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Title:
INTERMEDIATE DOCKING STATION FOR UNDERWATER VEHICLES
Document Type and Number:
WIPO Patent Application WO/2021/049949
Kind Code:
A1
Abstract:
Submersible intermediate docking station for at least one tethered or untethered underwater vehicle, adapted to be connected to a surface vessel through a tow cable. The submersible intermediate docking station and/or tow cable are controllable to attain one or more selectable equilibria between a resultant force vector of the tow cable acting upon the submersible intermediate docking station and a resultant force vector of the submersible intermediate docking station acting opposite of the tow cable, whereby motions of the submersible intermediate docking station are partly or largely decoupled from motions of the surface vessel.

Inventors:
SJÖBLOM HENRIK ALPO (NO)
SMINES VIDAR (NO)
Application Number:
PCT/NO2020/050232
Publication Date:
March 18, 2021
Filing Date:
September 11, 2020
Export Citation:
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Assignee:
KONGSBERG MARITIME CM AS (NO)
International Classes:
B63G8/00; B63B21/66
Domestic Patent References:
WO2019121743A12019-06-27
WO2015020529A12015-02-12
Foreign References:
JPH07223588A1995-08-22
US8145369B12012-03-27
CN111284633A2020-06-16
Attorney, Agent or Firm:
BRYN AARFLOT AS (NO)
Download PDF:
Claims:
CLAIMS

1. A system for operating a submersible intermediate docking station for at least one tethered or untethered underwater vehicle, wherein the submersible intermediate docking station is connectable to a surface vessel through a tow cable, and wherein the submersible intermediate docking station and/or tow cable are controllable to attain one or more selectable equilibria between a resultant force vector of the tow cable acting upon the submersible intermediate docking station and a resultant force vector of the submersible intermediate docking station acting opposite of the tow cable, whereby motions of the submersible intermediate docking station are partly or largely decoupled from motions of the surface vessel.

2. The system according to claim 1 , comprising at least one winch adapted for actively moving the tow cable at a specified speed, tension or length.

3. The system according to claim 1 or 2, comprising at least one self- traversing winch adapted for actively moving the docking station along a passive tow cable, or along an actively controlled tow cable, at a specified speed, tension or length.

4. The system according to one of claims 1-3, wherein the system is operable to maintain the tow cable in tension.

5. The system according to one of claims 1-4, wherein the intermediate docking station is provided with at least one of:

- control surfaces for lift control,

- control surfaces for drag control,

- buoyancy and ballast tanks for flotation or posture control,

- a movable tow cable termination point for posture control, - movable weights for posture control,

- through-flow control for drag and turbulence management,

- at least one propulsor,

- at least one hydro-acoustic or light-based position transponder, receiver or reflector, - at least one sensor for measuring inclination (posture),

- at least one sensor for measuring depth, or

- at least one sensor for measuring speed. 6. System according to one of claims 1-5, comprising a controller adapted for controlling at least one winch based on input parameters related to at least one of sea state conditions for the surface vessel, actual or predicted motions of the surface vessel, or actual or predicted motions of the docking station. 7. System according to claim 6, wherein the at least one winch is arranged on, or attached to, the surface vessel and/or the submersible intermediate docking station.

8. System according to one of claims 1-7, wherein the surface vessel comprising a dynamic positioning system and an associated propulsion system.

9. System according to claim 8, wherein a response time of the dynamic positioning system and the associated propulsion system is equal to or less than a natural wave induced surge motions of the surface vessel.

10. System according to claim 9, wherein input parameters to the dynamic positioning system and associated propulsion system comprising at least one of:

- sea state, such as wave heights or wave periods, or

- actual or predicted vessel motions, such as accelerations, velocities or motion periods.

11 System according to one of claims 1 -10, wherein the surface vessel comprising at least one ramp, dock, moonpool, crane or hoist for launch and/or recovery of the intermediate docking station.

12. System according to one of claims 1-11, wherein the surface vessel is a manned or unmanned vessel, and where the surface vessel is operated manually, partly- or fully remotely, automatically or autonomously.

13. System according to one of claims 1 -12, wherein the tow cable is an umbilical cable transmitting at least one of power, control signals or electronic data. 14. System according to claim 13, wherein the at least one of power, control signals or electronic data is transmittable in at least one of electrical, hydraulic, pneumatic or optical form.

15. System according one of claims 1-14, wherein the intermediate docking station serves as a protective enclosure or exoskeleton for one or more underwater vehicles.

16. System according to one of claims 1-15, wherein the underwater vehicle is an ROV or AUV, or multiple ROV and/or AUV vehicles.

17. Submersible intermediate docking station for at least one tethered or untethered underwater vehicle, wherein the submersible intermediate docking station is adapted to be connected to a surface vessel through a tow cable, and wherein the submersible intermediate docking station and/or tow cable are adapted to be controllable to attain one or more selectable equilibria between a resultant force vector of the tow cable acting upon the submersible intermediate docking station and a resultant force vector of the submersible intermediate docking station acting opposite of the tow cable, whereby motions of the submersible intermediate docking station are partly or largely decoupled from motions of the surface vessel.

18. Submersible intermediate docking station according to claim 17, wherein the submersible intermediate docking station and/or at least one winch is adapted for being dynamically controlled for controlling a tension of the tow cable based on input parameters related to at least one of actual or predicted motions of the surface vessel or actual or predicted motions of the submersible intermediate docking station.

19. Submersible intermediate docking station according to claims 17 or 18, wherein the submersible intermediate docking station is provided with at least one of:

- control surfaces for lift control, - control surfaces for drag control,

- buoyancy and ballast tanks for flotation or posture control,

- a movable tow cable termination point for posture control,

- movable weights for posture control,

- through-flow control for drag and turbulence management, - at least one propulsor,

- at least one hydro-acoustic position transponder, receiver or reflector,

- at least one sensor for measuring inclination (posture),

- at least one sensor for measuring depth, or

- at least one sensor for measuring speed.

20. Submersible intermediate docking station according to one of claims 17-19, further comprising a winch adapted for active winch control.

21. Submersible intermediate docking station according to one of claims 17-20, wherein the tow cable is an umbilical cable.

22. Submersible intermediate docking station according to one of claims 17-21 , further comprising a connector module or a wireless system for transmittal of at least one of power, control signals or other electronic data to or from the underwater vehicle.

23. Submersible intermediate docking station according to one of claims 17-22, further comprising a tether management system for a tethered underwater vehicle. 24. Submersible intermediate docking station according to one of claims 17-23, wherein the intermediate docking station is controllable from the surface vessel.

25. Submersible intermediate docking station according to one of claims 17-24, wherein the submersible intermediate docking station is serving as a protective enclosure or exoskeleton for one or more underwater vehicles. 26. A method for operating a submersible intermediate docking station for at least one tethered or untethered underwater vehicle, wherein the submersible intermediate docking station is connected to a surface vessel through a tow cable, and controlling at least one of the submersible intermediate docking station and/or tow cable to attain one or more selectable equilibria between a resultant force vector of the tow cable acting upon the submersible intermediate docking station and a resultant force vector of the submersible intermediate docking station acting opposite of the tow cable, whereby motions of the submersible intermediate docking station are partly or largely decoupled from motions of the surface vessel. 27. Method according to claim 26, further comprising dynamically controlling the submersible intermediate docking station and/or at least one winch for controlling a tension of the tow cable based on input parameters related to at least one of actual or predicted motions of the surface vessel and actual or predicted motions of the submersible intermediate docking station.

28. Method according to one of claims 27 or 28, further comprising controlling a speed, posture and/or position/depth of the submersible intermediate docking station during towing. 29. Method according to claim one of claims 26-28, further comprising stabilizing and/or altering at least one of a speed, posture and/or position/depth of the submersible intermediate docking station during towing by at least one of: a) affecting tow parameters at the surface vessel end of the tow cable, such as surface vessel speed and -heading, tow cable tension, paid out length-, reel velocity or exit point of cable; or b) affecting tow parameters at the docking station end of the tow cable, such as hydrodynamic lift, drag or thrust, buoyancy and centre of buoyancy, centre of gravity or tow cable termination point; or c) affecting tow parameters along the tow cable, such as catenary manipulation by means of buoyancy, drag, lift or weight; or d) affecting the tow geometry, i.e. vertical and horizontal components of the distance between docking station and surface vessel.

30. Method according to claim 29, further comprising countering wave induced motions of the surface vessel with a dynamic positioning system and an associated propulsion system, where a response time of the dynamic positioning system and the associated propulsion system is equal to or less than a natural wave induced surge motion of the surface vessel.

31. Method according to claim 30, wherein the input parameters to the dynamic positioning system and associated propulsion system comprising at least one of:

- sea state, such as prevailing, predicted or approaching wave heights or wave periods, or

- vessel motions, such as at least one of actual or predicted motions, accelerations, velocities or motion periods.

32. Method according to one of claims 26-31 , further comprising launching and/or recovering the submersible intermediate docking station from the surface vessel over a ramp, through a moonpool or over the side of the surface vessel.

33. Method according to one of claims 26-32, further comprising undocking or docking an underwater vehicle with the intermediate docking station during towing of the intermediate docking station by the surface vessel while the intermediate docking station is in a submerged state.

34. Method according to one of claims 26-33, further comprising operating an underwater vehicle while tethered to the intermediate docking station in a submerged state or while tethered to the intermediate docking station retrieved to the surface vessel, or while tethered directly to the surface vessel.

35. Method according to one of claims 26-34, further comprising operating the intermediate docking station as a submerged charging and/or electronic data exchange station for tethered or untethered underwater vehicles. 36. Method according to one of claims 26-35, further comprising operating the intermediate docking station as a survey platform while being submerged, e.g. by survey sensors fitted onto the intermediate docking station or by survey sensors fitted onto an underwater vehicle docked to the intermediate docking station. 37. Use of the submersible intermediate docking station according to one of claims 17-25,

- as a tether management system for an underwater vehicle, whether while stationary or under tow;

- for launch and recovery of underwater vehicles from and to a surface vessel; - for transmitting power and electronic data between a surface vessel and at least one submerged underwater vehicle;

- to carry payloads or instruments to serve at least one underwater vehicle, or to serve independently from, or as a substitute for, at least one underwater vehicle.

Description:
Intermediate docking station for underwater vehicles INTRODUCTION

The invention concerns a submersible intermediate docking station for underwater vehicles. The invention also concerns a system for operating a submersible intermediate docking station for at least one tethered or untethered underwater vehicle. The invention also concerns using said intermediate docking station for launch and recovery of underwater vehicles from and to a surface vessel, using said intermediate docking station for transmitting power and electronic data between a surface vessel and submerged underwater vehicles, or using said intermediate docking station to carry payloads or instruments, either to serve underwater vehicles, or to serve independently from, or as substitute for, underwater vehicles.

BACKGROUND

Launch and recovery of tethered or untethered underwater vehicles directly from a surface vessel require that the motions of the surface vessel and underwater vehicle are synchronized during undocking and docking. This could be particularly difficult in relation to small surface vessels exhibiting rapid/extensive motions in waves.

For tethered underwater vehicles such as ROVs, whether tethered directly to the surface vessel, or indirectly via a tether management system connected to the surface vessel with an umbilical, launch and recovery is most commonly performed via a moonpool or over the side while the surface vessel is largely stationary (apart from wave induced motions). During launch and recovery synchronization of the motions of the surface vessel and the underwater vehicle is performed solely by the tether or umbilical cable (as the case may be) from which the underwater vehicle is suspended. However, since a cable is not suitable to transfer shear forces, this method is only able to achieve a limited degree of synchronization in the lateral plane. Furthermore, since a cable is also not suitable to transfer pressure forces, synchronization along the vertical axis (i.e. principal direction of launch and recovery while surface vessel is stationary) is limited to that which can be performed with the cable in tension. Synchronization of the motions of the surface vessel and the underwater vehicle along the vertical axis is thus largely limited to velocities less than the (assisted or unassisted) sinking speed of the underwater vehicle. In the event that velocities, notably the downwards heave motions of the surface vessel, would exceed the sinking speed of the underwater vehicle, tension in the cable would be compromised. This would in turn compromise the required synchronization (notably loosing control of clearance between the surface vessel and the underwater vehicle) and pose a risk for excessive bending and breakage of the cable, or even collision between the surface vessel and the underwater vehicle.

For untethered vehicles such as AUVs, launch and recovery is most commonly performed over the side or stern, while the surface vessel is stationary or under way at slow speed, and while the underwater vehicle is at or close to the surface. The underwater vehicle is typically launched and recovered either with cranes or via ramps or docks. Recovery is typically contingent upon either having to catch and haul in the AUV with a cable or rope, or requiring the AUV to hone in on a ramp or dock under own propulsion. Either way entails considerable challenges and risks, since not only the surface vessel, but also the underwater vehicle, might experience extensive motions while at or near the surface, thereby posing a risk for failed or aborted docking, or even collision between surface vessel and underwater vehicle.

SUMMARY OF THE INVENTION

In a first aspect the invention provides a system for operating a submersible intermediate docking station for at least one tethered or untethered underwater vehicle. The submersible intermediate docking station is connectable to a surface vessel through a tow cable. The submersible intermediate docking station and/or tow cable are controllable to attain one or more selectable equilibria between a resultant force vector of the tow cable acting upon the submersible intermediate docking station and a resultant force vector of the submersible intermediate docking station acting opposite of the tow cable. The motions of the submersible intermediate docking station are partly or largely decoupled from the motions of the surface vessel. The system may comprise at least one winch adapted for actively moving the tow cable at a specified speed, tension or length. The system may include at least one self-traversing winch adapted for actively moving the intermediate docking station along a passive tow cable, or along an actively controlled tow cable, at a specified speed, tension or length. The at least one winch may be provided, arranged on, or attached to, the surface vessel and/or the submersible intermediate docking station.

The system may comprise a controller adapted for controlling at least one winch based on input parameters related to at least one of sea state conditions for the surface vessel, actual or predicted motions of the surface vessel, or actual or predicted motions of the intermediate docking station. The system may be operable to maintain the tow cable in tension. The intermediate docking station may be provided with at least one of: control surfaces for lift control; control surfaces for drag control; buoyancy and ballast tanks for flotation or posture control; a movable tow cable termination point for posture control; movable weights for posture control; through-flow control for drag and turbulence management; at least one propulsor; at least one hydro-acoustic or light-based position transponder, receiver or reflector; at least one sensor for measuring inclination (posture); at least one sensor for measuring depth; or at least one sensor for measuring speed.

The surface vessel may comprise a dynamic positioning system and an associated propulsion system. A response time of the dynamic positioning system and the associated propulsion system may be equal to or less than a wave induced surge motion of the surface vessel.

The input parameters to the dynamic positioning system and associated propulsion system may comprise at least one of: sea state, such as prevailing, predicted or approaching wave heights or wave periods; or vessel motions, such as actual or predicted accelerations, velocities or motion periods. The surface vessel may comprise at least one ramp, dock, moonpool, crane or hoist for launch and/or recovery of the intermediate docking station. The surface vessel may be a manned or unmanned vessel. The surface vessel may be operated manually, partly- or fully remotely, automatically or autonomously.

The tow cable may be an umbilical cable transmitting at least one of power, control signals or electronic data. The at least one of power, control signals or electronic data may be transmittable in at least one of electrical, acoustic, hydraulic, pneumatic or optical form.

The intermediate docking station may serve as a protective enclosure or exoskeleton for one or more underwater vehicles. The underwater vehicle may be an ROV or AUV, or multiple ROV and/or AUV vehicles.

In a further aspect, the invention provides a submersible intermediate docking station for at least one tethered or untethered underwater vehicle, wherein the submersible intermediate docking station is adapted to be connected to a surface vessel through a tow cable. The submersible intermediate docking station and/or tow cable are adapted to be controllable to attain one or more selectable equilibria between a resultant force vector of the tow cable acting upon the submersible intermediate docking station and a resultant force vector of the submersible intermediate docking station acting opposite of the tow cable. The motions of the submersible intermediate docking station are partly or largely decoupled from the motions of the surface vessel.

The submersible intermediate docking station and/or at least one winch may be adapted for being dynamically controlled for controlling a tension of the tow cable based on input parameters related to at least one of actual or predicted motions of the surface vessel and movements of the submersible intermediate docking station.

The submersible intermediate docking station may further comprise a winch adapted for active winch control. The tow cable may be an umbilical cable. The submersible intermediate docking station may be provided with at least one of: control surfaces for lift control; control surfaces for drag control; buoyancy and ballast tanks for flotation or posture control; a movable tow cable termination point for posture control; movable weights for posture control; through-flow control for drag and turbulence management; at least one propulsor; at least one hydro acoustic position transponder, receiver or reflector; at least one sensor for measuring inclination (posture); at least one sensor for measuring depth; or at least one sensor for measuring speed.

The submersible intermediate docking station may further comprise a connector module or a wireless system for transmittal of power, control signals or other electronic data to or from the underwater vehicle. The submersible intermediate docking station may further comprise a tether management system for a tethered underwater vehicle. The intermediate docking station may be controllable from the surface vessel. The submersible intermediate docking station may serve as a protective enclosure or exoskeleton for one or more underwater vehicles.

In a further aspect, the invention provides a method for operating a submersible intermediate docking station for at least one tethered or untethered underwater vehicle. The submersible intermediate docking station is connected to a surface vessel through a tow cable. The submersible intermediate docking station and/or tow cable is controlled to attain one or more selectable equilibria between a resultant force vector of the tow cable acting upon the submersible intermediate docking station and a resultant force vector of the submersible intermediate docking station acting opposite of the tow cable. The motions of the submersible intermediate docking station are partly or largely decoupled from the motions of the surface vessel.

The method may further comprise dynamically controlling the submersible intermediate docking station and/or at least one winch for controlling a tension of the tow cable based on input parameters related to actual or predicted motions of the surface vessel or actual or predicted motions of the submersible intermediate docking station. The method may further comprise controlling a speed, posture and position/depth of the submersible intermediate docking station during towing. The method may further comprise stabilizing and/or altering at least one of a speed, posture and/or position/depth of the submersible intermediate docking station during towing by at least one of: a) affecting tow parameters at the surface vessel end of the tow cable, such as surface vessel speed and -heading, tow cable tension, paid out length-, reel velocity or exit point of cable; or b) affecting tow parameters at the intermediate docking station end of the tow cable, such as hydrodynamic lift, drag or thrust, buoyancy and centre of buoyancy, centre of gravity or tow cable termination point; or c) affecting tow parameters along the tow cable, such as catenary manipulation by means of buoyancy, drag, lift or weight; or d) affecting the tow geometry, i.e. vertical and horizontal components of the distance between intermediate docking station and surface vessel. The method may further include countering wave induced motions of the surface vessel with a dynamic positioning system and an associated propulsion system, where a response time of the dynamic positioning system and the associated propulsion system is equal to or less than a natural wave induced surge motion of the surface vessel. This stabilizes the tow. The input parameters to the dynamic positioning system and associated propulsion system may include at least one of: sea state, such as prevailing, predicted or approaching wave heights or wave periods; or vessel motions, such as actual or predicted accelerations, velocities or motion periods. Launching and/or recovering the submersible intermediate docking station may be performed from/to the surface vessel over a ramp, through a moonpool or over the side of the surface vessel. Undocking or docking of an underwater vehicle with the intermediate docking station may be performed during towing of the intermediate docking station by the surface vessel while the intermediate docking station is in a submerged state. The method may further comprise operating an underwater vehicle while tethered to the intermediate docking station in a submerged state or while tethered to the intermediate docking station retrieved to the surface vessel, or while tethered directly to the surface vessel. The intermediate docking station may also be operating as a submerged charging and/or electronic data exchange station for tethered or untethered underwater vehicles. The intermediate docking station may also operate as a survey platform while being submerged. Survey sensors may then be fitted directly onto the intermediate docking station, and/or fitted onto an underwater vehicle docked to the intermediate docking station.

In a further aspect, the invention also provides uses of the submersible intermediate docking station as a tether management system for an underwater vehicle, whether while stationary or under tow; for launch and recovery of underwater vehicles from and to a surface vessel; for transmitting power and electronic data between a surface vessel and at least one submerged underwater vehicle; to carry payloads or instruments to serve at least one underwater vehicle, or to serve independently from, or as a substitute for, at least one underwater vehicle.

The invention provides a safe and efficient launch and recovery of underwater vehicles from a surface vessel, in particular for small surface vessels prone to wave induced motions. The underwater vehicle is launched from the surface vessel to the sea and recovered from the sea to onboard the surface vessel while docked to the intermediate docking station, with the option of the intermediate docking station serving as a protective enclosure or exoskeleton during launch and recovery. Use of an intermediate docking station provides a more robust procedure for launch and recovery of underwater vehicles. Undocking and docking of the underwater vehicle takes place while the intermediate docking station is submerged. As the intermediate docking station is partly or largely decoupled from the motions of the surface vessel, docking and undocking of the underwater vehicle from the intermediate docking station take place in an environment less affected by waves and any wave induced motions of the surface vessel as the case may be. This facilitates more controlled and safe maneuvering of the intermediate docking station and/or the underwater vehicle during the undocking and docking procedure. Use of an intermediate docking station also provides a flexible and cost efficient procedure for underwater operations. The intermediate docking station may have multiple uses as e.g. an independent survey platform, as a charging and/or electronic data exchange station for underwater vehicles, to carry payloads or instruments to an underwater vehicle. Such uses could reduce the need for more frequent launch and recovery of underwater vehicles from and to the surface vessel.

BRIEF DESCRIPTION OF DRAWINGS

Example embodiments of the invention will now be described with reference to the following drawings, where:

Figure 1 illustrates mainly forward and exterior of an example submersible intermediate docking station and an adjacent tethered underwater vehicle. At the forward of the body of the intermediate docking station, there is a termination point for the tow cable connecting the intermediate docking station to a surface vessel (surface vessel not shown). Suggested control devices are fitted onto the intermediate docking station;

Figure 2 illustrates example control devices fitted within the body of a submersible intermediate docking station;

Figure 3 illustrates mainly aft and interior of an exemplary submersible intermediate docking station and an adjacent exemplary tethered underwater vehicle. At an aft of the body, there is an entry opening for the underwater vehicle to enter the submersible intermediate docking station. Within the submersible intermediate docking station, there is a fitted space for an underwater vehicle to be docked, connection and securing interfaces for the underwater vehicle and a tether management system to pay out and reel in tether;

Figure 4 illustrates an example surface vessel carrying an example intermediate docking station containing an exemplary underwater vehicle. The intermediate docking station is placed on a ramp facing the stern of the surface vessel, from which launch and recovery may take place. The surface vessel may be provided with a winch to operate the tow cable;

Figure 5 illustrates an example intermediate docking station in a launched position towed behind a surface vessel;

Figure 6 illustrates an intermediate docking station 4 under tow while submerged, which may be below the wave base, and during which the intermediate docking station may be maintained in a steady state;

Figure 7 illustrates an intermediate docking station maintained in a steady state, during which the underwater vehicle undocks or docks;

Figure 8 illustrates an intermediate docking station, from which an exemplary tethered underwater vehicle has undocked while being under way. Power and control signals may be exchanged between the surface vessel and the underwater vehicle through the tow cable and tether. Positions a) and b) shows alternative configurations of the tether tow; Figure 9 illustrates an intermediate docking station being retrieved back to a surface vessel while the underwater vehicle remains submerged and underway. Positions a) and b) shows alternative configurations of the tether tow;

Figure 10 illustrates an intermediate docking station suspended under a surface vessel while in a stationary mode;

Figure 11 illustrates an intermediate docking station operating independently from an underwater vehicle, e.g. by performing seabed survey by means of survey equipment fitted directly onto the intermediate docking station (survey equipment not shown).

Figure 12 shows an illustrative overview of several prospective modes of operation for an intermediate docking station and underwater vehicle, as well as select positions of an intermediate docking station during undocking and docking the underwater vehicle from and to an intermediate docking station, as well as launch and recovery of an intermediate docking station from and to a surface vessel.

DETAILED DESCRIPTION Example embodiments of the invention are described with reference to the drawings. The same reference numerals are used for the same or similar features in all the drawings and throughout the description.

A submersible intermediate docking station 2 for underwater vehicles is shown in Figure 1. The intermediate docking station 2 is connected to a surface vessel via a tow cable 4. The tow cable may serve as an umbilical for power, communications, control signals and/or data. The intermediate docking station may typically be towed after the surface vessel (towed mode), either at the surface or submerged, but could also be submerged suspended below the surface vessel (stationary mode). The intermediate docking station 2 may provide a protective housing for one or more underwater vehicles, but could also solely be a lash-on point for underwater vehicles. The underwater vehicle may be a tethered or untethered underwater vehicle. The intermediate docking station 2 is controllable to various extents by affecting and managing the forces acting upon the intermediate docking station. Such forces may result from pull, thrust, gravity, inertia, buoyancy, hydrodynamic lift and -drag. The purpose of control is to be able to launch, submerge and recover the intermediate docking station from and to the surface vessel with a level of control. While submerged, a state is established where the motions of the intermediate docking station are partly or largely decoupled from the motions of the surface vessel. The motions of the intermediate docking station may instead be synchronized with the motions of the underwater vehicle 3 in order to facilitate undocking and docking of underwater vehicles with the intermediate docking station in a manner less contingent upon the motions of the surface vessel and less affected by surface waves. Launch and recovery of the intermediate docking station from and to the surface vessel would typically take place while the intermediate docking station is towed at or near the surface, and the underwater vehicle is docked in or to the intermediate docking station, with the possibility of also being protected by the intermediate docking station. Undocking and docking of the underwater vehicle from and to the intermediate docking station will typically take place while the intermediate docking station is towed submerged at some depth and maintained in a steady state partly or largely decoupled from the motions of the surface vessel. Further details of the intermediate docking station and a system for operating an intermediate docking station are provided in the following.

Figure 1 illustrates mainly a fore part and exterior sides of an intermediate docking station 2. The intermediate docking station 2 comprises a body 20, which may be closed (e.g. shell structure) or open (e.g. lattice structure). The body may serve as a protective housing or exoskeleton for all or part of an underwater vehicle 3. The underwater vehicle may e.g. be an ROV or AUV, or multiple ROV and/or AUV vehicles. In Figure 1 , the underwater vehicle 3 is tethered 5 to the intermediate docking station 2, however the underwater vehicle may also be untethered.

The intermediate docking station may have a body in the form of a shell structure with an elongated shape as illustrated in the example embodiment in Figure 1. The elongated shape may have substantially rectangular long sections and quadratic transverse sections, and is substantially symmetrical around a longitudinal axis. In alternative embodiments, alternative symmetrical or asymmetrical geometries could apply.

In the embodiment in Figure 1 , the fore part of the body 20 is tapered to reduce hydrodynamic resistance and turbulence while the intermediate docking station 2 is under tow, and to aid retrieval of the intermediate docking station into the surface vessel 1. The body and in particular the fore part of the body 20 may be ruggedized to allow for jolts and impacts during release and retrieval from and to the surface vessel. At the forward of the body 20, there is a termination point 21 for the tow cable 4. The termination point 21 may typically be located at or close to the central longitudinal axis of the intermediate docking station. The termination point may be movable to certain extent in either transverse direction to affect the posture (inclination) of the intermediate docking station while under tow. In alternative embodiments, the termination point may be fixed or movable only in certain directions.

One or more control devices may be fitted onto the body 20 of the intermediate docking station 2. Figure 1 shows the following suggested control devices fitted onto the body: control surfaces 23 (e.g. fins) for dive control; control surfaces 22 (e.g. flaps) for drag control; adjustable louvres 24 for through-flow control, drag and wake management. In alternative embodiments, the intermediate docking station may also be fitted with energized propulsors, such as propellers, rotors, pump jets, reaction thrusters or similar, which may partly or fully substitute other control devices or partly or fully substitute the tow force delivered by the surface vessel to the tow cable.

Figure 2 shows, indicated with thick lines, the following suggested control devices fitted within the body of the intermediate docking station: buoyancy 27 and/or ballast 28 tanks for flotation and posture control; movable fixed ballast 29 (weights) for posture (inclination) control; movable tow cable termination point 25 (e.g. revolving disks) for posture control. The intermediate docking station may also be provided with movable weights for posture control. Further to aforementioned movable control surfaces, the body may be fitted with (not shown) fixed control surfaces for directional stability, or movable control surfaces for sideways heading control (further to solely passive stabilization or alignment with the heading of the tow cable). In an alternative embodiment, the body of the intermediate docking station may itself be formed to serve as a control surface, i.e. generate a variable lift or drag, if the body is tilted e.g. by aforementioned methods of posture control. The intermediate docking station may typically be designed to be hydrodynamically stable, but could alternatively be hydrodynamically unstable. Protruding control devices as e.g. fins 23 and flaps 22, may be retractable or foldable flush with the ruggedized exterior of the intermediate docking station to reduce risk of damage during release and retrieval from and to the surface vessel.

The intermediate docking station 2 may also be provided with at least one of a hydro-acoustic or light-based position transponder, receiver or reflector, which may be arranged as appropriate to ensure line of sight between the docking station and underwater vehicle, and between the docking station and the surface vessel as the case may be. Further, the intermediate docking station may be provided with sensor(s) for measuring inclination (posture), sensor(s) for measuring depth and/or sensor(s) for measuring speed.

Figure 3 illustrates mainly the aft part and interior of the intermediate docking station and an adjacent tethered underwater vehicle. At the aft of the body, there is an opening for the underwater vehicle to enter or depart the intermediate docking station. The body of the intermediate docking station in Figure 3 serves as a protective housing or exoskeleton for the underwater vehicle. In alternative embodiments, if the body is not a protective housing or exoskeleton, the intermediate docking station may be provided with solely a lash-on point for the underwater vehicle to attach and detach to and from the intermediate docking station.

The interior 30 of the intermediate docking station in Figure 3 comprises a fitted space to accommodate the underwater vehicle while docked, securing/releasing devices and a tether management system 31 to pay out and reel in tether.

The embodiment of the intermediate docking station 2 show in Figure 3 is adapted to fully contain a substantially cuboid underwater vehicle. In alternative embodiments, the intermediate docking station may be adapted to serve multiple or different underwater vehicles simultaneously or successively, which may be in a variety of sizes and forms, or being contained fully or partially or not at all within the intermediate docking station. Accordingly, intermediate docking stations may be produced in a variety of sizes and forms, or with features allowing them to be configurable for the underwater vehicles in question.

Figure 4 illustrates an example surface vessel 1 carrying an intermediate docking station 2 containing an underwater vehicle 3. The intermediate docking station 2 is placed on a ramp 11 facing the stern of the surface vessel 1 , from which launch and recovery may take place. The entry to the ramp may be arranged with tapering sides or guides, which may be complimentary to, or a substitute for, the tapered forebody of the intermediate docking station to aid retrieval of the intermediate docking station onto the ramp 11. The ramp may also be located in other appropriate locations on the surface vessel, e.g. on the side or at the fore body of the surface vessel 1.

Active manipulators, such as movable tow cable guides or other catchment devices, may be used to further control the intermediate docking station during launch and recovery. Launch and recovery may also take place via a dock or moonpool, or the intermediate docking station may be hoisted into or out of the water with a crane or other hoist device. The surface vessel is provided with a tow cable connection point. The tow cable connection point may be movable between different positions. The tow cable 4 typically exits the surface vessel 1 via the ramp 11. Relevant rollers and guides may be fitted. Alternatively, the tow cable may be permanently or temporarily relocated to exit the surface vessel at another location, e.g. using slots or cable guides to relocate the exit point of the cable closer to midship, which may be preferable to minimize excitation of the tow cable e.g. from pitching of the surface vessel. The tow cable connection point may differ for different phases of operation, e.g. one launch and recovery position and another tow- or rest position. The surface vessel 1 may be fitted with a winch to operate the tow cable. The winch may be of a conventional reel type (i.e. with cable storage), or alternatively a traction winch or similar (i.e. with separate cable storage). The winch may be adapted for active winch control, including the ability to pay out or haul in cable to a given length or speed, and/or at a given tension. While not in use, cable may be stored on a winch drum, or a separate storage drum or similar. Alternatively, the intermediate docking station 2 may be provided with a winch for the tow cable 4 as a substitute for, or as a supplement to, a winch aboard the surface vessel 1. Subject winch could be a conventional reel type (i.e. with cable storage), or alternatively a self-traversing winch (i.e. without cable storage, but solely for traversing along a cable). A self-traversing winch may be adapted for actively moving the intermediate docking station along a passive tow cable, or along an actively controlled tow cable, at a specified speed, tension or length. A self- traversing winch operating on a passive cable could thus serve as a substitute for another winch for active tow cable control, whereas a self-traversing winch operating on an actively controleld cable could thus serve as a supplement or back-up for another winch for active tow cable control.

The intermediate docking station may also be controlled with multiple grouped cables or multiple separated cables rather than use of a single cable.

Figure 8 illustrates an overall system in a state of operation showing major components. A surface vessel 1 is towing a submerged intermediate docking station 2. The intermediate docking station 2 is connected to the surface vessel 1 through a tow cable 4. The tow cable 4 is typically heavier than water. A tethered underwater vehicle 3 is connected to the intermediate docking station 2 through a tether cable 5. The tether cable is typically neutrally buoyant in water. The tow- and tether cables respectively may be produced in metallic, non-metallic or composite materials.

One or more fixed or controllable devices may be fitted onto or within the tow cable 4 for the purpose of reducing drag, preventing vortex induced vibration and/or for catenary management, such as helical strake, hard-, ribbon- or hairy fairing, fixed or controllable weight or buoyancy elements, paravanes, birds or other devices for the purpose.

The tow cable 4 may be fitted with one or more propulsors, which could serve for the purpose of catenary management, or which could partly or fully substitute the tow force delivered by the surface vessel to the tow cable and in turn the intermediate docking station.

The tow- and tether cables respectively may also serve as umbilical cables, thereby enabling transmittal of power, control signals or other electronic data either from the surface vessel to the intermediate docking station, and/or from the intermediate docking station to the underwater vehicle, or vice versa. Transmittal would typically be electrical, but could also be either hydraulic, pneumatic, optical, acoustic or a combination thereof. Alternatively, a cable serving as umbilical may be separate from the tow cable.

The underwater vehicle 3 may be untethered, in which case the intermediate docking station need not include a tether management system.

Communications between the surface vessel 1 and intermediate docking station 2 and/or between the intermediate docking station 2 and underwater vehicle 3 may be wireless, such as, but not limited to, hydro-acoustic or light-based underwater communications. Light-based may e.g. be a laser based communication system.

The intermediate docking station 2 may be fitted with connection interfaces as an alternative or supplementary means for transmitting energy and exchange electronic data with tethered or untethered underwater vehicles while docked. The interface may be a connector module or a wireless system for transmittal of power, control signals or other electronic data to or from the underwater vehicle. A connector module or wireless system may typically be arranged in the interior of the intermediate docking station, in proximity or adjacent to a corresponding interface or transmitter fitted to the underwater vehicle.

In the embodiment shown in Figure 8, the tow itself delivers the entire force required for the intermediate docking station 2 to overcome hydrodynamic resistance and drag while underway. In alternative embodiments, the intermediate docking station may be fitted with energized propulsors, such as propellers, rotors, pump jets, reaction thrusters or similar, which may partly or fully substitute the tow force delivered by the tow cable In the embodiment shown in Figure 8, energy required to operate control devices of the intermediate docking station is supplied as electrical power via the tow cable 4 being an umbilical. In an alternative embodiment, energy required to operate control devices on the intermediate docking station 2 may be derived either from batteries or other forms of energy accumulators carried aboard the intermediate docking station 2, or energy derived from the intermediate docking station being towed through the water, such as hydrodynamic levers, micro-turbines or similar. Figure 4 illustrates an example surface vessel 1 carrying an intermediate docking station 2 containing an underwater vehicle 3. The intermediate docking station 2 is placed on a ramp 11 facing the stern of the surface vessel, from which launch and recovery may take place. The underwater vehicle 3 is securely docked to the intermediate docking station 2, and protected by its enclosing body 20. Any protruding control devices would typically be retracted or folded. The intermediate docking station 2 and underwater vehicle 3 collectively are typically neutrally or positively buoyant at this stage, but may be negatively buoyant. The surface vessel 1 is typically under way at some speed, but may in principle be stationary. Launch may take place by skidding or otherwise releasing or ejecting the intermediate docking station 2 into the water. The stern of the surface vessel 1 may be trimmed aft to aid release of the intermediate docking station 2 from the stern ramp 11. In alternative embodiments, the aft ramp 11 may be partially or fully submerged, or be arranged as a dock. A submerged ramp or dock may also be used to reduce the waterplane area of the surface vessel, which may reduce wave induced heave or pitch motions and/or create additional motion damping. Insofar the surface vessel 1 is under way, selection of an appropriate speed could help reduce wave induced dynamic motions of the surface vessel, such as due to resonance, and notably prevent parametric roll.

Since the intermediate docking station 2 containing the underwater vehicle 3 is towed behind the surface vessel 1 by means of a robust tow cable 4, it is typically possible to maintain a speed exceeding the self-propelled speed of the underwater vehicle 3, or exceeding the tow speed that would typically be permissible if towing a tethered underwater vehicle 3 directly, hence the tow speed may be chosen more freely.

Insofar the surface vessel 1 is under way, drag from the passing water will pull the intermediate docking station 2 away from the surface vessel upon entry to the water, while paying out tow cable 4 permits the intermediate docking station to move away from the surface vessel. It follows that the surface vessel 1 should be designed so as to avoid excessive reverse flow trailing the surface vessel to ensure separation between intermediate docking station and surface vessel.

Figure 5 illustrates an intermediate docking station 2 in a launched position towed behind the surface vessel 1 and readied for being submerged. The distance from the surface vessel 1 will typically be such that separation with the surface vessel is ensured, and such that the vertical component of the tow cable force acting upon the intermediate docking station 2 upon being submerged will be moderate or neutral, but could in principle be at no distance from the surface vessel 1. Relevant control devices on the intermediate docking station that were retracted or folded during release from the surface vessel, may typically be extended or unfolded. Thereafter, the intermediate docking station 2 may be submerged, either by reducing the buoyancy (or commence the launch with negative buoyancy), weight of the tow cable 4 and/or by applying negative hydrodynamic lift.

Figure 6 illustrates an intermediate docking station 2 under tow while submerged to a desired depth. The depth may be below the prevailing wave base, i.e. at a depth where surface waves would have negligible influence on the intermediate docking station 2 other than that which may be transferred via the tow cable 4.

While submerged, the intermediate docking station and/or tow cable may be controlled to attain one or more selectable equilibria between a resultant force vector of the tow cable acting upon the submersible intermediate docking station and a resultant force vector of the submersible intermediate docking station acting opposite of the tow cable. Such forces may result from pull, thrust, gravity, inertia, buoyancy, hydrodynamic lift and -drag. Suggested controllers to generate or respond to such forces have already been described with reference to Figures 1-4 and 8.

The resultant force vector of the submersible intermediate docking station is a sum of drag, lift, buoyancy gravity and inertia acting upon or enacted by the submersible intermediate docking station and control devices provided on or within the submersible intermediate docking station. The resultant force vector of the tow cable is a sum of pull, drag, lift, buoyancy, gravity and inertia acting upon or enacted by the tow cable and control devices provided on or within the tow cable. The forces enacted upon the tow cable are not restricted to forces enacted solely by the tow winch and any control devices downline of the tow winch. The forces enacted upon the tow cable also includes forces from the surface vessel itself such as by controlling the speed and thrust of the surface vessel.

The one or more selectable equilibria attained by the submersible intermediate docking station and/or tow cable result in the submersible intermediate docking station seeking or attaining either of the following motion states, all of which may be partly or largely decoupled from the motions of the surface vessel: a) A stationary state, whereby the depth, speed, heading and posture of the submersible intermediate docking station are maintained constant; b) A dynamic state, whereby either the depth, speed, heading or posture may change at a given rate; c) A transient state, whereby either depth, speed, heading or posture may change at a variable rate.

Docking and undocking of the underwater vehicle with the intermediate docking station may be performed in a number of ways. Docking and undocking should be carefully performed in order to avoid damaging the underwater vehicle and the intermediate docking station. There are primarily three ways of docking and undocking: i) Maintaining the intermediate docking station in a steady state and actively controlling the underwater vehicle in towards and finally entering or attaching to the submersed intermediate docking station. For undocking, the underwater vehicle is actively controlled to propel itself out and away from the submersed intermediate docking station; ii) Maintaining the underwater vehicle in a steady state and actively controlling the intermediate docking station in towards the underwater vehicle and finally enclosing or attaching to the underwater vehicle. For undocking, the reverse process is performed and the intermediate docking station is actively controlled to release the underwater vehicle and pull away from the underwater vehicle; iii) Actively controlling both the intermediate docking station and the underwater vehicle and maneuvering them towards each other for engagement and docking. For undocking, the reverse process is performed.

The motions of the intermediate docking station 2 are partly or largely decoupled from the dynamic motions of the surface vessel 1.To facilitate undocking and docking of the underwater vehicle 3 with the intermediate docking station 2 in a safer and more predictable manner, the intermediate docking station may be synchronized with the actual motions of the underwater vehicle 3 (e.g. an underwater vehicle 3 approaching the intermediate docking station 2) or the prospective motions of an underwater vehicle 3 (e.g. an underwater vehicle 3 departing the intermediate docking station 2). Such synchronization of the intermediate docking station and underwater vehicle, is far less contingent on ocean surface conditions and surface vessel motions than direct launch and recovery from or to the surface vessel 1 , while largely reducing or preventing the risk of unintended jolts or impacts during docking and undocking of an unprotected (and often fragile) underwater vehicle 3.

Synchronization may typically be achieved by maintaining both the intermediate docking station 2 and underwater vehicle 3 substantially in a stationary state, at which both the intermediate docking station 2 and underwater vehicle 3 maintains a similar speed, depth, heading and posture, with only a marginal difference in speed, enabling the intermediate docking station 2 and underwater vehicle 3 to move closer or move apart relative to each other. At stationary state, the average speed over ground of the intermediate docking station 2 will be same as for the surface vessel 1 (unless continuously paying out or reeling in tow cable 4). Alternatively, the intermediate docking station 2 could be controlled to exhibit a desired dynamic state or transient state, yet still partly or largely decoupled from the dynamic motions of the surface vessel 1. A dynamic state could be used e.g. to synchronize the intermediate docking station 2 with an underwater vehicle, e.g. a glider-type AUV utilizing variable buoyancy as means for propulsion, whereby the underwater vehicle 3 itself is not suitable to maintain a stationary steady state

To establish the desired state, the depth, speed, posture and heading of the submerged docking station 2 through the water may be stabilized and/or altered, with the assistance of the controllers described with reference to Figures 1-4 and 8, and by: a) Affecting tow parameters at the surface vessel end of the tow cable, such as surface vessel speed and -heading, tow cable tension, paid out length-, reel velocity or exit point of cable; or b) Affecting tow parameters at the intermediate docking station end of the tow cable, such as hydrodynamic lift, drag or thrust, buoyancy and centre of buoyancy, centre of gravity or tow cable termination point; or c) Affecting tow parameters along the tow cable, such as catenary manipulation by means of buoyancy, drag, lift or weight; or d) Affecting the tow geometry, i.e. vertical and horizontal components of the distance between intermediate docking station and surface vessel.

To approach a state where the motions of the submersible intermediate docking station are partly or largely decoupled from the surface vessel, the resultant force vector of the intermediate docking station 2 and the intermediate docking station control devices should be in equilibrium with the force vector of the tow cable acting upon the intermediate docking station. Any offset from such equilibrium will cause the intermediate docking station 2 to move in some direction at a speed and direction commensurate with the extent of such offset. In the steady state, the sum of the forces acting upon the intermediate docking station is zero, and the intermediate docking station will move at a constant speed and in a fixed direction. Any offset from this equilibrium will cause the intermediate docking station to accelerate/decelerate or change direction.

The control devices on the docking pod and the surface vessel may serve individually or in conjunction with each other. The system may serve with few or many control devices, but where more control devices would generally yield an enhanced level of control.

A more comprehensive level of control would typically apply in a towed mode, where the intermediate docking station is trailing a surface vessel under way, and where multiple control devices may be engaged.

A less comprehensive level of control would typically apply in a stationary mode, where the intermediate docking station is suspended below a surface vessel at a standstill, and where applicable control devices may be restricted solely to active heave compensation of the tow (i.e. suspension) cable, unless that which could alternatively be achieved by using energized propulsors on the intermediate docking station or tow cable.

The tow and associated control devices would typically be controlled by one or more processor units. The processor units receive input on the state of the control devices as well as the state of the tow, while being able to compute and send instructions back to the control devices on the intermediate docking station and the surface vessel to affect, notably tune, the tow.

The state of the tow could be assessed with sensors fitted to surface vessel 1 , tow cable 4 and/or intermediate docking station 2. Both absolute and relative reference systems could be applied, however, relative reference systems would be sufficient to achieve basic functionality. Relevant sensors would comprise depth sensors, speed sensors, heading- and posture sensors or other motion reference units, such as inertial devices, preferably with an accuracy and response time that would also allow for accurately deriving rate of change of the same parameters (such as accelerations). Given that the tow cable is not vertical and stiff as e.g. for a lift by a heave compensated crane, but has a catenary form (curvature), the cable itself is not a reliable indicator of the state of the intermediate docking station. For this reason, it will typically be required to fit multiple sensors on the intermediate docking station itself.

The underwater vehicles 3 would typically be provided with one or more relative positioning systems or transponders enabling the underwater vehicle 3 to locate the intermediate docking station 2 or vice versa.

The surface vessel 1 would typically be provided with a dynamic positioning system. The dynamic positioning system utilizes similar sensors as explained above to automate the operation of the surface vessel’s propulsion systems to automatically maneuver and navigate the surface vessel according to given criteria such as e.g. station keeping or auto-tracking.

The input parameters to the dynamic positioning system and associated propulsion system may include at least one of sea state, such as prevailing, predicted or approaching wave heights or wave periods; or vessel motions, such as actual or predicted accelerations, velocities or motion periods.

Prevailing conditions correspond to experienced (after the fact) conditions (such as a passing wave train). Predicted conditions is a forecast based on using experience or analytics to predict future conditions (e.g. using statistical or analytical wave models to predict the next one or two waves based on a wave train that has just passed). Approaching conditions are those that can be determined in advance with sensors (e.g. using a wave radar or wave buoy to monitor incoming waves).

Optionally, the response time of a dynamic positioning system, which is typically governed by the response time of power supply and propulsors, can be configured as equal to or less than the surface vessel’s natural wave induced surge motions. This would provide for using the dynamic positioning system to actively counteract wave induced motions of the surface vessel, and thereby stabilize the tow by reducing excitation of the tow cable originating from dynamic vessel motions. Input parameters for governing such optional feature would include the input parameters to the dynamic positioning system as described above.

The surface vessel may be a manned or unmanned vessel. The surface vessel may be operated manually, partly- or fully remotely, automatically or autonomously. The intermediate docking station may be controlled from the surface vessel. The control from the surface vessel may be automatically or autonomously or semi-autonomously. The intermediate docking station may also be operated semi-autonomously or autonomously based on preprogrammed mission data.

Additionally, sensors for tow cable tension and distance between surface vessel and intermediate docking station could be applied. Sensors may be local, such as sonar or inertial motion reference units, or utilize distributed technologies such as hydro acoustic or laser based transponder/reflector/sensor technologies for position reference. Parameters not directly obtained from sensors could alternatively be deduced analytically at various degrees of accuracy by formulae of physical dynamics.

The submersible intermediate docking station and/or at least one winch for the tow cable may be dynamically controlled by a controller based on input parameters related to at least one of actual or predicted motions of the surface vessel, or actual or predicted motions of the intermediate docking station, for controlling a tension of the tow cable. The sea state may also be used as an input parameter for predicting the motions of the surface vessel.

The processor algorithms could be logical and prescriptive, or a level of fuzzy logic or artificial intelligence and machine learning could apply. Fuzzy logic and machine learning could employ routines for parameter variation to hone in on favourable control settings in given circumstances that yield the desired result, e.g. such as minimizing dynamic motions, even without establishing a logical reasoning therefore. The processor algorithms could be devised to stabilize either end of the tow independently, i.e. single object optimization of surface vessel and intermediate docking station respectively, or could be devised to stabilize the entire tow collectively with presumed dependencies/transfer functions, i.e. multi object optimization of surface vessel, tow cable and/or intermediate docking station.

A processor-based control system may be substituted partly or fully by alternative control automation systems, which could even be a simple mechanical system, configured to hone in on- or self-stabilize at certain depth, speed and posture. Such an approach would be facilitated by the typical desired state of operation being a steady state equilibrium. A processor-based or other automated control system could be substituted partly or fully by manual control.

The tow cable could be more comprehensively controlled by a catenary management system. The system may analytically calculate the tow cable catenary form (curvature) utilizing basic input such as tow speed and tension in either end of the tow cable, or with further accuracy by using supplementary sensors such as tension gauges or position transponders along the cable, or current profilers to determine the hydro-mechanical environment surrounding the tow cable.

Figure 7 illustrates an intermediate docking station 2 maintained in a steady state, during which the underwater vehicle 3 undocks or docks. Upon establishing the desired state of the intermediate docking station, which is typically a stationary state, but may also be a dynamic or transient state as explained above, undocking of an underwater vehicle may take place either by the underwater vehicle being released and exiting the intermediate docking station by its own propulsion, or being pushed away with an ejection mechanism, or being dragged away by passing water, and/or the intermediate docking station being pulled away from the underwater vehicle, or a combination of any of the mentioned methods. In case water drag is used, this could be aided by employing adjustable louvres for through-flow control on the intermediate docking station 2 as described with reference to Figure 1. The speed through water of the underwater vehicle 3 will then be less than the intermediate docking station 2 during undocking, resulting in the underwater vehicle 3 moving away from the intermediate docking station in relative terms.

Docking may take place by mating the underwater vehicle 3 and intermediate docking station 2 with each other. This could be performed either by the underwater vehicle 3 honing in on the intermediate docking station 2 maintained in a steady state, or by the intermediate docking station 2 honing in on the underwater vehicle 3 maintained in a steady state, or by a combination thereof. In either case, the speed through water of the underwater vehicle 3 is higher than the intermediate docking station 2 during docking, resulting in the underwater vehicle 3 moving in on the intermediate docking station 2 in relative terms. In either case, the state of both the underwater vehicle 3 and intermediate docking station 2 will approach the same state, typically a steady state, but could be a dynamic state as earlier explained, at the moment of mating, with only a marginal difference in relative speed between the intermediate docking station 2 and the underwater vehicle 3 in order to come together.

Mating may also be assisted by a permanently or temporarily tethered underwater vehicle 3 being partly or fully pulled into the intermediate docking station 2 by means of a tensioned tether hauled in by a tether management system 31. In the event of a temporary tether, said tether may furthermore serve as a catchment device, in which case relevant fittings and methods for catchment would have to be devised.

Recovery of the intermediate docking station 2 to the surface vessel 1 , is substantially the reverse of the launch sequence. The intermediate docking station 2 may typically be brought to the surface at some distance from the surface vessel 1 while under way, either by increasing the buoyancy, increasing pull on the tow cable 4 and/or positive hydrodynamic lift. Upon having surfaced, the tow cable 4 may be used to haul the intermediate docking station towards the surface vessel, and ultimately to retrieve the intermediate docking station onto or into the surface vessel by such launch and recovery method- and features as already described with reference to Figure 4.

The intermediate docking station 2, inclusive of the underwater vehicle 3 as the case may be, will typically be only marginally buoyant at this stage to limit wave induced excitations of the intermediate docking station in the surface position. The intermediate docking station 2 may alternatively be made more or less buoyant to such extent that would yield the best synchronization of vertical motions between intermediate docking station 2 and surface vessel 1. Any protruding controllers on the body 20 of the intermediate docking station 2 would typically be retracted or folded. In the surface position, the intermediate docking station is marginally buoyant and will remain at the surface, but with limited buoyancy. Inertia will tend to keep motions less than the surface variability of passing waves.

Since the surface vessel 1 and intermediate docking station 2 are underway, and insofar the surface vessel maintains an adequate speed considering the wave induced motions of the surface vessel, which may exceed the self-propelled speed of the underwater vehicle as already described with reference to Figure 4, and which may also be such that would minimize wave induced motions of the surface vessel as further described with reference to Figure 3, the tow cable may be more reliably maintained in tension, such that intermediate docking station and surface vessel may naturally align and synchronize along the horizontal axis (i.e. principal direction of launch and recovery while surface vessel is underway). Separation is thus provided for, thereby reducing the risk of bending and breaking of the tow cable 4 and collision between surface vessel 1 and vehicle 2, even in inclement weather conditions.

Furthermore, insofar the intermediate docking station 2 is ruggedized as already described with reference to Figure 1 , and/or the underwater vehicle 3 is protected by the body 20 of the intermediate docking station 2 as already described with reference to Figure 3, and/or the receiving ramp 11 or cable exit is arranged with such tapering sides, guides or manipulators as already described with reference to Figure 1, then considerable jolts, impacts and misalignments could be permitted, thus enabling recovery in inclement weather conditions.

Recovery may take place with the underwater vehicle 3 securely docked to the intermediate docking station 2, or could alternatively take place with the intermediate docking station 2 empty, which may apply following release of an untethered underwater vehicle 3, or in case the intermediate docking station 2 will be retrieved whereas a tethered 5 underwater vehicle 3 will remain submerged, as illustrated in Figure 9.

The intermediate docking station 2 may be recovered via a moonpool 13 or hoist, in which case the recovery will typically take place while the surface vessel 1 is stationary, with the intermediate docking station 2 negatively buoyant, and by increasing pull on the tow cable. Such operation will be analogous to conventional launch and recovery, apart from the possibility that the underwater vehicle could be protected by the body of the intermediate docking station.

The intermediate docking station 2 as described herein may be used with various configurations and for various purposes, of which principal operating modes are outlined in the following.

Figure 8 illustrates typical two survey modes a) and b) with a tethered underwater vehicle 3 under way. Figure 8 a) shows the underwater vehicle 3 trailing the intermediate docking station 2, thereby distributing more of the tether drag to the intermediate docking station, which in turn may allow for higher speed (assuming the speed is restricted by the thrust of the underwater vehicle, which is commonly the case). Figure 8 b) shows the underwater vehicle 3 traversing alongside the intermediate docking station 2, thereby allowing more flexibility for relative motions between underwater vehicle 3 and intermediate docking station 2. This may in turn allow for maintaining operations in adverse weather conditions, where a steady state for the intermediate docking station 2 may not be ensured due to excessive motions of the surface vessel 1, but may compromise speed. Figure 9 illustrates alternative survey or stationary work modes a) and b) with a tethered underwater vehicle 3 in shallow water. In this case it could be impractical or of no benefit to maintain a intermediate docking station 2 in the water, since tether length and any associated drag, or length restriction of the tether as the case may be, may not pose a restriction on operation. In this case, the intermediate docking station 2 may be retrieved back to the surface vessel 1 , whereas the underwater vehicle 3 remains submerged while effectively tethered directly to the surface vessel 1. This could simplify operations, and be particularly suitable for works scopes that comprise successive stationary works and short repositionings.

Figure 10 illustrates a stationary work mode with a tethered underwater vehicle 3 in deep water, in which case it could be beneficial or required to maintain the intermediate docking station 2 in the water to conserve tether length 5 or to prevent excessive current induced drag on a lengthy tether 5. In this case, the intermediate docking station 2 may be suspended under a stationary surface vessel 1 , adjacent to the underwater vehicle 3. This would allow the underwater vehicle 3 to work with a short tether 5. This mode of operation is analogous to a conventional tether management system, but with the difference that the intermediate docking station 2 may serve as a protective housing and less weather dependent means to launch and recover underwater vehicles.

Figure 11 illustrates a survey mode with the intermediate docking station 2 operating independent from an underwater vehicle, by instead fitting e.g. survey equipment directly onto the intermediate docking station 2, and thus omitting the use of tethered or untethered underwater vehicles. The intermediate docking station may then operate as a survey platform. The intermediate docking station may carry an underwater vehicle for stand-by only, such as to inspect anomalies or objects of interest observed during said survey.

Figure 12 illustrates an overview of several prospective modes of operation for the intermediate docking station and underwater vehicle, as well as the select positions of the intermediate docking station during undocking and docking the underwater vehicle from and to the intermediate docking station, as well as launch and recovery of the intermediate docking station from and to the surface vessel, as explained above. It may also be envisaged that more than one intermediate docking station 2 and underwater vehicle 3 may be in the water at the same time. An intermediate docking station with underwater vehicle may e.g. be launched from the stern ramp and later when the surface vessel is in a stationary mode, another intermediate docking station and underwater vehicle may launched from the moonpool of the surface vessel.

Apart from the various tethered modes, the intermediate docking station 2 may be used to service untethered underwater vehicles 3, in which case the intermediate docking station may be used as a less weather dependent means to launch and recover underwater vehicles. Furthermore, the intermediate docking station 2 may be submerged as required, or remain submerged for prolonged periods, either towed under way or suspended below a stationary surface vessel 1 , during which underwater vehicles 3 could dock with the intermediate docking station 2 e.g. for electrical charging of batteries or exchange of electronic data back to the surface vessel and beyond, but without having to resurface. The intermediate docking station may similarly be used carry payloads or instruments and to deliver and retrieve payloads or instruments to and from submerged underwater vehicles. It may also be envisaged to operate battery powered underwater vehicles with a tether solely serving as a signal (not power) cable e.g. during select periods where high data transmission rates are required.

Having described embodiments of the invention it will be apparent to those skilled in the art that other embodiments incorporating the concepts may be used. These and other examples of the invention illustrated above are intended by way of example only and the actual scope of the invention is to be determined from the following claims.