 2, 3 or 4, wherein a beam emitted from the first laser interferometry sensing device is emitted in a direction extending out in front of the nose end of the AUV.

11. A method as claimed in claim 2, 3, 4 or 10, comprising:obtaining a first estimated velocity based on the first measurement and a second estimated velocity based on the second measurement; and determining a change of incline of the subsea surface relative to the AUV based on a difference between the first estimated velocity and the second expected velocity.

12. A method as claimed in claim 2, 3, 4, 10 or 11, wherein the pitch of the AUV is controlled such that the AUV remains substantially parallel to the subsea surface.

13. A method as claimed in any preceding claim, wherein the AUV comprises:a propulsion mechanism configured to propel the AUV, wherein the propulsion mechanism comprises a pair of counter-rotating propellers; andat least one of:a steering mechanism configured to rotate the propulsion mechanism in at least one of a pitch and a yaw direction, relative to the hull;wherein the method comprises:adjusting a position of the AUV using the steering mechanism; oran internal ballast located in the hull, and an internal ballast positioning mechanism configured to controllably move the internal ballast relative to the hull;wherein the method comprises:controlling a roll of the AUV by independently controlling the rotational speed of each propeller of the pair of counter-rotating propellers;controlling the roll of the AUV using the internal ballast positioning mechanism.

14. A method as claimed in any preceding claim, wherein the subsea surface is a seabed.

15. An autonomous underwater vehicle, AUV, for performing subsea operations, the AUV comprising:at least one laser interferometry sensing device configured to obtain a measurement relating to a position of the AUV using a laser beam directed at the subsea surface;a visibility sensing device configured to obtain a measurement relating to visibility below the AUV; anda controller in communication with the at least one laser interferometry sensing device and the visibility sensing device;wherein, when the AUV is cruising along the subsea surface, the controller is configured to:monitor a position of the AUV using the laser interferometry sensing device;determine a visibility below the AUV using the visibility sensing device; andcontrol a cruising height of the AUV relative to the subsea surface based on the visibility.

16. An AUV as claimed in claim 15, wherein the AUV comprises a hull having a nose end and a tail end, a first laser interferometry sensing device located towards the nose end, and a second laser interferometry sensing device located towards the tail end;wherein the controller is in communication with the first laser interferometry sensing device and the second laser interferometry sensing device; and wherein, when the AUV is cruising along the subsea surface, the controller is configured to:receive a first measurement relating to a velocity of the AUV relative to the subsea surface from the first laser interferometry sensing device; receive a second measurement relating to a velocity of the AUV relative to the subsea surface from the second laser interferometry sensing device;determine a change of incline of the subsea surface relative to the AUV based on the first measurement and the second measurement; and based on the change of incline, control a pitch of the AUV.

17. An autonomous underwater vehicle, AUV, for performing subsea operations, the AUV comprising:a hull having a nose end and a tail end;a first laser interferometry sensing device located towards the nose end; a second laser interferometry sensing device located towards the tail end; anda controller in communication with the first laser interferometry sensing device and the second laser interferometry sensing device;wherein, when the AUV is cruising along the subsea surface, the controller is configured to:receive a first measurement relating to a velocity of the AUV relative to the subsea surface from the first laser interferometry sensing device; receive a second measurement relating to a velocity of the AUV relative to the subsea surface from the second laser interferometry sensing device;determine a change of incline of the subsea surface relative to the AUV based on the first measurement and the second measurement; and based on the change of incline, control a pitch of the AUV.

18. An AUV as claimed in claim 17, comprising:at least one laser interferometry sensing device configured to obtain a measurement relating to a position of the AUV using a laser beam directed at the subsea surface; anda visibility sensing device configured to obtain a measurement relating to visibility below the AUV;wherein the controller is in communication with the at least one laser interferometry sensing device and the visibility sensing device;wherein, when the AUV is cruising along the subsea surface, the controller is configured to:monitor a position of the AUV using the at least one laser interferometry sensing device;determine a visibility below the AUV using the visibility sensing device; andcontrol a cruising height of the AUV relative to the subsea surface based on the visibility.- 42 -19. An AUV as claimed in claim 15, 16 or 18, wherein the controller is configured to continuously vary the cruising height based on the visibility.

20. An AUV as claimed in claim 15, 16 or 18, wherein the controller is configured to:decrease the cruising height if the visibility is below a first predetermined threshold; andincrease the cruising height if the visibility is above a second predetermined threshold that is greater than the first predetermined threshold.

21. An AUV as claimed in claim 15, 16, 18, 19 or 20, wherein the laser interferometry sensing device is used as the visibility sensing device.

22. An AUV as claimed in claim 16, 17 or 18, wherein the first laser interferometry sensing device is configured to emit a beam in a direction extending out in front of the nose end of the AUV.

23. An AUV as claimed in claim 16, 17, 18 or 22, wherein the controller is configured to:obtain a first estimated velocity based on the first measurement and a second estimated velocity based on the second measurement; anddetermine a change of incline of the subsea surface relative to the AUV based on a difference between the first estimated velocity and the second expected velocity.

24. An AUV as claimed in claim 16, 17, 18, 22 or 23, wherein the controller is configured to control the pitch of the AUV such that the AUV remains substantially parallel to the subsea surface.

25. An AUV as claimed in any of claims 15 to 24, wherein the AUV comprises:a propulsion mechanism configured to propel the AUV, wherein the propulsion mechanism comprises a pair of counter-rotating propellers; andat least one of:- 43 -a steering mechanism configured to rotate the propulsion mechanism in at least a pitch direction, relative to the hull; oran internal ballast located in the hull, and an internal ballast positioning mechanism configured to controllably move the internal ballast relative to the hull.

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