EP3436641B1

EP3436641B1 - Removing concrete structures from the seabed

Info

Publication number: EP3436641B1
Application number: EP17722121.5A
Authority: EP
Inventors: Christopher Allan HUTCHENS
Original assignee: Subsea 7 Ltd
Current assignee: Subsea 7 Ltd
Priority date: 2016-03-31
Filing date: 2017-03-29
Publication date: 2020-03-04
Anticipated expiration: 2037-03-29
Also published as: EP3436641A1; AU2017243197A1; GB2548874B; GB2548874A; WO2017168150A1

Description

This invention relates to the challenges of removing concrete structures such as concrete mattresses from the seabed, as may be required when decommissioning a subsea oil or gas installation.
It is common in the subsea oil and gas industry to stabilise, protect or separate subsea pipelines or other items of subsea infrastructure by installing concrete mattresses over and beside them. For example, mattresses may be placed over subsea pipelines in an area extending to a substantial radius around a surface installation such as a platform, where the risk of impact damage to the pipelines from falling objects is at its greatest. Mattresses may also be used to stabilise a soft seabed to mitigate scouring.
Concrete mattresses typically comprise a two-dimensional articulated array or matrix of concrete blocks joined by a net or mesh of flexible links or chords such as polypropylene ropes. Flexible mattresses like these are the most popular as they can drape over the subsea structures they are designed to protect. They can also follow the seabed contours more closely. This is important to reduce the risk of trawlers snagging their nets on a mattress. However, various other mattress designs have been used with varying degrees of success, including some in which concrete blocks are joined by steel wires or rods.
Under current regulations in the North Sea, for example, the default position is that all concrete mattresses must be removed as part of decommissioning procedures at the end of field life. This applies unless a thorough comparative assessment shows that a mattress recovery operation cannot be completed safely and efficiently. Even if permission can be obtained to leave such mattresses in situ when decommissioning, the operator retains perpetual liability for any items left on the seabed. Thus, the operator is required to pay for regular monitoring, including trawl sweeps to demonstrate that the mattresses do not present any hindrance to other users of the sea.
It has been estimated that nearly 40,000 concrete mattresses have been installed in the North Sea alone. This equates to at least 200,000 tonnes of concrete. The removal of such mattresses therefore presents a significant challenge to the subsea oil and gas industry.
As concrete mattresses were not designed to be recovered, their removal upon decommissioning can be complex, time-consuming and expensive. The complexity of removal operations is compounded by the variety of mattress designs and sizes that have been installed down the years. Also, mattresses in different locations will tend to degrade differently, depending upon their depth and their exposure to tidal currents, wave action, sunlight, marine growth and abrasive seabed soils. It is therefore difficult to predict the structural integrity of a mattress before an attempt is made to lift it.
Of course, mattress removal operations must be carried out in a way that minimises risk to personnel and the environment. For example, diver-intensive operations are undesirable when lifting potentially unstable loads from the seabed. Also, inadvertent damage to a subsea pipeline protected by a concrete mattress could release hydrocarbons into the sea.
For concrete mattresses that can be removed successfully, re-use presents the most cost-effective and environmentally-friendly method of disposal. For example, the material of such mattresses can be recycled to provide concrete aggregate for use in the construction industry.
In principle, concrete mattresses with sufficient structural integrity could be recovered by reversing the installation process, involving a multi-point crane lift of each individual mattress from the seabed to the surface. However, this requires a mattress to be properly rigged to a crane hook or to an intermediate lifting frame such as that disclosed in US 6406217 . This is difficult to achieve underwater in a way that ensures a stable balanced lift and that minimises the risk of dropping the mattress, or parts of the mattress, back onto the installation that the mattress was intended to protect.
Consequently, most concrete mattresses have been removed to date using subsea baskets or speed loaders. Subsea baskets may be embodied by half-height skips or steel cargo nets into which the mattresses are placed underwater before lifting. On the other hand, speed loaders are arranged to receive mattresses underwater stacked three- to five-deep and then to be recovered in one lift to the surface.
All such methods are diver-intensive. The use of divers introduces safety challenges and requires diver support vessels to be mobilised at great expense, and in any event is impractical if the water depth is greater than about 200m. ROVs can be used at greater depths but they suffer the same challenges of interacting underwater with concrete mattresses and with the equipment that will be used to lift the mattresses. The use of ROVs also ties up support vessels.
Subsea grabs are a diverless alternative but are not an efficient solution if numerous mattresses are being decommissioned. Their use also increases the risk of inadvertent damage to a subsea pipeline adjacent to a mattress and hence of a release of hydrocarbon pollutants.
GB 2486014 discloses a self-propelled underwater vehicle for removing concrete mattresses from the seabed according to the preamble of claim 1 and a method for recovering subsea material that comprises concrete mattresses from the seabed according to the preamble of claim 15. In particular, this document discloses a trolley that can lift and store mattresses for recovery. However, the size of mattresses that can be recovered is limited by the size of the trolley.
US 2016/175847 A1 relates to a subsea-operated vessel for crushing concrete mattresses.
With increasing decommissioning activities, it is beneficial for the subsea oil and gas industry to have a new solution to remove concrete mattresses from the seabed safely and efficiently, in a way that meets environmental requirements and that complies with national government policy. The invention solves this problem with a safe and effective technique that reduces decommissioning times, removes the requirement for a costly dive vessel and improves the safety and predictability of recovering mattresses with unknown integrity.
Against this background, the invention provides a remotely-operated vehicle for processing a concrete mattress so that the mattress can be removed easily from a subsea location. The vehicle of the invention has a concrete-crushing mechanism that is adapted for remote operation underwater, as water modifies the mechanical behaviour of rollers within the mechanism and changes the way in which concrete pieces and dust must be managed. For example, when mixed with seawater, concrete dust may become a muddy cloud. Additionally, concrete mattresses often contain other materials such as steel bars and polymer ropes that cannot be treated as at surface. Provision must also be made for removing debris from the seabed rather than leaving the debris behind, which may not be permitted by environmental legislation.
It is known to break rocks and to move seabed soil underwater during subsea trenching and mining operations. In subsea trenching as shown in WO 2013/061073 , for example, rotary cutters or excavating wheels may cut through rock when burying a cable, a pipeline or the like in a trench. However, the broken rock spoil is generally left on the seabed to the sides of the trench before eventually being moved by a conveyor, and no provision is made for recycling the spoil. Additionally, if used on a concrete mattress, rotary cutters could become entangled with the mattress and would be likely to damage subsea equipment under or beside the mattress. Such equipment may included a decommissioned pipeline under the mattress, therefore risking pollution of the sea by residual hydrocarbons.
Subsea mining may involve crushing mineral-bearing rocks underwater to extract mineral nodules that are then recovered to the surface. Here again, though, the majority of the rock spoil is left on the seabed. Conventional equipment for this purpose comprises a simple roller that crushes the rock on the seabed before the debris is pumped away as a slurry, as disclosed in WO 2011/156866 . The rocks being crushed underwater are typically much smaller than a concrete mattress.
The subsea mining system disclosed in US 2013/312296 requires the permanent presence of a support vessel at the surface, to which debris from the seabed is pumped through a hose for sorting and processing. Such a vessel would be prohibitively expensive to maintain for occasional use in the removal of concrete mattresses. WO 97/25488 discloses another subsea mining system that also shares the drawback of requiring permanent piping to transport debris to the surface. Such piping is practical only in shallow water, because deeper water would require either bonded flexible pipeline, which is very expensive, or rigid pipeline because the hydrostatic pressure at depth would collapse standard hoses. The present invention removes the need for a permanent hose and so allows working in deeper water.
In apparatus terms, the inventive concept is expressed as a self-propelled underwater vehicle for removing concrete mattresses from the seabed that comprises: a drive system adapted to drive the vehicle across a seabed on which the vehicle is supported in use; a crushing mechanism supported by a body, which mechanism is adapted to crush subsea material that comprises concrete mattresses; a self-loading system for feeding said material into the crushing mechanism to be crushed; and a disposal system for receiving crushed debris from the crushing mechanism, the disposal system comprising a container for holding the crushed debris onboard the vehicle. The body further comprises a housing surrounding the crushing mechanism, the housing having an intake opening communicating with the crushing mechanism for receiving said material from the self-loading system. The self-loading system comprises at least one articulated lifting arm that is mounted to the body and is capable of reaching across the intake opening and to the seabed in use.
Preferably, the container is removable from the vehicle for recovery of the crushed debris to the surface while the remainder of the vehicle remains on the seabed.
The housing preferably encloses an enclosed exit path for crushed debris leading from the crushing mechanism to the container.
The container may be positioned in a receiving position within the housing, beneath the crushing mechanism. In that case, a transport mechanism may be provided for moving the container from the receiving position to a recovery position outside the housing.
The recovery position of the container may be on the seabed, in which case a longitudinal channel set into an underside of the container can enable the container to straddle a pipeline laid on the seabed.
Alternatively, the container may be positioned outside the housing, offset horizontally from the crushing mechanism, with an inlet of the container being coupled to an outlet of the housing to extend the enclosed exit path.
The vehicle of the invention may further comprise a conveyor system for crushed debris, that system extending to the container from beneath the crushing mechanism.
The enclosed exit path may be defined partially by a shroud that covers an open top of the container.
Advantageously, a mass flow system is arranged to drive a flow of water within the housing in a flow direction that runs from the intake opening through the crushing mechanism and toward the container.
The intake opening is, conveniently, upwardly-facing and is positioned above the crushing mechanism. To allow the vehicle to straddle a pipeline laid on the seabed, the drive system suitably comprises laterally-spaced parallel tracks defining a longitudinal channel under the body between the tracks.
The lifting arm may comprise at least one tool for dismantling a subsea structure that comprises said subsea material. Conveniently, the lifting arm is mounted to the body aft of the intake opening of the housing, with respect to a direction of forward motion of the vehicle.
In method terms, the inventive concept is expressed as a method for recovering subsea material that comprises concrete mattresses from the seabed. The method comprises: self-loading said subsea material into a subsea vehicle at a seabed location by reaching an articulated lifting arm of the vehicle across an intake opening of the vehicle and lifting said subsea material from the seabed toward the intake opening; crushing said subsea material onboard the vehicle at the seabed location; and holding crushed debris in a container onboard the vehicle at the seabed location.
The method of the invention preferably further comprises: removing the container containing crushed debris from the vehicle while the vehicle remains at the seabed location; and raising the container to a surface location for recovery of the crushed debris.
The vehicle may be driven across the seabed toward subsea material to be recovered, preferably while straddling a subsea pipeline or other elongate subsea element.
To constrain the spread of particulate material, the method preferably comprises enclosing an exit path for crushed debris leading from a crushing mechanism of the vehicle to the container. The container may be held beneath the crushing mechanism in a receiving position to allowing crushed debris to fall into the container from the crushing mechanism. In that case, the container may be moved from the receiving position to a recovery position outside the vehicle. Alternatively, crushed debris may be conveyed from the crushing mechanism into the container horizontally offset from the crushing mechanism.
Particulate material can be kept within the housing and prevented from settling by propelling a flow of water through the crushing mechanism in a flow direction toward the container.
The container may be placed onto the seabed in a position straddling a subsea pipeline or other elongate subsea element.
Conveniently, the vehicle may be used to dismantle a subsea structure that comprises said subsea material, before self-loading the subsea material into the vehicle.
The container may be emptied of crushed debris at the surface location, in which case the emptied container may be returned to the seabed location to be replaced on the vehicle. Alternatively, the container may be left at the surface location and another empty container may be lowered to the seabed location to be coupled to the vehicle.
In specific embodiments, therefore, the vehicle may comprise remotely-operated displacement means and a chassis connected to the displacement means; the chassis carrying, at least, a mattress-handling device, a concrete-crushing unit and a disposal unit, all being remotely-operated. An outlet of the concrete-crushing unit is connected to an inlet of the disposal unit, and at least a basket, cassette or bin of the disposal unit can be removed.
The concrete-crushing unit may comprise a vertical inlet, crushing rollers and a collecting container located below the crushing rollers. A conveyor may transfer crushed pieces from the collecting container to the disposal unit. The disposal unit contains at least one basket, cassette or bin into which the conveyor transfers the crushed parts. Means may be provided for sorting the crushed pieces, for example as part of the conveyor.
It may be possible for the removable basket, cassette or bin of the disposal unit to be handled underwater by an ROV. The mattress handling device may comprise a robotic arm and/or a conveyor.
The method may include grabbing the mattress; transferring the mattress above a concrete-crushing unit of a machine; collecting crushed pieces in a container below the concrete-crushing unit; and conveying the crushed pieces to at least one disposal unit.
The disposal unit may be at least partially removable. The equipment is suitable to be used underwater and to be operated remotely.
In summary, the invention provides a self-propelled, self-loading subsea vehicle that fragments material and holds that material ready for recovery to the surface.
The vehicle of the invention provides a complete and integrated way of removing concrete structures such as concrete mattresses during offshore decommissioning works. The vehicle is operated remotely to obviate diver intervention and to improve safety during decommissioning. The invention combines an articulated tooling/excavator lifting arm with a crushing mechanism and a recoverable subsea basket into a single vehicle.
The self-loading capability of the vehicle differentiates the invention from other decommissioning technologies that are reliant on diver intervention or the use of cranes and that therefore suffer difficulty in recovering mattresses of unknown integrity. The self-propelled, tracked and mobile vehicle works along pipelines and other subsea structures, using its lifting arm to load concrete material into a hopper that directs the material into a crushing mechanism. The crushing mechanism may comprise large concrete-crushing rollers known from onshore applications, which are capable of breaking up concrete blocks and shearing rebar or polymer rope.
The concrete material gravity-feeds or is pulled into the crushing mechanism, where it is broken into fragments. The fragments are collected from the crushing mechanism by being conveyed under gravity or by conveyors to a storage location defined by a recoverable subsea basket. Thus, the fragments may fall directly into a basket under the crushing mechanism or may be transported into a basket offset from the crushing mechanism. Sediment and particulate material are kept contained within the vehicle during crushing and transport to the basket. The basket can then be recovered to a surface support vessel and an empty basket - either the same basket, after emptying, or a different basket - can be reinstated onto the vehicle for further collection and crushing of concrete material from the seabed.
Various design options for crushing mechanisms exist and could be interchangeable depending on the nature of the concrete mattress material to be recovered. For example, reciprocating impact hammers may be used in addition to, or instead of, rollers. Tools could also be interchanged on the lifting arm allowing concrete material to be broken up before being lifted into the crushing mechanism.
In order that the invention may be more readily understood, reference will now be made, by way of example, to the accompanying drawings in which:

Figure 1 is a schematic part-sectioned side view of a mattress-removal apparatus of the invention, in use on the seabed lifting a concrete mattress from over a subsea pipeline;
Figure 2 is a front view of the apparatus shown in Figure 1;
Figure 3 is a top view of the apparatus shown in Figure 1, omitting a lifting arm of the apparatus for clarity;
Figure 4 is a rear view of the apparatus shown in Figure 1;
Figure 5 is a schematic part-sectioned side view of another mattress-removal apparatus of the invention, in use on the seabed lifting a concrete mattress from over a subsea pipeline;
Figure 6 is a front view of the apparatus shown in Figure 5;
Figure 7 is a top view of the apparatus shown in Figure 5, again omitting a lifting arm of the apparatus for clarity;
Figure 8 is a rear view of the apparatus shown in Figure 5; and
Figure 9 is a schematic side view showing a system of the invention in use.

Referring firstly to Figures 1 to 4 of the drawings, a self-propelled vehicle 10 in accordance with the invention is shown on the seabed 12 straddling a subsea pipeline 14, along which the vehicle 10 can move while lifting concrete mattresses 16 that lie over the pipeline 14. The pipeline 14 is one example of an elongate subsea element; other examples include cables and service pipes that may also be laid across the seabed 12.
The vehicle 10 is designed to be overboarded and lowered to the seabed 12 from a surface support vessel, preferably using a launch and recovery system (LARS). The size and mass of the vehicle 10 may be similar to that of a subsea plough, although there is no need in this instance for the support vessel to exert a high bollard pull. This allows vehicles of the invention to be supported by a wide range of support vessels. The vehicle 10 comprises a body 18 and a recoverable subsea basket 20 that, in this example, is removably mounted to an aft end of the body 18. The basket 20 is separable from the body 18 for periodic recovery of crushed debris in the basket 20 to the surface while the remainder of the vehicle 10 remains on the seabed 12. The basket 20 maximises storage space for crushed debris to maximise the efficiency of the vehicle 10 by minimising the number of lifts of the basket 20 that are necessary to recover a given number of concrete mattresses 16.
A shroud 22 covers the otherwise open top of the basket 20 to minimise escape of particulate debris from the basket 20 into the sea. The shroud 22 may be attached to the basket 20 to move with the basket 20 during recovery, or may be attached to the body 18 of the vehicle 20 so that the basket 20 moves relative to the shroud 22 as the basket 20 is recovered.
An upper surface of the basket 20 is suitably fitted with lifting points 24 as shown, whereby lifting hooks or a lifting frame suspended from a crane on the support vessel can be coupled to the basket 20. The lifting points 24 may, for example, be similar to the ISO-approved design used on standard freight containers.
The body 18 of the vehicle 10 comprises a housing 26 supported by a steel chassis frame. The housing 26 is designed to contain the sediment and particles that are produced when crushing a concrete mattress 16. Thick steel panels may be employed for the housing 26 where there will be most contact with the material of the mattress 16 and thinner panels may be employed where lesser contact is expected. Some panels of plastics or composite materials can be used to complete the housing 26.
The members of the chassis frame of the body 18 are pressure-equalised or of open beam sections so as to avoid being crushed under hydrostatic pressure at the operating depth of the vehicle 10. The chassis frame is large enough to provide attachment points for the basket 20 and is designed to support the considerable weight of the crushed debris that will accumulate in the basket 20 in use of the vehicle 22.
The body 18 is supported by parallel tracks 28 that are driven electrically or hydraulically to advance the vehicle 10 across the seabed 12. The tracks 28 may be constructed and driven like those of existing ROVs used for subsea trenching and mining. In this example, the body 18 is connected directly to the tracks 28. This simple and low-cost arrangement suits use of the vehicle 10 on a generally flat, horizontal seabed 12.
The tracks 28 are wide so as to maximise the footprint of the vehicle 10 and hence to reduce the weight load of the vehicle 10 per unit area of the seabed 12. The size of the tracks 28 is selected in accordance with the expected soil conditions of the seabed 12 and the capacity of the basket 20.
The tracks 28 are spaced laterally from each other and raise the body 18 above the seabed 12. Conveniently, the resulting full-length longitudinal channel 30 under the body 18 between the tracks 28 accommodates the pipeline 14 along which the vehicle moves while lifting concrete mattresses 16 that lie over the pipeline 14. The body 18 helps to protect the pipeline 14 from debris that may fall as mattresses 16 are being lifted.
The housing 26 contains a crushing mechanism 32 that is situated under an intake opening 34 in the top of the housing 26. Otherwise, sealed panels of the housing 26 substantially enclose the crushing mechanism 32.
In this example, the crushing mechanism 32 comprises parallel opposed contra-rotating rollers 36 that extend longitudinally but could extend transversely instead. The rollers 36 are driven by an electrical or hydraulic drive system 38 that may comprise one or more motors and gearboxes. For example, the drive system 38 may use existing electrical motors rated for subsea use in deep water to drive a hydraulic drive via torque-multiplying gearing to apply sufficiently high torque to the rollers 36 to break the concrete of a mattress 16.
Each roller 36 of the crushing mechanism 32 suitably comprises a series of teeth or discs that are eccentric or have angularly-offset radial projections arranged to interleave in interlocking relation with their counterparts in the opposed roller 36. The cooperating edges of the interlocking teeth or discs apply opposing shear forces to crush the concrete blocks of the mattress 16 and to sever any polymer or steel elements of the mattress 16. In this way, the mattress 16 is turned into fragments of the concrete and other materials from which the mattress 16 was made.
In this example, the housing 26 further contains a conveyor system 40 comprising at least one conveyor that extends under the crushing mechanism 32 to receive fragmented material falling from the rollers 36 under gravity. The conveyor system 40 moves the fragmented material aft and is suitably inclined upwardly toward its aft end, as shown, to deposit the fragmented material into the top of the basket 20 under the shroud 22.
A mass flow system 42 comprises one or more impellers whose low-pressure suction side faces the intake opening 34 and whose high-pressure outlet side faces the basket 20. Thus, in use, the mass flow system 42 drives a flow of seawater within the housing 26 in a direction from the intake opening 34 toward the basket 20. This reduces pressure within the intake opening 34 to draw seawater into the intake opening 34. The mass flow system 42 is suitably situated downstream of the crushing mechanism 32 and upstream of the basket 20 as shown in Figure 1.
The incoming seawater flows through the crushing mechanism 32 and the conveyor system 40 to entrain particles of concrete and other materials. The flow driven by the mass flow system 42 keeps the particles mobile to prevent them clumping together and forming deposits that could otherwise clog the crushing mechanism 32 or the conveyor system 40. The flow also carries those particles into the basket 20 rather than allowing them to escape into the sea from the housing 26 through the intake opening 34. The particles can then be filtered out of the flow in the basket 20 as the seawater of the flow exhausts through openings in the basket 20.
The chassis frame of the body 18 supports a lifting arm 44 that is hinged to a mounting on top of the housing 26, preferably aft of the intake opening 34 as shown. The lifting arm 44 may be driven hydraulically, pneumatically or electrically. In use, the lifting arm 44 lifts and manipulates a load in the form of a concrete mattress 16.
In this example, the lifting arm 44 is a hydraulically-operated three-section arm like that of an onshore excavator. A hydraulic system drives the lifting arm 44 through a chosen range of reach and rotation about an upright pivot axis. The hydraulic system must withstand the hydrostatic pressure at the operating depth of the vehicle 10 while complying with environmental requirements.
In addition to supporting the mounting of the lifting arm 44, the chassis frame of the body 18 can support any ballast that may be required as a counterweight to counteract the turning moment of the lifting arm 44 when raising a concrete mattress 16.
The lifting arm 44 carries one or more interchangeable or integral tools at its free end to assist in removing concrete mattresses 16 and other concrete items and loading them into the crushing mechanism 32. The tools may, for example, comprise a manipulation or loading tool such as jaws or a grab 46 as shown and/or an additional tool such as a drill, a cutter, a circular saw, a diamond wire saw, shears or other cutting, suction or breaking tools. The grab 46 or other tools may be driven hydraulically, pneumatically or electrically.
Aside from the primary lifting function performed by a grab 46, such additional tools on the lifting arm 44 can be used to prepare a concrete mattress 16 for removal. For example, they may cut the joining chords between concrete blocks of a mattress 16 or break down the concrete into more manageable pieces for the grab 46 subsequently to lift them into the crushing mechanism 32.
Figure 1 shows a tool in the form of a grab 46 at the free end of the lifting arm 44 in the process of lifting a concrete mattress 16 from over the pipeline 14. The grab 46 is designed to limit damage to pipelines 14 and other subsea infrastructure but to maintain a firm grip on the mattress 16.
As the dashed lines in Figures 1 and 3 show, the concrete mattress 16, or a piece of the mattress 16, is being lifted toward the intake opening 34 on top of the housing 26. The mattress 16 is then released by the grab 46 to drop into and through the crushing mechanism 32 between the rollers 36. The fragmented material of the mattress 16 falls onto the conveyor system 40 and is thereby carried aft into the basket 20 for periodic recovery to the surface.
The inward and downward contra-rotation of the rollers 36, when viewed from above through the intake opening 34, engages the leading end of the concrete mattress 16. If the mattress 16 has sufficient structural integrity, its leading end draws the trailing portion of the mattress 16 through the intake opening 34 and into the crushing mechanism 32. The lifting arm 44 and the grab 46 help to feed the remainder of the mattress 16 through the intake opening 34. When no longer required for that purpose, the lifting arm 44 and the grab 46 can be positioned ready to lift the next mattress 16 from ahead of the vehicle 10 as the vehicle 10 advances along the pipeline 14.
Electrical, hydraulic and/or pneumatic power is supplied to the vehicle 10 by a tether 48. The tether 48 conveniently hangs from a support vessel at the surface but could instead be connected to a subsea source of power. The tether 48 also carries control signals to and from an operator at the surface who controls the operation of the vehicle 10. However, in principle, it would be possible for the vehicle 10 to operate autonomously without requiring continuous external control. In that case, it may be possible to omit the tether 48 and for the vehicle 10 to be self-powered.
Survey equipment 50 that may include cameras and lights is suitably mounted on the front of the body 18. This provides the operator with real-time video images to facilitate control of the vehicle 10, to identify a mattress 16 to be removed, to direct removal of the mattress 16 and to provide confirmation that the mattress 16 has been removed. Cameras, lights and other sensors may of course be positioned elsewhere on the vehicle 10 as necessary, for example to monitor ingress of material through the intake opening 34 on top of the housing 26.
The survey equipment 50 should allow the vehicle 10 to operate in blind conditions where there is significant sediment. For example, visibility issues when lifting items may be overcome using acoustic cameras, sonar, pipe tracker equipment or the like to identify existing structures.
Turning next to the vehicle 52 shown in Figures 5 to 8 of the drawings, this second embodiment of the invention shares several features with the first embodiment. Like numerals are used for like features.
In this example, the chassis frame of the vehicle 52 is connected to the tracks 28 indirectly, via a self-levelling hydraulic system comprising pairs of individually-operable jacks 54 spaced longitudinally and laterally. This more complex arrangement may be helpful where the vehicle 52 is to be used on a relatively uneven or inclined seabed 12.
When material is being crushed by the crushing mechanism 32, the recoverable basket 20 is held within the housing 26 in a receiving position, shown in solid lines in Figure 5, directly beneath the crushing rollers 36. Crushed debris falls from the crushing mechanism 32 into the basket 20 through the open top of the basket 20. This removes the requirement for a conveyor system 40 to transport the material into a basket 20 as shown in the first embodiment. However, again, a mass flow system 42 advantageously creates a flow of seawater from the intake opening 34 toward the basket 20 to carry particulate debris into the basket 20 rather than allowing it to escape into the sea from the housing 26 through the intake opening 34.
When the basket 20 is full, crushing is paused while the basket 20 is ejected from the housing 26 by being lowered and moved aft by a transport mechanism 56, eventually to lie in a recovery position on the seabed 12 aft of the vehicle 52 as shown in dashed lines. Here, the basket 20 is readily accessible to be recovered by a crane of a support vessel at the surface. In its place, an empty basket 20 can be placed in the recovery position on the seabed 12 aft of the vehicle 52, to be lifted by the transport mechanism 56 and moved forward into the receiving position within the housing 26 directly below the crushing rollers 36 ready for crushing to resume.
In this example, the underside of the basket 20 has a full-length longitudinal channel 58 to accommodate a pipeline 14 on the seabed 12. The basket 20 can thereby straddle the pipeline 14 and rest on the seabed 12 on both sides of the pipeline 14 without damaging the pipeline 14 or allowing the pipeline 14 to destabilise the basket 20.
Turning finally to Figure 9 of the drawings, this shows a support vessel 60 on the surface 62 of the sea interacting with a vehicle 10 of the invention to remove concrete mattresses 16 that lie over a subsea pipeline 14 on the seabed 12. The support vessel 60, the vehicle 10 and the depth of seawater between them are not shown to scale.
A full basket 20 that was attached to the vehicle 10 (in a position shown in dashed lines) is shown here in mid-water while being recovered by a crane 64 on the support vessel 60, which previously lowered lifting hooks or a lifting frame to the basket 20. A supporting work-class ROV 66 is used to engage the hooks or the frame with the basket 20 and then to monitor the lift and to support recovery of the basket 20 to the support vessel 60.
On-deck handling of the basket 20 on the support vessel 60 will depend on the number of mattresses 16 and hence on the quantity of material to be removed. For example, a full basket 20 can be left on a deck of the vessel 60 for removal back to shore for processing and eventual recycling of the debris within. In that case, the full basket 20 can be replaced by another, empty basket 20 stored on the deck of the vessel 60 that is lowered toward the seabed 12 and coupled to the vehicle 10 with the assistance of the ROV 66. Alternatively, the basket 20 can be emptied on deck for processing on the vessel 60 or can be emptied into a temporary store on the vessel 60, such as a half-height skip, for removal back to shore for processing and recycling. The emptied basket 20 can then be lowered back into the sea and coupled again to the vehicle 10 on the seabed 12.
Other variations are possible within the inventive concept. For example, a vehicle of the invention can support a magazine of interchangeable tools for attachment to the lifting arm. The lifting arm could access the magazine to change out tools without external assistance. Alternatively, a supporting ROV could be used to change out tools on the lifting arm, providing a fast change-over for efficient decommissioning of concrete mattresses. Further tools could be included on the ROV to help with recovering and breaking-up concrete mattresses.
The recoverable basket could be a trailer, or be mounted on a trailer, that is towed behind the vehicle. In that case, the vehicle may be regarded as an articulated vehicle. Alternatively, the recoverable basket could be a secondary vehicle, or be mounted on a secondary vehicle, that is driven to follow a primary vehicle in a master-slave relationship. Such a trailer or secondary vehicle could be equipped with tracks like those of the primary vehicle. In each case, the arrangement is, in effect, a single vehicle in two parts.
Baskets need not be lifted all the way to the surface by a crane but could instead be flown to, or toward, the surface by submersibles such as ROVs, with additional buoyancy being attached to the baskets or otherwise provided as necessary.
The vehicle of the invention removes the requirement for divers and eliminates risks arising from the removal of concrete mattresses of unknown integrity. The invention also provides an integrated system of lifting, crushing and recovery that improves on-deck handling of debris and safe recovery to onshore processing. Improved demobilisations reduce potentially dangerous multiple lifts.
Other benefits of the invention are increased speed of removal of mattresses, direct crushing of material and improved deck storage and transportation. Being quicker than lifting mattresses before crushing, the invention reduces vessel time and cost and may reduce or eliminate the need for onshore processing. Indeed, in principle, the recovered material could be sold directly for reuse.

Claims

A self-propelled underwater vehicle (10, 52) for removing concrete mattresses (16) from the seabed (12), comprising:
a drive system adapted to drive the vehicle (10, 52) across a seabed on which the vehicle is supported in use;

characterised in that, the vehicle further comprises a crushing mechanism (32) supported by a body (18), which mechanism is adapted to crush subsea material that comprises concrete mattresses (16);

a self-loading system for feeding said material into the crushing mechanism (32) to be crushed; and

a disposal system for receiving crushed debris from the crushing mechanism (32), the disposal system comprising a container (20) for holding the crushed debris;

wherein the body (18) further comprises a housing (26) surrounding the crushing mechanism (32), the housing (26) having an intake opening (34) communicating with the crushing mechanism (32) for receiving said material from the self-loading system; and

wherein the self-loading system comprises at least one articulated lifting arm (44) that is mounted to the body (26) and is capable of reaching across the intake opening (34) and to the seabed in use.
The vehicle of Claim 1, wherein the container (20) is removable from the vehicle (10, 52) for recovery of the crushed debris to the surface.
The vehicle of Claim 1 or Claim 2, wherein the housing (26) encloses an enclosed exit path for crushed debris leading from the crushing mechanism (32) to the container (20).
The vehicle of Claim 3, wherein the container (20) is positioned in a receiving position within the housing (26), beneath the crushing mechanism (32).
The vehicle of Claim 4, further comprising a transport mechanism (56) for moving the container (20) from the receiving position to a recovery position outside the housing (26).
The vehicle of any preceding claim, wherein a longitudinal channel (58) is set into an underside of the container.
The vehicle of Claim 3, wherein the container (20) is positioned outside the housing (26), offset horizontally from the crushing mechanism (32), and an inlet of the container is coupled to an outlet of the housing to extend the enclosed exit path.
The vehicle of Claim 7, further comprising a conveyor system (40) for crushed debris, that system extending to the container (20) from beneath the crushing mechanism (32).
The vehicle of Claim 7 or Claim 8, wherein the enclosed exit path is partially defined by a shroud (22) that covers an open top of the container (20).
The vehicle of any preceding claim, further comprising a mass flow system (42) arranged to drive a flow of water within the housing (26) in a flow direction that runs from the intake opening (34) through the crushing mechanism (32) and toward the container (20).
The vehicle of any preceding claim, wherein the intake opening (34) is upwardly-facing and is positioned above the crushing mechanism (32).
The vehicle of any preceding claim, wherein the lifting arm (44) is mounted to the body (26) aft of the intake opening (34) of the housing, with respect to a direction of forward motion of the vehicle (10, 52).
The vehicle of any preceding claim, wherein the lifting arm (44) comprises at least one tool (46) for dismantling a subsea structure that comprises said subsea material.
The vehicle of any preceding claim, wherein the drive system comprises laterally-spaced parallel tracks (28) defining a longitudinal channel (30) under the body (26) between the tracks (28).
A method for recovering subsea material that comprises concrete mattresses (16) from the seabed (12), the method comprising:
self-loading said subsea material into a subsea vehicle (10, 52) at a seabed location,

characterised in that said self-loading is achieved by reaching an articulated lifting arm (44) of the vehicle (10, 52) across an intake opening (34) of the vehicle (10, 52) and lifting said subsea material from the seabed (12) toward the intake opening (32); the method further comprising

crushing said subsea material onboard the vehicle (10, 52) at the seabed location; and

holding crushed debris in a container (20) onboard the vehicle (10, 52) at the seabed location.
The method of Claim 15, further comprising:
removing the container (20) containing crushed debris from the vehicle (10, 52) while the vehicle (10, 52) remains at the seabed location; and

raising the container (20) to a surface location for recovery of the crushed debris.
The method of Claim 16, comprising emptying the container (20) of crushed debris at the surface location and returning the emptied container (20) to the seabed location to be replaced on the vehicle (10, 52).
The method of Claim 16, comprising leaving the container (20) at the surface location and lowering another empty container to the seabed location to be placed on the vehicle (10, 52).
The method of any of Claims 15 to 18, further comprising driving the vehicle (10, 52) across the seabed (12) toward subsea material to be recovered.
The method of Claim 19, comprising straddling a subsea pipeline (14) or other elongate subsea element with the vehicle (10, 52).
The method of any of Claims 15 to 20, comprising enclosing an exit path for crushed debris leading from a crushing mechanism (32) of the vehicle (10, 52) to the container (20).
The method of Claim 21, comprising holding the container (20) beneath the crushing mechanism (32) in a receiving position and allowing crushed debris to fall into the container (20) from the crushing mechanism (32).
The method of Claim 22, comprising moving the container (20) from the receiving position to a recovery position outside the vehicle (10, 52).
The method of Claim 21, comprising conveying crushed debris from the crushing mechanism (32) into the container (20) horizontally offset from the crushing mechanism (32).
The method of any of Claims 21 to 24, comprising driving a flow of water through the crushing mechanism (32) in a flow direction toward the container (20).
The method of any of Claims 15 to 25, comprising placing the container (20) onto the seabed (12) while the container (20) straddles a subsea pipeline (14) or other elongate subsea element.
The method of any of Claims 15 to 26, comprising using the vehicle (10, 52) to dismantle a subsea structure that comprises said subsea material, before self-loading the subsea material into the vehicle (10, 52).