Abstract:
A method of underground laying a continuous elongated member in a bed of a body of water, wherein the continuous elongated member lies on the bed of the body of water along a given path; the method including the steps of:
       fragmenting a soil mass in the bed along the given path and under the continuous elongated member, so as to form in the bed two scarp slopes bounding the fragmented soil mass by two soil masses susceptible to slide;   advancing two supporting walls along the given path in an advancing direction, along the respective two scarp slopes, and transferring the fragmented soil mass between the two supporting walls, so as to promote sinking of the continuous elongated member between the two supporting walls.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. Nationalization of PCT International Application No. PCT/IB2009/006734 filed 3 Sep. 2009, which claims priority to Italian Patent Application No. MI2008A001586 filed 4 Sep. 2008, the entireties of both of the foregoing applications are incorporated herein by reference. 
     TECHNICAL FIELD 
     Embodiments of the present invention relates to a method of underground laying a continuous elongated member, such as an underwater pipeline, cable, umbilical, pipe and/or cable bundle, in the bed of a body of water. 
     BACKGROUND ART 
     In-bed laying an underwater pipeline normally comprises laying the pipeline along a given path on the bed of the body of water; fragmenting a soil mass along the path to a given depth; digging a trench or generally removing the fragmented soil mass; and possibly burying the pipeline. 
     More specifically, currently used known techniques comprise removing the fragmented soil mass to form a trench in the bed of the body of water; and laying the pipeline directly into the trench. The pipeline may later be covered over with the removed soil mass to fill in the trench and bury the pipeline. 
     Underwater pipelines carrying hydrocarbons are normally laid completely or partly underground for various reasons, some of which are discussed below. Underwater pipelines are normally laid underground close to shore approaches and in relatively shallow water, to protect them from damage by blunt objects, such as anchors or fishing nets, and are sometimes laid underground to protect them from natural agents, such as wave motion and currents, which may result in severe stress. That is, when a pipeline is laid on the bed of a body of water, it may span two supporting areas of the bed, i.e. a portion of the pipeline may be raised off the bed; in which case, the pipeline is particularly exposed to, and offers little resistance to the movements induced by, wave motion and currents. Underground laying may also be required for reasons of thermal instability, which result in deformation (upheaval/lateral buckling) of the pipeline, or to protect the pipeline from the mechanical action of ice, which, in particularly shallow water, may result in scouring of the bed. 
     To avoid damage, the pipeline often need simply be laid at the bottom of a suitably deep trench dug before laying (pre-trenching) or more often after laying the pipeline (post-trenching). At times, the protection afforded by the trench and eventual natural backfilling of the trench is not enough, and the pipeline must be buried using the fragmented soil mass removed from the trench, or any available soil mass alongside the trench. 
     The depth of the trench is normally such that the top line of the pipeline is roughly a meter below the surface of the bed, though severe environmental conditions may sometimes call for deeper trenches (of several meters). Trenching and backfilling are performed using digging equipment, and post-trenching (with the pipeline already laid on the bed) is the normal practice, to dig and backfill the trench in one go. 
     One method of in-bed laying underwater pipelines is described in Patent Application WO 2005/005736. This is a post-trenching method comprising the steps of fragmenting a soil mass in the bed to open the way; and drawing along the opening a huge plough, to form a trench, and vertical supporting walls connected to the plough and which respectively support two opposite soil masses bounded by two substantially vertical scarp slopes. 
     The above method has the drawback of being highly energy-intensive, due partly to the plough, and partly to friction between the supporting walls and the two soil masses. And energy consumption increases exponentially alongside an increase in trench depth. 
     Another method of in-bed laying underwater pipelines is described in Patent Application WO 2004/016366, which proposes fragmenting a soil mass in the bed, and removing the fragmented soil mass using a dredging unit on board a support vessel. That is, the fragmented soil mass is first sucked up from the bed along a dredging path up onto the support vessel, and then dumped back into the trench. 
     This method is also highly energy-intensive to draw the fragmented soil mass up onto the support vessel. Moreover, the scarp slopes are susceptible to slide; the method is unsuitable for in-depth laying underwater pipelines; and, in the event of slide, the pumps and conduits are called on to remove additional fragmented soil masses, thus further increasing energy consumption. 
     SUMMARY 
     One or more embodiments of the present invention provide a method of underground laying an underwater pipeline in the bed of a body of water, designed to eliminate the drawbacks of the known art. 
     One or more embodiments of the present invention provide a method enabling easy in-depth laying of underwater pipelines in the bed of a body of water. 
     According to an embodiment of the present invention, there is provided a method of underground laying a continuous elongated member in a bed of a body of water, wherein the continuous elongated member lies on the bed of the body of water along a given path; the method including the steps of:
         fragmenting a soil mass in the bed along the given path and under the continuous elongated member, so as to form in the bed two scarp slopes bounding the fragmented soil mass by two soil masses susceptible to slide;   advancing two supporting walls, along the given path in an advancing direction, along the respective two scarp slopes; and   transferring the fragmented soil mass between the two supporting walls, so as to promote sinking of the continuous elongated member between the two supporting walls.       

     Embodiments of the present invention provide for greatly reducing energy consumption by only removing the fragmented soil mass between the supporting walls preventing slide of the soil masses defined by the scarp slopes, thus enabling in-depth laying with the removal of only a small fragmented soil mass in relation to depth. 
     Another embodiment of the present invention provides a system for underground laying a continuous elongated member in the bed of a body of water. 
     According to an embodiment of the present invention, there is provided a system for underground laying a continuous elongated member in a bed of a body of water, wherein the continuous elongated member extends on the bed along a given path: the system comprising an underwater vehicle comprising a work assembly which is set into the bed and comprises:
         a fragmenting unit for fragmenting a soil mass in the bed along the given path and under the continuous elongated member, so as to form in the bed two scarp slopes bounding the fragmented soil mass by two soil masses susceptible to slide;   a sustaining unit comprising two supporting walls which are advanced, along the given path in an advancing direction, along the respective two scarp slopes; and   means for transferring the fragmented soil mass between the two supporting walls, so as to promote sinking of the continuous elongated member between the two supporting walls.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A number of non-limiting embodiments of the present invention will be described by way of example with reference to the accompanying drawings, in which: 
         FIG. 1  shows a partly sectioned side view, with parts removed for clarity, of a system for underground laying an underwater pipeline in the bed of a body of water; 
         FIG. 2  shows a cross section of the bed when digging a trench in which to lay the underwater pipeline; 
         FIG. 3  shows an isometric view, with parts removed for clarity, of an underwater vehicle of the  FIG. 1  system; 
         FIG. 4  shows a plan view, with parts removed for clarity, of the  FIG. 3  underwater vehicle; 
         FIG. 5  shows a larger-scale front view, with parts removed for clarity, of the  FIG. 3  underwater vehicle; 
         FIG. 6  shows an isometric view, with parts removed for clarity, of the  FIG. 3  underwater vehicle in another configuration; 
         FIG. 7  shows a partly sectioned isometric view, with parts removed for clarity, of the  FIG. 3  underwater vehicle; 
         FIG. 8  shows a side section, with parts removed for clarity, of the  FIG. 3  underwater vehicle; 
         FIG. 9  shows a larger-scale isometric view, with parts removed for clarity, of a detail of the  FIG. 3  underwater vehicle; 
         FIG. 10  shows a front section, with parts removed for clarity, of a detail of the  FIG. 3  underwater vehicle; 
         FIG. 11  shows a larger-scale side view, with parts removed for clarity, of a detail of the  FIG. 3  vehicle; 
         FIG. 12  shows a larger-scale section, with parts removed for clarity, of a detail of the  FIG. 11 . 
     
    
    
     DETAILED DESCRIPTION 
     Underwater Pipeline Underground Laying System 
     Number  1  in  FIG. 1  indicates as a whole a system for underground laying underwater pipelines in a bed  2  of a body of water  3  of level SL. 
     In the following description, the term “body of water” is intended to mean any stretch of water, such as sea, ocean, lake, etc., and the term “bed” is intended to mean the concave layer of the earth&#39;s crust containing the mass of water in the body. 
     Underground laying system  1  provides for underground laying an underwater pipeline  4 , which has an axis A 1 , extends along a given path P on bed  2 , and has been laid beforehand by a known laying vessel not shown in the drawings. Underground laying system  1  comprises a support vessel  5 ; and a convoy  6  comprising a number of underwater vehicles  7 ,  8 ,  9 ,  10  advanced in an advancing direction D 1  along path P. 
     Though the present description refers specifically to an underwater pipeline, underground laying system  1  provides for underground laying continuous elongated members of all types, such as cables, umbilicals, pipe and/or cable bundles, not shown in the drawings. 
     Underwater vehicles  7 ,  8 ,  9 ,  10  are guided along path P by support vessel  5 . More specifically, support vessel  5  serves to guide vehicles  7 ,  8 ,  9 ,  10  along path P, and to supply vehicles  7 ,  8 ,  9 ,  10  with electric power, control signals, compressed air, hydraulic power, etc., so each underwater vehicle  7 ,  8 ,  9 ,  10  is connected to support vessel  5  by a cable bundle  11 . 
     Each vehicle  7 ,  8 ,  9 ,  10  is designed to:
         fragment a respective soil layer of bed  2  to form two soil masses  12  bounded by respective opposite, substantially vertical scarp slopes  13 , as shown clearly in  FIG. 2 , and a fragmented soil mass  14  between the two scarp slopes  13 ;   support soil masses  12  along scarp slopes  13  ( FIG. 2 );   transfer the fragmented soil mass  14  between the two opposite scarp slopes  13  ( FIG. 2 );   guide pipeline  4 ; and   bury pipeline  4  with the removed fragmented soil mass  14 .       

     Underwater vehicles  7 ,  8 ,  9 ,  10  are kept close together to seamlessly sink pipeline  4 . 
     In the  FIG. 1  example, underwater vehicle  10  performs no fragmenting function. 
     The fragmented soil mass  14  is bounded at the bottom by bottom faces  15 ,  16 ,  17  increasing gradually in depth in the opposite direction to direction D 1 . 
     In other words, underwater vehicles  7 ,  8 ,  9 ,  10  dig a trench  18 , on the bottom face  17  of which pipeline  4  is laid and covered with fragmented soil mass  14 . 
     With reference to  FIG. 2 , for the purpose of this description, the term “scarp slope” is intended to mean a surface connecting rock formations, sediment or terrains at different heights, and, in the example shown, scarp slopes  13  are substantially vertical. 
     Depending on the depth of trench  18  and the nature of soil mass  12 , soil masses  12  bounded by respective scarp slopes  13  must be supported to prevent soil masses  12  from sliding. 
     For example, a soil mass of granular material, such as sand or gravel, tends to settle into a surface (natural slope) at a given angle, known as natural slope angle, to the horizontal. 
     If bed  2  is made solely of cohesive rock, on the other hand, there is practically no risk of soil masses  12  sliding at scarp slopes  13 . Nevertheless, underground laying system  1  ( FIG. 1 ) is designed to cope with any type of problem, regardless of the geological structure of bed  2 . 
     Underwater Vehicles 
     The following is a detailed description of underwater vehicle  9 , with reference to  FIGS. 3-10 . Underwater vehicles  7 ,  8 ,  10  in  FIG. 1  are not described in detail, but are structurally similar to underwater vehicle  9 , from which they differ solely as regards the size of certain component parts. Accordingly, the reference numbers used with reference to underwater vehicle  9  also apply to corresponding parts of underwater vehicles  7 ,  8 ,  10  in  FIG. 1 . 
     In  FIG. 3 , underwater vehicle  9  extends along an axis A 2 , and comprises a work assembly  19  which is set into bed  2 ; two drive assemblies  20  which rest on bed  2  and advance work assembly  19  in direction Dl ( FIG. 1 ); and two connecting assemblies  21 , each for connecting a respective drive assembly  20  to work assembly  19 , and for adjusting the relative positions of drive assemblies  20  and work assembly  19 . 
     Work assembly  19  comprises a supporting frame  22 ; a sustaining unit  23 ; a fragmenting unit  24 ; a dredging unit  25 ; and an auxiliary dredging unit  26 . 
     Supporting frame  22  substantially comprises a number of beams, each of which is inverted-U-shaped, as shown more clearly in  FIG. 7 . 
     Sustaining unit  23  comprises two opposite supporting walls  27  fixed to frame  22  and parallel to axis A 2 . As shown more clearly in  FIG. 10 , each supporting wall  27  comprises a base structure  28 ; a number of panels  29  connected elastically, preferably by rubber fasteners, to base structure  28 ; and a number of actuators  30  for inducing vibration in panels  29 , preferably in a vertical direction D 2  crosswise to axis A 2  and parallel to supporting walls  27 . 
     With reference to  FIG. 5 , fragmenting init  24  comprises a number of vertical cutters  31  for fragmenting a soil mass cross section of a width substantially equal to the distance between opposite walls  27 . Fragmenting unit  24  also comprises two arms  32 , each of which supports half the number of cutters  31  and rotates, with respect to frame  22 , about a vertical axis A 3  (parallel to supporting walls  27 ) to set cutters  31  to a work position, in which arms  32  are perpendicular to supporting walls  27  and cutters  31  connect opposite supporting walls  27 , and a rest position, in which arms  32  are parallel to supporting walls  27 , so the pipeline can be placed between the two arms  32  and respective cutters  31 . 
     Dredging unit  25  comprises two dredging devices  33 . As shown more clearly in  FIG. 8 , each dredging device  33  is fitted to underwater vehicle  9  and located at least partly between walls  27 . In the example shown, each dredging device  33  comprises a suction conduit  34  having a suction port  35  located at the bottom of supporting wall  27  and, in use, under pipeline  4  ( FIG. 1 ); a delivery hose  36  for unloading the fragmented soil mass  14  downstream from convoy  6  ( FIG. 1 ); and a pump  37  between suction conduit  34  and hose  36 . 
     With reference to  FIG. 7 , auxiliary dredging unit  26  comprises two pumps  38  (only one shown in  FIG. 7 ) located on opposite sides of sustaining unit  23 ; and a number of conduits  39  extending between and directly over supporting walls  27 . Each conduit  39  comprises two branches  40  respectively adjacent to the inner faces of opposite supporting walls  27 ; and a header  41  communicating with both branches  40  and having an outlet port  42 . Each branch  40  comprises a suction port  43  located close to the bottom of respective supporting wall  27  and facing the opposite supporting wall  27 . 
     Pumps  38  are connected to each branch  40  by a respective hose  44  which generates an upward jet in respective branch  40 , so that each conduit  39  acts as an ejector pump between suction ports  43  and outlet port  42 . 
     With reference to  FIG. 4 , each drive assembly  20  comprises a supporting body  45 ; and a powered track  46  looped about supporting body  45  and movable about supporting body  45  by known means not shown in the drawings. Supporting body  45  is at least partly hollow, and comprises a control device  47  in turn comprising valves and a pump (not defined in detail), and a pipe  48  connected to the laying vessel to feed/expel air to/from body  45  and so alter the buoyancy of drive assembly  20  and underwater vehicle  9  as a whole. In other words, supporting body  45  is a variable-buoyancy body. 
     Each connecting assembly  21  comprises two articulated joints  49 , each of which comprises a bracket  50  fitted to supporting body  45  to rotate about an axis A 4 ; an arm  51  hinged to bracket  50 ; and an actuator  52 , in particular a hydraulic cylinder, hinged to bracket  50  and arm  51  to form, with bracket  50  and arm  51 , a variable-configuration triangle. Arm  51  is in turn hinged to a connecting member  53  fitted to work assembly  19  as shown in  FIG. 9 . 
     With reference to  FIG. 9 , connecting member  53  comprises a fork  54 ; and a dove-tail prismatic body  55  with a threaded central hole. 
     With reference to  FIG. 6 , connecting assembly  21  also comprises four tracks  56  which, in the example shown, are grooves extending along supporting walls  27  in direction D 2  ( FIG. 1 ). More specifically, each supporting wall  27  has two tracks  56 ; and two actuators  57 , each located at a respective track  56  and connected to connecting member  53  to move connecting member  53  ( FIG. 9 ) with respect to supporting wall  27 . 
     With reference to  FIG. 9 , each track  56  has a seat having a dove-tailed cross section and engaged in sliding manner by prismatic body  55 . 
     With reference to  FIG. 8 , each actuator  57  is fitted to frame  22 , and comprises an electric motor  58 ; and a threaded bar  59  housed in the seat of track  56  and engaging the threaded hole in prismatic body  55  so as to form, with prismatic body  55 , a screw-nut screw mechanism. 
     With reference to  FIG. 6 , each connecting assembly  21  comprises two tow bars  60  fitted to a respective pair of connecting members  53  and adjacent to a respective supporting wall  27 . Each tow bar  60  of underwater vehicle  9  is connected to the respective tow bars of adjacent underwater vehicles  8  and  10 , as shown in  FIG. 1 . 
     With reference to  FIG. 1 , hoses  36  of dredging devices  33  all extend downstream from the last underwater vehicle  10  in conveyor  6 , and have outlet ports  61  located over pipeline  4 , so the material removed by dredging devices  33  is fed back into trench  18  once pipeline  4  is sunk. 
     With reference to  FIG. 8 , the work assembly also comprises a number of carriages  62  fitted to frame  22  and located between supporting walls  27  to push pipeline  4  downwards and so aid in sinking pipeline  4 . 
     With reference to  FIG. 11 , each panel  29  has an outer face  63 ; an inner face  64  ( FIG. 12 ); and vertical ribs  65  and horizontal ribs  66  for stiffening panel  29 . 
     Panel  29  is equipped with a lubricating device  67  for forming a water film along outer face  63  of panel  29 , and which comprises a number of nozzles  68  equally spaced along outer face  63 ; conduits  69  at vertical ribs  65  ( FIG. 12 ); and a pump (not shown) connected to conduits  69  by hoses  70  ( FIG. 12 ). 
     Nozzles  68  are housed in recesses  71  in panel  29 , so as not to project from outer face  63 . 
     With reference to  FIG. 12 , each nozzle  68  is oriented to emit a jet at a 20° angle with respect to outer face  63  and in the opposite direction to advancing direction D 1  ( FIG. 11 ). 
     With reference to  FIG. 11 , the size of the jets and the number of nozzles are selected to cover the whole of outer face  63  with a film of water and so reduce friction between panel  29  and scarp slope  13  ( FIG. 2 ). 
     Operation of system  1  will be clear from the above description. 
     Advantages 
     In addition to the energy-saving advantages already mentioned, the fragmented soil mass is removed by dredging unit  25  and auxiliary dredging unit  26 . In many applications, dredging unit  25  is unable to remove all the fragmented soil mass  14  on its own, so the rest of fragmented soil mass  14  is removed by auxiliary dredging unit  26 . 
     Soil masses  12  are prevented from sliding at the fragmenting, removal, and sinking stages, by being confined by supporting walls  27 ; and friction between supporting walls  27  and soil masses  12  is greatly reduced by vibrating panels  29  contacting soil masses  12  along scarp slopes  13 . 
     Underwater vehicles  7 ,  8 ,  9 ,  10  are highly versatile, and can adjust the position of work assembly  19  with respect to drive assemblies  20  and hence the depth of the work assembly in bed  2 . 
     The distance between drive assemblies  20  and work assembly  19  can also be adjusted. For example, in sandy beds, it is best to keep drive assemblies  20  as far away as possible from work assembly  19 , to prevent the weight of drive assemblies  20  from inducing slide of soil masses  12  and so further increasing friction between soil masses  12  and supporting walls  27 . 
     Conversely, in rocky beds, where the above drawback does not apply, it is best to keep drive assemblies  20  as close as possible to work assembly  19 , so as to provide greater forward thrust to fragmenting unit  24 , which encounters considerable resistance in rocky terrain. 
     Because of the play between each track  56  and respective connecting member  53  and independent actuators  57 , work assembly  19  can be tilted slightly with respect to drive assemblies  20 . 
     Independent actuators  57  enable the two drive assemblies  20  to be set to two different heights with respect to work assembly  19 , and therefore to operate at two different levels on either side of work assembly  19 , while keeping work assembly  19  vertical. 
     Because cutters  31  can be set to a work position and a rest position, underwater vehicles  7 ,  8 ,  9 ,  10  can be withdrawn from the trench without interfering with pipeline  4  being sunk. 
     The above feature enables one or more underwater vehicles  7 ,  8 ,  9 ,  10 —for example, underwater vehicle  10  in FIG.  1 —to be used solely for removal, support and sinking work. 
     Removal and setup of underwater vehicles  7 ,  8 ,  9 ,  10  are also made easier by the variable buoyancy of supporting bodies  45 . 
     Clearly, changes may be made to the embodiment of the present invention as described herein without, however, departing from the scope of the accompanying Claims.