GB2543764A - Bed level maintenance in sediment-floored water areas using autonomous underwater vehicle technology - Google Patents

Abstract

An autonomous underwater vehicle (AUV) to erode or disperse build-up of sediment on the bed of an aquatic zone. The AUV is unmanned and moves along a pre-set route close to the sea or river bed, and uses hydraulic or pneumatic means to erode sediment on the bed surface. The eroded sediment is enclosed into a duct (g) and directed upwards to be discharged in a plume (y) high into the water above the vehicle, where it is dispersed by natural currents. In one embodiment the plume may be directed by a flexible chute of fabric (h) on a whip support (i). The AUV may be free-floating, on robotic legs, or on wheels (c). The bed disturbance is induced hydraulically by pump, fan or propeller (e), or pneumatically by air injection. There is a buoyancy adjusting compartment (b) to keep the vehicle at the correct height above the surface. Power is supplied by a rechargeable battery or fuel cell (j) housed in a watertight compartment (k). The vehicle uses an underwater positioning system (l) and is controlled by an on-board computer system (m) in a watertight compartment (n).

Description

Bed level maintenance in sediment-floored water areas using autonomous underwater vehicle technology

Aquatic zones with anthropogenic uses such as navigation, flood relief, water storage or power generation suffer from sedimentation problems and usually require periodic maintenance dredging for effective long-term use. Sedimentation commonly occurs because man, in adopting an aquatic zone for his use, modifies the natural balance between sediment accretion and erosion processes, usually by reducing exposure to erosive processes (strong currents, waves) as a result of channel deepening or breakwater construction. Dredging is costly, disruptive and often not environmentally friendly. As the dredging process requires the mobilisation of expensive, large and powerful plant, it is undertaken as infrequently as possible (eg at 1-10 year intervals), compounding the problem as high amounts of energy are often required to dig out the consolidated bed sediments that form during the inter-dredge period, particularly where mud is present.

The invention described here provides a new approach to this problem, by effectively reintroducing a persistent, low-level erosive process to accreting bed areas in order to check the build-up of sediment. This chronic erosion is applied using a specially designed autonomous underwater vehicle (AUV). The application of newly-emerging AUV technology to this task is novel. The invention is not a new dredging method, it is a system for preventing or reducing the need for maintenance dredging. An autonomous vehicle can sweep the bed of a water area on a daily basis, thus keeping sediment moving. Frequent sweeping prevents sediment consolidation, thus significantly reducing the energy used.

The invention offers many advantages for water area bed-level management. Frequent, low-level power consumption allows renewable energy sources to be effectively utilised. Autonomous operation, requiring no human intervention, allows optimum sweeping cycles to be used (unaffected by human work shifts) and reduces labour costs to maintenance and emergency response requirements only. Autonomy, simple AUV design and low renewable power consumption enable very economic operation. A bed-hugging design will enable the AUVs to work simultaneously with other water area users, thus eliminating disruption to other users of the area. By moving only the surface veneer of sediment, and generating low-concentration sediment plumes, the device will also have negligible impact on benthic ecology and water quality.

The description of the invention is illustrated by the accompanying two figures. Figure 1 shows the basic design and typical mode of operation (top left is a section view of a wheeled type, top right a section of a free-floating type, bottom is an oblique view of a free-floating AUV working a site) and figure 2 describes in more detail the component elements of an example idealised wheeled vehicle. In the figure sections, the wavy line is the water surface and the black band the sediment bed.

The device is a submersible vehicle designed to enable controlled remobilisation of recently accumulated aquatic bed sediments at all points within a defined work area on a regular basis (typically diurnally or semi-diurnally). The device is autonomously operated and navigated, therefore allowing sediment resuspension activity which is both persistent (to prevent sediment consolidation) and carefully timed (so that remobilised sediments are ultimately dispersed by natural currents into zones where accumulation is acceptable or natural processes continue the dispersion processes).

The device is bed-hugging (on robotic legs, on wheels or free-floating, the two latter configurations illustrated in Figure 1), thus both facilitating the process of bed disturbance and minimising interference with other vessels using the water area.

Bed disturbance (Figure 1 [v]) will be induced hydraulically (pump, fan) or pneumatically (air injection, air lift), and does not involve any mechanical (bed digging or scraping) activity, thus affecting only the bed-surface veneer of naturally mobile, recently-accumulated sediment [w] (mud or sand). A carefully planned trackplot [x] will ensure all parts of the work area are subject to resuspension by the AUV during each work cycle. The plume [y] generated by this resuspension action will be directed up into the water column above the unit, with design objectives of creating a dilute sediment plume and a plume that reaches as high as is practicable towards the water surface, both factors increasing the amount of time that the sediment remains in-transit suspended above the bed (lowering settling velocity, increasing settling depth respectively) thus maximising the lateral movement of the resuspended sediment out of the worked water area. The latter dispersion is effected by careful planning that the timing of disturbance utilises advantageous naturally occurring currents [zj.

Every work environment is likely to be different in its sediment management needs. A detailed appraisal of the sedimentary regime at each site will always be necessary, and the AUV mechanical configuration and operational programme designed to provide an optimal sediment cleaning regime. An example of an idealised AUV mechanical configuration is shown in Figure 2 (section at top, plan at bottom), for explanatory purposes, and where component elements are lettered [a-n].

To operate effectively the device needs to be a near-constant distance above the bed (bed gap). In the example shown in Figure 2 this gap [a] is simply maintained by having a just negatively buoyant vehicle (controlled via a buoyancy-adjusting compartment, [b]) and wheels [c] that keep the equipment at the optimum height above the bed. With a free-floating device this distance would need to be dynamically controlled using both buoyancy control and hydroplanes, with the bed-gap constantly sensed using a precision altimeter.

The device would be driven across the bed at speeds of up to two knots. In the example shown in Figure 2, drive is via the two front or all wheels [c]. Negative buoyancy and wheel design (grip) would need to provide sufficient traction, without over-compacting the bed. A legged (walking) device is another option which may better address these issues. With other configurations, and specifically a free-floating device, movement would be induced by a propeller or a system of thrusters.

Efficient turning is a requirement for both accurate navigation and effective cover of the bed. With the example shown in Figure 2, tight controlled turns would be achieved by independently operating the wheels [c], and may also involve a retractable spud device ([d] one on each bow) around which the turn could pivot. With a free-floating device the turns would have to be dynamically controlled, and are liable to be less precise.

Bed erosion will be induced by water flow, generated by a pump, a propeller or by an air-lift system. The flow rate will be controllable, so that the correct level of bed erosion and plume sediment concentration can be achieved for each work site. In the example shown in Figure 2 the flow is induced by a large diameter horizontally mounted propeller [e] which pulls in water immediately below the periphery of a flange [f] enclosing the propeller, the shear stress caused by this flow inducing bed erosion under the flange. The flow downstream of the propeller is enclosed in a duct [g] which directs the plume of sediment laden water [y] vertically upwards.

It is an advantage that the upward flowing plume of sediment-laden water passes as high as possible into the overlying water before the plume's energy is lost and sediment particles begin to settle back to the bed. This will optimise the travel vectors of the particles before they reaccumulate. A rigid vertical pipe containing the plume will be impractical in most cases, causing drag and snagging. A flexible chute of light nylon fabric [h] on a whip support [i], that will break off if snagged, is one solution, as illustrated in Figure 2. Other methods may be used such as air bubble entrainment or rising vortex generation.

Power will be supplied via a rechargeable battery or fuel cell [j], housed in a watertight compartment [k], and suitably insulated. The capacity of this power source will depend on the size of the vehicle and the duty cycle (ratio of working hours to recharging hours). The AUV will preferably have the ability to self-navigate to a recharging station and automatically connect to the recharging power source. The AUV will preferably run on renewably produced energy.

For most applications the AUV will require to know its position accurately and constantly. This may sometimes be achieved via a water-surface breaking wand with a global positioning antenna. Usually an underwater range-range or range-bearing acoustic positioning system will be relied on [I], backed up by a dead-reckoning system (involving compass, inertia sensors and odometer/Doppler speed-over-ground sensing). Another positioning alternative is a scanning sonar system, that produces a map of solid features/laid targets, with software algorithms matching this map to a supplied baseline map (Simultaneous Location & Mapping, SLAM software). In all cases work areas will normally be pre-mapped using acoustic methodology (multibeam and side-scan) to provide a baseline map and allow mission planning, providing a foreseen trackplot to be followed by the AUV in order to effectively cover the bed, remain within the work area and avoid obstacles. AUVs may also have a forward-looking sonar dedicated to obstacle avoidance, to provide for new obstacles that may arise.

Preferably each AUV will operate a monitoring system that logs environmental and performance data (such as bed levels and turbidity) and also vehicle operational parameters (eg power condition, pitch and roll, water ingress, position).

Mission control will be managed via an on-board computer system [m] housed in a separate watertight compartment [n]. The primary function will be navigation, interpreting the location data and translating this into guidance signals. The control system will also manage start and stop times, buoyancy, docking, power management, emergency responses and data logging. In complex environments, it will be preferable to have an acoustic data link between the AUV and an onshore receiving station, so that autonomous operations can be monitored and overridden if necessary.

Claims (1)

1. CLAIM An autonomously operating bed-hugging mobile device for applying hydraulically a chronic element of low-level erosion to sediment-floored aquatic areas (with the intention of establishing a long-term balance between processes of sediment deposition and erosion), working a defined bed area at frequent intervals (typically diurnally or semi-diurnally), mobilising the bed-surface veneer of recently accumulated sediment and dispersing the eroded sediment upwards into the overlying water body as a low-concentration plume, with the automated timing of the operation utilising advantageous dispersion by natural water-flow.