Abstract:
A seabed plow capable of over-the-stern release and retrieval is usable in any of boulder clearing, trench cutting and backfill modes. In its boulder clearing mode, the plow uses its skid in torquing configuration followed by moldboards to push boulders out of the path to be trenched. In the trench cutting mode, the plow uses the skid in the same configuration with its share and moldboards for initial trench depth and the same share and moldboards with moldboard extensions for increasing trench depth in subsequent passes. In the backfill mode, the plow uses its skid in trench-straddling configuration following a blade with a passage designed to discharge fragmented spoil directly onto the pipe being covered. Thus, a single plow can be reconfigured for all modes for release and retrieval to and from a relatively small vessel without use of heavy lifting equipment.

Description:
BACKGROUND OF THE INVENTION  
       [0001]    This invention relates generally to the offshore laying of pipe and cable and more particularly concerns equipment used in the preparation and trenching of the seabed to receive the pipe or cable and in the backfilling of the trench once the pipe or cable has been laid. 
         [0002]    Present pipe laying methods include a few basic seabed trenching tasks performed using long-accepted, time-consuming, budget-eating practices and equipment. 
         [0003]    One problem is that sometimes, before trenching can begin, it may be necessary to clear the seabed of boulders on, or partially buried in, the intended pipeline path. Presently, the boulder removal process involves dragging the boulders, one at a time, at cables-end from a transport/towing vessel. In some boulder fields, this can be a lengthy and tedious process. It always requires one or more divers, a remotely operated vehicle (ROV), or other boulder handling mechanisms to connect the cable to the boulder. 
         [0004]    Another problem is that, when the time for trench cutting arrives, a trench cutting plow must be lowered to the seabed. Launching a trench cutting plow typically requires a large vessel carrying a crane and supporting equipment to lift the plow from the vessel, to swing the plow clear of the deck and to lower the plow into the sea. After its final trench cutting pass, retrieval of the trench cutting plow from the seabed to its storage area on the towing vessel again requires use of the crane and supporting equipment. In operation on the seabed, known trench cutting plows have additional problems. For example, many require skids which straddle the width of the trench being cut, so the number of possible passes that can be made and the depth of the trench that can be cut is limited. 
         [0005]    Similar problems are experienced in the backfilling of the trench to cover the pipe. First, a heavy backfill plow must be lowered to the seabed. As with launching trench cutting plows, launching known backfill plows typically requires a large vessel and a crane and supporting equipment to raise the plow from the vessel, to swing the plow clear of the deck and to lower the plow into the sea. After its final pass the backfill plow must be retrieved from the seabed and returned to its place on the towing vessel by use of the crane and supporting equipment. In operation of the backfill plow on the seabed, one or more passes of the plow are necessary to cover the pipeline and fill the trench. Typical known backfill plows have chasses with front skids which travel in the trench and straddle the pipeline, followed by moldboards which are angled forwardly and away from the chassis to collect the spoil in their path and deposit it in the trench to the sides of the pipeline. Since the skids ride in the trench in close proximity to the pipeline, there is significant risk that contact with the skids will compromise the integrity of the pipe. Also, since the mixture of seawater and spoil, which is more dense than the hollow pipe, is pushed by the moldboards to the outer limits of the trench and discharged to the sides of the pipe, there is a significant risk that, as the spoil settles, the pipe will “float,” resulting in inadequate burial of the pipeline. 
         [0006]    Much of the seabed trenching task equipment and operating methods are, in terms of time and money, very inefficient and beg for improvement. But the inadequacy of the individual plows and their operating methods is dwarfed by the need for a large, heavily equipped vessel to transport, launch and retrieve these plows instead of a much smaller vessel which could otherwise be used for operating purposes. Typically, the cost of known trench cutting and backfill plows is in a range of $8,000,000 each. The cost of the transport/towing vessel with the crane and supporting equipment is in a range of $500,000,000. The rental fee for the vessel and plows ranges from $150,000 to $600,000 per day. 
         [0007]    Accordingly, it is an object of this invention to reduce the weight and cost and increase the efficiency of the plows, equipment and vessels used in the offshore laying of pipe and cable. 
       SUMMARY OF THE INVENTION 
     Single And Multi-Mode Chasses 
       [0008]    In accordance with the invention, a seabed-plow chassis is provided in which an elongated member is adapted for mounting a skid on one of its ends to support that end above the seabed and is adapted for mounting one or more tools on its other end to perform a variety of seabed trenching tasks. 
         [0009]    In a first mode of operation in which the chassis is a part of a boulder clearing plow, the tool consists of moldboards for clearing boulders which are initially pushed by the skid outward of the path traveled by the skid further outward from the path as the skid leads the moldboards along the seabed. 
         [0010]    In a second mode of operation in which the chassis is a part of a trench cutting plow, the tool consists of a plow share and moldboards for sequentially cutting and moving spoil to create a trench as the skid leads the plow share and moldboards along the seabed. 
         [0011]    In a third mode of operation in which the chassis is a part of a backfill plow, the tool consists of a blade and moldboards which cooperate as the blade and moldboards lead the skid along the seabed to sequentially collect, funnel inward and release downward into the trench spoil lying outside of the trench. 
         [0012]    Different chasses can be adapted to accommodate each of the modes or the same chassis can be adapted to interconnect any of the tools with the skid according to the desired mode of operation. 
         [0013]    The elongated member of the chassis may have one or more permanent transition surfaces or one or more attachments providing transition surfaces. The transition surfaces are configured to extend between the skid and the various tools which may be mounted on the chassis. The transition surface contours are shaped and located so that the appropriate transition surface makes contact with and pivots about a fulcrum on the stern of a plow transporting/towing vessel as the plow crosses that fulcrum during its release from the vessel into the sea and during its retrieval from the sea onto the vessel. The shapes and locations of the transition surfaces and the weight of the elongated member are coordinated so as to resist roll of the chassis about a transition axis as the plow moves on the deck or across the fulcrum. 
         [0014]    Preferably, the vertical longitudinal cross-sections of the transition surfaces are concave, the fulcrum is a roller and the paths defined by continuous symmetrically opposite points of the transition surfaces are contoured to maintain contact with the roller as the plow crosses the roller. 
         [0015]    In a preferred embodiment of the chassis for use in more than one mode of operation, a first transition surface is configured to extend between the skid and the tool in the first/boulder clearing and third/backfill modes and a second transition surface is configured to extend between the skid and the tool in the second/trench cutting mode. 
       Boulder Clearing Plow And Methods 
       [0016]    For clearing boulders from a seabed, the plow includes the chassis, a skid mounted on and supporting one end of the chassis above the seabed and moldboards mounted on and oriented in angular relationship to the other end of the chassis. The trailing moldboards clear boulders initially pushed outwardly by the leading skid further outward as the skid leads the moldboards along the seabed. 
         [0017]    In a preferred embodiment of the boulder clearing plow, a head is mounted on a leading end of the skid. The head has leading faces angled rearward from a vertical, longitudinal center plane of the skid and tapered rearward from its top edges, enabling the head to torque boulders partially buried in the seabed away from the skid. The chassis transition surface extends between the skid and the moldboards and is contoured to maintain contact with and pivot about the fulcrum/roller on the stern of the vessel as the plow crosses the fulcrum during launch and recovery. 
         [0018]    The boulder clearing plow may also include keel plates, at least one keel plate extending under each moldboard. The heel plates&#39; primary function is to ensure the lateral stability of the plow during operation by resisting departure of the plow from its intended path even when the plow encounters seabed obstructions or uneven amounts of spoil. 
         [0019]    The plow components are, in weight and in their contact surfaces with the fulcrum, coordinated to resist roll of the plow about a launch and recovery transition axis of the boulder clearing plow. 
         [0020]    Pulling points for connection of a pulling line to the boulder clearing plow are symmetrically arranged in relation to the longitudinal axis of the chassis and are displaced from the bottom of the boulder clearing plow by a height less than a radius of the vessel roller to facilitate passage of the plow contact surface over the vessel roller. 
         [0021]    The method of clearing boulders from a path on the seabed includes the steps of positioning the plow bow-forward in the direction of an initial seabed path and then propelling the plow along the initial seabed path to push boulders lying in the initial seabed path to the port and starboard sides of the plow. After the initial path is cleared the method continues, if a wider path is necessary, with the step of repositioning the plow bow-forward in a direction opposite the initial seabed path direction and on a second seabed path along one of the port and starboard sides of the initial seabed path. Once so repositioned, the method continues with the step of propelling the plow along the second seabed path to push boulders from the second seabed path further to one of the starboard and port sides of the plow, respectively. After the second path is cleared the method continues, if an even wider path is necessary, with the step of repositioning the plow bow-forward in the direction of the initial seabed path and on a third seabed path along the other of the starboard and port sides of the initial seabed path. Once so repositioned, the method continues with the step of propelling the plow along the third seabed path to push boulders from the third seabed path further to the other of the starboard and port sides of the plow, respectively. If a still wider path is necessary, the method can further include repeating the above widening steps in relation to the path resulting from the contiguity of the initial, second and third paths. The method anticipates repeating these steps for successively contiguous paths until a single path of desired width has been cleared along the seabed. 
         [0022]    For the over-the-stern boulder clearing plow herein disclosed, the method of clearing boulders from the seabed path is preceded by the steps of propelling the plow on the deck of the vessel toward and across the fulcrum on the stern of the vessel, allowing the plow to rotate about the fulcrum as the plow crosses the fulcrum and is released from the fulcrum into the sea and lowering the released plow at tow-line end toward the seabed. Furthermore, the method of clearing boulders from the seabed path is followed by the steps of raising the plow at tow-line end toward the fulcrum on the stern of the vessel at the other end of the tow line and pulling the plow across the fulcrum onto the deck of the vessel. 
       Backfill Plow And Methods 
       [0023]    For backfilling spoil into a seabed trench, the plow includes the chassis, a skid supporting the aft end of the chassis above the seabed, moldboards mounted on the chassis forward of the skid and a blade mounted on and spanning the bottom edges of the moldboards. The blade collects the spoil in its path as the plow travels forward on the seabed. The moldboards are sized and oriented to span the trench and funnel the collected spoil toward the center of the blade as the plow travels forward on the seabed. The blade has a passage at its rear apex which is configured to dispense the collected and funneled spoil onto the top of a pipe disposed in the trench below the passage. 
         [0024]    In a preferred embodiment, the backfill plow further includes a flapper board aft of the passage which fragments spoil discharged through the passage. The flapper board consists of a plate swinging below a horizontal shaft with a weight biasing the plate toward a vertical orientation. 
         [0025]    The skid is configured to straddle the trench and includes a crossbar mounted on the rear end of the chassis, a pair of skid posts, one on each end of the crossbar, and a pair of skis, one on the bottom of each post. The front surface of the crossbar may be adapted to the level spoil which has been discharged into the trench. 
         [0026]    The backfill plow may also include at least two keel plates spaced apart under the blade. The primary function of the keel plates is to ensure the lateral stability of the plow during operation by resisting departure of the backfill plow from its intended path even when the plow encounters seabed obstructions or uneven amounts of spoil. 
         [0027]    The plow has at least one transition surface between the skid and the moldboards which is contoured to contact and pivot about the fulcrum on the stern of a vessel as the backfill plow crosses the fulcrum during launch and retrieval of the backfill plow from and to the vessel. 
         [0028]    The plow components are, in weight and in contact surfaces with the fulcrum, coordinated to resist roll of the backfill plow about the transition axis of the plow. 
         [0029]    Pulling points for connection of a pulling line to the backfill plow are symmetrically arranged in relation to the longitudinal axis of the chassis and are displaced from the bottom of the backfill plow by a height less than the radius of the roller to facilitate passage of the plow contact surfaces over the roller. 
         [0030]    The method of backfilling spoil into a seabed trench includes the steps of propelling the blade to travel forward on the seabed and collect spoil along the sides of the trench, funneling the collected spoil toward a rear apex of the blade and allowing the funneled spoil to be discharged through an opening in the blade apex and onto a top surface of a pipe disposed in the trench. The method may further include one or both of the steps of fragmenting the discharged spoil before the discharged spoil reaches the pipe and/or leveling the spoil after it is discharged into the trench. 
         [0031]    For the over-the-stern backfill plow herein disclosed, the method of backfilling spoil into the trench is preceded by the steps of propelling the backfill plow on the deck of the vessel toward and across the fulcrum on the stern of the vessel, allowing the backfill plow to rotate about the fulcrum as the plow crosses the fulcrum and is released from the fulcrum into the sea and lowering the released plow at tow-line end to the seabed. Furthermore, the method of backfilling spoil into the trench can be followed by the steps of raising the backfill plow at tow-line end toward the fulcrum on the stern of the vessel at the other end of the tow line and pulling the plow across the fulcrum onto the deck of the vessel. 
         [0032]    As a result of the above plow structures and methods, the vessels needed for transport, launch, recovery and operation of the plows are smaller and presently plentiful. They are available to the user at rental fees ranging from $10,000 to $100,000 per day. This is a huge savings in comparison to the $150,000 to $600,000 per day rental fees presently paid for vessels required by the old plow structures and methods. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0033]    Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings in which: 
           [0034]      FIG. 1  is a top, left, rear perspective view illustrating a chassis adapted for use in any of boulder clearing, trench cutting and backfill modes; 
           [0035]      FIG. 2  is a bottom, right, front perspective view of the chassis of  FIG. 1 ; 
           [0036]      FIG. 3  is a side elevation view of the chassis of  FIG. 1 ; 
           [0037]      FIG. 4  is a top plan view of the chassis of  FIG. 1 ; 
           [0038]      FIG. 5  is a bottom plan view of the chassis of  FIG. 1 ; 
           [0039]      FIG. 6  is a top, left, front perspective view of a transition attachment for use with the chassis of  FIG. 1 ; 
           [0040]      FIG. 7  is a bottom, left, front perspective view of the transition attachment of  FIG. 6 ; 
           [0041]      FIG. 8  is a top plan view of the transition attachment of  FIG. 6 ; 
           [0042]      FIG. 9  is a side elevation view of the transition attachment of  FIG. 6 ; 
           [0043]      FIG. 10  is a top, left, front perspective view illustrating a skid configured for use in the boulder clearing mode and the first pass of a plow in trench cutting plow mode; 
           [0044]      FIG. 11  is a top, left, front perspective view illustrating a crossbeam usable to convert two skids as seen in  FIG. 10  into a skid configured for use in the backfill plow mode; 
           [0045]      FIG. 12  is a front elevation view illustrating a skid configured for use in the second and subsequent passes of a plow in the trench cutting plow mode; 
           [0046]      FIG. 13  is a top, right, rear perspective view illustrating the chassis of  FIG. 1  used in the boulder clearing plow mode; 
           [0047]      FIG. 14  is a bottom, right, front perspective view of the boulder clearing plow of  FIG. 13 ; 
           [0048]      FIG. 15  is a top plan view of the boulder clearing plow of  FIG. 13 ; 
           [0049]      FIG. 16  is a side elevation view of the boulder clearing plow of  FIG. 13 ; 
           [0050]      FIG. 17  is a front elevation view of the boulder clearing plow of  FIG. 13 ; 
           [0051]      FIG. 18  is a top, left rear perspective view of a typical keel plate for use with the boulder clearing plow of  FIG. 13  and the backfill plow of  FIG. 37 ; 
           [0052]      FIG. 19  is a side elevation view of the boulder clearing plow of  FIG. 13  during release from/retrieval to a vessel with the plow skid pivoting on the stern of a vessel; 
           [0053]      FIG. 20  is a side elevation view of the boulder clearing plow of  FIG. 13  during release from/retrieval to a vessel with the plow chassis angled portion pivoting on the stern of a vessel; 
           [0054]      FIG. 21  is a side elevation view of the boulder clearing plow of  FIG. 13  during release from/retrieval to a vessel with the plow transition attachment pivoting on the stern of a vessel; 
           [0055]      FIG. 22  is a side elevation view of the boulder clearing plow of  FIG. 13  during release from/retrieval to a vessel with the plow keel plates pivoting on the stern of a vessel; 
           [0056]      FIG. 23  is a top plan view illustrating the boulder clearing plow of  FIG. 13  positioned to clear a path through a field of boulders; 
           [0057]      FIG. 24  is a graphic representation of a typical boulder clearance route pattern of the boulder clearing plow of  FIG. 13 ; 
           [0058]      FIG. 25  is a side elevation view of the boulder clearing plow of  FIG. 13  in operation; 
           [0059]      FIG. 26  is a top, left, rear perspective view illustrating the chassis of  FIG. 1  used in the trench cutting plow mode; 
           [0060]      FIG. 27  is a top plan view of the trench cutting plow of  FIG. 26 ; 
           [0061]      FIG. 28  is a side elevation view of the trench cutting plow of  FIG. 26 ; 
           [0062]      FIG. 29  is a front elevation view of the trench cutting plow of  FIG. 26 ; 
           [0063]      FIG. 30  is a top, left, front perspective view of a detachable share for use with the trench cutting plow of  FIG. 26 ; 
           [0064]      FIG. 31  is a bottom, right, rear perspective view of the detachable share of  FIG. 30 ; 
           [0065]      FIG. 32  is a vertical, longitudinal, center cross-sectional view of the detachable share of  FIG. 30 ; 
           [0066]      FIG. 33  is a side elevation view of the trench cutting plow of  FIG. 26  during release from/retrieval to a vessel with the skid post passing over the stern of a vessel; 
           [0067]      FIG. 34  is a side elevation view of the trench cutting plow of  FIG. 26  during release from/retrieval to a vessel with the chassis angled portion passing over the stern of a vessel; 
           [0068]      FIG. 35  is a side elevation view of the trench cutting plow of  FIG. 26  during release from/retrieval to a vessel with the chassis transition surface and share attachment plates passing over the stern of a vessel; 
           [0069]      FIG. 36  is a side elevation view of the trench cutting plow of  FIG. 26  during release from/retrieval to a vessel with the moldboards passing over the stern of a vessel; 
           [0070]      FIG. 37  is a top, left, front perspective view illustrating the chassis of  FIG. 1  used in the backfill plow mode; 
           [0071]      FIG. 38  is a top plan view of the backfill plow of  FIG. 37 ; 
           [0072]      FIG. 39  is a side elevation view of the backfill plow of  FIG. 37 ; 
           [0073]      FIG. 40  is a front elevation view of the backfill plow of  FIG. 37 ; 
           [0074]      FIG. 41  is a top, left, rear perspective view illustrating a spoil collecting blade for use with the backfill plow of  FIG. 37 ; 
           [0075]      FIG. 42  is a top, right, rear perspective view illustrating a flapper board for use with the backfill plow of  FIG. 37 ; 
           [0076]      FIG. 43  is a side elevation view of the backfill plow of  FIG. 37  during release from/retrieval to a vessel with the plow skid pivoting on the stern of a vessel; 
           [0077]      FIG. 44  is a side elevation view of the backfill plow of  FIG. 37  during release from/retrieval to a vessel with the chassis angled portion pivoting on the stern of a vessel; 
           [0078]      FIG. 45  is a side elevation view of the backfill plow of  FIG. 37  during release from/retrieval to a vessel with the plow transition attachment pivoting on the stern of a vessel; 
           [0079]      FIG. 46  is a side elevation view of the backfill plow of  FIG. 37  during release from/retrieval to a vessel with the plow keel plates pivoting on the stern of a vessel. 
           [0080]      FIG. 47  is a top plan view of the backfill plow of  FIG. 37  in operation; 
           [0081]      FIG. 48  is a side elevation view of the backfill plow of  FIG. 37  in operation; 
           [0082]      FIG. 49  is a top plan view illustrating the boulder clearing plow of  FIG. 13  positioned to backfill a wide trench on a typical wide trench backfill route pattern; and 
           [0083]      FIG. 50  is a side elevation view illustrating a plow suspended below the stern roller of a vessel. 
       
    
    
       [0084]    While the invention will be described in connection with preferred embodiments thereof, it will be understood that it is not intended to limit the invention to those embodiments or to the details of the construction or arrangement of parts illustrated in the accompanying drawings. 
       DETAILED DESCRIPTION  
     Single Mode And Multi-Mode Chasses 
       [0085]    Turning first to  FIGS. 1-5 , a seabed-plow chassis  10  for use as a component of various seabed plows has an elongated member  11  adapted for mounting a skid on one of its ends  13  and one or more tools on its other end  15 . 
         [0086]    As seen in  FIGS. 13-17 , the chassis  10  is used in a first mode of operation as part of a boulder clearing plow  100 . In the first mode  100 , boulders B on or partially buried in the seabed are initially pushed by the skid  40  outward of the path P traveled by the skid  40 . The tool includes moldboards  90  which push the boulders B initially pushed away by the skid  40  and other boulders B in the path of the moldboards  90  further outward as the skid  40  leads the moldboards  90  along the seabed S. 
         [0087]    As seen in  FIGS. 26-29 , the chassis  10  is used in a second mode of operation as part of a trench cutting plow  200 . In the second mode  200 , the tool includes a plow share  210  and moldboards  90  which sequentially cut and move spoil M to create a trench T as the skid  40  leads the plow share  210  and the moldboards  90  along the seabed S. 
         [0088]    As seen in  FIGS. 37-40 , the chassis  10  is used in a third mode of operation as part of a backfill plow  300 . In the third mode  300 , the tool includes a blade  310  and moldboards  90  which cooperate, as the blade  310  and moldboards  90  lead the skid  40  along the seabed S, to sequentially collect spoil M lying outside of the trench, funnel the collected spoil M inward, and release the funneled spoil M downward into the trench T. 
         [0089]    The chassis  10  is uniquely configured to facilitate over-the-stern launch and retrieval of a plow  100 ,  200  or  300  from and to, respectively, the deck D of a vessel V and to and from, respectively, the seabed S. The movement of the plow  100 ,  200  or  300  from or to a resting place on the deck D of the vessel V to or from a point at which all contact of the plow  100 ,  200  or  300  with the vessel V is terminated is herein referred to as “transition.” Looking at  FIGS. 15 ,  27  and  38 , the plows  100 ,  200  or  300  described herein have longitudinal axes  101 ,  201  and  301 , respectively. As shown, the longitudinal axes  101 ,  201  and  301  are aligned in parallel with their anticipated directions of movement on the seabed S. Looking at  FIGS. 19-22 ,  33 - 36  and  43 - 46 , the plow axes  103 ,  203  and  303  are aligned in the direction of “transition” of the plows  100 ,  200  and  300 , respectively, on the deck D. As shown, the longitudinal axes  101 ,  201 , and  103  of  FIGS. 15 ,  27  and  38  are aligned with the transition axes  103 ,  203 , and  303  of  FIGS. 19-22 ,  33 - 36  and  42 - 46 , respectively. The plows  100 ,  200  and  300  need not, however, be aligned on the deck D in the same orientation they assume in operation on the seabed S. Therefore, as used herein, a “transition” axis” is any axis, longitudinal or not, which extends through a plow  100 ,  200  or  300  in a direction parallel to the anticipated direction of movement  39  of the plow during launch or retrieval. 
         [0090]    It is preferred that the plows  100 ,  200  or  300  will have their weight distribution and the location of their surfaces which contact the deck D and the fulcrum/roller R on the stern of the vessel V during release or retrieval so coordinated as to resist roll of the plows  100 ,  200  or  300  about their respective transition axes  103 ,  203  and  303 , respectively. As shown and described, the chassis  10 , skid  40  and skid posts  45 , transition attachment  70 , moldboards  90  and keel plates  110  and  370  have various surfaces contoured to support their plows in sliding contact with the deck D and to pivot about the fulcrum/roller R on the stern of the vessel V as the plow  100 ,  200  or  300  crosses the fulcrum/roller R during release/retrieval of the plow. Other components can be used or specially added for the purpose. 
       Chassis Structure 
       [0091]    Returning to  FIGS. 1-5 , a preferred embodiment of the chassis  10  can be used in any of the plow modes  100 ,  200  and  300  seen in  FIGS. 13 ,  26  and  37 , respectively. As best seen in  FIGS. 1 and 2 , in the preferred embodiment of the chassis  10 , the skid and tool ends  13  and  15  of the elongated member  19  are substantially horizontal and joined by a midsection  27  which angles down from the skid end  13  to the tool end  15 . A post receptacle  19  extends vertically through the skid end  13 . Fork lift receptacles  21  extend widthwise across the top of the tool end  15  of the elongated member  11 . One receptacle  21  is at the junction of the tool end  15  with the angled portion  17  of the elongated member  11 . The other receptacle  21  is further to the rear of the elongated member  11  and immediately in front of a spaced pair of share connection plates  23  which extend above the elongated member  11 . 
         [0092]    A transition member  25  extends above the tool end  15  of the elongated member  11  between the fork lift receptacles  21 . As best seen in  FIG. 3 , the top surfaces of the receptacles  21 , the share connection plates  23  and the transition member  25  form a substantially continuous transition surface  27  useful for launch and recovery purposes as hereinafter explained in relation to the second/trench cutting mode  200 . 
         [0093]    Side extension plates  29  taper downwardly from the tool end  15  and a back flange plate  31  caps the tool end  15  of the elongated member  11 . A share connection slot  33  extends through the bottom of the tool end  15  of the elongated member  11  between the share connection plates  23 . 
         [0094]    Looking at  FIGS. 13-17 ,  26 - 29  and  37 - 40 , each of the plows  100 ,  200  and  300  has pulling points  65 , as shown on tow bars  67  extending laterally from the skid end  13  of the elongated member  11 , for connection of a pulling line L to the plow  100 ,  200  and  300 . Preferably, the pulling points  65  are symmetrically arranged in relation to the central longitudinal axes  101 ,  201  and  301  of the plows  100 ,  200  and  300  and are displaced from the points of contact of the plows  100 ,  200  and  300  with the deck D or roller R by a height less than a radius of the roller R to facilitate passage of the points of contact across the roller R. 
       Transition Attachment 
       [0095]    Turning to  FIGS. 6-9 , a transition attachment  70  is configured to extend between the skid and tool ends  13  and  15  on the bottom of the elongated member  11  of the chassis  10  when the chassis is used in either of its first/boulder clearing or third/trench cutting modes  100  or  300  as seen in  FIGS. 13 ,  26  and  37 . 
         [0096]    As shown in  FIGS. 6-9 , the transition attachment  70  extends in a generally horizontal wishbone shape with its tines  71  opening from its front to its rear ends  73  and  75 . The top surface  77  of the transition attachment  70  is contoured to mate against the bottom surface  31  of the elongated member  11  of the chassis  10  against which the transition attachment  70  will be secured by pinning the transition member  25  of the chassis  10  between the transition clevis plates  83 , as is best seen in  FIGS. 16 and 39 . The bottom surface  79  of the transition attachment  70  is contoured to make contact with and pivot about fulcrum R on the stern of a plow transporting/towing vessel V, seen in  FIGS. 21 and 22  and  44  and  45 , as the plow  100  or  300  crosses the fulcrum/roller R during its release from the vessel into the sea and during its retrieval from the sea onto the vessel. 
         [0097]    The shape of the attachment bottom surface  79  and the weight of the elongated member  11  and attachment  70  are coordinated so as to resist roll of the chassis  11  about the plow transition axis  103  or  303  as the plow  100  or  300  moves on the deck D toward or away from the fulcrum R. 
         [0098]    Preferably, the fulcrum R is a roller and, as best seen in  FIG. 9 , the vertical longitudinal cross-sections of the attachment bottom surface  79  are concave. Looking at  FIGS. 16 ,  39  and  50 , the radius of the concavity  79  is greater than the radius of the fulcrum R so as to facilitate passage of the transition attachment  70  across the fulcrum R during release and retrieval of the plows  100  and  300 . Looking at  FIG. 8 , the concavity  79  is symmetric about a longitudinal vertical plane centered on the attachment  70 . The surface  79  can have any shape as long as it provides paths which facilitate the over-the-stern release and retrieval of the plow  100  or  300 . The paths may be linear or planar and are preferably symmetrically defined by continuous opposite points of the attachment bottom surface  79 . 
         [0099]    As shown, the front end  73  of the attachment  70  has a leading face  81  which is angled to smooth the transition to and from the skid end  13  of the elongated member  11  of the chassis  10 . Back plates  85  are provided on the ends of the tines  71  for connection to the moldboards  90 . The gap  87  between the tines  71  functions as a passageway for debris in the third/backfill mode  300 , as is hereinafter explained. 
       Skid 
       [0100]    Turning to  FIGS. 10 and 11 , a preferred embodiment of the skid  40  is adaptable for use in any of the plow modes  100 ,  200  and  300  seen in  FIGS. 13 ,  26  and  37 , respectively. 
         [0101]    In  FIG. 10 , the skid  40  is shown configured for use in the first/boulder clearing and second/trench cutting modes  100  and  200 , seen in  FIGS. 13 and 26 , respectively. When used in the first/boulder clearing or second/trench cutting mode  100  or  200 , the parallel outer skis  41  of the skid  40  are in close proximity to each other, bolted on opposite sides of a center ski  43 . In this bolted configuration, a head  51  can be mounted on the front of the skis  41  and  43  in either the first/boulder clearing mode or for a first pass, the second/trench cutting mode  100  or  200 . Alternatively, as shown in  FIG. 12 , the outer skis  41  can be pivotally mounted on the center ski  43  using linkages  48  so that the outer skis  41  can be canted laterally upward from the center ski  43 , provided the head  51  is not attached to the skis  41  and  43 . The use of canted outer skis  41  is specially applicable to second and subsequent passes in this second/trench cutting mode  200 , enabling the canted skis  41  to conform to the side walls of the trench T and facilitating the deepening of the trench T by a second and subsequent passes of the trench cutting plow  200 . Thus, deeper trenches can be cut without need for a larger trenching plow. 
         [0102]    In of the above bolted or pivotal configurations for the outer skis  41 , whether or not the head  51  is used, a post  45 , which is pinned in a receptacle  47  in the center ski  43 , extends upward to a top  49  which is convex from front to back. As shown, the outer skis  41  have receptacles  47  which are the same as the center ski receptacles  47 . When used in the first/boulder clearing mode  100 , the boulder clearing head  51  is preferably added to the leading end of the skid  40  across the fronts of the skis  41  and  43 . As shown, the leading faces  53  of the head  51  are angled rearward from a vertical, longitudinal center plane of the skid  40  and are tapered rearward from their top edges  55 . The angled and tapered faces  53  will torque partially buried boulders out of the seabed and away from the skid  40  and, if necessary, allow the plow  200  to ride over a boulder B which strikes the head  51  below its top edges  55 . 
         [0103]    When used in the second/trench cutting mode  200 , either the bolted configuration of the skis  41  or the pivotal configuration of the skis  41  in an uncanted condition can be used, preferably with the head  51  in place for the first pass of the plow  200 . For subsequent passes, it is preferred that the pivotal configuration of the skis  41  be used in the canted condition without the head  51 . In the second/trench cutting mode  200 , trenches up to  25  meters wide can be cut using multiple passes. 
         [0104]    In  FIG. 11 , a crossbeam  57  is shown for converting the outer skis  41  of the skid  40  shown in  FIG. 10  for use in the third/backfill mode  300 . In the backfill mode  300 , a crossbeam  57  spaces a pair of open ended receptacles  63  apart from a center post  59  extending upward from the midpoint of the crossbeam  57 . As seen in  FIG. 37 , two posts  45  are seated, one in each of the receptacles  47  of the two outer skis  41  as shown in  FIG. 10 . The posts  45  extend up from their respective outer skis  41 , pass through their respective open ended receptacles  63  in the crossbeam  57  and are pinned with the skis  41  at the desired distance below the crossbeam  57 . The crossbeam center post  59  is pinned in the chassis post receptacle  19  to set the desired height of the chassis  10  above the skis  41 . The crossbeam  57  as shown has a front surface  61  configured to also serve as a spoil leveler in the backfill mode  300 . 
       Boulder Clearing Plow And Methods 
       [0105]    Looking at  FIGS. 13-17 , the boulder clearing plow  100  includes the chassis  10 , the skid  40 , the transition attachment  70  and the moldboards  90 . The skid  40 , in the configuration shown in  FIG. 10  with the head  51  is mounted on and supports the skid end  13  of the chassis  10  above the seabed S. The moldboards  90  include primary, secondary and tertiary moldboards  91 ,  93  and  95  mounted on the tool end  15  of the chassis  10 . The transition attachment  70  is mounted under the chassis  10  between the skid  40  and the primary moldboards  91 . 
         [0106]    As best seen in  FIG. 13 , in the first/boulder clearing mode  100  the chassis  10  is oriented upside down in comparison to its orientation as shown in  FIGS. 1 and 2 . That is, in the boulder clearing plow  100 , the skid end  13  is lower than the tool end  15  of the elongated member  11  and the skid post  45  extends upward through the receptacle  19  in the skid end  13  of the chassis  10 . 
         [0107]    As seen in  FIGS. 13-17 , the primary moldboards  91 , which may be permanently or detachably mounted on the tool end  15  of the chassis  10 , are angled outward and rearward from the tool end  15  of the chassis  10  and the transition attachment  70 . The secondary moldboards  93  are mounted below the primary moldboards  91  and the transition attachment  70  to increase the overall depth of the moldboards  91 . The tertiary moldboards  95  are used when wider paths are to be cleared of boulders B. They are mounted at the free ends of the primary and secondary moldboards  91  and  93  and increase the length of the moldboards  90  for the full depth of the combined primary and secondary moldboards  91  and  93 . 
         [0108]    As best seen in  FIGS. 13-15 , when the tertiary moldboards  95  are used, a chassis extension  33  is connected by its front flange  35  to the back flange plate  31  of the chassis  10 . A supporting structure  37  of beams and struts connects the chassis extension  33  to the tertiary moldboards  95 . Recovery fins  97  are appended to the free ends of the moldboards  90 . The fins  97  have divergingly arcuate ends  99  for contact with the roller R during launch and recovery. 
         [0109]    The boulder clearing plow  100  may also include keel plates  110 , shown in detail in  FIG. 18 . The keel plates  110  have a vertical center plate  111  and horizontal base plates  113  which extend laterally from the center plate  111 . The base plates  113  and the center plates  111  support a vertical mounting plate  115  at an angle complementary to the angle of the moldboards  90 . This structure is reinforced by small and large vertical support plates  117  and  119 . At least one keel plate  110  mounted on the front of and extending under each set of moldboards  90 . As best seen in  FIGS. 14 ,  15 ,  17  and  18 , the keel plates  110  are mounted in parallel at the junctions of the secondary and tertiary moldboards  93  and  95 . The primary function of the keel plates  110  is to steady the path of the boulder clearing plow  100  as the head  51  and moldboards  90  encounter boulders B, spoil M and/or other obstacles on the seabed S. 
         [0110]    Turning to  FIGS. 19-22  and  50 , the over-the stern release ( FIGS. 19-22 ) and retrieval ( FIGS. 22-19 ) of the boulder clearing plow  100  from a vessel V to the seabed S or from the seabed S onto a vessel V, respectively, are illustrated. During release, the plow  100  is preferably and as shown initially positioned on the deck D with its moldboards  90  aft and the longitudinal axis  101  of the plow  100  aligned on the transition axis  103  of the plow  100 . The skid  40  and keel plates  110  provide the initial contact points or surfaces of the plow  100  with the deck D. As seen in  FIG. 19 , as the plow  100  is propelled, by winch or other suitable push/pull equipment (not shown), along the deck D of the vessel V toward and across the fulcrum/roller R at the stern of the vessel V, when the keel plates  110  have cleared the fulcrum/roller R, the plow  100  drops onto the moldboard  90  and slides on the moldboards to the concave surface of the transition attachment  70 , at which point the moldboards slide to the transition attachment  70  and the the skid  40  begins to rise from the deck D. All contact between the plow  100  and the vessel V has transferred to the transition attachment  70  and the fulcrum/roller R of the vessel V. Looking at  FIG. 20 , as the transition attachment  70  travels travel sternward beyond the fulcrum/roller R, all contact between the plow  100  and the vessel V remains on the concave transition surface  79  of the attachment  70  and the fulcrum/roller R of the vessel V and the plow  100  continues to tip toward the sea and the skid  40  continues to rise. As seen in  FIG. 21 , as the attachment  70  travels further sternward on the fulcrum/roller R, all contact between the plow  100  and the vessel V is still between the transition attachment  70  and the fulcrum/roller R but the skid  40  is nearly vertical. Looking at  FIG. 22 , as the plow  100  continues to rotate on and travel across the fulcrum/roller R, the buoyancy of the sea water and the travel speed of the vessel V limit rotation of the plow  100 . As the transition attachment  70  slides off the fulcrum/roller R, the skid  40  becomes the final contact with the fulcrum/roller R until the plow  100  is fully released at the end of a pulling line L to the seabed S. 
         [0111]    Retrieval of the boulder clearing plow  100  at the end of the pulling line L from the seabed S is accomplished by reversal of the release method. As seen in  FIG. 22 , as the plow  100  is raised at the end of the line L toward the fulcrum/roller R on the stern of the vessel V, the skid  40  will first contact the fulcrum/roller R. As noted above, the pulling points  65  of the plow  110  are located so as to assure that the head  51  and skis  41  and  43  of the sled  40  will not hang-up on the fulcrum/roller R. Further contact with the fulcrum/roller R transfers sequentially to the transition surface  79  of the transition attachment  70  as seen in  FIG. 21 , the concave portion of the attachment transition surface  79  as seen in  FIG. 20  and then along the bottom of the moldboards  90 , as seen in  FIG. 19 , until they contact the keel plates  110  and the plow  100  has been pulled fully across the fulcrum/roller R and comes to rest with the skid  40  and keel plates  110  as the contact points on the deck D of the vessel V. 
         [0112]    Turning to  FIG. 23-25 , the use of the boulder clearing plow  100  to clear boulders B from a path P on the seabed is illustrated. As seen in  FIG. 24 , the plow  100  is positioned with the plow  100  bow-forward in the direction of an initial seabed path P 1  which will be at the center of the intended final path P. The pattern of the final path P spirals out from the initial path P 1 . The plow  100  is then propelled, perhaps at the end of the pulling line L, powered by a winch or by the travel of the vessel, along the initial path P 1  to clear boulders B from the initial path P 1  to the port and starboard sides of the plow  100 . After the initial path P 1  is cleared, boulder clearing continues, if a wider path P is necessary, by repositioning the plow  100  bow-forward, for travel in a direction opposite the initial seabed path direction, on a second seabed path P 2  along, as shown, the starboard side of the initial path P 1 . The plow  100  is then propelled along the second path P 2  to clear boulders in the second path P 2  further away from the path P 1 . After the second path P 2  is cleared, boulder clearing continues, if a wider path P is necessary, by repositioning the plow  100  bow-forward, for travel in the direction of the initial seabed path direction, on a third seabed path P 3  along the port side of the initial path P 1 . The plow  100  is then propelled along the third path P 3  to clear boulders from the third path P 3  further away from the first path P 1 . If a still wider path P is necessary, boulder clearing can further include repeating the widening along the path P resulting from the contiguity of the initial, second and third paths P 1 , P 2  and P 3 , as shown along paths P 4  and P 5 . The boulder clearing process anticipates repetition of the widening steps to widen successively contiguous paths P n  until a single path P of desired width has been cleared along the seabed. 
         [0113]    Looking at  FIG. 25 , as the head  51  strikes one or more boulders B in its initial path P 1 , the boulders B will be torqued out of the seabed and around the port or starboard side of the head  51 , depending on which side of the head  51  strikes the boulders B. The trailing moldboards  90  will torque and push the boulders B further to port or starboard away from the plow  100 . On ensuing paths P 2-n , only the outside of the head  51  and the outside moldboards  90  are on a path to strike the boulders B, pushing them further away from the initial path P 1 . As seen in  FIG. 22 , boulders B which have been pushed aside will be deposited in a small spoil heap H created aft of the plow  100  by the partial penetration of the seabed by the moldboards  90 . 
       Trench Cutting Plow And Methods 
       [0114]    Looking at  FIGS. 26-29 , the trench cutting plow  200  includes the chassis  10 , the skid  40 , the moldboards  90  and the share  210 . The skid  40 , in the configuration shown in  FIG. 10 , is mounted on and supports the skid end  13  of the chassis  10  above the seabed. The moldboards  90  initially include only the primary moldboards  91  mounted on the tool end  15  of the chassis  10 . If more than one pass of a trench cutting plow  200  is to be performed, the secondary and tertiary moldboards  93  and  95  can be added. Wedges (not shown) can be positioned between the chassis  10  and the moldboards  90  to angle the moldboards at a desired angle upward and rearward from the chassis  10  for second and subsequent passes of the plow. The transition attachment  70  is not used. As shown, the head  51  may optionally be attached to the skid  40  in the first pass of the second/trench cutting mode  200 . 
         [0115]    As best seen in  FIG. 20 , in the second/trench cutting mode  200  the chassis  10  is oriented right-side up as shown in  FIGS. 1 and 2 . That is, in the trench cutting plow  200 , the skid end  13  is higher than the tool end  15  of the elongated member  11  and the skid post  45  extends upward through the receptacle  19  in the skid end  13  of the chassis  10 . 
         [0116]    The plow share  210  may be permanently or detachably mounted on the chassis  10 . A preferred embodiment of the share  210  shown in  FIGS. 26-32  includes a shoe box  211  joining the bottoms of center ribs  213  and side plates  215  which support the parting plates  217  of the share  210 . A vertical plate  219  aligned with the shoe box  211  extends upwardly above the parting plates  217  and is inserted between the share connection plates  23  on the chassis  10 . A pin  221  inserted through a boss  223  on the vertical plate  219  and the connection plates  23  secures the share  210  to the chassis  10 . 
         [0117]    Turning to  FIGS. 33-36 , the over-the stern release ( FIGS. 36-33 ) and retrieval ( FIGS. 33-36 ) of the trench cutting plow  200  from the vessel V to the seabed S and from the seabed S onto the vessel V, respectively, are illustrated. During release, the plow  200  herein described is initially positioned upside down on the deck D with moldboards  90  aft and the longitudinal axis  201  of the plow  200  aligned on the plow&#39;s transition axis  203 . The arcuate top  49  of the skid post  45  and the free ends of the moldboards  90  provide the initial contact points or surfaces with the deck D. As seen in  FIG. 36 , as the plow  200  is propelled, by winch or other suitable push/pull equipment (not shown), along the deck D of the vessel V toward and across the fulcrum/roller R at the stern of the vessel V, only the moldboards  90  and the arcuate top  49  of the post  45  will remain in contact with the fulcrum/roller R until the share connection plates  23  reach the fulcrum/roller R. As seen in  FIG. 35 , as the plow  200  continues sternward travel, only the tops of the share connection plates  23  followed by the trailing top or transition surface  27  of the transition member  25  and the arcuate top  49  of the post  45  will remain in contact with the fulcrum/roller R. As seen in  FIG. 34 , as the center of gravity of the plow  200  has passed the fulcrum/roller R, the cantilevered weight of the plow  200  causes the plow  200  to pivot on the transition surface  27  of the transition member  25 , allowing the moldboards  90  to drop toward the seabed S and the skid post  45  to rise from the deck D. At this point in transition, all contact between the plow  200  and the vessel V transfers to the angled portion  17  of the chassis elongated member  11  and the fulcrum/roller R of the vessel V. Looking at  FIG. 33 , after the angled portion  17  of the chassis elongated member  11  has travelled sternward beyond the fulcrum/roller R, the plow  200  will have rotated further toward the seabed S and all further contact between the plow  200  and the vessel V will have transferred to the arcuate top  49  of the skid post  45  and the fulcrum/roller R of the vessel V. The arcuate top  49  of the skid post  45  provides the final contact with the fulcrum/roller R as the plow  200  is fully released at the end of a pulling line L to the seabed S. 
         [0118]    Retrieval of the trench cutting plow  200  at the end of the pulling line L from the seabed S is accomplished by reversal of the release method. As seen in  FIG. 33 , as the plow  200  is raised at the end of the line L toward the fulcrum/roller R on the stern of the vessel V, the arcuate top  49  of the skid post  45  will first contact the fulcrum/roller R. As noted above, the pulling points  65  of the plow  110  are located so as to assure that the post  45  will not hang up on the fulcrum/roller R. Further contact with the fulcrum/roller R transfers sequentially to the angled portion  17  of the chassis elongated member  11  as seen in  FIG. 34 , to the transition surface  27  as seen in  FIG. 35 , and to the tops of the share connection plates  23  as seen in  FIG. 36 . When the plow  200  has been pulled fully across the fulcrum/roller R, it will come to rest with the arcuate top  49  of the skid post  45  and the tops of the free ends of the moldboards  90  as the contact points on the deck D of the vessel V. 
       Backfill Plow And Methods 
       [0119]    Turning to  FIGS. 37-40 , for backfilling spoil into a seabed trench, the backfill plow  300  includes the chassis  10 , the skid  40  configured to straddle the trench being backfilled, the moldboards  90  mounted on the chassis  10  forward of the skid  40  and the blade  310  mounted on and spanning the bottom edges of the moldboards  90 . 
         [0120]    As best seen in  FIG. 37 , in the third/backfill mode  300 , the chassis  10  is oriented upside down in comparison to its orientation as shown in  FIGS. 1 and 2 . That is, in the backfill plow  300 , the skid end  13  is lower than the tool end  15  of the elongated member  11 , as in the first/boulder clearing mode  100  shown in  FIG. 13 , and the crossbeam center post  59  extends upward through the receptacle  19  in the skid end  13  of the chassis  10 , similar to the post  45  in the first/boulder clearing mode  100  shown in  FIG. 13 . However, the chassis  10  is oriented in reverse in comparison to the first/boulder clearing mode  100  shown in  FIG. 13 , so that the skid  40  is at the trailing end of the backfill plow  300 . In comparison to the first/boulder clearing mode  100 , the skis  41  are also reversed in the third/boulder clearing mode  300  for forward travel in a trailing position. 
         [0121]    As seen in  FIGS. 37-40 , in the third/backfill mode, the moldboards  90 , including the primary moldboards  91 , the secondary moldboards  93  and the tertiary moldboards  95 , are mounted on the chassis  10  in the same way as described in relation to the first/boulder clearing mode  100  of  FIGS. 13-17  by use of the chassis extension  33  and supporting structure  37 . The transition attachment  70  is also mounted to the chassis  10  in the same manner as described in relation to the first/boulder clearing mode  100  of  FIGS. 13-17 . The recovery fins  97  are appended to the free ends of the tertiary moldboards  95  as described in relation to the first/boulder clearing mode  100  of  FIGS. 13-17 . 
         [0122]    Looking at  FIG. 41 , the blade  310  has a passage  311  at its rear apex  313 . The passage  311  is configured to dispense the spoil collected by the blade  310  and funneled by the moldboards  90  onto the top of a pipe or cable C disposed in the trench T below the passage  311 . The side edges of the blade  310  are secured to the lower portions of their respective moldboards  90  by use of side plates  315  and to the chassis extension  33  by use of an upright mounting structure  317 . The mounting structure  317  is centered on the leading edge  319  of the blade  310  and, as shown, extends from the blade edge  319  to the passage  311 . The blade  310  may be stiffened by ribs  321 . As shown, the passage  311  is slightly greater than semi-circular with a diameter  323  parallel to the blade leading edge  319 . The stiffening ribs  321  fan out from points along the passage circumference  325  to respective points along the blade leading edge  319 . 
         [0123]    Looking at  FIG. 42 , the backfill plow  300  preferably further includes a flapper board  340  aft of the passage  311 . The flapper board  340  includes a plate  341  fixed to and swinging below a horizontal shaft  343 . The shaft  343  is journalled to reciprocate on an axis parallel to the passage diameter  323 . A weight  345  biases the plate  341  toward a vertical orientation. The slapping action of the flapper board  340  fragments spoil discharged through the blade passage  311 . Large and small stiffeners  347  and  349  reinforce the plate  341 . The reciprocating swing of the plate  341  on its shaft  343  is caused as water and spoil discharging through the passage  311  swings the plate  341  toward the rear and the weight  345  causes the plate  341  to swing back toward vertical. 
         [0124]    The backfill plow  300  may also include keel plates  370 , at least one keel plate  370  extending on opposite sides of the spoil passage  311 . The keel plates  110 , shown in  FIG. 18  for use in the first/boulder clearing mode  100 , can be used in the third/backfill mode  300  except that, in the backfill mode  300 , they are mounted on the front of the moldboards  90  and extend under the blade  310 . As seen in  FIGS. 38-40 , the keel plates  370  are mounted in parallel at the junctions of the secondary and tertiary moldboards  93  and  95 . The primary function of the keel plates  370  is to steady the path of the backfill plow  300  as the blade  310  and moldboards  90  encounter and collect spoil M on the seabed S. 
         [0125]    Turning to  FIGS. 43-46 , the over-the stern release ( FIGS. 46-43 ) and retrieval ( FIGS. 43-46 ) of the backfill plow  300  from a vessel V to the seabed S or from the seabed S onto a vessel V, respectively, are illustrated. During release, the plow  300  described herein is initially positioned on the deck D with moldboards  90  aft and the longitudinal axis  301  of the plow  300  aligned on the transition axis  303  of the plow  300 . The skid  40  and bottoms of the keel plates  370  provide the initial contact points with the deck D. As seen in  FIG. 46 , as the plow  300  is propelled, by winch or other suitable push/pull equipment (not shown), along the deck D of the vessel V toward and across the fulcrum/roller R at the stern of the vessel V, the keel plates  370  clear the fulcrum/roller R, allowing the moldboards  90  to drop toward the seabed S. The plow begins to pivot on the transition surface  79  and the skid  40  begins to rise from the deck D. At this point in transition, all contact between the plow  300  and the vessel V has transferred to the transition attachment  70  and the fulcrum/roller R of the vessel V. Looking at  FIG. 45 , as the plow  300  travels further sternward across the fulcrum/roller R, all contact between the plow  300  and the vessel V has transferred to the concave portion of the transition surface  79  of the attachment  70  and the fulcrum/roller R of the vessel V. As seen in  FIG. 44 , as the attachment  70  has travelled sternward beyond the fulcrum/roller R, all contact between the plow  300  and the vessel V remains on the angled portion  17  of the chassis elongated member  11  and the fulcrum/roller R of the vessel V. The plow  300  has tipped so that the skid  40  nears vertical. Looking at  FIG. 43 , as the plow  300  continues to rotate on and travel across the fulcrum/roller R, the buoyancy of the sea water and the travel speed of the vessel V limit rotation of the plow  300  as the transition attachment  70  slides off the fulcrum/roller R. The shape of the attachment  70  affords a smooth transition from the transition surface  79  to the skid  40 . The skid  40  becomes the final contact with the fulcrum/roller R until the plow  100  is fully released at the end of a pulling line L to the seabed S. 
         [0126]    Retrieval of the backfill plow  300  at the end of the pulling line L from the seabed S is accomplished by reversal of the release method. As seen in  FIG. 43 , as the plow  300  is raised at the end of the line L toward the fulcrum/roller R on the stern of the vessel V, the skid  40  will first contact the fulcrum/roller R. As noted above, the pulling points  65  of the plow  300  are located so as to assure that the head  51  and skis  41  and  43  of the sled  40  will not hang up on the fulcrum/roller R. Further contact with the fulcrum/roller R transfers sequentially to the angled portion  17  of the chassis elongated member  11  as seen in  FIG. 44 , to the attachment transition surface  79  as seen in  FIG. 45  and to the bottom of the keel plates  370  as seen in  FIG. 46 . When the plow  300  has been pulled fully across the fulcrum/roller R, it will come to rest with the skid  40  and keel plates  370  as the contact points on the deck D of the vessel V. 
         [0127]    Turning to  FIGS. 47 and 48 , in backfilling spoil M to cover a pipe P laid in a seabed trench T, the backfill plow  300 , with the blade  310  forward, is propelled to travel on the seabed S and collect spoil M along the sides of the trench T. The moldboards  90  funnel the collected spoil M toward the rear apex  313  of the blade  310  and the funneled spoil M is discharged through the passage  311  in the blade apex  313  and onto the top surface of the pipe P disposed in the trench T. Preferably, the discharged spoil M is fragmented, as shown by the flapper board  340 , before the discharged spoil M reaches the pipe P and the spoil M discharged onto the pipe P and into the trench T is leveled by the front surface  61  of the skid crossbar  57 . The use of the passage  311  to discharge the spoil M directly onto, rather than to the sides of, the pipe P reduces the likelihood that the more dense spoil M will lift the pipe P in the trench T during backfilling. 
         [0128]    Turning to  FIG. 49 , for trenches wider than the widest span of the moldboards  90 , the boulder clearing plow  100  seen in  FIG. 13  can be used to narrow the width of the trench T. This is accomplished by aligning the longitudinal axis  101  of the plow  100  outside of the spoil M one side of the trench T, as shown with only the starboard moldboards  90  pushing the spoil M. On the first pass P a  the spoil M in the path of the starboard moldboards will be pushed toward or into the trench T. When the first pass P a  is completed, the plow  100  is aligned on the other side of the trench T, as shown again with only the starboard moldboards  90  pushing the spoil M. On the second pass P b , the spoil M in the path of the starboard moldboards will be pushed toward or into the trench T. When the second pass P b  is completed, the process can be repeated for passes P n  until the trench T is filled or until the trench T is sufficiently filled to complete backfilling using the backfill plow  300 . 
         [0129]    Turning to  FIG. 50 , the plows  100 ,  200  or  300  can be retrieved by use of the tow line L connected to the retrieval fins  97 . Depending on which of the plows  100 ,  200  or  300  is retrieved, the orientation of the plow  100 ,  200  or  300  can be rotated 180° about the axis of the tow line L to a suitable retrieval position. In either 180° orientation, the arcuate ends  99  of the fins  97  will enable the plow  100 ,  200  or  300  to pass over the fulcrum/roller R. 
         [0130]    The plows  100 ,  200  and  300  are made using steel plates which are welded, bolted or pinned, depending on the intended permanency or detachability of the components being connected. The same chassis  10 , skids  40 , transition attachment  70 , moldboards  90 , and keel plates  110  can be configured into three different modes of operation and the share  210  and blade  310  added as needed for their respective modes. The need for a large vessel, cranes and supporting equipment is eliminated since the plow modes  100 ,  200  and  300  are all capable of over-the-stern launch and recovery. 
         [0131]    While the convention apparatus and methods have been described in relation to laying pipe, it is also applicable to laying cable. Furthermore, all of the plows  100 ,  200  and  300  can be adapted for use with a remotely operated vehicle (ROV) which can “dock” on the plow to provide additional mechanical functionality such as skid height adjustment or electrical functionality such as cameras, lights, and load measurement. 
         [0132]    Thus, it is apparent that there has been provided, in accordance with the invention, a multi-mode seabed plow and plow release, operation and retrieval methods that fully satisfy the objects, aims and advantages set forth above. While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications and variations as fall within the spirit of the appended claims.