Field of invention

The invention relates to offshore surveying method, and more specifically to subsea surveying methods using autonomous shuttle systems.

Background

Research Disclosure 662093 (published 20 May 2019) describes a subsea shuttle system, using autonomous subsea shuttles for multipurpose transportation and storage purposes. Research Disclosure 677082 (published 21 August 2020) provides further detail regarding possible shuttle structure and support, applications, e.g., on/offloading of a payload, and the propulsion system of the subsea shuttle.

Statement of invention

According to a first aspect of the invention, there is provided an unmanned underwater vehicle, comprising an outer hull defining an internal volume, a pressure communication channel between the internal volume and the exterior of the outer hull, arranged to control the pressure in the internal volume based on the exterior pressure, a buoyancy and trim control system, a probe provided within the internal volume of the vehicle, a port at the bottom side of the vehicle, providing a channel between the internal volume nd the exterior of the vehicle, for the probe to extend through the port for collecting material or data.

The unmanned underwater vehicle may further comprise one or more position detectors.

The probe may in particular be a cone penetration testing (CPT) probe, and the buoyancy control system may be arranged to adjust the buoyancy in response to the force onto the CPT probe when being inserted into the seabed.

The unmanned underwater vehicle may be landed on the seabed or remain suspended above the seabed during insertion of the probe into the seabed. The unmanned underwater vehicle may comprise a top hatch for extending the CPT probe during use.

The probe may alternatively be a core sampling probe or a vibro-coring probe, arranged to collect samples from the seabed. In yet another alternative, the probe comprises a flexible hose terminating in a collection device, and the flexible hose is connected to a suction pump for drawing seabed samples into the unmanned underwater vehicle.

According to a second aspect of the invention, there is provided a method of collecting samples or data with an unmanned underwater vehicle, the method comprising: providing the unmanned underwater vehicle comprising an outer hull defining an internal volume, and a pressure communication channel between the internal volume and the exterior of the outer hull, arranged to control the pressure in the internal volume based on the exterior pressure, extending a probe from the internal volume through a port at the bottom side of the vehicle, and into the seabed, and controlling the buoyancy and trim.

The method may further comprise measuring the position and/or orientation of the unmanned underwater vehicle with respect to the seabed.

The method may further comprise carrying out cone penetration testing by driving the probe into the seabed and measuring the resistance of the seabed to the probe. During that step, the buoyancy may be adjusted to counter the force onto the probe by the seabed.

The method may further comprise landing the unmanned underwater vehicle on the seabed, or maintaining a predetermined distance to the seabed, prior to and during the step of extending the probe into the seabed.

The method may further comprise carrying out seabed sample collection with a core sampling probe or with a vibro-coring probe. The probe may comprise a flexible hose terminating in a collection device, wherein the flexible hose is connected to a suction pump, and the method further may further comprise collecting seabed samples by extending the flexible hose to the seabed and drawing seabed samples into the unmanned underwater vehicle with the suction pump.

Drawings

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 schematic vertical cross section of a subsea shuttle;

Figure 2 is a perspective view of a drawing of a partially opened up shuttle;

Figure 3 is a perspective view of a drawing of a partially opened up shuttle; ad

Figure 4 is a flow diagram of a method.

Specific description

Surveying methods relying on surface vessels are very weather dependent and, in some regions, surveying can only be carried out during the summertime. An autonomous subsea shuttle is provided with equipment for one or more of: seabed site surveys, inspections, environmental analysis, and geotechnical mapping. This enables an unmanned autonomous underwater shuttle to perform operational activities regardless of weather conditions because below the surface the conditions are much more stable.

The subsea shuttles described in the earlier-mentioned publications comprise a relatively lightweight outer hull, when compared to a submarine hull, because the pressure within the hull is the same as or slightly higher than the pressure of the surrounding seawater. The hull typically includes pressure communication valves to enable the pressure to be equalised, but the hull may also include hatches or doors to enable instrumentation to extend outside the hull during measurements. The material of the hull can also be chosen such that it is effectively transparent for the acoustic or electromagnetic detection methods, enabling all instrumentation to be contained within the hull.

The versatility of the shuttle enables a broad range of detection methods to be implemented.

Figure 1 illustrates a schematic underwater vehicle. The underwater vehicle (or ‘shuttle’) may be an autonomous underwater vehicle (AUV), or a remotely operated underwater vehicle (ROV). The vehicle comprises an outer hull 10, having a hydrodynamic shape to reduce drag. An elliptical outer hull 10 is shown in Figure 1, but other hydrodynamic shapes known in the art are suitable. Within the outer hull 10, a cargo container or payload 12 is arranged. The payload may be a fluid tank, or could be instrumentation for surveying the subsea environment, or any other desirable payload. The cargo 12 may be fixed within the outer hull 10 using a frame or other rigid supports (not shown). In this way, a space between the outer hull 10 and cargo 12 is formed. The outer hull 10 has a channel 14, which is in fluid communication, or pressure communication, with the surrounding seawater when the vehicle is submerged. When the vehicle is submerged, the space between the outer hull 10 and cargo 12 at least partially filled with seawater, depending on the net buoyancy requirements. In some embodiments, a part of the outer hull 10 volume is occupied by one or more compartments for containing gas (e.g. ballast tanks). In some embodiments, the channel 14 is selectively closed (e.g. via operation of a valve) to allow or block fluid communication through the channel.

Although the word ‘sea’ and ‘seawater’ are used throughout, these may equally be understood as ‘lake’ and ‘freshwater’, respectively, 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.

At the stern (back end) of the vehicle, a propeller 16 is provided. The propeller is coupled to a power source and control unit (not shown) to enable autonomous and/or remote operation of the vehicle. An electric power source is preferably used. In some embodiments, the vehicle includes a buoyancy controller 18. The movement in the horizontal direction is generally controlled by the propeller, while the depth position is generally controlled by the buoyancy controller 18. Additional propellers or other drivers, such as water or gas jets, may be provided for further control of the movement and position of the shuttle.

The vehicle structure shown in Figure 1 is different from that of conventional submarines because instead of having a pressure hull maintained at or near to atmospheric pressure to accommodate personnel, the inner structure is maintained at a pressure similar to the external hydrostatic pressure. An advantage of the pressure communication is that a relatively light-weight hull structure can be used. By “similar” to the external hydrostatic pressure, it is meant that the internal pressure is kept suitably close to the external pressure so that the pressure differential dP (i.e. overpressure or under-pressure) is not too large. A slight overpressure may be maintained to ensure that the hull keeps its desired shape and is not deformed by currents or other external pressures.

The shuttle may land on the seabed by creating negative buoyancy. A dedicated landing area may be provided which has been prepared in advance, for example by removing obstacles or levelling the seabed. Alternatively, the shuttle may land in an area which has not been prepared for landing in advance. A landing arrangement 20 is provided at the bottom of the shuttle to enable the shuttle to land in different conditions.

Seismic surveying methods can be used, whereby an energy source for generating a shock wave is carried by the shuttle. The detectors may also be carried by the shuttle, or may be installed by the shuttle. The installation by the shuttle involves the laying of a network of acoustic listening devices on the seabed, possibly inter-connected by electric cables. The laying of networks by a shuttle provides an improvement over the laying of networks by a surface vessel given the avoidance of surface or weather effects, but also avoiding a need for cranes and other equipment required to lift devices over board of a surface vessel.

Examples of equipment for standard survey and inspection methods are: multibeam echo sounders, sidescan sonars, profilers, sparkers, magnetometers, and other typical equipment for such methods. This equipment can be carried by the shuttle. The technical details and operation methods of these examples as such will be known to the skilled person.

The inventors have further appreciated that the shuttle structure itself can be used as a resonator to amplify an acoustic signal emitted, or to improve detection of received signals. If used as a resonator to amplify emission, the acoustic sources are rigidly connected to structural parts of the shuttle. Design principles known from loudspeaker enclosures can be used for the placement of the sources and shape of the relevant shuttle parts. Basic loudspeaker designs such as dipole enclosures, horn enclosures, or quarter wave enclosures can be used for designing equivalent resonators in the shuttle body. If used as a resonator to amplify received acoustic signals, a distributed network of stress sensors is used within the shuttle body to detect the deformation of the shuttle body under the influence of the received acoustic signals. The sensor outputs are sent to signal processors for further analysis.

The seismic sources are powered from the shuttle and can replace air guns used by conventional seismic surface vessels. The shuttle source can also vary the frequency band for optimal data acquisition. This setup enables the shuttle to perform seismic data acquisition during the whole year. A further advantage of using the shuttle to collect data is that the subsea environment is less noisy when compared to the sea surface environment and a better signal to noise ratio can be achieved. The use of seismic sources powered from the shuttle is more environmentally friendly to marine life then use of air guns from surface. This setup can also be used for seismic data acquisition in ocean-bottom cable systems and trenched permanent reservoir monitoring (PRM) systems. The Shuttle may include long baseline (LBL) transponder systems and arrange for them to be positioned to improve the accuracy of the positioning systems. One or more smaller autonomous underwater vehicles may be used for deployment and recovery of equipment.

A further example of a surveying method whereby the characteristics of the shuttle can be used to improve a conventional detection method is cone penetration testing, CPT. In a CPT method, a cone with instrumentation is driven into the soil at a controlled rate. The resistance to the movement of the conical tip as well as to the string supporting the cone is measured and analysed to determine properties of the soil. Additional tools can be carried by the cone to determine formation parameters such as fluorescence, conductivity, pH, temperature, etc.

At the on-shore surface, CPT is typically carried out from a dedicated truck with a large weight, and/or anchoring means to the soil, to have sufficient counter-force to drive the cone into the soil. The inventors have realised that the buoyancy control of the shuttle can be used for generating a counter-force when driving the cone into the seabed or when withdrawing the cone from the seabed. A negative buoyancy can be created to drive the cone into the seabed, while a positive buoyancy can be created to assist with pulling the cone out of the seabed.

An example is provided of buoyancy fluctuations due to changing payload of a subsea shuttle. In a shuttle, the capacity of a cargo tank may be approximately 10,000 litres. The mass when filled with hydrocarbons would then approximately be 4.2 tonnes. The mass of seawater in the cargo tank, after offloading all the fluid cargo and replacing it with seawater, would approximately be 10.04 tonnes. There would therefore be a net change in mass of approximately 5.84 tonnes from displacing the hydrocarbon with seawater. The net, negative buoyancy change would be around 60kN. A typical thrust of a conventional CPT device, attached to the seabed by suction or by attachment to a larger structure, is also around the value of 60kN. During a changing payload, the shuttle may be at rest on the seabed. The shuttle will have a buoyancy control system to compensate for a change of such scale in order to maintain neutral buoyancy before taking off. The buoyancy control system includes dedicated volumes within the shuttle, which can be filled with gas or light fluids to displace a corresponding volume of water. A process of changing payload may not be combined with a surveying method, but this example illustrates a range of forces a shuttle can generate or handle.

A subsea shuttle may be specifically designed for surveying without an additional functionality of transporting large amounts of fluids such as CO2, as envisaged in the before-mentioned publications. Figure 2 illustrates a perspective view of a surveying shuttle 21. The overall outer shape of the shuttle is hydro dynamically smooth to reduce drag during movement. A propeller 22 is provided at the stern as the main driver for transporting the shuttle. Additional propellers or jets may be provided (not illustrated) for more precise positioning during a surveying operation. Rudders 23 are provided at the rear end as well. The shuttle’s side is partially opened up on the side for showing the inside arrangement, but in some embodiments there is no side opening. Alternatively, side or bottom hatches can be provided which not only act as doors to the openings, but which can also be extended out further to act as landing surfaces and create a larger area of contact between the shuttle and the seabed during landing. The inside cavity used to accommodate equipment may have general dimensions similar to those of a standard shipping container. The shuttle has one or more top hatches 24. The top hatches can be opened up for loading equipment. The top hatch can also be opened up during a CPT operation, as illustrated in Fig. 2, wherein a string 25 emerges through the top. The string is an example of a probe, but other examples such as a flexible hose may also be used. During transport, the string is stored horizontally within the shuttle. The string extends through a port in the bottom part of the shuttle, and part 26 of the string is visible before extending into the seabed. The port has a hatch to close it during transport or when the port is not being used. The advantage of the equal pressure between the inside and outside of the hull is that the hatched can be relatively simple arrangements which do not necessarily need to withstand large pressure differentials. The CPT tool itself may be a standard tool attached to the shuttle within the inside cavity, or it may be specifically developed for the shuttle.

As illustrated in Fig. 2, the shuttle is stationary with respect to the seabed without being connected to the seabed, other than by way of the string 26. The conditions below the sea surface tend to be stable, thereby reducing the challenges for maintaining a stationary position when compared to a surface vessel. The main force acting on the shuttle is the back action of the drill string when it is driven into the seabed, and the buoyancy is adjusted to provide a counterforce. The buoyancy adjustment thereby becomes the main counterforce for the CPT operation. A plurality of sensors is provided to monitor the position of the shuttle with respect to the seabed, and the output of the sensors is received by a controller unit which sends a control signal to the one or more propeller and/or jets. The trim is also controlled, which corresponds to the angle or rotation with respect to the seabed, and the shuttle is preferable maintained in a horizontal orientation while avoiding roll.

Alternatively, the shuttle may be landed on the seabed before the CPT operation is carried out. An excess negative buoyancy is then provided to keep the shuttle in the landed position, and the negative buoyancy may be increased further during the CPT operation if needed.

By way of example of further detail of the CPT tool, the seabed penetration depth is around 50m, the diameter is between 3 and 10 cm, the data transmission takes place acoustically through the drill string, and/or fibre optic cables can be used. The push capacity mentioned above is 60kN, although higher capacities up to 200kN are also achievable. The speed at which the probe is driven into the seabed is a few centimetres per second. The resistance by the seabed is measured with piezo-electric elements to determine cone resistance, pore pressure, and/or friction against a sleeve or the string. Alternative cones can be used for determining electrical conductivity, temperature, shear wave velocity, natural gamma rays, magnetic flux density or the magnetic field.

The overall shuttle arrangement used for the CPT operation can also be used for other purposes. For example, core sampling can be carried out from the shuttle when stationary above or on the seabed. Core sampling involves the collection of cylindrical cores of formation from the seabed with a specialised drill. Although the back action of core sampling is not as high as for CPT, the same overall approach can be used. A further example is vibro-coring, which is typically used for unconsolidated sediments and soils.

Yet a further example is the hoovering up of samples from the top layer of the seabed with a hose and a reduced pressure pump for providing suction. The hose can be dragged along the seabed, and comprise a collection device arranged to collect sediments from the top layer of the seabed. The collection device may comprise a weight to urge the collection device against the seabed, and comprise a sharp edge to act like a scraper. The hose may be flexible.

Figure 3 illustrates the shuttle in more detail, without showing the measurement tools. Stern and bow buoyancy and trim control sections 31 are arranged around a cargo section 32, which has a standard shipping container size. Power supplies 33 and propulsion and control machinery 34 are provided in the stern section. The bow section 35 may include further sensors. Figure 4 is a flow diagram of the main steps of a method of collecting samples or data with an unmanned underwater vehicle: S1, providing the unmanned underwater vehicle comprising an outer hull defining an internal volume, and a pressure communication channel between the internal volume and the exterior of the outer hull, arranged to control the pressure in the internal volume based on the exterior pressure; S2 extending a probe from an inner cavity through a port at the bottom side of the vehicle, and into the seabed; S3 controlling the buoyancy and trim.

The different surveying methods described herein may be combined and could be carried out at the same time. For example, while carrying out the CPT method, an acoustic or electromagnetic method may be carried out, even though the CPT method may cause some noise in the acoustic measurements. The shuttle may also collect some data over a much longer period of time, such as a continuous subsea temperature measurement process, and in the meantime carry out a CPT method.

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.