Underwater burial apparatus

Apparatus for locally disturbing an underwater bed to enable the burial of an elongate member (39) comprises a mounting (30,31), resilient means (88-93) such as brush filaments coupled to the mounting, and means (9) for moving the resilient means with respect to the mounting. The resilient means is constructed such that on moving the mounting along the line of the elongate member while moving the resilient means with respect to the mounting, the underwater bed is disturbed by the resilient means to enable the elongate member to be buried.

The present invention relates to a method and apparatus to enable the
 underwater burial of an elongate member, such as a cable or pipe.
 Post lay burial of underwater telecoms cables is normally carried out in
 soft silt or sand soils (hereinafter "soil") by means of water jetting.
 This method is employed in preference to mechanical soil cutting devices
 as low pressure water jets will not damage the polythene outer covering of
 the cable and has the advantage of requiring little mechanical contact
 with the cable. There are various types of water jetting tools in use but
 they all work on essentially the same principle. Jetting swords or nozzles
 are directed at the seabed on each side of the cable and fluidise the soil
 allowing the cable to be imbedded. The disadvantage of water jetting
 systems is that they are inefficient. For every kW of water power directed
 at the seabed an equal amount has to be directed in the opposite direction
 to balance the reactive forces. Typically, powers of greater than 75 kW
 are absorbed by the tool alone to bury a cable in soft soils and sand at
 an acceptable rate. The neutrally buoyant Remotely Operated Vehicle (ROV)
 to which the tool is attached requires at least the same amount of power
 again in order to effectively transport it in deep water.
 Mechanical soil cutting devices are only used in soils too hard to
 effectively fluidise, such as consolidated clays of greater than 50 kPa
 shear strength, chalk and rock. These tools are generally toothed wheels
 or chain cutters and in use require that the cable is first "loaded" into
 a protective chute and depressor to prevent contact between the cutting
 surfaces and the cable. The fact that the cable needs to be loaded creates
 the need for complicated subsea robotics both for loading and unloading
 the cable as well as mechanisms for automatic ejection, in the event of
 loss of vehicle power. Because of the size and weight of conventional
 mechanical cutting tools and their associated equipment, they can only be
 deployed from heavy subsea tractors, typically weighing greater than 12
 tonnes. Whilst necessary for burial of pre-laid cables in hard materials,
 they are of limited use in areas of soft seabed as they tend to sink into
 the soil.
 Both of the solutions above are of necessity powerful and hence expensive.
 Consequently they are both, together with their launching and control
 systems, very large (in excess of 100 tonnes), they are expensive to
 transport and can only be launched from cableships and offshore vessels
 with strong, spacious decks.
 The emerging single span and festoon cable systems business however is
 dictating the need for lower cost shallow water cable deburial, retrieval
 and reburial solutions. The maintenance requirements for such low cost
 cable systems will not stand the cost of permanently based vessels of size
 necessary to deploy a modern 250 hp, 130 tonne cable maintenance ROV nor
 indeed will they support the cost of such an ROV on standby.
 It would therefore be advantageous to devise a method for imbedding cable
 in soft soils using low power.
 In accordance with a first aspect of the present invention there is
 provided apparatus for locally disturbing an underwater bed in a line so
 as to enable the burial of an elongate member, the apparatus comprising a
 mounting, resilient means coupled to the mounting, and means for moving
 the resilient means with respect to the mounting, wherein the resilient
 means is constructed such that on moving the mounting along the line while
 moving the resilient means with respect to the mounting, the underwater
 bed is disturbed by the resilient means to enable the elongate member to
 be buried.
 In most cases the resilient means will move soil mechanically but it has
 been found that in addition the resilient means agitates the water and
 creates an eddy effect which at least partially disturbs and fluidises the
 bed (which may be a seabed, riverbed, lakebed etc.) in the region of a
 cutting face. In some cases the eddy current effect could be used alone
 with no contact between the resilient means and the bed.
 It has been recognised that jetting with low power such as 9 kw is not
 practical to trench depths in excess of 600 mm which leaves some form of
 mechanical method as the only approach. It has also been recognised that a
 conventional mechanical tool would not be feasible because the associated
 loading/unloading and ejection equipment would be large and costly.
 It has been found that the disadvantages of the conventional mechanical
 approach (which uses rigid toothed wheels or chain cutters) can be avoided
 by the provision of resilient means. The resilient means is generally
 stiff enough to cause disturbance of the water adjacent the cutting face
 and apparent fluidisation of the disturbed soil at or adjacent the
 advancing cutting face, whilst being flexible enough to contact the
 elongate member, such as a cable, without causing any damage.
 The resilient means may comprise any suitable resilient member or members.
 For instance the resilient means may comprise one or more strips of rubber
 which extend from a rotating axle. Preferably however the resilient means
 comprises a plurality of elongate resilient members such as brush
 filaments. The density, length and flexibility of the resilient members
 can be suitably chosen for the type of bed material, type of elongate
 member (e.g. telecommunications cable), cutting depth and cutting speed
 required. The brush filaments may be formed from any suitable material
 such as metal or stiff plastic. The stiffness of the filaments of a
 conventional varnish stripping brush has been found to be suitable. In one
 example nylon brush filaments are used with a diameter in the range of 1-2
 mm, and a length in the range of 20-30 mm.
 The resilient means may be oscillated with respect to the mounting, or
 mounted on a conveyor belt. However preferably the resilient means are
 rotated with respect to the mounting, and typically extend from an axle
 rotatably coupled to the mounting. In this case the means for moving the
 resilient means with respect to the mounting comprises drive means for
 rotating the axle. This provides a simple low power solution. Where the
 resilient means comprises a plurality of elongate resilient members such
 as brush filaments the filaments preferably extend radially from the axle.
 The axle may be inclined or perpendicular to a vertical plane passing
 through the line of disturbance. However preferably the axle is mounted
 and deployed (for instance from an ROV) such that, in use, the axle lies
 substantially parallel to a vertical plane passing through the line of
 disturbance. This ensures that when the elongate member is buried at the
 same time, it is not damaged by rigid rotating components.
 The resilient means may be uniformly distributed about the axle but
 preferably is arranged as a plurality of arms circumferentially spaced
 around the axle. We believe that the spaced arms are more efficient in
 effecting fluidisation of the underwater bed material by means of a
 "fanning" effect, resulting in a lower power requirement.
 In one example the resilient means is arranged as a plurality of groups
 axially spaced along the axle. In a preferred embodiment the resilient
 means is arranged as twenty axially spaced groups. In an alternative
 example, the resilient means may be in the form of one or more spirals
 extending along the length of the axle. The radial length of each group
 (eg the length of the brush filaments) and the axial spacing (eg. the
 spacing between the groups or the pitch of the spiral) can be suitably
 chosen for the depth of trench required and the angle of deployment of the
 axle. In preferred examples the groups are spaced by 50-100 mm and the
 spiral has a pitch of 18-38 mm.
 Typically the resilient means extends downwards in use into the underwater
 bed, and in a preferred embodiment the resilient means is non-uniformly
 distributed such that, in use, the density of the resilient means
 increases downwards into the underwater bed. For instance the axial
 spacing between groups of resilient means or the pitch of the spiral may
 decrease along the length of the axle. This results in improved burial
 efficiency since the majority of the fluidisation work is carried out in a
 lower region of the underwater bed.
 Preferably the apparatus further comprises a second axle rotatably coupled
 to the mounting substantially parallel to the first axle and having
 resilient means extending therefrom. In this case the pair of rotating
 resilient means disturbs the underwater bed on either side of the elongate
 member, forming a trench which receives the elongate member. The walls of
 the trench collapse behind the apparatus and bury the elongate member to
 an extent dependent on the stiffness of the underwater bed material. The
 apparatus is particularly suited for operation in soft seabed materials
 such as soft soil or sand.
 The apparatus typically further comprises means for guiding an elongate
 member (eg cable) in use into the underwater bed. The means for guiding an
 elongate member may comprise, for example, one or more depressors such as
 depressor rollers.
 Typically the apparatus further comprises deployment means such as a
 hydraulic cylinder for deploying the mounting between a raised position
 and a lowered position. The mounting may be deployed by rotating the
 mounting or by sliding the mounting between its raised and lowered
 positions. Slidably deploying the mounting has the advantage that the
 mounting can be arranged at a fixed optimum angle which does not vary with
 the depth of burial.
 The apparatus preferably comprises a mechanical depressor (such as one or
 more depressor rollers) which applies a downward pressure to the elongate
 member to force the elongate member into the trench being excavated.
 However in the case where there are two axles and both are rotated so as
 to create a downward force between the axles, the downward force created
 by the resilient means and the associated fluid flow may remove the
 requirement for a mechanical depressor. This reduces the resistance to
 movement along the line of disturbance, further reducing the power
 requirement for the vehicle drive. It also reduces the post burial tension
 in the buried elongate member.
 In one example the apparatus further comprises means for delivering a fluid
 (eg. a liquid such as water) in the region of the resilient means. This
 provides a lubricating effect which improves the efficiency of the
 apparatus. Preferably the lubricating water is delivered in the region of
 a lower end of the apparatus since the seabed will generally be dryer in
 this region.
 The apparatus is typically mounted to an ROV. The ROV may be a neutrally
 buoyant, free-swimming vehicle. Alternatively the ROV may be a negatively
 buoyant vehicle.
 In accordance with a second aspect of the present invention there is
 provided a method of locally disturbing an underwater bed in a line to
 enable the burial of an elongate member, the method comprising moving
 apparatus according to the first aspect of the present invention along the
 line while moving the resilient means with respect to the mounting whereby
 the underwater bed is disturbed by the resilient means to enable the
 elongate member to be buried.
 The method may be carried out before the elongate member is laid on the
 underwater bed, providing a trench in which the elongate member is laid.
 Preferably however the underwater bed is disturbed along the line of a
 previously laid elongate member.
 Preferably the underwater bed is formed with material of less than
 approximately 30 kPa shear strength, such as soft soil or sand.

Referring to FIGS. 1 and 2, an underwater ROV 30 has a cable burial device
 29 mounted on its underside. The cable burial device comprises a pair of
 hollow mounting arms 31,32 which are pivotally attached to the ROV via a
 bracket. The bracket has an upper plate 21 and lower plate 22 which define
 a space which receives the arms 31,32. The upper plate 21 and lower plate
 22 each have four elongate slots, the slots 23-26 formed in the upper
 plate being shown in FIG. 2. The arms 31,32 are fixed in place with bolts
 which pass through the arms 31,32 and the corresponding slots in the upper
 and lower plates 21,22. Two of the bolts 27,28 are indicated in FIGS. 1
 and 2. The slots 23-26 enable the lateral spacing between the arms 31,32
 to be varied as desired. The bracket is rotatably mounted to the ROV 30 at
 a pair of hinge points 2,2'. An hydraulic deployment cylinder 3 is
 pivotally mounted to the bracket at 4 and to the main body of the ROV at
 4' and can be actuated to deploy the arms 31,32 from a horizontal raised
 position (FIG. 1), to an angled operative position (shown in FIG. 5).
 The arms 31,32 each carry three rotary bearings 80-82. Two axles 5,6 (shown
 in cross-section in FIG. 3) are rotatably mounted in the bearings 80-82.
 The axles 5,6 are each driven by respective hydraulic motors, the
 left-hand hydraulic motor 9 being illustrated in FIG. 1. The axle 6 has a
 respective hydraulic motor hidden by the motor 9 in FIG. 1.
 Twenty paddle brush assemblies 20 (some of which are labelled in FIGS. 1
 and 2) are mounted on the axles 5,6, adjacent pairs of brush assemblies
 being separated by nylon spacers 85,86 etc. Referring to FIG. 3, each
 brush assembly 20 comprises an annular brush holder 87 threaded onto a
 respective one of the axles 5,6 and fixed in place with grub screws or
 keyways (not shown). Each brush holder 87 has six slots which each retain
 a radially extending brush 88-93. The brush holder has a body diameter 95
 of 60 mm. The overall diameter 94 of the brush assembly is 110 mm. The
 brushes 88-93 are each made up of grit impregnated nylon filaments with a
 filament diameter of 1.5 mm and a trim length 96 of 25 mm. The brush
 length (i.e. the length of the brushes 88-93 parallel to the axles 5,6) is
 25 mm. The nine brush assemblies between the two rear bearings 80,81 are
 more closely spaced and have an axial spacing 97 of 50 mm. The remaining
 eleven brush assemblies are more widely spaced with an axial spacing 98 of
 100 mm.
 The arms 31,32 also carry depressor support brackets 150-152,154-156 which
 carry depressor bearings 100-105. Nylon depressor rollers 72-74 are
 rotatably mounted in respective pairs of the depressor bearings 100-105.
 The raised rear depressor roller 72 is shown in FIG. 4.
 FIG. 5 is a side view of the ROV 30 burying a cable 39 which has been
 previously laid on the seabed. Power and control is supplied to the ROV 30
 from a surface vessel via an umbilical (not shown). The ROV 30 advances in
 the direction 132 along the seabed 37 on a pair of tracks (right-hand
 track 35 being shown in FIGS. 1 and 5) along the line of the cable 39 to
 be buried.
 The angle of deployment 36 is suitably adjusted for the required burial
 depth and cable bend radius. In this example the angle of deployment 36 is
 45.degree. and the depth of burial is 1000 mm. As can be seen in FIG. 5,
 the spacing of the upper brush assemblies and the trim length of the brush
 filaments is chosen such that at the chosen angle of 45.degree., when
 viewed in the direction of travel 132 the brush assemblies overlap
 slightly.
 The cable 39 in advance of the device 29 lies on the seabed 37. As the ROV
 advances, a cutting face 38 is continuously mechanically cut away and
 disturbed or fluidised by the action of the brushes to form a trench 40.
 The cable 39 is guided into the trench by the depressor rollers 72-74. The
 rear depressor roller 72 is mounted higher than the other two rollers
 73,74 to maintain a minimum bend radius of 1.5 metres for the cable 39.
 The soft soil or sand forming the walls of the trench collapses almost
 immediately behind the device to bury the cable 39 behind the device.
 The rotation of the brushes generates turbulence in the trench 40. The
 brushes also come into contact with the cutting face. The combined effect
 of the agitation of the water near the cutting face and the mechanical
 contact of the brushes with the cutting face 38 causes the cutting face to
 be cut away or at least partially fluidised to enable the cable 39 to be
 buried. If the brushes come into contact with the cable 39 their
 flexibility ensures that the cable is not damaged. The resilience of the
 brushes ensures that the brushes which have contacted the cable return to
 their original shape.
 The axles 5,6 may each be rotated clockwise or anticlockwise at up to 500
 revolutions per minute. In one example the axles are rotated such that the
 left-hand axle (viewed from the front or the rear) is rotated clockwise
 and the right-hand axle is rotated anticlockwise. In a second example the
 axles are rotated such that the left-hand axle is rotated anti-clockwise
 and the right-hand axle is rotated clockwise. It has been observed that
 the power consumption is slightly lower when the left-hand axle is rotated
 anti-clockwise and the right-hand axle is rotated clockwise.
 In an alternative arrangement (not shown) the arms 30,31 may be slidably
 mounted to the ROV at a desired angle and deployed as indicated at 140 in
 FIG. 5.
 FIG. 6 is a cross-sectional view of the device 29 burying a cable 39.
 However, in the example of FIG. 6 the arms 31,32 have been moved inwardly
 to the maximum extent permitted by the slots 23-26. The axles 5,6 are both
 driven inwardly downwards as indicated at 13,14. As a result the brushes
 which contact the cable 39 apply a downwards pressure which guides the
 cable 39 into the trench 40. This may remove the requirement for the
 depressors 72-74.
 FIGS. 7 and 8 illustrate an alternative cable burial device. In this case
 the hollow arms 31,32 each have five holes 110-114 formed between the two
 rear bearings 80,81. Water is pumped along the interior of the arms 31,32
 at low pressure, and is emitted from the holes 110-114 in the region of
 the rear brush assemblies. This provides a lubricating effect in the lower
 part of the trench 40 which improves the trenching efficiency.
 FIG. 9 is a schematic side view and FIG. 10 is a section along line D--D of
 an alternative cable burial device 60 with spiral brushes. The device 60
 is identical in all other respects to the device 29 shown in FIGS. 1-6 and
 identical components are indicated with the same reference numerals. In
 the embodiment of FIGS. 9 and 10, the brush filaments extend radially from
 the axles 5,6 and are arranged in a continuous spiral 61,62. The brush
 filaments are mounted in a spiral groove in three nylon holders 120-122.
 The overall diameter 123 is 140 mm, the trim length of the brush filaments
 is 15-20 mm, and the pitch 124 between adjacent turns of the spiral is
 18-38 mm. The pitch of the spiral groove on the rear nylon brush holder
 120 may be reduced to increase the brush density towards the lower end of
 the apparatus.
 Both spirals 61,62 are right-handed spirals and both axles are rotated in
 the same sense. In a first example, both axles 5,6 are driven
 anti-clockwise (as viewed from the left of FIG. 9). In this example the
 auger effect of the spiral brushes causes material to move upwards away
 from the base of the trench. This has the advantage of enabling the
 removal of fluidised soil from the base of the trench. In a second example
 the axles 5,6 are both rotated clockwise. In this case the auger effect of
 the spiral brushes causes material to move downwards towards the base of
 the trench. This has the advantage of delivering water to the base of the
 trench and thereby providing a lubricating effect.