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CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     This application is a United States National Stage Application of, and claims the benefit pursuant to 35 U.S.C. §371 of International Patent Application Serial No. PCT/GB2012/052667, filed on Oct. 26, 2012, which claims priority to and the benefit of Great Britain Application No. 1118503.0, filed on Oct. 26, 2011, both which are incorporated herein in their entirety by reference. 
     The present invention relates to an apparatus for forming a trench in the bed of a body of water, such as the bed of a lake or the seabed. Such trenches are commonly required for burying underwater pipelines or cables, in order to protect the pipeline or cable from damage. A variety of trenching apparatus suitable for use in an underwater environment is known. Hereinafter “subsea” is used to refer to environments beneath the sea or beneath a lake, and “seabed” is used to refer to the bed of a lake or the seabed. 
     The present invention relates in particular to the formation of a trench in hard seabed soils. More especially the present invention relates to forming a seabed trench using a tracked vehicle, and to such tracked vehicles. 
     BACKGROUND 
     Tracked vehicles are known for use on land and for use in the subsea environment. Such vehicles are usually carried on endless articulated tracks which may, for example, be driven by sprocket wheels engaging internal parts of the endless track. Articulated vehicle tracks have the advantage of spreading the weight of the vehicle over a large contact area. 
     In the subsea environment, especially in connection with laying of pipes or cables in a trench vehicles with endless articulated tracks have some disadvantages. For example, when, during trenching operations, it is necessary to correct the course of the trenching vehicle (i.e. to change its direction) conventional skid steering reduces the tractive effort of the vehicle, and can cause sinkage of the tracks into the seabed, and loss of trench cutting performance. Conventional skid steering changes the relative speed of the articulated tracks at opposed sides of the vehicle in order to effect the change in direction. 
     Skid steering often causes pitching of the vehicle which can result in disadvantageously high loading of the trench forming tool carried by the vehicle and also problems in the grading of the trench (i.e. the inclination of the trench base). These problems can also occur as the tracked vehicle passes over objects (for example rocks or debris) on the seabed which may similarly cause pitching of the vehicle. 
     A further problem can occur when a subsea trench cutting apparatus is lowered from a service vessel (for example) to the seabed. If the approach speed of the apparatus is too great, the apparatus can be damaged on contact with the seabed. 
     The present invention seeks to overcome or alleviate some or all of the above problems. 
     BRIEF SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention there is provided an underwater trenching apparatus comprising: 
     at least one trenching tool; 
     a lifting attachment point configured for attachment of a lifting means for lowering the trenching apparatus to, or raising the trenching apparatus from, its location of operation; at least two steerable endless track units for transmitting tractive effort to move the underwater trenching apparatus in use; and
 
for each steerable endless track unit,
 
a first mounting arrangement permitting rotation of the track unit about a vertical axis, and
 
a first actuator configured to move the track unit about the vertical axis, and
 
control means configured to control the action of the actuator.
 
     According to a second aspect of the invention there is provided an underwater trenching apparatus comprising 
     at least one trenching tool; 
     a lifting attachment point configured for attachment of lifting means for lowering the trenching apparatus to, or raising the apparatus from, its location of operation; 
     at least two endless track units for transmitting tractive effort to move the underwater trenching apparatus in use; 
     a main body portion to which the lifting attachment portion is attached and to which the endless tracks are mounted; 
     height adjustment means for adjusting the vertical spacing between the main body portion and the respective endless tracks; and 
     control means configured to control the action of the height adjustment means. 
     Preferably in the first aspect of the invention the apparatus further comprises 
     a main body portion to which the lifting attachment portion is attached and to which the endless track units are mounted, 
     height adjustment means for adjusting the vertical spacing between the main body portion and the respective endless track units; and 
     control means configured to control the action of the height adjustment means. 
     Preferably in the second aspect of the invention the apparatus further comprises: 
     at least two steerable endless track units for transmitting tractive effort to move the underwater trenching apparatus in use; and 
     for each steerable endless track unit, 
     a first mounting arrangement permitting rotation of the track unit about a vertical axis, and 
     a first actuator configured to move the track unit about the vertical axis, and 
     control means configured to control the action of the actuator. 
     Preferably the apparatus of the first or second aspect further comprises, for each endless track unit 
     a second mounting arrangement permitting rotation of the endless track unit about a horizontal axis arranged substantially perpendicularly to the direction of movement of the track unit, and a second actuator configured to move the endless track unit about the horizontal axis and
 
control means adapted to control the action of the second actuator.
 
     In preferred embodiments the lifting attachment point is centrally located with respect to the endless track units. 
     Preferably the height adjustment means includes one or more dampers configured to reduce contact loads on the endless track units when the endless track units contact the seabed on lowering of the trenching apparatus to the seabed. 
     In preferred embodiments the trenching tool is mounted centrally with respect to the endless track units. 
     In further embodiments the control means is operable to control the respective first actuators of the endless track units independently of the other endless track unit(s). 
     In some preferred embodiments the underwater trenching apparatus comprises two steerable endless track units. 
     In other preferred embodiments the underwater trenching apparatus comprises two steerable endless track units and two non-steerable endless track units. 
     In other preferred embodiments the underwater trenching apparatus comprises four steerable endless track units. 
     In other preferred embodiments each endless track unit is independently steerable. 
     In further preferred embodiments, each endless track unit comprises a single endless track mounted on a subframe and wherein said first actuator is connected to said subframe. 
     In other preferred embodiments each endless track unit comprises a further actuator configured to move the track unit about the vertical axis. 
     Preferably said first actuator and said further actuator cooperate to move said endless track unit about the vertical axis. 
     Preferably the height adjustment means and control means of a given endless track unit are operable to adjust the vertical spacing between the main body portion and a given endless track unit independently of the spacing between the main body portion and a different endless track unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the invention and to show how the same may be carried into effect, reference will be made, by way of example only, to the following drawings, in which: 
         FIG. 1  is a perspective view of a subsea trench cutting apparatus according to the present invention; 
         FIG. 2  is a side view of the apparatus of  FIG. 1 ; 
         FIG. 3  is a plan view of the apparatus of  FIG. 1 ; and 
         FIG. 4  is a front view of the apparatus of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings, there is shown a trenching apparatus  10  comprising a main body portion  12  and a trenching tool  14 . The trenching tool  14  may, in principle, be any suitable tool for cutting a subsea trench as known in the art, the most suitable tool being selected by the person of skill in accordance with, for example, the subsea ground conditions. Suitable trenching tools ( 14 ) include (but are not limited to) jetting tools, chain cutters, augers, rockwheels and the like. 
     The trenching apparatus  10  as illustrated in  FIGS. 1 to 4  includes four endless track units  16 , indicated at  16   a ,  16   b ,  16   c  and  16   d . In variations of the invention, other numbers of endless track units may be provided, for example, 2, 6 or 8 endless track units. Each endless track unit  16  comprises an endless track  18  mounted on a subframe  20  and usually connected to a drive arrangement  22  which moves the endless track  18  around the subframe  20  and so—with the endless tracks in contact with the ground—propels the apparatus  10  over the ground. In some cases, separate propulsion means may be provided, such as a subsea tractor or a tow line from a surface vessel, but these are less preferred. 
     Each endless track  18  consists of a number of individual track elements  24 , each of which elements  24  is articulatedly joined to its neighbour so to form an endless loop. Articulated tracks  18  and their drive means  22  are known per se in the art and any suitable design configured for subsea use may be employed with the apparatus  10  of the present invention. 
     Each endless track unit  16  is attached to the respective frame or leg  26  which is, in turn attached to the main body portion  12 . In particular, each endless track unit  16  is attached to a first end  26   a  of a respective leg  26 , an opposed second end  26   b  of which is attached to the main body portion  12 . 
     Each endless track unit  16  is attached to its respective first end  26   a  of leg  26  in such a manner that the endless track unit  16  can rotate with respect to the leg  26  about an axis  28 . Axis  28  is nominally horizontal with respect to the orientation of the apparatus  10  shown in the Figures. 
     Leg  26  and endless track unit subframe  20  are most preferably configured such that nominally horizontal axis  28  is located centrally with respect to the endless track unit  16 . That is, the axis  28  crosses the endless track unit  16  at a point at, or about, half way along the length of the endless track unit  16  (the length being the maximum horizontal front-to-back dimension of the endless track unit). 
     Because of the articulation of the endless track unit  16  with respect to its leg  26 , the apparatus  10  of the present invention is better able to accommodate uneven-ness in the ground (seabed) traversed by the apparatus  10 . 
     In particular embodiments, an actuator (e.g. a hydraulic actuator) with an associated control means may be disposed between the leg  26  and the subframe  20  of the endless track unit  16 , thereby to control the rotation of the endless track unit  16  about its respective nominally horizontal axis  28 . 
     As noted above, second end  26   b , which is the upper end as illustrated, of each leg  26  is connected to the main body portion  12 . The connection or joint between each leg  26  and the main body portion  12  is such that the leg  26  can rotate with respect to the main body portion  12 . In particular, the leg  26  can rotate about an axis  30  with respect to the main body portion. Axis  30  is configured to be nominally horizontal (with respect to the orientation of the apparatus as illustrated and perpendicular to the normal direction of travel of the apparatus  10 ). By rotation of the leg  26  about axis  30 , the height of the endless track unit  16  with respect to the main body portion  12  varies. That is, rotation of leg  26  about axis  30  results in an increase or decrease (in accordance with the direction of rotation) in the vertical spacing of the endless track unit  16  from the main body portion  12 . Here, “vertical” is with respect to the orientation of the apparatus  10  as shown in the Figures. 
     Adjacent an upper part of each leg  26 , an arm or shoulder  34  extends outwardly from leg  26 . Shoulder  34  and arm  32  are preferably arranged in substantially the same vertical plane. An actuator  36  extends between the arm  32  and the shoulder  34 . The actuator  36  is pivotally connected at its first (upper) end  38  to a distal portion (with respect to the main body portion  12 ) of the arm  32  and is pivotally connected at its second (lower) end  40  to shoulder  34 . Actuator  36  may be of any suitable type for underwater use. A hydraulic actuator (as illustrated) is preferred. 
     As will be readily apparent form the Figures, extension of actuator  36  will cause movement of leg  26  about pivot axis  30  such that the vertical spacing of the endless track unit  16  with respect to the main body portion  12  is increased. Correspondingly, retraction of the actuator  36  causes movement of leg  26  about pivot axis  30  in the opposite sense, so that the vertical spacing of the endless track unit  16  with respect to the main body portion  12  is decreased. A control means (typically an electronic control means) is provided, by means of which an operator can control the action or operation of actuator  36 . 
     In such embodiments the apparatus  10  of the present invention is better equipped to accommodate the uneven surface of a subsea environment as each endless track unit  16  may adopt a range of orientations by rotating about its nominally horizontal axis  28 . Furthermore, each respective endless track unit may thus adopt a different orientation to another respective track unit by rotating about said nominally horizontal axis. In addition, the vertical spacing of each respective endless track unit (i.e.  16   a ,  16   b ,  16   c  and  16   d ) from the body portion  12  can be changed as the apparatus  10  traverses the seabed by movement of leg  26  about pivot axis  30 . 
     In further preferred embodiments of the invention, at least some of the endless track units  16  are steerable. That is, at least some of the endless track units are moveable about a nominally vertical axis whereby their alignment with respect to the main body portion  12  is changed or adjusted. Preferably two of the endless track units  16  are steerable, the steerable units being those arranged side-by-side with respect to the direction of travel of the apparatus  10 . 
     Where the apparatus  10  has more than two endless track units  16 , in particular where the apparatus  10  has four endless track units  16 , preferably at least two of the endless track units  16  are steerable. More preferably, all of the endless track units  16  are steerable. Thus, in a preferred form of the apparatus  10 , the apparatus  10  has four endless track units  16  all of which are steerable. 
     Each endless track unit  16  is attached to leg  26  through a pair of connection brackets  42   a ,  42   b . Each connection bracket  42   a ,  42   b  is attached to leg  26  such that connection bracket  42   a ,  42   b  pivots about nominally horizontal axis  28 . Connection bracket  42   a  is connected to a lower end portion  26   a  at an outside edge thereof. Connection bracket  42   b  is connected to a lower end portion  26   a  at an inside edge thereof. The terms “inside” and “outside” are used in relation to the apparatus  10  as a whole (and not in respect of an individual leg  26  taken in isolation). Rotation or pivoting of the endless track unit  16  about nominally horizontal axis  28  as described above is thereby achieved. 
     Extending between the connection brackets  42   a ,  42   b  and intersecting the endless track unit  16  is a carrying member  46 . Endless track unit  16  is mounted on the carrying member  46 . At its respective ends the carrying member  46  is fixedly connected to the connection brackets  42   a ,  42   b . The carrying member  46  may be formed integrally with the respective connection brackets  42   a ,  42   b.    
     Endless track units  16  which are steerable are mounted on the carrying member  46  such that the endless track unit  16  is rotatable with respect to the carrying member  46  about a nominally vertical axis (with respect to the orientation of the apparatus  10  as shown in the Figures). Typically, the subframe  20  of the endless track unit  16  is pivotally attached to the carrying member  46  to allow said rotation of the endless track unit  16  about the vertical axis. 
     Steerable endless track units  16  further comprise a pair of actuators  44   a ,  44   b  which extend between the connection bracket  42   a  and the subframe  20  of the endless track unit  16 . Each actuator  44   a ,  44   b  is pivotally connected at a first end  44   a ′,  44   b ′ to the connection bracket  42   a  and at a second end  44   a ″,  44   b ″ is pivotally connected to the subframe  20  of the endless track unit  16 . Actuators  44   a ,  44   b  associated with a given endless track unit  16  operate as a pair to effect steering of the endless track unit  16 . Thus, extension of actuator  44   a  is accompanied by a corresponding retraction of actuator  44   b  so that the endless track unit  16  is turned in an anti-clockwise (counter-clockwise) sense, as observed from above ( FIG. 3 ). Likewise, extension of actuator  44   b  is accompanied by a corresponding retraction of actuator  44   a , so that the endless track unit  16  is turned in a clockwise sense. 
     Actuators  44   a ,  44   b  may be of any type suitable for underwater use, but are preferably hydraulic actuators. 
     Control means are provided to control the motion or operation of the actuators  44   a ,  44   b  whereby an operator can adjust the angular position (about the vertical axis) of the steerable endless track unit  16  with respect to the main body  12 . The apparatus  10  can thus be steered by an operator. In other words, the direction of travel of the apparatus  10  can be adjusted by an operator, for example in the light of operational circumstances such as ground conditions. 
     In some advantageous embodiments of the invention, each steerable endless track unit  16  is independently steerable. That is, the control means provides for extension/retraction of the actuators  44   a ,  44   b  associated with an endless track unit  16  independently of the state of operation of the actuators  44   a ,  44   b  associated with a different endless track unit  16 . 
     In embodiments wherein the steerable track units  16  are independently steerable, the track units are moveable about a nominally vertical axis such that the alignment of each respective track unit  16  may be changed or adjusted with respect to the main body portion  12 . Thus track unit  16   a , for example, may be moved about a nominally vertical axis by a greater or lesser extent in comparison to track units  16   b ,  16   c  and  16   d . Track unit  16   a  will therefore adopt a different alignment with respect to the main body portion  12  in comparison to track units  16   b ,  16   c  and  16   d . Hence each track unit may respectively adopt a range of orientations in a horizontal plane independently of the configuration of the other track units. Advantageously the provision of independently steerable track units enhances the range of movement permissible for the underwater trenching apparatus. As the track units  16  can each independently rotate about their respective nominally vertical axes each track unit may rotate to a different extent to change the heading of the underwater trenching apparatus improving its manoeuvrability. For example, when the apparatus moves in a circular path the inwardly located track units with respect to the centre of the circle as defined by the circular path may rotate to a different extent compared to the track units outwardly located with respect to the centre of the circle defined by the circular path. By rotating the respective inwardly and outwardly located track units an appropriate amount, it is possible for embodiments of the apparatus with independently steerable track units to perform this manoeuvre in a tighter turning circle than an apparatus without independently steerable track units. 
     In other advantageous embodiments, endless track units  16  may be steerable in pairs. That is, for a given pair of track units, substantially the same steering operation is applied to each endless track unit  16  of the pair, so that the angular position of the each of the said endless track units  16  in the pair is substantially the same at any given time. Endless track units arranged side-by-side (such as  16   a  and  16   b , or  16   c  and  16   d ) conveniently form steering pairs. 
     In other advantageous embodiments, all of the steerable endless track units are steered in unison by the control means, so that each adopts substantially the same angular position, and the control means is configured to operate accordingly. 
     The control means may further be configured to allow an operator to select any of the above steering configurations, that is, fully independent steering of the endless track units  16 , paired steering of endless track units  16 , or steering in unison of all the steerable endless track units  16 . 
     Independent steering ability for each of the endless track units  16  further allows the apparatus  10  to adopt a so-called “crab-steering” configuration. In this configuration the control means (typically via an operator input) sets the respective actuators  44   a ,  44   b  such that each of the endless track units  16  is arranged in a substantially parallel configuration with respect to the other endless track units  16 . The apparatus  10  thus moves “sideways” along an essentially straight path. For example, if the normal “straight ahead” direction of travel of the apparatus  10  is indicated in  FIG. 3  by arrow  52  which is aligned with the longitudinal axis of the apparatus  10 , then a “crab steering” setting of the endless track units  16  results in movement of the apparatus along a path such as that indicated by arrow  54  at an angle θ to the straight ahead direction of arrow  52 . The longitudinal axis of the main body portion  12  remains aligned with arrow  52 . 
     Main body portion  12  of apparatus  10  is provided with a lifting attachment point or formation  48 . The lifting attachment formation  48  is used for the attachment of lifting cables by means of which the trenching apparatus  10  is lowered to the seabed (from a service vessel at the surface) and is raised from the seabed for return to the surface. Lifting attachment formation  48  is most preferably secured directly to the main body portion  12  of apparatus  10  and is provided with suitable components or fittings to which lifting cables may conveniently be attached. 
     Lifting attachment formation  48  is preferably arranged substantially centrally with respect to the main body portion, and preferably at or adjacent a top surface  50  of the main body portion  12 . 
     Substantially central arrangement of the lifting attachment formation  48  with respect to the main body portion  12  is most preferably such that—at least as so far as is practical—the lifting attachment formation  48  is spaced substantially equidistantly from the extremities of the main body portion  12  and/or is arranged substantially equidistantly from each endless track unit  16  (or, where more than four endless track units are provided, the lifting attachment formation  48  is arranged substantially equidistantly from those endless track units  16  disposed at or near outer corners of the apparatus  10 , when viewed in plan). 
     The above substantially central arrangement of the lifting attachment formation is achievable in the present invention because of the provision of the steerable endless track units  16 . 
     The provision of steerable endless track units  16  as described herein is also advantageous in allowing a central location of the trenching tool  14  with respect to main body portion  12  (“central” having the same meaning as in respect of the lifting attachment formation defined above). Such central location of the trenching tool  14  is advantageous in ensuring that cutting loads (derived from the trenching operation with the trenching tool  14 ) are evenly distributed through the apparatus  10  whereby even pressures of each endless track on the ground (seabed) are achieved. That is, the pressure exerted by each track on the seabed is substantially the same at any time during trenching operations. 
     In further advantageous embodiments, the apparatus  10  of the present invention provides a damping action or effect on landing of the apparatus  10  on the seabed. That is, a damping effect is provided at the point when the apparatus  10  contacts the seabed as the apparatus  10  is lowered. A noted, the apparatus  10  is attached to lifting cables at lifting attachment formation  48  during lifting and lowering operations. The damping action achieved with the apparatus  10  significantly reduces the loading on the apparatus  10  during the landing operation thereby avoiding damage to the apparatus  10 . 
     The damping effect or action is achieved by adjustment of the “ride height” of the apparatus  10 , that is, by suitably adjusting the angular position of legs  26  about pivot axes  30  using actuators  36 . The adjustment of the legs  26  takes into account the known, or expected, ground conditions of the seabed at the landing point. 
     Each of the legs  26  is most preferably individually adjustable, that is, without reference to the angular position of the other legs  26 , An advantage of this arrangement is that the vertical spacing of each endless track unit  16  with respect to the main body portion  12  can be independently adjusted—for example in the light of particular ground conditions encountered during trenching operations—so that an even track pressure is achieved. 
     Furthermore, individual adjustment of the “ride height” (that is, the vertical spacing of each endless track unit  16  from the main body portion  12 ) can be advantageous in ensuring that the trenching tool  14  is at, or is maintained at, a desired angle. Typically the desired angle is vertical, or such angle of the tool  14  as is required to obtain a trench with the vertical sides. Deflection from the desired tool angle may occur due to ground conditions, such as unevenness of the seabed, and the independent adjustment of the legs  26  compensates accordingly. 
     As noted, legs  26  are operated under the control of a control means. The control means may operate automatically, or may be under the control of an operator, or may have automatic functions and operator functions, and may have a manual override of automatic functions. The same applies to each or any of the control means for the actuators  44   a ,  44   b  and actuators controlling the angular position of endless track units  16  about horizontal axis  28 . Of course, a single control means may be provided to control all of, or any group or combination of, actuators provided on the apparatus  10 . 
     The, or each, control means is preferably an electronic control means. Elements of such control means for use by an operator are desirably located at the surface (in a service vessel) or, in conjunction with suitable remote control equipment, may be located on land. Such elements can, for example, include an electronic control panel or the like. 
     Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of the words, for example “comprising” and “comprises”, means “including but not limited to”, and is not intended to (and does not) exclude other moieties, additives, components, integers or steps. 
     Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise. 
     Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith.

Summary:
An underwater trenching apparatus comprises at least one trenching tool; a lifting attachment point configured for attachment of a lifting means for lowering the trenching apparatus to, or raising the trenching apparatus from, its location of operation; at least two steerable endless track units for transmitting tractive effort to move the underwater trenching apparatus in use; and for each steerable endless track unit, a first mounting arrangement permitting rotation of the track unit about a vertical axis, and a first actuator configured to move the track unit about the vertical axis, and control means configured to control the action of the actuator. In alternative forms, in addition to, or alternative to the steerable endless track units, the apparatus includes height adjustment means for adjusting the vertical spacing between the main body portion and the respective endless tracks; and control means configured to control the action of the height adjustment means.