Patent Description:
Known methods of conducting seismic surveys are disclosed in <CIT>; <CIT>; <CIT>; and <CIT>.

<CIT> discloses a method for cycling autonomous underwater vehicles (AUVs) that record seismic signals during a marine seismic survey. The method includes deploying plural current AUVs on the ocean bottom; recording the seismic signals during the marine seismic survey with plural current AUVs; releasing from an underwater base a new AUV to replace a corresponding current AUV from the plural current AUVs; recovering the current AUV; and continuing to record the seismic signals with the new AUV.

A first aspect of the invention provides a method of deploying autonomous underwater vehicles (AUVs) according to claim <NUM>.

A further aspect of the invention provides a method of retrieving autonomous underwater vehicles (AUVs) according to claim <NUM>.

The towed deployment/retrieval method of the present invention enables the AUVs to be deployed or retrieved quickly and efficiently over a large area. The towing motion of the device can be beneficial, assisting in ejecting the AUVs from the device or loading them into the device. For example the towing motion may cause a flow of water through a deployment channel of the device, this flow generating a motive force which assists in ejecting the AUV out of the device (optionally in combination with operation of a thruster of the AUV).

The AUVs may be deployed or retrieved one-by-one by the submerged device as it is towed by the surface vessel, or multiple AUVs may be deployed or retrieved simultaneously.

A third aspect of the invention provides a submersible device according to claim <NUM>.

Various preferred features of the invention are set out in the dependent claims.

A method of deploying autonomous underwater vehicles (AUVs) 1a-c with a deployment/retrieval device <NUM> is shown in <FIG> and <FIG>. The device <NUM> will be described in detail below, but in general comprises a pair of carousels 3a,b, each carousel carrying a stack of thirty six AUVs. The device <NUM> is loaded with seventy two AUVs on the deck of a surface vessel <NUM>. The device <NUM> carrying the AUVs is then lowered into the water by a crane <NUM> and a tether <NUM> as shown in <FIG> until it is at a required depth. At this point the surface vessel <NUM> may be stationary or it may be moving.

After the device <NUM> containing the AUVs has been submerged as in <FIG>, the surface vessel <NUM> is driven to the left as shown in <FIG> so that it tows the submerged deployment device containing the AUVs. The AUVs are then deployed one-by-one from the device <NUM> as it is towed by the surface vessel. The towing speed is typically between <NUM>/s and <NUM>/s, and most preferably between <NUM>/s and <NUM>/s. For example the towing speed may be <NUM>/s. Each carousel has six platforms, each platform carrying six AUVs. As the surface vessel moves, a transfer device (not shown) within the device <NUM> unloads the AUVs one-by-one from the platforms, and moves between the platforms and a deployment funnel in order to transfer the AUVs one-by-one from the platforms to the deployment funnel. The AUVs are then deployed one-by-one from the deployment funnel. As shown in <FIG>, a thruster of each AUV 1a-c is operated after it has been deployed so that it moves horizontally away from the towed device <NUM>.

After the AUVs have been deployed as shown in <FIG>, they descend autonomously to the seabed, and land at precisely controlled locations where they acquire seismic data during a seismic survey. When the survey is complete, the AUVs return to the surface vessel <NUM> where they are retrieved by essentially the reverse process to deployment, as shown in <FIG> and <FIG>. Thrusters of the AUVs are operated so that the AUVs form a line in front of the device in a retrieval zone <NUM> as shown in <FIG>. The submerged device <NUM> is towed through the retrieval zone <NUM> by the surface vessel <NUM>, and the AUVs are loaded one-by-one into a retrieval funnel of the device as it is towed through the retrieval zone <NUM> by the surface vessel. After the AUVs have been loaded into the towed device <NUM>, the device <NUM> containing a full payload of the AUVs is lifted out of the water and onto the surface vessel by the crane <NUM> as shown in <FIG>.

The submersible/retrieval device <NUM> will now be described in detail. The device <NUM> has a chassis or cage <NUM> shown in <FIG> divided into four segments: two segments <NUM>, <NUM> at the bottom of the chassis for storing the AUVs and two segments <NUM>, <NUM> at the top of the chassis for retrieving and deploying the AUVs. The two carousels 3a, b are mounted to the chassis <NUM> so that they can be rotated about a vertical axes relative to the chassis <NUM>. <FIG> shows one of the carousels 3a in detail, without any AUVs. Each carousel comprises a vertical shaft <NUM> rotatably mounted to the chassis, and three removable pallets, one of which is shown in <FIG>. Each pallet comprises a pallet chassis with an upper mounting part <NUM> and a lower mounting part <NUM>, each having a pair of holes <NUM>. The pallet chassis is mounted onto the shaft <NUM> by inserting pins <NUM> into the holes <NUM>, and the pallet can be removed from the shaft <NUM> by lifting it off the pins <NUM>. Each pallet chassis carries six platform segments <NUM> arranged in a vertical stack. Each platform segment <NUM> can accommodate two AUVs. The three platform segments <NUM> on each level of the stack together constitute a platform which can accommodate six AUVs (two AUVs per platform segment <NUM>).

A transfer mechanism <NUM> shown in <FIG> is mounted to the chassis and arranged to load and unload the AUVs from the platforms. A transfer device <NUM> is mounted on a pair of vertcial rails <NUM> in a channel between the pair of carousels. The transfer device <NUM> can be driven up and down on the rails <NUM> by a lead screw <NUM> driven by an electric or hydraulic motor (not shown).

The transfer device <NUM> supports an AUV 1a as shown in <FIG> by gripping the AUV between a lower jaw <NUM> underneath the AUV and an upper jaw <NUM> above the AUV. The jaws <NUM>, <NUM> are slidably mounted on a support frame <NUM>, and can be driven horizontally by an electric or hydraulic motor <NUM> and a pair of drive cogs <NUM>, <NUM> between a retracted position shown in <FIG>, <FIG>, <FIG>, <FIG> and an extended position shown in <FIG> and <FIG>.

In order to unload an AUV from a platform, the motor is first operated to rotate the lead screw <NUM> and drive the transfer device <NUM> down to a selected vertical level. The support frame <NUM> is rotated (if necessary) about a vertical axis by a motor (not shown) and drive cog <NUM> so that it faces a selected one of the carousels 3a,b. So for instance in <FIG> the support frame <NUM> is pointing left so it is facing the carousel on the left-hand side of <FIG>, but it can be rotated by <NUM> ° by the drive cog <NUM> so that is facing the carousel on the right-hand side of <FIG>. The selected carousel is also rotated on its shaft <NUM>, if required, so that the platform segment <NUM> facing the transfer device is not empty.

Each level of the stack has an associated guard <NUM> carried by an actuator <NUM> (a solenoid or hydraulic ram). The guards <NUM> can be individually moved between an extended (closed) position and a retracted (open) position. <FIG> shows all of the guards <NUM> on the left-hand side in their extended (closed) position, and all of the guards <NUM> on the right-hand side in their retracted (open) position.

When the transfer device <NUM> has reached the selected vertical level of the stack and is pointing in the correct direction, then the appropriate guard <NUM> is retracted. Then the motor <NUM> is operated so that the jaws <NUM>, <NUM> move horizontally to their extended position. The lower jaw <NUM> comprises a pair of arms 211a, b which are received in slots 130a, b in the platform segment <NUM> underneath the AUV.

The lower jaw <NUM> is suspended on a pair of struts <NUM> which are telescopically mounted within struts <NUM> suspended from the upper jaw <NUM>. The lower jaw <NUM> can be driven up and down by an actuator <NUM>, and as it does so the struts <NUM> slide in and out of the struts <NUM>. As the jaws <NUM>, <NUM> move horizontally to their extended position, a curved pad <NUM> contacts the side of the AUV as shown in <FIG>. The actuator <NUM> then drives the lower jaw <NUM> up so that the AUV becomes clamped between the jaws <NUM>, <NUM>.

After the AUV has been gripped, the motor <NUM> is operated so that the jaws <NUM>, <NUM> carrying the AUV retract back into the transfer channel. Then the support frame <NUM> is rotated (if necessary) by the drive cog <NUM> so that it faces in the deployment direction (rather than the retrieval direction). Next the lead screw <NUM> is rotated to drive the transfer device <NUM> carrying the AUV up the transfer channel until it reaches the position shown in <FIG>, <FIG>, <FIG> and <FIG>.

As mentioned above, the chassis <NUM> has two segments <NUM>, <NUM> at the top of the chassis for retrieving and deploying the AUVs. A retrieval funnel <NUM> (<FIG>) is mounted within the segment <NUM> at the front of the device <NUM> and a deployment funnel <NUM> is mounted within the segment <NUM> at the rear of the device <NUM>. Each funnel has a wide opening facing out of the device, and a narrow opening facing into the device. So as shown in <FIG> the retrieval funnel <NUM> has a wide forward-facing opening <NUM> for receiving AUVs during the retrieval process of <FIG>, a narrow rear-facing opening <NUM> for feeding the AUVs towards transfer device <NUM>, and a retrieval channel <NUM> between the openings <NUM>, <NUM>. Similarly the deployment funnel <NUM> has a narrow forward-facing opening <NUM> for receiving the AUVs from the transfer device <NUM>, a wide rear-facing opening <NUM> for deploying AUVs during the deployment process of <FIG>, and a deployment channel <NUM> between the openings <NUM>, <NUM>.

During the deployment process, when the transfer device <NUM> has reached the narrow opening <NUM> of the deployment funnel <NUM>, the jaws are released and the AUV is forced out of the wide opening <NUM> of the deployment funnel by the action of the water flowing through the deployment channel <NUM>. That is - the towing motion causes a flow of water through the deployment channel <NUM> of the deployment funnel and this flow generates a motive force which ejects the AUV out of the device. Optionally the AUV may also operate its thrusters to assist its ejection from the deployment funnel <NUM>.

Four homing devices <NUM>, such as acoustic transmitters, are arranged to output homing signals <NUM> (such as acoustic signals) which guide the AUVs to the retrieval funnel <NUM> during the retrieval process as shown in <FIG>.

During the retrieval process, the transfer device <NUM> receives the AUVs one-by-one at the narrow opening <NUM> of the retrieval funnel. It then grips the AUV and transfers it down to a vacant platform. A selected carousel 3a, b is rotated, if required, so that the platform segment facing the transfer device is vacant. The appropriate guard <NUM> is then retracted, the motor <NUM> is operated so that the jaws <NUM>, <NUM> move horizontally to their extended position, the AUV is released so that it drops onto the platform, and the jaws <NUM>, <NUM> are retracted.

The AUV may optionally operate its thrusters as shown in <FIG> to force it into the retrieval funnel <NUM>, or it may be stationary and "swallowed up" by the towed device <NUM>. The towing motion causes a flow of water through the retrieval channel <NUM> of the retrieval funnel which guides the AUV towards the narrow opening <NUM> of the retrieval funnel.

When the device <NUM> is full, it is lifted up onto the deck of the surface vessel as shown in <FIG>. Two pairs of doors <NUM> are then opened as shown in <FIG>, and the six full pallets are lifted off their mounting pins <NUM> and removed. Six empty pallets are then immediately loaded onto the device <NUM> which is then submerged and towed to retrieve a further batch of seventy two AUVs.

A similar process is followed during deployment. That is: the device <NUM> is lowered into the water with a full payload of AUVs as shown in <FIG>; the AUVs are deployed as in <FIG>; the empty device <NUM> is lifted up onto the deck of the surface vessel; the doors <NUM> are opened; the six empty pallets are lifted off their mounting pins and removed; and six full pallets are then immediately loaded onto the device <NUM> which is submerged and towed to deploy a further batch of seventy two AUVs.

The device has four ducted propellers <NUM> mounted at its four corners and oriented at <NUM>° to the towing direction. Propellers <NUM> are used to control the yaw angle of the device <NUM> as it is towed so it adopts the orientation shown in <FIG> and <FIG>. The tether <NUM> is attached to the device <NUM> by a towing arm <NUM> which is rotatably mounted to the chassis at a pivot <NUM>. A pair of damping devices <NUM> act on the arm <NUM> to provide a damping action. The arm <NUM> rotates about the pivot <NUM> so that the device <NUM> adopts a level pitch during towing as shown in <FIG> and <FIG>.

To sum up: the submersible device <NUM> can be used to deploy and/or retrieve AUVs. The device has two carousels 3a,b, each carousel having six platforms arranged in a vertical stack, each platform being configured to carry six AUVs. Each platform is divided into three removable sub-platforms <NUM>. The transfer mechanism of <FIG> loads or unloads the AUVs one-by-one onto or from the platforms using a transfer device <NUM>. The platforms are stacked in a vertical stacking direction, and a lead screw <NUM> of the transfer mechanism is arranged to move the transfer device <NUM> in the vertical stacking direction in order to transfer the AUVs between the platforms and the deployment and retrieval funnels.

The device <NUM> receives electric power from the tether <NUM>. If electric motors and actuators are used then they receive this power directly - if hydraulic motors and actuators are used then the device <NUM> will have a hydraulic power unit which converts the electrical power transmitted down the tether <NUM> into hydraulic power.

The AUVs 1a-c are illustrated schematically in <FIG>, <FIG> and <FIG>, but <FIG> show an exemplary one of the AUVs 1a in detail. The AUV comprises a body with a nose <NUM> and a tail <NUM> at opposite ends of the AUV. The body of the AUV comprises a cylindrical pressure vessel <NUM> (<FIG>) contained within a housing formed by upper and lower shells <NUM>, <NUM>. The pressure vessel <NUM> contains batteries <NUM> and three orthogonally oriented seismic sensors <NUM> (<FIG>). Starboard and port horizontal thrusters 310a,b are carried by the body and can be operated to propel the AUV forward and backwards. A single vertical thruster <NUM> is also carried by the body and can be operated to control the pitch angle of the AUV and effect a vertical take-off from the seabed as will be described in further detail below. Each thruster 310a,b, <NUM> comprises a propeller housed within a respective duct.

The pressure vessel and thrusters are contained within a housing formed by the upper and lower shells <NUM>, <NUM> which meet at respective edges around the circumference of the AUV. The upper shell <NUM> forms a downward-facing cup and the lower shell <NUM> forms an upward-facing cup. The shells <NUM>, <NUM> together provide a hydrodynamic hull of the AUV, including a port shroud <NUM> (<FIG>) which shrouds the port thruster 310b, a starboard shroud <NUM> which shrouds the starboard thruster 310a, and a vertical shroud <NUM> which shrouds the vertical thruster <NUM>.

The shells <NUM>, <NUM> together provide three ducts which contain the three thrusters 310a,b, <NUM>. A vertical duct <NUM> (<FIG>) contains the vertical thruster <NUM> as shown in <FIG>. The vertical duct <NUM> has an opening <NUM> in the upper shell and an opening <NUM> in the lower shell, and provides a vertically oriented channel for water to flow through the vertical thruster <NUM> when it is generating vertical thrust. The vertical duct <NUM> is bounded by a wall <NUM> which is circular in cross-section transverse to the flow direction through the duct. Each shell <NUM>, <NUM> also has four recesses formed in its edge where it meets the other shell, the eight recesses together providing four openings <NUM>-<NUM> for port and starboard horizontal ducts <NUM>, <NUM> (<FIG>) which contain the horizontal thrusters. Each horizontal duct has a respective forward opening <NUM>, <NUM> (<FIG>) at a forward end of the duct and an aft opening <NUM>, <NUM> (<FIG>) at an aft end of the duct. As shown in <FIG>, the horizontal ducts <NUM>, <NUM> are circular in cross-section transverse to the flow direction through the duct. The port duct <NUM>, <NUM>, <NUM> provides a channel for water to flow through the port thruster 310b, and the starboard duct <NUM>, <NUM>, <NUM> provides a channel for water to flow through the starboard thruster 310a.

The lower shell <NUM> includes a planar disc <NUM>. The disc <NUM> acts as a base for the AUV, with a substantially planar downward-facing external surface which can provide a stable platform for the AUV when it is sitting on a platform segment <NUM> or on the seabed. The upper shell incudes an upper skin <NUM> opposite the disc <NUM> with a substantially planar upward-facing external surface. Thus the AUV can land upside down if necessary. The disc <NUM> and upper skin <NUM> also have substantially planar internal faces - this maximises the internal space of the AUV.

The batteries <NUM> can be moved relative to the rest of the AUV in a fore-aft direction <NUM> to control a pitch angle of the AUV. The batteries <NUM> slide fore-and aft on rails <NUM> shown in <FIG> and <FIG>. In <FIG> the batteries <NUM> are positioned fully aft but they can be moved forward until they engage a plate <NUM> towards the front of the pressure vessel in order to reduce the angle of pitch of the AUV. The range of travel of the batteries <NUM> is sufficient to adjust the pitch of the AUV from <NUM>° (level) to <NUM>° (nose up). When the batteries are positioned fully aft as in <FIG> the pitch angle is <NUM>° (with the nose <NUM> pointing up).

The batteries are moved by an actuation system comprising a motor <NUM> which engages a lead screw <NUM>, rotation of the motor <NUM> driving the motor <NUM> and the batteries <NUM> fore and aft.

The horizontal thrusters 310a,b are spaced apart in a port-starboard direction <NUM> shown in <FIG> and <FIG>. Each horizontal thruster is oriented to generate a thrust force in a fore-aft direction <NUM> perpendicular to the port-starboard direction <NUM>. The port and starboard ducts <NUM>, <NUM> are aligned parallel with this fore-aft thrust direction <NUM>. The vertical thruster <NUM> is oriented to generate a thrust force in a height direction <NUM> (<FIG>) perpendicular to the fore-aft and port-starboard directions <NUM>, <NUM>. The vertical duct <NUM> is aligned parallel with this vertical thrust direction <NUM>.

The horizontal thrusters 310a,b are each reversible (i.e. they can be spun clock-wise or anti-clockwise) so that their thrust forces can be switched between being directed forward and being directed aft. As shown in <FIG>, the pressure vessel <NUM> carries the horizontal thrusters on struts 325a,b on the starboard and port sides of the pressure vessel <NUM>. The struts 325a,b are fixed, so the orientations of the horizontal thrusters 310a,b are fixed relative to the pressure vessel and the rest of the AUV. Therefore their thrust forces cannot be re-oriented relative to the rest of the AUV at an angle from the fore-aft direction <NUM>. The horizontal thrusters 310a,b can be driven together to drive the AUV forwards or backwards, or driven differentially to control its yaw angle.

In an alternative embodiment (not shown) the horizontal thrusters 310a,b may be thrust-vectored like the thrusters in <CIT> - that is, their thrust forces can be re-oriented at an angle from the fore-aft direction (for instance to effect vertical take-off). However this is less preferred because it would make them more complex, and more difficult to shroud compactly.

A typical mission profile for the AUV is shown in <FIG>. The AUV has a centre of gravity (G) below its centre of buoyance (B). During deployment (<FIG>) the batteries <NUM> are positioned fully forward so the pitch angle of the AUV is <NUM>°, and the horizontal thrusters generate a thrust T which can either drive the AUV backwards (tail first) out of the deployment/retrieval device <NUM> as shown in <FIG>, or forwards (nose first). On descent (<FIG>) the batteries <NUM> are moved aft so the pitch angle of the AUV increases to <NUM>°, and the horizontal thrusters are operated to generate a thrust T which drives the AUV backwards (tail first). On arriving at the seabed <NUM> (<FIG>) the batteries <NUM> are moved forward so the pitch angle of the AUV returns to <NUM>° and the AUV rests stably on the seabed. To take off (<FIG>) the batteries <NUM> are moved aft and a vertical thrust T from the vertical thruster <NUM> causes the AUV to lift off and pitch nose up. On ascent (<FIG>) the vertical thruster <NUM> is turned off and the horizontal thrusters generate a thrust T which drives the AUV forwards (nose first) with its nose up. Finally, the AUV is retrieved by the device <NUM> as in Figure 3f with its batteries <NUM> moved forward so the pitch angle is <NUM>°.

The vertical thruster <NUM> is positioned so that its thrust force is offset forward from the centre of gravity (G) and centre of buoyancy (B), so that as well as being used to effect vertical take-off as in <FIG> it can also be used to achieve fine pitch control. However this method of pitch control is not efficient over a long period, hence the use of a moving mass (in this case, the batteries <NUM>) as a more efficient method of controlling the steady state pitch of the AUV during descent and ascent. The moving mass allows the centre of gravity to be moved near to the centre (level pitch) for deployment and recovery (<FIG>) and when the AUV is on the seabed (<FIG>). Having the centre of gravity central on the seabed means the moment arm acting on the AUV from ocean currents is the same regardless of the direction of the ocean current.

The AUV is designed to travel efficiently both forwards and backwards. If this was not the case, the AUV would need to be capable of adjusting its pitch from -<NUM>° to <NUM>° during a mission instead of from <NUM>° to <NUM>°. This would increase the amount of space required for the moving mass system and hence would increase the maximum fore-aft length of the AUV.

The AUV includes a buoyancy control system (not shown) for controlling its buoyancy during the mission. The buoyancy control system is preferably housed in the space between the pressure vessel <NUM> and the upper and lower shells <NUM>, <NUM>. The buoyancy control system may be, for example, an active system which is operated to make the AUV neutrally buoyant during deployment/retrieval (<FIG>), negatively buoyant during descent (<FIG>) and during a seismic survey (<FIG>), and positively buoyant during ascent (<FIG>).

<FIG> is a schematic view of a control system for controlling the thrusters and moving mass. The pressure vessel <NUM> contains a controller <NUM> which is programmed to autonomously control the thrusters 310a, 310b, <NUM> and the motor <NUM> in order to follow the mission profile shown in <FIG>. That is, the controller <NUM> is arranged to operate the horizontal thrusters to generate forward thrust to drive the AUV forwards with the nose leading during ascent, and also arranged to operate the thrusters to generate reverse thrust to drive the AUV backwards with the tail leading during descent. The batteries <NUM> supply power to the thrusters 310a, 310b, <NUM> and the motor <NUM>.

The AUV has a maximum length L in the fore-aft direction as shown in <FIG> and <FIG>. The nose <NUM> and a tail <NUM> at opposite ends of the AUV are spaced apart in the fore-aft direction <NUM> by this maximum length L. Each horizontal thruster is housed within a respective horizontal duct <NUM>, <NUM> with a forward duct opening <NUM>, <NUM> at a forward end of the duct and an aft duct opening <NUM>, <NUM> at an aft end of the duct. Each horizontal duct provides a channel for water to flow through its respective thruster in the fore-aft direction <NUM> during operation of the thruster. The motor <NUM> moves the batteries <NUM> relative to the body (forwards or backwards) to control a pitch of the AUV. The AUV has a fore-aft mid-plane <NUM> (shown in <FIG> and <FIG>) which is perpendicular to the fore-aft direction <NUM> and lies half way between the nose <NUM> and the tail <NUM>. The mid-plane <NUM> is also a perpendicular bisector of a fore-aft line between the nose and the tail.

The propellers of the horizontal thrusters are positioned on this mid-plane <NUM>, and the mid-plane <NUM> also passes through both horizontal ducts <NUM>, <NUM> as shown in <FIG> (which is a cross-section taken along the mid-plane <NUM>). This amidships position of the horizontal thrusters (and their associated ducts) enables them to operate relatively efficiently whether they are driving the AUV forwards or backwards.

Although the horizontal thrusters 310a, b are positioned symmetrically (i.e. on the mid-plane <NUM>) the horizontal thrusters 310a,b themselves are not symmetrical and they are more efficient when directing a thrust force which moves the AUV forwards. Since they must overcome gravity when the AUV is ascending, the horizontal thrusters are therefore used to drive the AUV forwards when it is ascending and backwards when it is descending (rather than vice versa).

In an alternative embodiment the horizontal thrusters 310a,b could be positioned towards the tail of the vehicle, or they could actuated so that they move to the nose or tail of the vehicle depending on the direction of travel. Although these thruster positions would be more efficient, the thrusters would be more difficult to shroud and they would need to protrude from the body of the AUV.

The vertical thruster <NUM> is also reversible (i.e. it can be spun clock-wise or anti-clockwise) so its thrust force can be switched between being directed up and down. However, it works most efficiently when the thrust is directed up to propel the nose of the AUV up as in Figure 3d to effect vertical take-off from the seabed. As shown in <FIG>, the pressure vessel <NUM> carries the vertical thruster on a strut <NUM> at the forward end of the pressure vessel <NUM>. The strut <NUM> is fixed, so the orientation of the vertical thruster <NUM> is fixed relative to the pressure vessel <NUM> and the rest of the AUV.

Therefore its thrust force cannot be re-oriented at an angle from the vertical direction <NUM>.

In an alternative embodiment (not shown) the vertical thruster <NUM> may be thrust-vectored - that is, its thrust force can be re-oriented at an angle from the vertical direction relative to the pressure vessel <NUM> and the rest of the body of the AUV. However this is less preferred because it would make it more difficult to shroud compactly.

The overall shape of the AUV is a circular disc, and various significant aspects of its shape will now be discussed.

The port and starboard shrouds <NUM>, <NUM> have a convex planform external profile when viewed from above in the height direction as in <FIG>. Similarly the vertical shroud <NUM> at the tail of the AUV has a convex planform external profile when viewed from above in the height direction as in <FIG>.

As can be seen in <FIG>, the AUV (including the shrouds <NUM>, <NUM>, <NUM>) has a substantially circular planform external profile when viewed from above in the height direction, except where the shells <NUM>, <NUM> are cut away to provide the openings for the horizontal thrusters (these cut-away regions presenting a straight planform profile as indicated in <FIG> at <NUM>, rather than a circular planform profile).

As can also be seen in <FIG> the AUV has a maximum length L in the fore-aft direction which is approximately equal to its maximum width W in the port-starboard direction. In other words the length-to-width aspect ratio (L/W) of the AUV is approximately one. This aspect ratio provides a number of advantages. Firstly - it enables the AUVs to be packed together efficiently when they are stored in the deployment/retrieval device <NUM>, on the deck of the surface vessel <NUM>, or at another storage location. Secondly - it enables the AUV to be easily rotated about a vertical axis in a confined space. Thus the AUV can be rotated without being removed from the pallet of <FIG> on the deck of the surface vessel in order to place it in the correct orientation for connecting a charging cable to a charging socket (not shown) in the side of the AUV. It also enables the AUV to rotate within the confined space of the thin end of the deployment funnel <NUM> during underwater deployment - operating its horizontal thrusters differentially to orient it in the correct direction with its nose or tail pointing out of the deployment funnel. Thirdly, when the AUV arrives at the seabed it can land in any orientation regardless of the direction of ocean currents. This can be contrasted with an AUV with a higher aspect ratio (L>>W) which would present a higher drag profile to width-wise (port-starboard) currents than to length-wise (fore-aft) currents and hence must land with its length running parallel with the ocean currents to prevent it from being disturbed by them during the seismic survey.

Note that the AUV has no protruding parts such as fins, control surfaces, thrusters etc. which protrude from the side, front or back of the body of the AUV. Any such protruding parts might break during operation of the AUV. If such protruding parts are included in an alternative embodiment, then the length-to-width aspect ratio (L/W) of the AUV - including the protruding parts - may deviate from unity by up to <NUM>%. In other words, in such an alternative embodiment <NUM><L/W<<NUM>. Alternatively the AUV may remain with no protruding parts but be shaped with a more elongated planform profile.

The AUV has a relatively small height relative to its length and width. In other words the AUV has a maximum height H in the height direction, and the maximum width (W) and maximum length (L) are both higher than the maximum height H. So with reference to <FIG> the AUV has a maximum height H between the disk <NUM> at the base of the AUV and the upper skin <NUM>, a maximum width W between the port and starboard extremities of the shrouds <NUM>, <NUM>, and the width-to-height aspect ratio (W/H) is approximately <NUM>. Similarly, with reference to <FIG>, the AUV has a maximum length L between the nose <NUM> and tail <NUM>, and the length-to-height aspect ratio (L/H) is approximately <NUM>. This relatively small height provides the benefit of presenting relatively low drag to ocean currents when the AUV is stationed on the seabed, and also makes it less likely to being disturbed on the seabed by trawls and dredges.

Note that the AUV has no protruding parts such as fins, control surfaces, thrusters etc. which protrude from the top or bottom of the body of the AUV. Any such protruding parts might break during operation of the AUV. If such protruding parts are included in an alternative embodiment, then the height - including the protruding parts - may increase so the aspect ratios L/H and W/H may reduce to as low as <NUM>. Alternatively the AUV may remain with no protruding parts but be shaped with a more heightened profile.

The body <NUM>, <NUM>, <NUM> of the AUV, and preferably the AUV as a whole (that is, including any shrouds, fairings, fins, control surfaces, thrusters or other protruding parts) has a planform external profile (that is, an external profile when viewed from above as in <FIG>) with two lines of symmetry: a fore-aft line of symmetry running between the nose <NUM> and the tail <NUM>, and a port-starboard line of symmetry running between the shrouds <NUM>, <NUM>. This provides a symmetrical hydrodynamic profile with similar drag characteristics regardless of whether the AUV is moving forwards or backwards.

Similarly the body <NUM>, <NUM>, <NUM> of the AUV, and preferably the AUV as a whole (that is, including any shrouds, fairings, fins, control surfaces, thrusters or other protruding parts) has an external profile when viewed from the side (as in <FIG>) with at least two lines of symmetry: a fore-aft line of symmetry <NUM> shown in <FIG> running between the nose <NUM> and the tail <NUM>, and a vertical line of symmetry running vertically from top to bottom (in the mid-plane <NUM>). This also provides a symmetrical hydrodynamic profile with similar drag characteristics regardless of whether the AUV is moving forwards or backwards.

The openings <NUM>-<NUM> in the horizontal ducts have peripheral edges which are swept by <NUM>° relative to the port-starboard direction (as can be seen by the <NUM>° angle of the line <NUM> in <FIG>) so that they are visible around their full circumference when viewed in the port-starboard direction as in <FIG>. Similarly the top and bottom openings of the vertical duct have peripheral edges which lie at an angle to the fore-aft direction so that they are visible around their full circumference when viewed in the fore-aft direction as in <FIG>.

Claim 1:
A method of deploying autonomous underwater vehicles (AUVs), the method comprising loading the AUVs (1a, 1b, 1c) into a deployment device (<NUM>), the deployment device comprising a deployment funnel (<NUM>) with a wide opening facing out of the deployment device (<NUM>) and a narrow opening facing into the deployment device (<NUM>); submerging the deployment device containing the AUVs after the AUVs have been loaded into the deployment device; towing the submerged deployment device containing the AUVs with a surface vessel (<NUM>); moving the AUVs to the deployment funnel (<NUM>) of the submerged deployment device; deploying the AUVs from the deployment funnel (<NUM>) of the submerged deployment device as it is towed by the surface vessel; and operating a thruster (310a, 310b) of each AUV after it has been deployed so that it moves away from the submerged deployment device.