GB2573588A

GB2573588A - Subsea apparatus

Info

Publication number: GB2573588A
Application number: GB1815478.1A
Authority: GB
Inventors: Anthony Crowther Paul; Andrew Stevenson James
Original assignee: Atlantic Marine and Aviation LLP
Current assignee: Atlantic Marine and Aviation LLP
Priority date: 2018-05-08
Filing date: 2018-09-24
Publication date: 2019-11-13
Also published as: WO2019215444A1; GB201815478D0

Abstract

A subsea clearance apparatus 1 comprising a chassis 2, fore and aft skids 3, 4 spaced along a principal axis P of the apparatus 1, a pair of flanks 22a, 22b with a plurality of tines 5 depending from the flanks 22a, 22b. The tines 5 are spaced transversely with respect to the principal axis P and pitched toward the fore of the apparatus. In use, the chassis 2 is spaced from the seabed by the skids as the apparatus is propelled therealong such that the tines 5 engage and deflect away from the principle axis obstructions which are larger than a predetermined size and which protrude by a predetermined amount from the seabed. The orientation of the fore skids is adjustable to steer the apparatus and their height is adjustable to adjust the space between the tines and the seabed. Optionally, the adjustable support has a column rotatably received within a receptacle of the chassis and a travelling support mounted to the column for rotation therewith, the actuation means connect to the column for rotating the column within the receptacle to adjust the orientation of the adjustable support.

Description

This invention relates generally to a subsea apparatus. More specifically, although not exclusively, this invention relates to a subsea clearing apparatus, e.g. for clearing large obstacles, such as boulders, from a seabed whilst minimising the environmental impact thereof and to a method of clearing and/or preparing a seabed.

Prior to installing a structure on a seabed, it is important to ensure that the seabed is as flat and regular as possible. In the case of laying a subsea pipeline for example, irregularities and obstructions on the seabed will increase the risk of pipeline spanning and overstressing.

Prior to installing a structure on the seabed, a seabed preparation and clearance operation is carried out. In some cases, this operation is performed by removing large boulders individually using a subsea grab. This has the disadvantage in that it is a time consuming and expensive process and may lead to significant disruption of benthic habitats.

It is also known to use a subsea plough, which is pulled behind a vessel along the seabed. Such a device has moldboards configured to deflect boulders out of its path, clearing the requisite area (e.g. an intended pipeline path). The use of a subsea plough has the disadvantage in that when clearing boulders from the seabed, the device also scrapes away and deflects an upper layer of the seabed. This leads to unnecessary disruption of the seabed, which can be detrimental to the subsea environment.

Additionally, in order to clear large boulders, subsea ploughs must be robust and heavy. As such, subsea ploughs are often launched from a vessel, for example by propelling them from the deck, which can cause further environmental damage as the plough comes to rest on the seabed.

It is therefore a first non-exclusive object of the invention to provide a subsea clearance apparatus that overcomes, or at least mitigates the drawbacks of the prior art.

Accordingly, a first aspect of the invention provides a subsea apparatus comprising a chassis, fore and aft supports spaced along a principal axis of the apparatus and actuation means operable to adjust the height and/or orientation of at least one support, wherein the chassis is spaced, in use, from the seabed by a distance corresponding to the height of the supports as the apparatus is propelled therealong and such that the direction of travel corresponds to the orientation of the adjustable support.

The subsea apparatus may comprise a subsea clearance apparatus, which may comprise one or more engaging elements depending from the chassis. The engaging element(s) may be arranged transversely with respect to the principal axis. The chassis may be spaced, in use, from the seabed by the supports as the apparatus is propelled therealong such that the engaging elements engage and/or deflect away from the principal axis obstructions which are larger than a predetermined size and/or protrude by a predetermined extent from the seabed.

Another aspect of the invention provides a subsea clearance apparatus comprising a chassis, fore and aft supports spaced along a principal axis of the apparatus and one or more engaging elements depending from the chassis and arranged transversely with respect to the principal axis, wherein the chassis is spaced, in use, from the seabed by the supports as the apparatus is propelled therealong such that the engaging elements engage and/or deflect away from the principal axis obstructions which are larger than a predetermined size and/or protrude by a predetermined extent from the seabed.

By limiting the extent to which the apparatus engages the seabed, the environmental damage resulting from the clearing operation can be minimised.

The or each engaging element is preferably spaced, in use, from the seabed, e.g. to engage and deflect away from the principal axis obstructions which are greater than a predetermined size and/or protrude by a predetermined extent from the seabed. The one or more engaging elements may comprise a plurality of engaging elements. The or each engaging element may comprise a board or plate.

Preferably, the or each engaging element comprises a tine, tooth, prong or spike, hereinafter tine. The engaging elements may comprise a plurality of spaced tines. The engaging elements or tines may be spaced transversely with respect to the principal axis. The engaging elements or tines may be configured to engage obstructions which are larger than a predetermined size and/or protrude by a predetermined extent from the seabed.

Another aspect of the invention provides a subsea clearance apparatus comprising a chassis, fore and aft supports spaced along a principal axis of the apparatus and a plurality of tines depending from the chassis and spaced transversely with respect to the principal axis, wherein the chassis is spaced, in use, from the seabed by the supports as the apparatus is propelled therealong such that the tines engage and deflect away from the principal axis obstructions which are larger than a predetermined size and/or protrude by a predetermined extent from the seabed. The tines may be configured to engage obstructions protruding from the seabed a distance greater than a predetermined amount.

The engaging element(s) or tine(s) may be configured to engage and/or deflect away from the principal axis only obstructions which are larger than a predetermined size and/or protrude by a predetermined extent from the seabed. The obstructions may comprise boulders or other elements or seabed debris. The predetermined size or threshold or predetermined extent of protrusion may be 10mm or more, for example 200mm or more, such as 300mm or 400mm.

The chassis may comprise one or more, e.g. a pair of, transverse members, transverse elements or flanks, hereinafter flanks. The engaging element(s) ortine(s) may depend from the flanks. The flanks may provide or form part of a tool chassis or tool chassis portion, for example a chevron or v-shaped tool or tool chassis, e.g. when viewed in plan.

The flanks and engaging element(s) or tine(s) may together form a tool, e.g. a clearing tool. The flanks may each be angled or swept, e.g. toward the aft of the apparatus. The or at least one of the flanks may extend at an oblique or right angle relative to the principal axis of the apparatus. The flanks may be collinear with one another. Preferably, the flanks diverge from one another, e.g. from the principal axis and/or toward the aft of the apparatus.

In embodiments, the angle or sweep of the flanks may be adjustable, e.g. to adjust the effective span or swept width thereof. The angle or sweep of the flanks may be pneumatically or hydraulically adjustable.

A cross-bracing or supporting member may be connected between the flanks, for example, to interconnect and/or reinforce or brace the flanks.

At least one or each flank and/or the cross-bracing may comprise one or more, for example a plurality of, segments, e.g. modular segments. The segments may be arranged such that the effective span or swept width of the apparatus is adjustable. The segments may be attached or attachable, e.g. releasably attached or attachable, to each other and/or to an end of the or each flank.

The engaging element(s) or tine(s) may be angled or pitched toward the fore of the apparatus. In embodiments, at least one or some, e.g. all, of the tines may be pivotally mounted to the chassis. In such embodiments, the apparatus may comprise a locking means or mechanism, e.g. for locking the engaging element(s) or tine(s) against pivotal movement and/or in a predetermined position or orientation.

In embodiments, some or all of the engaging elements or tines may be pivotally adjustable between a first position in which the engaging element(s) or tine(s) are angled or pitched toward the fore of the apparatus and a second position in which the engaging element(s) or tine(s) extend substantially vertically and/or orthogonal to the chassis or flank. The engaging element(s) or tine(s) may be pivotally adjustable to a third position in which they are angled or pitched toward the aft or the apparatus. The engaging element(s) or tine(s) may be stepwise or infinitely adjustable between the first, second and/or third positions. In the first position and/or third position, the engaging element(s) or tine(s) may be angled or pitched 45 degrees relative to the apparatus, chassis or flanks. Other angles are also envisaged without departing from the scope of the invention, for example between 0 and 90 degrees, preferably between 30 and 60 degrees, such as between 40 and 50 degrees.

In embodiments, at least some of the engaging elements or tines may be hydraulically adjustable, for example pivotally, relative to the apparatus, chassis or flanks.

At least some or all of the engaging elements or tines may be configured to rotate, for example, about a longitudinal axis thereof. At least some or all of the engaging elements or tines may comprise a belting, screw profile or worm drive configured to rotate or spin.

In use, the engaging element(s) or tine(s) may be spaced from the seabed or the engaging element(s) or tine(s) and seabed may describe a space therebetween. The engaging element(s) or tine(s) may be angled or pitched so as to be spaced, in use, from the seabed. The space may be more than 0mm, for example at least 50mm, in particular at least 100mm or at least 150mm. The space may be 500mm or less, for example 400mm or less, 300mm or less or 250mm or less. The space may be between 50mm and 500mm or between 50mm and 400mm, such as between 50mm and 300mm. Preferably, the space is between 100mm and 300mm, for example between 150mm and 250mm, in particular about 200mm. The space is preferably greater than 0mm, for example greater than 10mm, greater than 50mm or greater than 100mm. It is envisaged that the space could be more than 1m without departing from the scope of the invention.

The space described between adjacent engaging elements or tines may be more than 200mm, for example at least 400mm, 600mm, 800mm or 1000mm. The space described between adjacent engaging elements or tines may be 100mm or less, for example 50mm or less or 10mm or less. The spacing of the engaging elements or tines may be adjustable, for example, to adjust the predetermined size, a predetermined width or the predetermined extent of obstructions to be engaged.

The engaging element(s) or tine(s) may be replaceable or may comprise or each comprise a replaceable end, tip or leading edge. The engaging element(s) or tine(s) may comprise a material of greater strength, wear resistance and/or hardness relative to the chassis. The end(s), tip(s) or leading edge(s) of the engaging element(s) or tine(s) may comprise the material, which may be stronger, more wear resistant and/or harder than a main body thereof. The engaging element(s) or tine(s) may comprise a wear resistant coating, for example, Hardox (RTM).

At least some or all of the engaging elements or tines may be hollow and/or perforated, e.g. for reducing their weight and/or for allowing debris of a predetermined size to pass therethrough. The main body of the engaging elements or tines may comprise a mesh having holes or grills. The main body of the engaging elements may comprise a flat or concave leading edge or front face. The main body may comprise a leading edge or front face formed of metal, such as steel.

The space between the engaging element(s) or tine(s) and the seabed may be adjustable, for example, by adjusting or altering the height of the fore and/or aft support. The actuation means may be operable to adjust, in use, the height of at least one of the supports to control the distance between the tines and the seabed.

Additionally or alternatively, the space may be adjustable by adjusting the angle or pitch of the engaging element/s) or tine(s), for example by pivoting at least some of the tines relative to the chassis. In some embodiments, the space may be adjustable by adjusting a combination of the height of the fore and/or aft support and the angle or pitch of the tines.

The adjustable support may be rotatable about a steering axis, e.g. for steering the apparatus. The steering axis may be vertical or substantially vertical in use.The actuation means may be operable to rotate the adjustable support about its steering axis. The actuation means may comprise a steering actuator, which may be operable to rotate the adjustable support about its steering axis.

The apparatus may comprise a pair of laterally spaced fore supports. The actuation means may be operable to adjust the orientation of each fore support. The actuation means may comprise a respective actuator for each support. The respective actuators may be suitable for adjusting or operable to adjust the orientation of each of the fore supports independently. Alternatively, each of the fore supports may be connected to a steering frame, e.g. for synchronising their orientation relative to one another.

The adjustable support may comprise a column, which may be rotatably received within a receptacle of the chassis. The adjustable support may comprise a travelling support, which may be mounted to the column for rotation therewith. The actuation means may be operatively connected to the column, e.g. for rotating the column within the receptacle to adjust the orientation of the adjustable support.

The adjustable support may comprise an extendable column, e.g. for adjusting the height of the adjustable support. The actuation means may be operable to extend and/or retract the extendable column, e.g. to adjust or for adjusting the height of the adjustable support. The extendable column may comprise an extendable post or telescopic member or other assembly, such as a rack and pinion mechanism.

The rotatable column may comprise the extendable column. The chassis may comprise a first tubular segment, which may be secured thereto. The first tubular segment may provide the receptacle. The rotatable column may comprise a second tubular segment, which may be rotatably received within the first tubular segment. The rotatable column may comprise an inner column, which may be reciprocable within the second tubular segment, e.g. to provide the extendable column.

The inner column may be splined in the second tubular segment, e.g. to inhibit rotation therebetween. The inner column may be non-round and/or cooperate with a corresponding non-round inner surface of the second tubular segment, e.g. to provide the spline. The apparatus may comprise a third, non-round tubular segment, which may be secured within the second tubular segment, e.g. to provide the non-round inner surface. The non-round inner column and/or inner surface of the second tubular segment may comprise a substantially square or rectangular cross-section. The inner column and/or third tubular segment may comprise a box section tubular segment.

The actuation means may comprise a height actuator, which may be mounted to and/or at least partially with, e.g. housed at least partially within, the extendable column.

The actuation means or one or more of the actuators may comprise an actuator, e.g. a hydraulic or pneumatic actuator. The actuator may comprise a cylinder, e.g. with a piston reciprocable therein. The cylinder of the height actuator may be connected and/or mounted to or at least partially within the second segment and/or the piston may be connected to the first segment or vice versa. The cylinder of the steering actuator may be connected and/or mounted to the chassis and/or the piston may be connected to a lug projecting from the rotatable column or vice versa.

Another aspect of the invention provides a steering apparatus comprising any one or more features of the support steering arrangement of the aforementioned subsea apparatus.

Another aspect of the invention provides a height adjustment apparatus comprising any one or more features of the support height adjustment arrangement of the aforementioned subsea apparatus.

Another aspect of the invention provides a connection assembly for connecting a support to a chassis of a subsea apparatus, the assembly comprising any one or more features of the support height adjustment arrangement and/or support steering arrangement of the aforementioned subsea apparatus.

In alternative embodiments, the fore and/or aft supports may each comprise a post or member extending from, into or through the chassis, for example an aperture in the chassis. The post or member may be releasably and/or adjustably connected, mounted or secured to the chassis by a mounting, securing or locking means or mechanism. In embodiments, the post or member is connected to the chassis by a locking pin or member, which may be receivable in and/or engageable with one or more, e.g. a plurality of, holes, recesses or receptacles in one or each of the post and chassis. The height of the support may be stepwise adjustable, for example the post or member may comprise a series of holes, recesses or receptacles each configured to receive the locking pin or member.

At least one or each of the fore and aft supports may comprise a plurality, e.g. two or more, supports. In embodiments, one or each of the fore and aft supports comprises a pair of supports. The supports of the or each support pair may be arranged either side of the principal axis of the apparatus. The supports of the or each support pair may be equidistant to the principal axis of the apparatus.

At least one or each support may comprise a travelling support, such as a skid or roller, preferably a skid. At least one or each of the supports, e.g. the fore supports, may be canted, cantable, tilted, tiltable or pivotable, for example about an axis substantially parallel with or along the seabed and/or for accommodating irregularities or undulations in the seabed. At least one or each of the supports, e.g. the fore supports, may be pivotally connected to the extendable member or post. The inner column may comprise a clevis, which may cooperate with a tang of the travelling support. The pivotal connection may comprise a pin received by the clevis and tang.

Each support of the support pair may be adjustable, e.g. independently. Each support of the support pair may comprise a respective actuator, which may be operable to adjust, e.g. independently, the height of the chassis portion to which the support is connected. In embodiments, the apparatus may comprise a controller, for example a feedback controller. The controller may be operable to identify the state, for example state of extension or retraction, of each actuator.

At least one or each support or travelling support may comprise a track, for example a continuous track, i.e. a tank track or a caterpillar track. At least one or each support may comprise a wheel. Each track or wheel may comprise a, be connected or be connectable to a drive unit, for example a motor. The apparatus may be configured to be propelled, for example driven, by the one or more drive units.

In embodiments, the chassis may comprise a pair of longitudinal members, which may be arranged either side of and/or extend substantially parallel to the principal axis. Each longitudinal member may comprise a respective fore support, e.g. mounted or connected thereto. Each longitudinal member may be connected or secured to one of the flanks, for example an intermediate portion of the flanks, e.g. between the ends of the flanks. Each flank may comprise a respective aft support, e.g. mounted or connected thereto or connected at or adjacent a rearmost and/or outermost end thereof.

The or each skid, roller, wheel or track may be configured to rotate relative to their respective support. Each skid, roller, wheel or track may be configured to rotate relative to their respective support about the steering axis. The or each skid, roller, wheel or track may be rotatable about the steering axis via a respective hydraulic or pneumatic actuator.

The support(s), e.g. the fore support(s), may be configured to rotate about their respective steering axis independently or in concert. The support(s), e.g. the fore support(s), may be configured to rotate in the same direction and/or to the same extent for steering the apparatus. The support(s), e.g. the fore support(s), may be connected and/or linked to move together and/or in concert. The apparatus may comprise one or more steering actuator(s), e.g. for rotating the support(s), e.g. the fore support(s), about their steering axes. The steering actuator(s) may be hydraulic or pneumatic and/or may be connected to one or each of the support(s), e.g. both of the fore supports. In embodiments, each of the support(s), e.g. the fore support(s), may comprise an actuator connected or coupled thereto. The controller may be operatively connected to and/or operable to control the steering actuator(s). The controller may comprise a wireless communication means or module, for example such that steering may be carried out remotely from the apparatus by way of wireless communication.

The pair of fore supports may be connected to the steering frame either side of the principal axis of the apparatus. The steering frame may be pivotally or rotatably connected to the chassis. The steering frame may be rotatable or pivotable, for example about a steering axis, for steering or for facilitating steering of the apparatus, e.g. as it is propelled, in use, along the seabed.

The pair of fore and/or aft supports may be pivotally or rotatably fixed, for example relative to one another and/or the steering frame. The pair of fore and/or aft supports may be configured to rotate in concert via the steering frame. The or each fore and/or aft support may comprise a hydraulic cylinder, for example a passive hydraulic cylinder, in particular a viscous damper. The or each hydraulic cylinder may be connected between a respective support and the steering frame. The or each hydraulic cylinder may be configured to dampen loading, for example shock loading about the steering axis of the steering frame.

In some embodiments, the or each hydraulic cylinder may comprise a hydraulic actuator. The or each hydraulic actuator may be actuable to effect rotation of the steering frame about its steering axis, e.g. for steering or facilitating steering of the apparatus.

The apparatus may comprise a tow line, e.g. for towing the apparatus such as by a vessel. The apparatus may comprise one or more bridle lines, which may be connected to the fore support(s). The apparatus may comprise a pair of bridle lines each connected to a respective fore support. The tow line may be connected to the apparatus or to the or each fore support, e.g. by respective bridle lines. Each of the respective bridle lines may be connected or connectable to the or a tow line, for example at a common point. In embodiments, the bridle lines comprise lengths or portions of a single line, which may be slideably connected to a tow line, e.g. thereby to maintain substantially the same tension in each bridle line.

The one or more bridle lines may be connected to the steering frame. The steering frame may be rotatable about its steering axis via the one or more bridle lines. The or each hydraulic cylinder may be configured to dampen shock loading imparted to the steering frame by the one or more bridle lines.

Alternatively, each of two or more fore supports may be connected to and rotatable about their respective steering axis via a respective bridle line. The chassis or each of the respective bridle lines may comprise a tension adjusting mechanism so as to maintain a substantially constant tension in the bridle lines as the apparatus is steered. The chassis or each respective bridle line may comprise a sensor to measure the tension of the bridle lines.

The apparatus may comprise one or more keel plates. In embodiments, the or at least one or each aft support comprises a keel plate, e.g. a respective keel plate. The or each keel plate may be configured to extend, in use, into and/or substantially perpendicularly to the seabed. The or at least one or each keel plate may be adjustable, e.g. height adjustable.

The apparatus may be at least partially hollow and/or floodable. The chassis may comprise a hollow and/or floodable portion. The hollow and/or floodable portion may comprise or be a ballastable portion. The chassis or hollow and/or floodable portion may comprise a drain. Additionally or alternatively, the hollow and/or floodable portion may comprise a pump, for example a ballast pump. The hollow and/or floodable portion may comprise one or more, e.g. two or more or a plurality of, chambers each individually floodable and/or ballastable. Additionally or alternatively, the apparatus may comprise one or more buoyancy modules or buoyancy tanks. The one or more buoyancy modules or buoyancy tanks may each individually be floodable or ballastable. The one or more buoyancy modules or buoyancy tanks may be mounted or connected to the chassis.

The apparatus may comprise integral power sources or units, for example, hydraulic, pneumatic and/or electronic configured to provide electronic, telemetric, hydraulic and/or hydraulic signals to the apparatus. The integral power sources or units may be configured to operate one or more actuators, e.g. the height adjustment and/or steering actuators, of the apparatus. The integral power sources or units may be configured to operate one or more drive units, e.g. for propelling the apparatus. The integral power sources or units may be configured to operate one or more sensors and/or send/receive signals, for example control signals, to/from a remote apparatus, computer or server, e.g. on the surface.

The apparatus may comprise one or more lifting points, for example one or more eyelets or other elements which may be for connection with a crane or other lifting means, for lifting and/or transporting the apparatus. The chassis or tool or flank(s) may comprise the lifting point, which may be connected thereto or integral therewith. The apparatus may comprise a cradle, receptacle, dock or basket, which may be configured to receive and/or locate one or more remotely operated vehicles (ROV) or ROV units (hereinafter ROV). The or at least one of the ROVs may also be a WROV, a ROGE ROV, utility ROV or a zodiac ROV.

The apparatus may comprise an ROV, which may be mounted or received, e.g. releasably mounted or received, in or on the cradle, receptacle, dock or basket. The ROV may be configured to provide one or more signals, e.g. electronic, telemetric, hydraulic and/or pneumatic signals to the apparatus, e.g. for operating one or more features or actuators, e.g. the height adjustment and/or steering actuators, of the apparatus. The apparatus may comprise electrical or fluidic connectors or connections for connection to the or an ROV. The connections may provide electrical and/or fluid communication between the ROV and the actuators.

Another aspect of the invention provides a subsea apparatus comprising a dock for releasably connecting a remotely operated vehicle (ROV) to one or more operating features or systems of the apparatus.

The apparatus or dock may be configured to connect the remotely operated vehicle to the actuation means for the operation thereof. The dock may comprise one or more connectors, e.g. quick release connectors, for connection with the ROV. The apparatus may be configured to receive one or more signals from the ROV and to operate one or more features, systems or actuators of the apparatus in response thereto. The apparatus may comprise the ROV. The ROV may be configured to control one or more ancillary devices simultaneously or separately or independently of the apparatus. The dock may comprise a cradle, receptacle or basket.

The ROV may be configured to provide electronic, telemetric, hydraulic and/or pneumatic power and/or signals to the apparatus instead of, or in addition to, the integral power sources or units.

The ROV may comprise a remote hydraulic, pneumatic or electrical power supply. The ROV may be configured to provide image data, for example camera feeds. The ROV may be configured to provide sensor or telemetric data, for example heading, pitch, roll heave, sonar, altitude and/or depth data. The ROV may be configured to relay data to a remote apparatus, computer or server, e.g. on the surface.

The ROV may provide a bridge between, for example facilitate a connection between, an external hydraulic, pneumatic and/or electrical power source and the apparatus.

The ROV may be configured to propel the apparatus along the seabed.

Another aspect of the invention provides a method of steering and/or adjusting the height of an apparatus, for example an apparatus as described above.

The method may comprise adjusting the orientation of one or more supports, e.g. as the apparatus is propelled along the seabed. The method may comprise adjusting the height of one or more supports, e.g. as the apparatus is propelled along the seabed.

Another aspect of the invention provides a method for clearing a seabed, the method comprising deploying a subsea clearance apparatus, e.g. as described above, from a vessel to a seabed, propelling the apparatus along the seabed such that one or more engaging elements engage and/or deflect away from a principal propelling axis obstructions which are larger than a predetermined size and/or protrude by a predetermined extent from the seabed.

Propelling the apparatus along the seabed may comprise pulling, driving or pushing the apparatus. Propelling the apparatus along the seabed may comprise pulling the apparatus using a tow line and/or one or more, e.g. a pair of, bridle lines. The tow line may be pulled by a vessel, e.g. the vessel from which the apparatus was deployed or a different vessel.

Propelling the apparatus along the seabed may comprise pulling, driving or pushing the apparatus using or via a ROV, subsea excavator or any other suitable subsea vehicle. Propelling the apparatus along the seabed may comprise driving the apparatus using one or more drive units.

Propelling the apparatus along the seabed may comprise a combination of any of the aforementioned methods.

The method may comprise maintaining the engaging element(s) in a spaced relation relative to the seabed as the apparatus is propelled along the seabed. The method may comprise maintaining in a spaced relation relative to the seabed a chassis from which the one or more engaging elements depend as the apparatus is propelled along the seabed. The chassis may be maintained in spaced relation relative to the seabed by fore and aft supports.

The space between the engaging element(s) and the seabed may be adjusted. In embodiments, the space may be adjusted by adjusting the height of the fore and/or aft support and/or adjusting an angle or pitch of the engaging element(s).

The method may comprise steering the apparatus as it is propelled along the seabed, for example by rotating one or more fore support(s) about respective steering axes. The method may comprise steering the apparatus as it is propelled along the seabed, for example by rotating one or more skids, rollers, wheels or tracks steering axis of one or more support(s), for example fore supports. The method may comprise steering the apparatus as it is propelled along the seabed, for example by rotating the or a steering frame about its steering axis. The method may comprise rotating the or a steering frame about its steering axis by changing the angle of tow. The method may comprise rotating the or a steering frame about its steering axis by actuating one or more hydraulic actuators. The method may comprise steering the apparatus remotely via wireless communication. The method may comprise rotating the fore supports using an actuator, which may be hydraulic or pneumatic.

The method may comprise receiving, mounting, locating or coupling an ROV on or within a cradle of the apparatus, e.g. before or after lowering the apparatus toward and/or onto the seabed. The method may comprise providing signals, e.g. electronic, telemetric, hydraulic or pneumatic signals, from the ROV to the apparatus. The method may comprise relaying data, for example images and/or sensor data, to a remote apparatus, computer or server, e.g. on the surface, which may be collected by the ROV. The sensor data may comprise telemetric data, for example, heading, pitch, roll, heave, altitude and/or depth data. The image data may comprise data from camera feeds.

The method may comprise receiving, mounting, locating or coupling more than one ROV or ROV unit on or within a cradle of the apparatus.

The method may comprise lifting the apparatus, for example using a lifting point thereof or thereon. The method may comprise deploying the apparatus using a lifting means or equipment. The method may comprise deploying the apparatus using a crane, for example a vessel crane. Deploying the apparatus may comprise engaging the lifting point with the lifting means, equipment or device, e.g. the crane. Deploying the apparatus may comprise placing and/or releasing or dropping the apparatus onto or into the sea.

The method may comprise deploying the apparatus with a hollow and/or floodable portion at least partially empty or vacant, e.g. for reducing line tensions of the lifting equipment. The method may comprise flooding or at least partially flooding the apparatus, e.g. the hollow and/or floodable portion, before, during or after the apparatus is deployed to the seabed. Additionally or alternatively, the method may comprise at least partially or gradually flooding the apparatus, e.g. the hollow and/or floodable portion, before or as the apparatus is deployed to the seabed.

The method may comprise retrieving the apparatus, for example to the surface, e.g. after a region of the seabed has been cleared. The method may comprise detaching or decoupling an ROV, if present, from the apparatus before, during or after retrieval of the apparatus to the surface. For example, the method may comprise detaching or decoupling an ROV from the apparatus when the apparatus is on the seabed.

The method may comprise evacuating at least part of the hollow and/or floodable portion, e.g. prior to or during retrieval of the apparatus. Alternatively, the method may comprise gradually exhausting the hollow and/or floodable portion as the apparatus is retrieved. Evacuating or exhausting the hollow and/or floodable portion may comprise operating a pump, for example a ballast pump.

For the avoidance of doubt, any of the features described herein apply equally to any aspect of the invention. For example, the apparatus may comprise any one or more features relevant to the method and/or the method may comprise any one or more features or steps relevant to one or more features of the apparatus.

Another aspect of the invention provides a computer program element comprising and/or describing and/or defining a three-dimensional design for use with a simulation means or a three-dimensional additive or subtractive manufacturing means or device, e.g. a threedimensional printer or CNC machine, the three-dimensional design comprising an embodiment of at least one of the apparatus, chassis, flanks, tines and/or skids described above.

A further aspect of the invention provides a computer program element comprising computer readable program code means for causing a processor to execute a procedure to implement one or more steps of the aforementioned method.

A yet further aspect of the invention provides the computer program element embodied on a computer readable medium.

A yet further aspect of the invention provides a computer readable medium having a program stored thereon, where the program is arranged to make a computer execute a procedure to implement one or more steps of the aforementioned method.

A yet further aspect of the invention provides a control means or control system or controller comprising the aforementioned computer program element or computer readable medium.

For purposes of this disclosure, and notwithstanding the above, it is to be understood that any controller(s), control units and/or control modules described herein may each comprise a control unit or computational device having one or more electronic processors. The controller may comprise a single control unitor electronic controller or alternatively different functions of the control of the system or apparatus may be embodied in, or hosted in, different control units or controllers or control modules. As used herein, the terms “control unit” and “controller” will be understood to include both a single control unitor controller and a plurality of control units or controllers collectively operating to provide the required control functionality. A set of instructions could be provided which, when executed, cause said controller(s) or control unit(s) or control module(s) to implement the control techniques described herein (including the method(s) described herein). The set of instructions may be embedded in one or more electronic processors, or alternatively, may be provided as software to be executed by one or more electronic processor(s). For example, a first controller may be implemented in software run on one or more electronic processors, and one or more other controllers may also be implemented in software run on or more electronic processors, optionally the same one or more processors as the first controller. It will be appreciated, however, that other arrangements are also useful, and therefore, the present invention is not intended to be limited to any particular arrangement. In any event, the set of instructions described herein may be embedded in a computer-readable storage medium (e.g., a non-transitory storage medium) that may comprise any mechanism for storing information in a form readable by a machine or electronic processors/computational device, including, without limitation: a magnetic storage medium (e.g., floppy diskette); optical storage medium (e.g., CD-ROM); magneto optical storage medium; read only memory (ROM); random access memory (RAM); erasable programmable memory (e.g., EPROM ad EEPROM); flash memory; or electrical or other types of medium for storing such information/instructions.

Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and/or in the following description and drawings, and in particular the individual features thereof, may be taken independently or in any combination. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination, unless such features are incompatible. For the avoidance of doubt, the terms “may”, “and/or”, “e.g.”, “for example” and any similar term as used herein should be interpreted as non-limiting such that any feature so-described need not be present. Indeed, any combination of optional features is expressly envisaged without departing from the scope of the invention, whether or not these are expressly claimed. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and/or incorporate any feature of any other claim although not originally claimed in that manner.

Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which:

Figure 1 is a plan view of an apparatus according to an embodiment of the invention;

Figure 2 is a side elevation of the apparatus of Figure 1;

Figure 3 is a front elevation of the apparatus of Figures 1 and 2;

Figure 4 is an enlarged view of part of Figure 2 showing a selection of tines;

Figure 5 is an enlarged view of part of Figure 1 showing a selection of tines;

Figure 6 is a side elevation of a support according to an embodiment of the invention;

Figure 7 is a front elevation of the support of Figure 6; and

Figure 8 is a perspective view of the support of Figures 6 and 7.

Referring now to Figures 1 to 5, there is shown a subsea clearance apparatus 1 according to an embodiment of the invention. The apparatus 1 includes a chassis 2 formed of a plurality of hollow, floodable beams. The chassis 2 is supported by a pair of fore supports in the form of leading skids 3 and a pair of aft supports in the form of trailing skids 4, which are spaced from the leading skids 3 along the principal axis P of the apparatus 1. The apparatus 1 also includes a plurality of spaced tines 5 depending from the chassis 2 for engaging obstructions, such as boulders, on the seabed S.

The chassis 2 includes a pair of longitudinal members 21a, 21b located either side of and extending parallel to the principal axis P. A cross-brace 20 interconnects the longitudinal members 21a, 21b at a leading side of the apparatus 1 to provide a substantially U-shaped configuration. The longitudinal members 21a, 21b extend rearwardly of the cross-brace 20 and connect to a chevron-shaped tool portion 22. The tool portion 22 includes a pair of flanks 22a, 22b, which diverge from one another and from the principal axis P and are swept toward the aft of the apparatus 1.

Each flank 22a, 22b includes an end portion 23a, 23b, to which one of the trailing skids 4 is mounted, and an intermediate, modular extension portion 24a, 24b for selectively extending the effective length of the flanks 22a, 22b. The modular extension portions 24a, 24b are interconnected by cross-brace 24, which increases the strength and rigidity of the tool portion 22. The modular portions 24a, 24b are of equal length and increase the effective span W of the chassis 2 and, consequently, the swept area of the apparatus 1. The crossbrace 24 has a ladder-type structure which extends perpendicular to the principal axis P and is extendable in this embodiment to accommodate modular extension portions 23a, 23b of different lengths.

Each of the longitudinal members 21a, 21b includes a respective bridle connection 25 connecting to its leading end. Each bridle connection 25 is rotatable about a vertical pivot 25a and is connected to the end of a bridle line B. The bridle connection 25 transfers the pulling force from the tow line T via the bridle line B to the apparatus 1. The chassis 2 also has a lifting eye 26 for attachment to a lifting apparatus (not shown). The lifting eye 26 is located proximate the centre of gravity of the apparatus 1 to enable the entire apparatus 1 to be lifted from a single lifting point.

The apparatus 1 may also include a pair of tension adjusters (not shown), which may be in the form of winches. Each respective tension adjuster may be located within one of the bridle connections 25 and may be configured to reel in and pay out the respective bridle lines B so as to maintain substantially constant tension therein as the angle of tow varies. Each bridle connection 25 may have a sensor (not shown) for measuring the tension of a bridle line B attached thereto. Alternatively, the bridle lines B may be provided by portions of the same line to which the tow line T may be slidably connected, thereby to maintain the constant tension as the angle of tow varies.

The leading skids 3 are attached to the chassis 2 either side of the principal axis P, adjacent the leading end of a respective one of the longitudinal members 21a, 21b. Each of the leading skids 3 may be pivotable or rotatable with respect to the chassis about a steering axis 30 to facilitate steering of the apparatus 1. Each of the leading skids 3 is also tiltable about an axis 31 parallel to the seabed S, such that they can be canted upwardly and downwardly to accommodate surface irregularities of the seabed S.

Each leading skid 3 has a base plate 32 for contacting the seabed S and distributing the weight of the apparatus 1. Each skid includes a pair of leading edges 33 that converge to a leading apex, with a lip 34 extending from the leading edges 33. The lip 34 is inclined upwardly toward the fore of the apparatus 1. The taper of the leading edge 33 and the lip 34 forms a bow which deflects matter and obstructions out of the path of the skids 3 when the apparatus 1 is run along the seabed S. The leading skids 3 may also have a steering actuator, e.g. a hydraulic actuator (not shown), to rotate the skids 3 about the steering axis 30.

Alternatively, in a particularly preferred embodiment, the leading skids 3 may be attached to the chassis 2 via a steering frame (not shown). Each of the leading skids 3 may be fixed with respect to the steering frame and one another about their steering axis. The steering frame (not shown) may be pivotable or rotatable about a steering axis with respect to the chassis 2. The steering frame (not shown) may have one or more hydraulic cylinders (not shown) attached between itself and each of the leading skids 3. Each hydraulic cylinder may be a viscous damper.

Each trailing skid 4 includes a vertical plate 40 extending from a respective end portion 23a, 23b of one of the flanks 22a, 22b and a pair of base plates 41, which extend perpendicularly therefrom and rest on the seabed S to provide a flat, sliding contact with the seabed S. A keel plate 42 is attached by riveting to each vertical plate 40 in this embodiment and extends downwardly therefrom. Each keel plate 42 depends perpendicularly from the base plates 41 and penetrates, in use, into the seabed S to inhibit inadvertent lateral movement as the tines 5 engage obstructions.

In this embodiment, each of the leading skids 3 and each of the trailing skids 4 is rigidly connected to the chassis 2. However, it is also envisaged that one or more of the skids 3, 4 may be adjustably connected to the chassis 2. In such embodiments, the adjustable connection may be adjusted via one or more actuators, e.g. hydraulic actuators (not shown). Each actuator may be operable to adjust the height of the chassis 2 in the region of that actuator.

The tines 5 are spaced from one another along each of the flanks 22a, 22b so as to extend across the entire span W of the apparatus 1. The tines 5 depend from each flank 22a, 22b, are angled or pitched towards the fore of the apparatus 1 and extend parallel to each other and to the principle axis P. The spacing of the tines 5 along the flanks 22a, 22b is such that a gap Y is provided between adjacent tines 5. The tines 5 extend from a root at the chassis 2 to a reinforced tip 51. The reinforced tip 51 is of a material of greater wear resistance than a body 52 of the tines 5. In this embodiment, the angle of the tines 5 is adjustable relative to the chassis 2 using an actuator (not shown) in order to vary the distance X between the tips 51 of the tines 5 and the seabed S.

In this embodiment, the apparatus 1 also includes a support frame 6 extending from the cross-brace 20 and spanning the longitudinal members 21a, 21b. The support frame 6 has a cradle 61 for receiving and supporting an ROV. The support frame 6 includes a series of connections (not shown) for connecting the actuators to an ROV. An ROV may be received within the cradle 61 and connected to the connections to provide hydraulic inputs to the hydraulic actuators.

In this embodiment, the apparatus 1 is configured to be deployed to the seabed S and pulled therealong by a vessel (not shown) connected to the chassis 2 by the bridle lines B and tow line T. Any obstructions protruding from the seabed S a distance greater than the space X described between the tines 5 and the seabed S and having a width greater than the spacing Y described between adjacent tines 5 are deflected out of the path of travel of the apparatus 1.

Prior to deployment of the apparatus, the hollow chassis members 20, 21a, 21b, 22a, 22b, 23a, 23b, 24a, 24b are empty or at least partially empty. The bridle lines B are connected to the bridle connection 25 at one of their ends and to the tow line T at a common point at the other of their ends. The apparatus 1 is connected to a vessel crane (not shown) at lifting eye 26 and the apparatus 1 is lifted from the vessel by the crane. The apparatus 1 is then placed into the sea such that the hollow chassis members 20,21a, 21b, 22a, 22b, 23a, 23b, 24a, 24b fill with water as the apparatus is lowered to the seabed S.

The apparatus 1 is lowered such that the contact surfaces 32 and 41 of the leading and trailing skids 3, 4 contact the seabed S, causing minimal disruption thereto. The chassis 2 includes water inlet ports (not shown) for admitting water therein at a predetermined rate. In some embodiments, the water inlet ports may be selectively opened and closed using valve means to control the speed at which the chassis 2 is flooded, thereby controlling the decent of the apparatus 1 to the seabed S. Once on the seabed S, flooding of the hollow chassis members 20, 21a, 21b, 22a, 22b, 23a, 23b, 24a, 24b is complete.

An ROV is received by the cradle 61 and connected to the hydraulic system via the connections. The hydraulic actuators of the tines 5 are operated by the ROV so as to adjust the distance X between the tines 5 and the seabed S. The apparatus 1 is then pulled by a vessel via the tow line T to carry out the clearing operation. While the apparatus 1 is being towed, a controller on the surface can send steering signals to the ROV. The bridle connection 25 transfers the pulling force from the tow line T via the bridle line B to the apparatus 1. As the angle of tow varies, the bridle lines B pivot about vertical pivot 25a so as to effect steering by virtue of a change in direction of force.

In a preferred embodiment, the bridle lines B are connected to the steering frame (not shown). As the angle of tow varies, the steering frame (not shown) is rotated or pivoted about its steering axis. The leading skids 3, fixed with respect to the steering frame (not shown) and one another rotate by virtue of rotation of the steering frame (not shown) so as to effect steering of the apparatus. The hydraulic cylinders connected between the steering frame (not shown) and each leading skid 3, dampens any shock loading imparted to the steering frame (not shown) due to sudden change of angle of tow or take up in bridle line B tension.

Alternatively, the ROV can operate hydraulic actuators associated with the leading skids 3 so as to rotate the skids about their respective steering axis and steer the apparatus 1. The ROV may operate hydraulic actuators connected between the steering frame (not shown) and leading skids 3.

In embodiments which include a sensor in each of the bridle connections 25, the tension in the bridle lines B is monitored. As the apparatus 1 is steered or the vessel changes the angle of tow and unequal bridle line tensions are measured, a controller fed by the sensors operates the winches in order to equalize the bridle line tensions.

As the apparatus 1 is pulled along the seabed S, any obstructions in the path of the leading skids 3 are deflected out of the path thereof by the leading edge 33 and the lip 34. In addition, any obstructions protruding from the seabed S by a distance greater than the space X between the tines 5 and the seabed S and having a width greater than the space Y described between adjacent tines 5, are deflected out of the path of the apparatus 1. Once the clearing operation is complete the ROV operates a ballast pump (not shown) to evacuate water from the hollow chassis members 20, 21a, 21b, 22a, 22b, 23a, 23b, 24a, 24b. The vessel crane then retrieves the apparatus 1 to the surface.

Figures 6 to 8 illustrate a support 100 used to connect each of the leading skids 3 to the chassis 2. The support 100 is shown in a retracted state. The support 100 includes a first tubular section 110, a second tubular section 120 rotatable relative to the first tubular section 110, a third tubular section 130 and a fourth tubular section 140 translatable relative to the third tubular section 130.

The first tubular section 110 is a hollow section with a circular cross-section, which is rotationally fixed within the chassis 2 in this embodiment. The second tubular section 120 is a hollow tubular section with a circular-cross section having a smaller diameter than the first tubular section 110. The second tubular section 120 is received within the first tubular section 110, with an outer surface 121 thereof spaced by a small lubrication gap G from the inner surface of the first tubular section 110. The second tubular section 120 is rotatable relative to the first tubular section 110 and the chassis 2.

A portion of the second tubular section 120 protrudes from an end of the first tubular section 110. A lug 122 is welded to the outer surface 121 such that the second tubular section 120 can be attached to a strut H of a steering frame or steering system (not shown). Torque is imparted to the second tubular section 120 via the lug 122.

A first end cap 123 is welded to a free end of the second tubular section 120 and a second end cap 124 is welded to the other free end of the second tubular section 120. The first end cap 123 and second end cap 124, together with the second tubular section 120 describe an interior volume, split into two separate volumes, first volume 125a and second volume 125b by mounting plate 126 in this embodiment.

The third tubular section 130 is a box section in this embodiment and is received within the second tubular section 120, in particular the second volume 125b. The third tubular section 130 is welded to the first end cap 123 at a free end thereof. The third tubular section 130 is fixed rotationally relative to the second tubular section 120 by virtue of the first end cap 123.

A gap is described between the second tubular section 120 and the third tubular section 130, the gap being filled by a plurality of packers 131 in this embodiment. The packers 131 take the form of elastomeric bushings to prevent relative movement between the second tubular section and the third tubular section.

The fourth tubular section 140 is a box section in this embodiment. The fourth tubular section 140 is received within the third tubular section 130. The third tubular section 130 and fourth tubular section 140 comprise a key and keyway arrangement. The fourth tubular section 140 has an end plate 141 welded to a free end thereof. The end plate 141 is sized such that, in the retracted state shown, it is circumscribed by the first end cap 123.

A pair of spaced flanges 142 depend from the end plate 141, each flange 142 having a clevis 143 formed therein. The pair of clevises 143 are configured to receive a pin Z to pivotally connect the skid 3 thereto.

A hydraulic actuator 150 has a piston 151 extending from actuator cylinder 152 and connected to the other free end of the fourth tubular section 140 via a pair of spaced depending arms 144. The fourth tubular section 140 is moveable longitudinally relative to the third tubular section 130 along the principle axis of the third tubular section 130 due to activation of the actuator 151. The hydraulic actuator 150 is housed within the second tubular section 120, in particular, the first volume 125a. The hydraulic actuator 150 is supported by the mounting plate 126.

In this embodiment, the principle axis of the first, second, third and fourth tubular sections are colinear.

In use, to rotate the skid 3 a torque is applied to the second tubular section 120. The second tubular section 120 is free to rotate relative to the first tubular section 110 and therefore rotates relative to the chassis 2.

The first end cap 123 is welded to the second tubular section 120 and is therefore fixed rotationally relative thereto. The first end cap 123 rotates with the second tubular section 120. The third tubular section 130 is also welded to the first end cap 123 and is therefore rotated along with the second tubular section 120.

Due to the geometry of the third tubular section 130 and fourth tubular section 140, rotation imparted to the third tubular section 130 causes rotation of the fourth tubular section 140,

i.e. the fourth tubular section 140 is rotationally dependent on the third tubular section 130.

The skid 3 is attached to the fourth tubular section 140 via the clevises 143 attached to the end plate 141. As the end plate 141 is welded to the fourth tubular section 140 rotation thereof also imparts rotation to the skid 3.

In use, to adjust the height of the skid 3, pressurised fluid in the actuator cylinder 152 causes the piston 151 to extend away from the mounting plate 126.

The fourth tubular section 140, connected to the piston 151 via the pair of spaced, depending arms 144, moves longitudinally relative to the third tubular section 130 along the principle axis thereof. The skid 3, connected to the fourth tubular section 140 via the pin P and clevises, also moves along with the fourth tubular section 140 away from the chassis.

In the present embodiment, both rotation and height adjustment of the skid 3 can be performed simultaneously.

It will be appreciated by those skilled in the art that several variations to the aforementioned embodiments are envisaged without departing from the scope of the invention. For example, the apparatus need not include tines 5 and may instead include one or more engaging elements having a different configuration. The tines 5 or other engaging element(s) may be fixed to the chassis 2. The skids 3, 4 may be replaced with rollers or any io other suitable support means. Whilst not stated above, the tines 5 could be configured to engage the seabed S, although this is preferably avoided to minimise the impact to the seabed S. In some embodiments, the apparatus 1 may include one or more buoyancy tanks. The one or more buoyancy tanks may be floodable or ballastable. It will also be appreciated by those skilled in the art that any number of combinations of the 15 aforementioned features and/or those shown in the appended drawings provide clear advantages over the prior art and are therefore within the scope of the invention described herein.

Claims

1. A subsea apparatus comprising a chassis, fore and aft supports spaced along a principal axis of the apparatus and actuation means operable to adjust the height and/or orientation of at least one of the supports, wherein the chassis is spaced, in use, from the seabed by a distance corresponding to the height of the supports as the apparatus is propelled therealong and such that the direction of travel corresponds to the orientation of the adjustable support(s).

2. A subsea apparatus according to claim 1, wherein the adjustable support is rotatable about a steering axis for steering the apparatus, the actuation means being operable to rotate the adjustable support about its steering axis.

3. A subsea apparatus according to claim 2, wherein the actuation means comprises a steering actuator operable to rotate the adjustable support about its steering axis.

4. A subsea apparatus according to claim 2 or claim 3 comprising a pair of laterally spaced fore supports, wherein the actuation means is operable to adjust the orientation of each fore support.

5. A subsea apparatus according to claim 4, wherein the actuation means comprises a respective actuator for each support for adjusting the orientation of each of the fore supports independently.

6. A subsea apparatus according to claim 4 comprising a steering frame connected to each of the fore supports for synchronising their orientation relative to one another.

7. A subsea apparatus according to any preceding claim, wherein the adjustable support comprises a column rotatably received within a receptacle of the chassis and a travelling support mounted to the column for rotation therewith, the actuation means being operatively connected to the column for rotating the column within the receptacle to adjust the orientation of the adjustable support.

8. A subsea apparatus according to any preceding claim, wherein the adjustable support comprises an extendable column for adjusting the height of the adjustable support, the actuation means being operable to extend and/or retract the extendable column to adjust the height of the adjustable support.

9. A subsea apparatus according to claim 8 when dependent upon claim 7, wherein the rotatable column comprises the extendable column.

10. A subsea apparatus according to claim 9, wherein the chassis comprises a first tubular segment secured thereto which provides the receptacle, the rotatable column comprising a second tubular segment rotatably received within the first tubular segment and an inner column reciprocable within the second tubular segment to provide the extendable column.

11. A subsea apparatus according to claim 10, wherein the inner column is splined in the second tubular segment to inhibit rotation therebetween.

12. A subsea apparatus according to claim 11, wherein the inner column is non-round and cooperates with a corresponding non-round inner surface of the second tubular segment to provide the spline.

13. A subsea apparatus according to claim 12 comprising a third, non-round tubular segment secured within the second tubular segment to provide the non-round inner surface.

14. A subsea apparatus according to any one of claims 8 to 13, wherein the actuation means comprises a height actuator housed at least partially within the extendable column.

15. A subsea apparatus according to any preceding claim, wherein the apparatus comprises a subsea clearance apparatus having a plurality of tines depending from the chassis and spaced transversely with respect to the principal axis for engaging and deflecting away from the principle axis obstructions which are larger than a predetermined size and/or which protrude by a predetermined amount from the seabed as the apparatus is propelled along the seabed, wherein the actuation means is operable to adjust, in use, the height of at least one of the supports to control the distance between the tines and the seabed.

16. A subsea clearance apparatus according to claim 16, wherein the tines are angled or pitched toward the fore of the apparatus.

17. A subsea clearance apparatus according to claim 17, wherein the distance between the tines and the seabed is adjustable by adjusting the angle or pitch of the tines.

18. A subsea clearance apparatus according to any one of claims 15, wherein the chassis comprises a pair of flanks and the tines depend from the flanks, the fore and aft supports each comprising a pair of skids on either side of the principal axis of the apparatus, the chassis comprising a pair of longitudinal members each having one of the fore skids mounted thereto and each aft skid being mounted to one of the flanks.

19. A subsea clearance apparatus according to claim 18, wherein each of the aft supports comprises a keel plate.

20. A subsea clearance apparatus according to claim 18 or claim 19, wherein each of the pair of fore supports is tiltable about an axis parallel, in use, with the seabed.

21. A subsea clearance tool according to any preceding claim comprising a dock configured to receive a remotely operated vehicle and to connect the remotely operated vehicle to the actuation means for the operation thereof.