Abstract:
A seabed trenching plow has a chassis, a sled and a towing assembly. The towing assembly has a pair of wings extending laterally from each side of the chassis. The wings are aligned on an axis transverse to the chassis and adapted for connection to a towing line. The transverse axis is forward of the center of gravity of the plow and rearward of the sled, affording an over the stern releasable and retrievable trenching plow of sufficient weight and strength to excavate a three meter trench in a single pass. To assess the protective capabilities of the trench, a threshold signal indicative of a desired composition of seabed-trench soil is compared with a real-time data signal indicative of the actual to produce an alarm signal when the real-time data signal is not protective-capability compliant with the threshold signal.

Description:
BACKGROUND 
       [0001]    This invention relates generally to seabed plows and more particularly concerns a deep digging over-the-stern trenching plow with instrumentation for assessing the protective capabilities of a seabed trench. 
         [0002]    The present practices and equipment, typically requiring cranes and associated heavy equipment and structures, used to release and retrieve a plow from a vessel into the sea and from the sea onto the vessel typically limit the weight of the plow to a maximum of approximately 20 tons. The trenching depth and strength of known plows are compromised accordingly. 
         [0003]    The depth achievable in the first trenching pass of these known 20 ton trenching plows is at best 1.4 meters. Deeper trenches can be dug by multiple passes, but the deeper the trench and the greater the number of passes, the greater the forces applied to the limited strength plow. Therefore, even when multiple passes of known trenching plows are run, a trench depth of approximately 2.7 meters is the most that can be expected. But, in many applications, trenches three meters deep may be insufficient to protect their buried contents. Consider, for example, the impact forces that might be applied to a pipeline buried in a trench located in an iceberg zone. 
         [0004]    On the other hand, there is a plow weighing 200 tons that requires use of an A-frame or crane for launch and retrieval and can achieve a first pass depth of 2.0 meters and a maximum total depth of 2.7 meters. The maximum depth of 2.7 meters is dictated because the configuration required of the plow for launch and retrieval by A-frame or crane does not afford a plow of sufficient strength to withstand the forces that will be incurred in excavating a trench greater than 3.0 meters in depth, regardless of the number of passes used for the purpose. 
         [0005]    Assuming that a suitable seabed trench can be excavated, the capability of the trench to protect pipelines, cables and other objects laid or buried in a seabed trench is a foremost concern. For example, the likelihood that damage may be caused by icebergs and other undersea objects drifting or otherwise moving in the vicinity of the trench is a function of the composition of the soil in which the object is laid or buried and the depth at which the object is laid or buried in the soil. 
         [0006]    Plow tip sensors are presently used to measure the shearing force applied by the tip of the plow to the seabed. Load cells are also presently used to measure the total tow force applied to the trenching plow. It is presently understood that the difference between the measured shearing and total tow forces will be generally indicative of the non-tip forces applied to the plow. Such information is useful to understanding the orientation of and the forces applied to the plow during the trenching process but does not afford an assessment of the protective capabilities of a trench. 
         [0007]    The assessment is complicated because the composition of the soil may change considerably along the path of the trench and the depth of the trench along its path may vary somewhat from the depth expected from a given design and adjustable configuration of the trenching plow. 
         [0008]    It is, therefore, an object of this invention to provide a trenching plow capable of digging trenches deeper than can be dug by known trenching plows. It is also an object of this invention to provide a method for over-the-stern release and retrieval of a deep digging trenching plow from a vessel into the sea and from the sea onto the vessel. It is another object of this invention to provide a method and instrumentation for assessing, on a real-time basis, the ability of a trench to protect objects laid or buried in the trench from damage by the impact of external objects. 
       SUMMARY OF THE INVENTION 
       [0009]    In accordance with the invention a seabed trenching plow has a chassis, a sled connected to a forward end of the chassis by uprights and a towing assembly. The towing assembly has a pair of wings extending laterally from each side of the chassis. The wings are aligned on an axis transverse to the chassis and adapted for connection to a towing line. The transverse axis is forward of the center of gravity of the plow and rearward of the connection point of the sled uprights to the chassis. 
         [0010]    The method of releasing the seabed plow from a deck of a vessel having a stern roller includes the steps of connecting the plow to a towing line at a point forward of a center of gravity of the plow and rearward of the sled uprights, causing the plow to traverse along the deck and over the stern roller and allowing the plow to rotate by gravitation about the stern roller until the plow is suspended by the towing line from the vessel aft of the stern roller. 
         [0011]    The method of retrieving the seabed plow from the deck of the vessel includes the steps of raising the plow from the seabed to the stern roller at the end of a towing line connected to the plow at a point forward of a center of gravity of the plow and rearward of the sled uprights and pulling the chassis to traverse against and rotate about the stern roller until the plow is resting on the deck of the vessel. 
         [0012]    Also in accordance with the invention, a method for assessing the protective capabilities of a seabed trench includes the steps of generating a threshold signal indicative of a desired composition of seabed-trench soil for a specific application, pulling a trenching plow having a plow share with a soil-analyzing tip along an intended trench path in the seabed, generating a real-time data signal in response to the composition of the soil analyzed by the soil-analyzing tip along the intended trench path and comparing the real-time data signal to the threshold signal to produce an alarm signal when the real-time data signal is not protective-capability compliant with the threshold signal. 
         [0013]    The step of generating the real-time data signal may include the sub-steps of measuring the force required to pull the soil-analyzing plow tip through the soil, the sleeve friction of the soil, the pore pressure of the soil and the total pull force applied by the pulling mechanism to the plow and combining the measured data according to an algorithm predetermined to produce a signal indicative of the composition of the soil being analyzed by the soil-analyzing plow tip. 
         [0014]    The sub-step of measuring may also include measuring the depth of the soil-analyzing plow tip. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings in which: 
           [0016]      FIG. 1  is a perspective view of an over-the-stern trenching plow utilizing a towing line assembly according to the invention; 
           [0017]      FIG. 2  is a side elevation view of the over-the-stern trenching plow of  FIG. 1 , 
           [0018]      FIGS. 3A-3H  are side elevation views of the over-the-stern trenching plow of  FIG. 1  in sequential transition orientations during retrieval of the over-the-stern trenching plow from the sea to the stern deck of a transporting/towing vessel; 
           [0019]      FIG. 4  is a side elevation view of the plow of  FIG. 1  equipped with trench soil assessment instrumentation in accordance with the invention; and 
           [0020]      FIG. 5  is a schematic diagram of the trench soil assessment instrumentation of  FIG. 4 . 
       
    
    
       [0021]    While the invention will be described in connection with a preferred embodiment 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 
       [0022]    Turning first to  FIGS. 1 and 2 , a trenching plow weighing as much as 100 tons or more includes a chassis  10 , a sled  30 , a plow share  50 , moldboards  60  and a towing assembly  80 . 
         [0023]    The chassis  10  shown has three elongated vertical plates  11  spaced by transverse vertical reinforcing plates  13  and extending from a nose plate  15  to an end plate  17 . The bottom of the chassis  10  lies in fore and aft horizontal planes  19  and  21  with an intermediate plane  23  angled downwardly fore to aft. The chassis  10  has a convex nose  25  beginning at the top edge of the nose plate  15  and transitioning into a downwardly angled midsection  27  followed by a horizontal end section  29  extending to the top edge of the end plate  17 . 
         [0024]    The sled  30  is mounted on the chassis  10  below its nose  25 . Uprights  31  are pivotally pinned between the sled skids  33  and brackets  35  mounted on the underside of the nose  25  and a reinforcing strut  37  is pivotally pinned between the uprights  31  and the angled midsection  27  of the chassis  10 . The uprights  31  and reinforcing strut  37  are apertured and pinned to permit adjustment of the vertical distance between the chassis nose  25  and the angle of the uprights  31  with respect to vertical. The sled uprights  31  are pinned to the chassis nose brackets  35  on a common axis  39 . 
         [0025]    The plow share  40  as shown is mounted in any known manner against the bottom of the horizontal end portion  29  of the chassis  10 , as shown under the aft section  17  of the chassis  10 , with the tip  41  of the plow extending forward to approximately a point below the junction of the angled midsection  27  of the chassis  10  with the horizontal end portion  29  of the chassis  10 . The plow share  40  is in shape generally similar to known plow shares. However, its tip  41  is considerably further below its chassis  10  than the tips of known plow shares, the present plow tip  41  being as much as three meters below the chassis  10  in comparison to known plow tips which are no more than 1.4 meters below their chassis. Its weight is significantly greater than the weight of most known plow shares, the present plow share  40  weighing as much as 100 tons or more in air in comparison to known plow shares which weigh no more than 40 tons in air. Its width may be, but is not necessarily, wider than the width of known plow shares, the present plow share  40  being as much as nine meters wide in comparison to known plow shares which are no more than 4.2 meters wide. 
         [0026]    The moldboards  50  are mounted in any known manner against the outer aft-most faces of the outer vertical elongated plates  11 . The moldboards  50  are generally similar to known moldboards, though their weight may be, but is not necessarily, significantly greater than the weight of known moldboards, the present moldboards  50  weighing as much as ten tons in comparison to known moldboards which weigh no more than two tons. Preferably, each of the moldboards  50  is divided into proximal and distal sections  51  and  53  joined by hinge pins  55  at angled-cut ends  57 . Wedges  59  can be inserted above or below the hinge pins  55  so that the bottom of the moldboard distal sections  53  can be locked in either a horizontal or upwardly angled condition relative to the bottom of the proximal sections  51  of the moldboards  50 . As seen in  FIG. 1 , the moldboards  50  are preferably provided with rollers  61  so as to reduce friction when the moldboards  50  traverse the deck of a vessel and a connecting frame  63  providing reinforcement between the distal sections  53  of the moldboards  50 . 
         [0027]    As shown, a towing assembly  70  is located on the downwardly angled midsection  27  of the chassis  10  aft of the connection point of the sled uprights  31  to the chassis nose  25 . In the embodiment shown, the towing assembly  70  includes wings  71  mounted against the outer faces of the outer vertical elongated plates  11 . Each wing  71  carries mounting rings  73  aligned on a common axis  75  to facilitate connection, perhaps in a clevis fashion, to a tow line (not shown). 
         [0028]    Looking at  FIG. 2 , the key parameters of the present trenching plow are the locations of its center of gravity  81 , of the connecting axis  39  between the sled uprights  31  and the chassis nose brackets  35  and the common axis  75  of the towing assembly mounting rings  73 . In accordance with the invention, the common axis  75  of the towing assembly mounting rings  73  must fall between the sled upright connecting axis  39  and the plow center of gravity  81 . 
         [0029]    Preferably, and as shown, the center of gravity  81  of the present plow, which weighs as much as 100 tons or more, is approximately 15 meters aft of the sled upright connecting axis  39  and the common axis  75  of the towing assembly mounting rings  73  is approximately midway between the center of gravity  81  and the sled upright connecting axis  39 . In comparison, known trenching plows have a center of gravity approximately 5-6 meters aft of the nose of the plow, about ⅓ to ⅖ the distance of the present plow, and a tow line connection point forward of the uprights. Therefore, the present plow results in a moment as much as 12.5 to 15 times that of known plows. 
         [0030]    In practice, the towing line connection assembly  70  can be located to position the common axis  75  of the towing assembly mounting rings  73  anywhere between the center of gravity  81  and the sled upright connecting axis  39 . However, the closer the common axis  75  of the mounting rings  73  is to the center of gravity  81  the better, so long as it is forward of the center of gravity  81 . 
         [0031]    The configuration and weight of the chassis  10 , sled  30 , plow share  50 , moldboards  50  and towing assembly  70  are coordinated to position the center of gravity  81  of the plow at a location affording a resulting moment suitable to a given 20 to 100 ton or more trenching plow application. 
         [0032]    Looking at  FIGS. 3A-3H , assume a plow weight of 96 tons and a center of gravity  81  approximately 15 meters aft of the sled upright connecting axis  39 . The transition of the over-the-stern trenching plow across the stern roller R of a transporting/towing vessel V during retrieval from the sea W is sequentially shown from a point P 1  of first contact with the roller R to a point P 8  at which the plow has entirely traversed the roller R and is at rest on the deck D of the vessel V. 
         [0033]    Beginning with  FIG. 3A , the plow has been retrieved at the end of a winch driven tow line L to the point P 1  with the plow oriented for contact between the roller R and the top surface of the nose  25 . The towline L remains turned on the roller R. In this orientation, the moment of the plow about the roller R is near minimal. 
         [0034]    As is seen in  FIG. 3B , the plow has been further retrieved to a point P 2  at which the apex of the convex nose  25  is in contact with the roller R, the towline L remains turned on the roller R and the center of gravity  81  of the plow has rotated the slightly astern of its position in  FIG. 3A . In this orientation, because of the convex structure of the nose  25  and the sternward shift of the center of gravity  81 , the moment of the plow about the roller R is greater but still near minimal. 
         [0035]    As is seen in  FIG. 3C , the plow has been further retrieved to a point P 3  at which the common axis  75  of the towing assembly mounting rings  73  is above the contact point P 3  and below the high point of the roller R, so that the towline L is slightly turned on the roller R. Also, the contact point P 3  has shifted to the downwardly angled midsection  27  of the chassis  10 . The center of gravity  81  of the plow has rotationally shifted further slightly sternward but very little net shift of the center of gravity  81  has occurred because of the angled midsection  27  of the chassis  10 . Therefore, in this orientation, the moment of the plow about the roller R is substantially the same as in  FIG. 3B , which is still near minimal. 
         [0036]    As is seen in  FIG. 3D , the plow has been further retrieved to a point P 4  at the junction of the downwardly angled midsection  27  and the horizontal end section  29  of the chassis  10 . The common axis  75  of the towing assembly mounting rings  73  has shifted above the roller R. The towline L no longer contacts the roller R and has levered the chassis  10  at the fulcrum point P 4  to shift the center of gravity  81  of the chassis  10  to approximately 2.334 meters  83  astern of the fulcrum point P 4 , creating a total moment of 224.1 metric ton-meters. 
         [0037]    As is seen in  FIG. 3E , the continued pull of the towline L has caused the horizontal end section  29  of the chassis  10  to advance slightly on the roller R and has significantly levered the chassis  10  at the fulcrum point P 5  to further shift the center of gravity  81  of the chassis  10  to approximately 3.768 meters  85  astern of the fulcrum point P 5  creating a total moment of 362 metric ton-meters. 
         [0038]    As is seen in  FIG. 3F , further continued pull of the towline L has caused the horizontal end section  29  of the chassis  10  to advance more significantly on the roller R, levering the chassis  10  at the fulcrum point P 6  to further shift the center of gravity  81  of the chassis  10  to approximately 4.518 meters  87  astern of the fulcrum point P 6 , creating a total moment of 433.9 metric ton-meters, the maximum total moment of the retrieval process. 
         [0039]    As is seen in  FIG. 3G , further continued pull of the towline L has caused the chassis  10  to advance until the center of gravity  81  of the chassis  10  is substantially but not quite directly above the contact point P 7 , reducing the total moment of the plow about the roller R once again to near minimal. 
         [0040]    Finally, looking at  FIG. 3H , further continued pull of the towline L has caused the chassis  10  to advance until the plow is entirely forward of the stern roller R and the plow is resting on the deck D of the vessel V. 
         [0041]    The release of the plow from the deck D of the vessel V into the sea S is essentially the reverse of the retrieval process illustrated in  FIGS. 3A-3H , except that independent winch lines are used to pull the plow in the opposite direction across the stern roller R, as by a block-and-tackle assembly, against the tension of the towing line L. 
         [0042]    Turning now to  FIG. 4 , in order to assess the protective capabilities of a seabed trench dug by a trenching plow such as the plow of  FIG. 1 , the plow share  40  is equipped with a soil-analyzing tip  41 . The soil-analyzing tip  41  includes load pins  43 , a pressure sensor  45  and a friction sensor  47 . The load pins  43  measure the tip reaction force  83  which is the force required to pull the soil-analyzing plow tip  41  through the soil. For example, the plow design may anticipate a tip reaction force  83  up to 650 tons. The pressure sensor  45  measures the pore pressure  85  of the soil passing under the plow tip  41 . The friction sensor  47  measures the sleeve friction  87  of the soil passing under the plow tip  41 . A load cell  49  is located on the plow or elsewhere in a position suitable to measure the total pull force  89  applied to the plow via the tow line L by its pulling mechanism, such as one or more vessels or winches. 
         [0043]    For example, the plow design may anticipate that a total pull force  89  on the plow will be in a range of 200 to 250 tons. Since the total pull force  89  is measured and the offsetting tip reaction, sleeve and friction forces  83 ,  85   87  are also measured, the forces exerted on the plow share  40  between the plow tip  41  and the bottom of the chassis  10 , a distance in the range of 3 meters, is calculable. 
         [0044]    Turning to  FIG. 5 , data transfer units  91  powered by batteries  93  are cable-connected to the sensors  43 ,  45  and  47  in the plow tip  41  and collect data to be received by remote data receiving units  95  which may, for example, be located on remote operated vehicles  97  in communication with a vehicle controller  99 , a plow controller  101  and a GPS device  103 . The data transfer units  91  may, for example, be SENTOOTH 100® data transfer units. 
         [0045]    In operation, the method for assessing the protective capabilities of a seabed trench includes the steps of generating a threshold signal indicative of a desired composition of seabed-trench soil for a specific application. The trenching plow, which has a plow share  40  with a soil-analyzing tip  41 , is pulled along an intended trench path in the seabed. As the plow is pulled along the intended path, a real-time data signal is generated in response to the composition of the soil analyzed by the soil-analyzing tip  41 . The data signal is herein identified as being a real-time signal because the amplitude of the signal is coordinated to the position of the plow along the length of the trench. If and when the soil is backfilled into the trench to further increase the protective capability of the trench, within reasonable limitations, the backfilled soil will be the soil that was excavated and analyzed during trenching, so that the data signal substantially accurately indicates the varying composition of the soil along the backfilled trench. If the trench is not backfilled, the data signal will even more closely indicate the varying composition of the soil defining the trench. 
         [0046]    The real-time data signal is then compared to the threshold signal to produce an alarm signal when the real-time data signal is not protective-capability compliant with the threshold signal. 
         [0047]    The step of generating a real-time data signal may include two sub-steps. The force required to pull the soil-analyzing plow tip through the soil, the sleeve friction of the soil, the pore pressure of the soil and the total pull force applied by the pulling mechanism to the plow are all measured as the plow is pulled along the intended trench path. The measured data is combined according to an algorithm predetermined to produce a signal indicative of the composition of the soil being analyzed by the soil-analyzing plow tip. The algorithm may be standardized or unique to a given application so as to weigh the measured data according to the desired predominance of its importance in a given protective capability analysis. 
         [0048]    The sub-step of measuring may also include measuring the depth of the soil-analyzing plow tip for inclusion in the measured data being combined according to the algorithm so as to enable accounting for depth variations that may occur along the length of the trench. 
         [0049]    By way of example, a suitable algorithm might weigh the plow tip reaction force, the sleeve friction of the soil, the pore pressure of the soil, the total pull force applied to the plow and the deviation of the depth of the trench from a predetermined depth as 70%, 10%, 10%, 5% and 5%, respectively. 
         [0050]    Thus, it is apparent that there has been provided, in accordance with the invention, an improved over-the-stern trenching plow and a method of releasing and retrieving the plow from the vessel into the sea and from the sea onto the vessel and a method and instrumentation for assessing the protective capabilities of a seabed trench that fully satisfy the objects, aims and advantages set forth above. While the invention has been described in conjunction with a specific embodiment thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art and 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.