Field of the invention

The invention relates to autonomous underwater vehicles and the operation thereof, and more specifically the utilisation of autonomous underwater vehicles for transporting gas, liquids or objects.

Background

The offshore production process of hydrocarbons involves transport of materials, both of hardware components used for the process and production or injection fluids. Well- established means of transport include surface vessels, umbilicals, flowlines and pipelines. The following publication discloses an alternative system of transport based on subsea shuttles: Research Disclosure database number 662093, published digitally on 20 May 2019. The subsea shuttles provide multipurpose storage and transportation services, as described in this publication. The inventors have realised shortcomings in the technology described in this publication, in particular related to the loading and unloading system, and have developed improvements as described in more detail below.

Statement of invention

According to a first aspect of the invention there is provided an assembly for loading or unloading an autonomous underwater vehicle, AUV, the assembly comprising: a flexible conduit anchored to the seabed, at least one buoyancy element attached to the flexible conduit, a connector for connecting to the AUV, a driver for moving the connector to the AUV.

The driver may be a remotely operated vehicle, ROV. The ROV may either be received by the AUV when the ROV is not in operation, or by a surface vessel or an ROV garage located on the seabed.

Alternatively, the driver may be permanently attached to the conduit and comprise a steering module and a communication module. The steering module may comprise one or more propellers for displacing water. The flexible conduit may be part of a bundle, the bundle further comprising one or more of: one or more further conduits, a power cable, a communication cable.

A plurality of beacons may be located on the seabed for aiding navigation of the AUV, wherein the beacons emit an acoustic, optical or electromagnetic signal, and wherein the AUV comprises one or more corresponding receivers.

The connector may comprise one or more of: a mating connector, a stab-in connector, a screw connector or a bayonet connector.

According to a second aspect of the invention there is provided a method for loading or unloading an AUV, the method comprising: landing the AUV on the seabed in the vicinity of a flexible conduit; steering an end of the flexible conduit to the AUV; connecting the end of the flexible conduit to the AUV.

The step of steering may be carried out by an ROV, and/or by a steering module at the end of the flexible conduit.

The step of landing the AUV on the seabed may be carried out either before or after both steps of steering and connecting an end of the flexible conduit.

The buoyancy of the AUV may be adjusted to change the orientation of the main longitudinal axis of the AUV from a horizontal orientation to a vertical orientation. The AUV may be used as a separator of a multiphase liquid when oriented in a vertical orientation.

The method may further comprise adjusting the buoyancy of the AUV to change the orientation of the main longitudinal axis of the AUV from a horizontal orientation to an orientation of the main longitudinal axis between 45 degrees and 90 degrees with respect to the horizontal plane.

The method may further comprise operating a plurality of AUVs simultaneously. Figures

Some embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which:

Figure 1 is a perspective view of autonomous underwater vehicles in operation;

Figure 2 is a perspective view of an autonomous underwater vehicle in transit;

Figure 3 is a schematic view of autonomous underwater vehicles carrying an external load; and

Figure 4 is a perspective view of an autonomous underwater vehicle connected to a conduit.

Detailed description

The inventors have realised that the loading and unloading of fluids at the seabed can be improved by using a flexible conduit comprising one or more buoyancy elements. The buoyancy elements may be separate devices which are attached to the conduit, or may be part of the conduit itself, for example the wall of the conduit when made of a buoyant material. The contents of the conduit may also be taken into account when establishing a positive buoyancy. For example, if the conduit is permanently filled with gas during use then the buoyancy elements need to contribute less buoyancy than if the conduit if filled with liquid phases. The flexible conduit is at one end anchored to the seabed and at the other end connected to the one or more buoyancy elements. Alternatively, buoyancy elements may be connected to the conduit at intermittent locations along the length of the conduit. From the anchoring point, the same conduit or a second conduit such as a pipeline continues to a subsea manifold which routes the flow of liquids further towards or away from the well or other destinations. A plurality of flexible conduits can be provided, all connected to the same manifold or to different manifolds. Although manifolds are given as an example, any other source or destination of the shuttle contents may form an end point of the conduit. The multiple conduits and manifold are preferably separated by a distance sufficient to ensure safe manoeuvring and landing of the subsea shuttles, but the distance may also be much larger, for example to allow the shuttle to land in a convenient location when the manifold is not near a convenient location for landing.

The expressions ‘shuttle’, ‘subsea shuttle’, ‘autonomous underwater vehicle’ or ‘AUV’ are used interchangeably to indicate the same object.

Although the word ‘sea’ is used throughout, this may equally be understood as ‘ocean’ or ‘lake’ and the invention is envisaged to be used in any large body of water. Similarly, when the words ‘seabed’ or ‘sea surface’ are used, this is not intended to be limited to a sea in a strict sense but should also be understood to cover ‘ocean bed’ or ‘ocean surface’, or similar terms for any large body of water.

A bundle of conduits is used in an alternative embodiment, instead of a single conduit. The plurality of conduits can be used for transporting the same fluids in the same direction, thereby creating redundancy, or for transporting different fluids. The bundle may include cables for power and signal transmission towards or away from the subsea shuttle. By way of example, the power cables can be used for charging the shuttle’s batteries for electric propulsion, while the cargo such as CO ₂ is being (un loaded.

When the conduit is not connected to a subsea shuttle, the buoyancy force of the buoyancy element is sufficient to overcome the gravitational forces of the conduit with or without fluids inside. The conduit then extends upwards from the seabed. Alternatively, the buoyancy elements do not lift the conduit completely upwards, for example to avoid the conduit reaching the surface in shallow waters. Thus the conduit may take any shape in the water both in a connected and unconnected state. The subsea shuttle generally operates at a constant depth below the sea surface, avoiding the variable conditions of weather, wave motion, temperature fluctuation and tides at the sea surface. The conduit also does not extend to the surface, like a conventional riser does, and requires less additional infrastructure such as a mooring platform for a ship. The subsea shuttle can land on the seabed adjacent the conduit. More specifically, a dedicated landing area can be provided. The dedicated landing area has a generally flat surface for receiving the shuttle, which is created by levelling the seabed or by laying one or more support elements to level out the seabed. The supporting elements may be structures, constructions or seabed elevations/excavations. The support elements are such that the outer hull of the shuttle can be landed and rested steady thereon without suffering damage. The dedicated landing area can also include acoustic, optical or electromagnetic beacons to mark the area for the shuttle. A remotely operated vehicle (ROV) can be used to take the end of the conduit to the subsea shuttle and move and attach the conduit to a connector on the subsea shuttle. Alternatively, the conduit itself includes a driving unit such as a propeller, control unit and a communication unit to enable remote operation of the driving unit.

The operating steps of first landing the shuttle and then connecting to the conduit are reversed in a different embodiment. The operation of moving the connector to the shuttle requires some work, mainly due to the buoyancy forces of the buoyancy elements which need to be overcome. This work is reduced by connecting the connector while the shuttle is close to the free end of the conduit before landing. However, this method can only be carried out if the currents are not too high because the challenges of stabilising the shuttle and keeping the shuttle in one position may outweigh the benefits of reduced effort for connecting closer to the end of the conduit.

Fig. 1 illustrates a subsea arrangement with a subsea shuttle 1 in transit and a second subsea shuttle 2 which has landed near a flexible conduit 3. The same conduit 3 is illustrated in two different orientations, a first orientation A when attached to the landed shuttle and a second orientation B when free from the shuttle. The conduit is anchored to the seabed at a position indicated with reference number 4, from where the conduit continues to a manifold 5. A second flexible conduit 6 is illustrated as being connected to the manifold and being anchored to the seabed generally opposite the first conduit such that there is sufficient space for a second shuttle to land on the seabed near the second conduit while a first shuttle has landed near the first conduit. An advantage of using the shuttle below the sea surface is that heave and swell of the wave motion and tides are avoided. However, there will still be currents below the surface, whether tidal or due to other reasons. An advantage of landing the shuttle on the seabed is that such currents may be less strong near the seabed and the contact with the seabed anchors the shuttle. If the current is particularly strong, separate anchors can be used, but in a typical application scenario the function of landing is sufficient to prevent the shuttle from moving. This landing option is a further advantage over surface vessels, which require a conventional anchor. The landing anchoring strength can be controlled, adjusted as needed by adjusting the shuttle buoyancy to becoming more or less negative.

In the specific embodiment illustrated in Fig. 1 , each conduit has three buoyancy modules 7 attached near the free end of the conduit to keep the conduit raised from the seabed when free from a shuttle. The length of the conduit is less than the distance to the sea surface, such that the benefits of operating in the stable conditions near the sea bed also apply to the conduit. The modules 7 have a fixed volume and therefore a fixed buoyancy, but alternatively a more intricate arrangement may be used with a flexible inflatable volume in combination with gas compressors or gas pumps to actively control the buoyancy.

A connector 8 is provided at the free end of the conduit (the free end refers to the end which is only free when not attached to a shuttle), which can be attached to a corresponding connector 9 on the shuttle, as illustrated for shuttle 2 which has landed in Fig. 1. Existing technology known to the skilled person can be used for the connectors. Mating connectors, stab-in connectors, screw connectors or bayonet type connectors are examples of possible connectors. Connectors for power or signal transfer may be of a wet mate type, or alternatively be based on inductive coupling. In the embodiment of a bundled conduit, existing multi-bore connectors may be used.

A remotely operated vehicle (ROV) 10 is used to take the free end of the conduit and move it to connector 9 on the shuttle such that the connection can be made and signals, power, fluids or gas can flow between the manifold and the shuttle. The shuttle may have a hatch for receiving the ROV when it is not required for connecting the conduit to the shuttle. In other embodiments, the ROV is controlled and/or stored at a surface supporting vessel, or a subsea installation such as a manifold or a dedicated ROV parking installation, e.g a garage that houses the ROV. Fig 1 also illustrates a landing area, for example a disc with a 25m diameter, whereby the conduit can be connected within that area, as well as an area with cables where the shuttle is not allowed to land.

Fig. 2 is a perspective view of a larger area with shuttle 1 in transit between the conduits 3 and 6 illustrated in Fig. 1 on the left and a similar conduit 21 on the right hand side of the figure, whereby conduit 21 is connected to a vessel at the sea surface. Domes 23 illustrate the range of the acoustic signals.

A plurality of acoustic beacons 22 are provided on the seabed along the path between the first set of conduits and conduit 21 . The shuttle comprises an acoustic receiver and signal processors which are used to navigate the shuttle based on the received acoustic signal. Alternatively, optical signals can be used for navigation if the subsea visibility in the relevant wavelength range is good enough. Electromagnetic signals may also be used for navigation. The AUV comprises one or more corresponding receivers. The AUV has a considerable length, typically over 100m, and a plurality of receivers along the length of the AUV can be used for triangulation or other methods of accurately determining the location of the beacons.

Figure 3 illustrates further use of the shuttle described above. Besides carrying a load within the main body of the shuttle, a load 31 can also be carried underneath the shuttle as illustrated in Fig. 3A. A specific example of a load can be a cable reel 32 carried underneath the main body of the shuttle as illustrated in Fig. 3B. Although the use of a cable reel for laying a cable from a floating structure is known, the use in combination with a subsea shuttle provides advantages over the known methods due to the proximity to the seabed and the stable subsea conditions. The heave and sway motion of the waves or tides is avoided, thereby avoiding stress on the cable during laying of the cable. Flowever, also the proximity to the seabed means that the distance 33 of cable between the reel and the seabed is much shorter than when extending from a surface vessel, which reduces forces on the cable from water flow as well as the weight of the cable itself. The shorter distance also provides more accurate control over the position of the cable on the seabed. An additional booster engine 34 is provided to overcome the additional resistance of the cable reel and cable laying process. When landing of the shuttle is required while it is carrying a cable reel, landing on a flat surface would be difficult and therefore a landing frame is provided on the seabed which extends from the seabed to the shuttle and creating a space between the shuttle and the seabed for accommodating the cable reel. Alternatively or in addition, an opening can be excavated in the seabed for receiving the cable reel during landing. The shuttle can be used for a variety of purposes. Fluids can be transported to a well or from a well. Examples of fluids and related applications are:

• CO ₂ for injection into a well and storage;

• MEG or Methanol as a chemical supply to a process, for example as hydrate inhibitor;

• oil and gas, the shuttle can transport the oil and gas from the well to a facility downstream in the production process;

• fresh water;

• toxic waste, either to be stored within a formation, or to be removed from the site;

• multiphase fluids, for separation of oil, gas, water and sand, discussed in more detail below;

• septic waste or bio-mass, for storage within a formation below the seabed;

• rig fluids such as mud, brine, drill cuttings etc., which need to be transported away from a well.

CO ₂ capture is a particular area of interest because it addresses the environmental problem of CO ₂ production. Capture typically takes place at a land facility, but can also take place offshore. The captured CO ₂ is loaded into the shuttle. The pressure inside the shuttle is balanced with the pressure outside the shuttle, and at the constant depth of operation of the shuttle below the water the pressure is sufficient to liquefy the gas in an embodiment. After landing of the shuttle and connection to the conduit which is again connected to a disposal well, the C02 is injected into a reservoir for permanent storage.

The shuttle can also be used for construction. Examples are: module transport, cable installation as discussed above, pipe installation decommissioning, installation of subsea structures, oil spill emergency support, or environment clean-up.

The shuttle can further be used for energy storage: the internal volume can be filled with bio fuel such as ethanol; diesel, heli-fuel or ammonia; or the internal volume can be filled with a battery bank for temporary storage of electrical energy. The internal volume could even be used to store live fish and the shuttle can be used for transporting the fish. The pressure within the shuttle is controlled such that the pressure difference across the outer hull is small and a lightweight construction can be used. A slight overpressure may also be used to keep the outer hull in a slightly inflated shape. The pressure within internal storage tanks is also kept either the same, or at a slight overpressure with respect to the pressure outside the storage tanks. The temperature of the cargo may also be actively controlled. For example, well fluids are typically at a higher pressure than seawater. When the tanks are loaded with well fluids, the temperature may be allowed to adjust slowly to the ambient temperature during transport while the pressure is controlled to compensate for volume reduction under a temperature reduction. Alternatively, heat exchangers are used to actively control the temperature, for example to reduce the temperature of well fluids before or after loading.

The general method described above is illustrated in Fig. 4, and includes the steps of (S1) landing the AUV on the seabed in the vicinity of a flexible conduit, (S2) steering an end of the flexible conduit to the AUV, and (S3) connecting the end of the flexible conduit to the AUV. As noted above, the order of the steps may be altered.

Fig. 5 illustrates a different arrangement of the shuttle during (un)loading. As the shuttle is unmanned, there is no restriction to the orientation of the shuttle and the shuttle is moved to a vertical orientation in this example. The same effects may be achieved with an orientation deviating from a precise vertical orientation, and under the influence of currents there is also likely to be some movement. After docking, the shuttle rises from a horizontal orientation to a vertical orientation as illustrated in Fig 5A, instead of the horizontal position on the seabed shown in Fig. 5B. The change in orientation can be achieved by adjusting the buoyancy in specific buoyancy modules placed along the length of the shuttle. When placed vertically, the shuttle may not need to be anchored in a subsea region without much current, but otherwise an anchoring line (not shown) could be used to avoid strain on the conduit. In a specific example, the conduit itself also includes an anchoring line such as a strong steel cable attached to the seabed which carries out the function of anchoring and which prevents large forces on other components of the conduit such as control lines, power cables and fluid lines. When the shuttle receives a multiphase well fluid during loading in vertical orientation it can act as a vertical separator with gas accumulating at a top section of an internal container and fluid at the bottom of the container during filling. While the container fills up, water which is used as a buffer liquid is displaced and released into the sea, to another shuttle, a surface vessel or to a dedicated storage tank. The separated gas can be bled of at the top, and compressed and/or cooled into a well in an injection operation for reservoir pressure support. Alternatively, the separated gas can be loaded into another shuttle for transportation away from the loading site. Alternatively, the fluid phase, such as oil, may be taken away from the lower end of the shuttle, while leaving the gas inside the shuttle containers.

The shuttle may include a plurality of containers, all in a vertical position, whereby some of the containers can be dedicated for separation while other containers are used for storage of separated fluid phases. Different permutations of filling containers can be used, as can be envisaged by the skilled person.

Some wells produce a multiphase liquid including a combination of hydrocarbons and water, where water separation is also of interest. Water separation can be carried out in a horizontal orientation of the shuttle, but in principle water separation can also be carried out in a vertically arranged separation shuttle as described above.

The shuttle separator can be disconnected when fully loaded and move to a facility upstream in the industrial process. Alternatively, a dedicated separator shuttle can be stationary and transfer all separated fluid phases to another shuttle as illustrated in Fig 5C. The conduits preferably (but not necessarily) include multiple flow lines such that the separator shuttle can receive multiphase fluids and release separated fluids at the same time. If a flowline is available then separated fluids can be transferred to the flowline. Unlike conventional stationary separators, the shuttle can easily be transported between locations. It is also envisaged that different separator shuttles with different configurations are used, and that a separator shuttle can be swapped for a different one if the well requirements change or for maintenance or repair. In one embodiment, the separator shuttle is capable of moving to and from a location driven by its own propulsion system. In another embodiment, a dedicated separator shuttle does not have its own independent propulsion system but needs to be towed to and from the location, for example by another AUV, a cargo shuttle, a submarine or a surface vessel. Although the invention has been described in terms of preferred embodiments as set forth above, it should be understood that these embodiments are illustrative only and that the claims are not limited to those embodiments. Those skilled in the art will be able to make modifications and alternatives in view of the disclosure which are contemplated as falling within the scope of the appended claims. Each feature disclosed or illustrated in the present specification may be incorporated in the invention, whether alone or in any appropriate combination with any other feature disclosed or illustrated herein.