Abstract:
A method for cutting an underwater structure below the level of the seabed on which it is installed is described. The method has the following steps: positioning cutting means and penetrating means associated with the cutting means in close proximity to the underwater structure to be cut, at the intersection region between the structure and the seabed; causing both the penetrating means and the cutting means associated therewith to penetrate into the seabed in a substantially longitudinal direction to a portion of the structure which is embedded in the seabed; and actuating and driving the cutting means and the penetrating means in a substantially perpendicular direction to the embedded portion so as to cut the structure below the level of the seabed. An apparatus for carrying out the method is also described.

Description:
[0001]    The present invention relates to a method and apparatus for cutting underwater structures. 
         [0002]    Currently, submerged structures are cut employing different methods and using apparatuses of different nature with the purpose of removing and/or replacing the same. 
         [0003]    Some of the known cutting devices make use of fixed blades, such as that described in document U.S. Pat. No. 3,056,267, while others are provided with several rotary blades coordinated in their action, such as that described in document U.S. Pat. No. 4,180,047. However, both apparatuses have the drawback of operating only on the emerging, free end of the submerged structure. 
         [0004]    In document EP-B-0540834 there are disclosed a method and device for cutting underwater structures which make use of a diamond cable as a cutting means; the device allows the underwater structure to be cut at any level between the seabed and the surface. 
         [0005]    Nowadays, the in-force provisions governing the field and applied worldwide, which are extremely more strict as for what concerns the environmental protection, require that submerged structures are cut below the level of the seabed, without leaving potentially dangerous stumps of the cut structure emerging from the seabed. Under these conditions and employing the presently available means, it is required to remove a considerable amount of seabed around the base of the structure to be cut. This additional operation is cost- and time-consuming, and it often damages the environment which should be safeguarded instead. 
         [0006]    From document GB-A-409754, an apparatus is known to demolish an underwater structure. Said apparatus comprises a boring tool, namely an auger, which is guided with a guiding tube; the boring tool creates a series of holes diagonally into the structure, even below the level of the seabed, in order to weaken the structure as well as to allow the removal thereof. Alternatively, the hole created by the boring tool can be used for positioning a charge of explosive. However, such a method does not appear neither safe nor easy to be used. 
         [0007]    Document EP-B-1373645 (hereinafter referred to as “document &#39;645”) discloses a method and apparatus for cutting an underwater structure below the level of the seabed on which it is installed. The method of document &#39;645 includes the sequential steps of determining the ideal plane of cutting having taken in account characteristic features of the structure such as its morphology and positioning on the seabed, the shape and consistency of the seabed, and the depth below the level of the seabed at which the cutting has to be performed; positioning and anchoring cutting means in the proximity of the cutting area; boring at least one hole through the seabed in the proximity of the structure, at least up to the depth at which the cutting has to be performed, along a direction parallel to the cutting direction and lying on the cutting plane; introducing said cutting means into said hole, and cutting the structure, said cutting being performed outside of said hole. In document &#39;645, the apparatus adapted for carrying out the method of the same patent includes boring means to bore the hole; cutting means comprising a cutting frame and a movable cutting unit which, in use, at least partially radially projects from said hole, means for positioning said boring means and said cutting means; and anchoring means. 
         [0008]    However, while the method and apparatus as disclosed in document &#39;645 achieve the intended goal of performing the cutting below the seabed, they still have certain drawbacks. Indeed, since the practice has shown that cutting is performed substantially obliquely to the structure to be cut, cutting is more difficult to be initiated and it takes more time than if it were possible to cut substantially perpendicularly to the structure. Finally, the on-field practice has shown that the movable cutting unit often suffers from clogging problems with the debris of the seabed which limit the performance thereof. 
         [0009]    Therefore, an object of the present invention is to provide a method for cutting an underwater structure which allows to obtain said cutting below the level of the seabed with a minimum expenditure of means and of energies, with the least impact as possible on the surrounding marine environment, and with an increased efficiency and easiness when compared to the methods of the prior art. 
         [0010]    A further object of the present invention is to provide an apparatus adapted for carrying out the above-described cutting method. 
         [0011]    Therefore, an object of the present invention is a method for cutting an underwater structure, typically a support pier of an offshore production platform, below the level of the seabed on which it is installed, comprising the following steps: a step of positioning cutting means and penetrating means associated with said cutting means in close proximity to the underwater structure to be cut, at the intersection region between said structure and the seabed; a step of causing both said penetrating means and said cutting means associated therewith to penetrate into said seabed in a substantially longitudinal direction to a portion of said structure which is embedded in the seabed; and a cutting step which includes actuating and driving said cutting means and said penetrating means in a substantially perpendicular direction to said embedded portion so as to cut said structure below the level of the seabed. 
         [0012]    In a preferred embodiment of the method of the present invention, said penetrating step includes causing said penetrating means and said cutting means to penetrate into the seabed in a substantially longitudinal direction to said embedded portion of said structure and, simultaneously or sequentially, moving said penetrating means and said cutting means towards said embedded portion so as to partially wind said cutting means around said embedded portion, said cutting means being of a flexible type. 
         [0013]    In a further preferred embodiment of the method of the present invention, said cutting step includes exerting a stretching action on said flexible cutting means while they are actuated and driven, along with the penetrating means associated therewith, in a substantially perpendicular direction to said embedded portion so to obtain a substantially perpendicular transversal cut of said structure below the level of the seabed. 
         [0014]    In another preferred embodiment of the method of the present invention, said cutting step includes applying a swinging movement to both said penetrating means and said cutting means while they are actuated and driven in a substantially perpendicular direction to said embedded portion so to obtain a substantially perpendicular transversal cut of said structure below the level of the seabed. 
         [0015]    In yet another preferred embodiment of the method of the present invention, said penetrating step includes causing said penetrating means and said cutting means to penetrate into the seabed in a longitudinal direction to said embedded portion of said structure and, simultaneously or sequentially, moving said penetrating means and said cutting means towards said embedded portion so as to partially wind said cutting means around said embedded portion, said cutting means being of a flexible type, and wherein said cutting step includes first exerting a stretching action on said flexible cutting means and then applying a swinging movement to both said penetrating means and said cutting means while they are actuated and driven in a substantially perpendicular direction to said embedded portion so to obtain a substantially perpendicular transversal cut of said structure below the level of the seabed. 
         [0016]    In a further variant embodiment, while the cut is being performed, the cutting means can be moved along the effective direction of penetration and in the way opposite to the way of penetration so as to cut the structure with a non-coplanar cut geometry in order to prevent a potential slipping of the portion of the structure being cut. 
         [0017]    Another object of the present invention is an apparatus adapted for carrying out the method of the present invention, said apparatus being characterized as comprising a penetrating/cutting frame, an anchoring frame, and a swinging frame which connects the penetrating/cutting frame to the anchoring frame. 
         [0018]    Preferably, the penetrating/cutting frame of the apparatus according to the present invention comprises a diamond-edged cable driven along a closed circuit by a series of pulleys between two parallel, mutually spaced tubular columns which, at one end, are transversally connected by a rigid back plate in such a manner that the penetrating/cutting frame has a fork shape, and which, at the free end, carry respective perforating devices each preferably comprising a cutting head and a respective motor drive unit. 
         [0019]    More preferably, said series of pulleys, which are accommodated within the columns, comprises an aligned pair of swivel pulleys which are positioned in the proximity of the free ends of the respective columns so as to subtend a cutting portion of the diamond-edged cable, and a pulley which is provided with a motor drive unit for moving the diamond-edged cable along the closed circuit at a certain speed. 
         [0020]    Even more preferably, the apparatus according to the present invention comprises a compensating device for changing the geometry of said cutting portion of the diamond-edged cable from a rectilinear configuration to a curved or angled configuration, and vice versa. 
         [0021]    In a preferred embodiment of the apparatus according to the present invention, the apparatus further comprises auxiliary ejecting nozzles to assist the swinging frame in its swinging movement to drive the active portion of the penetrating/cutting frame towards the underwater structure to be cut, said auxiliary ejecting nozzles preferably comprising ejecting nozzles arranged on the penetrating/cutting frame and supplied with a pressurized fluid from supply lines. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]      FIG. 1  is a side elevation view of the apparatus according to a preferred embodiment of the present invention; 
           [0023]      FIG. 2  is a front elevation view of the penetrating/cutting frame of the apparatus shown in  FIG. 1 ; 
           [0024]      FIG. 3  is a front elevation sectional view of the penetrating/cutting frame shown in  FIG. 2  with the cutting means in a first configuration; 
           [0025]      FIG. 4  shows a cross-section of the penetrating/cutting frame with the cutting means in the first configuration as shown in  FIG. 3 ; 
           [0026]      FIG. 5  is a front elevation sectional view of the penetrating/cutting frame shown in  FIG. 2  with the cutting means in a second configuration; 
           [0027]      FIG. 6  shows a cross-section of the penetrating/cutting frame with the cutting means in the second configuration as shown in  FIG. 5 ; 
           [0028]      FIG. 7  is a front elevation view of the anchoring frame of the apparatus shown in  FIG. 1 ; 
           [0029]      FIG. 8  is a side elevation view of the anchoring frame shown in  FIG. 7 ; 
           [0030]      FIGS. 9 and 10  show top plan views of the anchoring frame shown in  FIGS. 7 and 8  in two different configurations; 
           [0031]      FIG. 11  is a side elevation view of the swinging frame having mounted thereto the penetrating/cutting frame; 
           [0032]      FIGS. 12 and 13  are front elevation views of the two half-frames comprising the swinging frame shown in  FIG. 11 ; 
           [0033]      FIGS. 14 and 15  are side elevation views of the swinging frame having mounted thereto the penetrating/cutting frame in two further different configurations; 
           [0034]      FIGS. 16 and 17  are side elevation views showing another preferred embodiment of the swinging frame having mounted thereto the penetrating/cutting frame; 
           [0035]      FIG. 18  is a sectional plan view of the apparatus shown in  FIG. 1 ; 
           [0036]      FIGS. 19-22  show the major embodiment steps of the embodiment method for the apparatus according to the present invention; and 
           [0037]      FIGS. 23 ,  24  and  25  are respectively a side elevation view, a front elevation view and a cross-section of the penetrating/cutting frame having mounted thereto the auxiliary ejecting means according to a preferred embodiment of the apparatus of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0038]    Referring now to the present invention in greater detail,  FIG. 1  shows an overall side view of a cutting apparatus  1  according to a preferred embodiment of the present invention, comprising a penetrating/cutting frame  2 , an anchoring frame  3 , and a swinging frame  4  which connects the penetrating/cutting frame  2  to the anchoring frame  3 . Particularly, also referring to  FIG. 2  and assuming that like reference numerals will denote like elements throughout the different figures, the penetrating/cutting frame  2  includes two parallel, mutually spaced columns  21 ,  21  which, at one end, are transversally connected by a rigid back plate  22  in such a manner that the penetrating/cutting frame  2  has a fork shape. At the free ends, the columns  21 ,  21  carry respective perforating devices  23 ,  23 . At the back plate  22 , each column  21  carries a series of sliding rollers  24 . 
         [0039]    Also referring to  FIGS. 3-6 , the columns  21 ,  21  are constituted of tubular uprights accommodating pulleys  61 ,  61 ,  62  and  63  adapted for driving cutting means, in the form of a diamond-edged cable  5 , along a closed circuit partially running through the columns  21 ,  21 . Pulleys  61 ,  61  are positioned in reciprocal alignment with each other in the proximity of the free ends of the respective columns  21 ,  21 , and they subtend an active cutting portion  51  of the diamond-edged cable  5 . 
         [0040]    For reasons that will appear clear hereinafter, said cutting portion  51  can take on two alternate configurations, i.e. a rectilinear configuration ( FIGS. 3 and 4 ) and a curved configuration ( FIGS. 5 and 6 ) and, to this end, both the pulleys  61 ,  61  are swivel-mounted on pins  610 ,  610 . One of the two columns  21 ,  21  has accommodated therein, in opposition to the pulley  61 , a compensating device  64  which allows the configuration of said cutting portion  51  of the diamond-edged cable  5  to be changed from said rectilinear configuration to said curved configuration, and vice versa. Said compensating device  64  includes the driven pulley  62  mounted to a slide  65  driven along a rectilinear guide  66  arranged inside the column  21  which also accommodates a linear actuator  67  adapted for translating the assembly comprising the pulley  62  and the slide  65  along the rectilinear guide  66 . Instead, the pulley  63 , which is accommodated inside the other column  21  and provided with a motor drive unit  630 , is adapted for slidably driving the diamond-edged cable  5  along the closed circuit at the desired work speed. 
         [0041]    In order to allow the diamond-edged cable  5  to pass from one column  21  to the other, there are provided passage windows  210 ,  210  at the free ends of the columns and passage windows  220 ,  221  at the connected ends thereof. Obviously, the window  221  has such a length to allow the cable to extend longitudinally while the configuration of its cutting portion  51  is changed from the rectilinear configuration to the curved configuration, and vice versa. Each of the perforating devices  23  includes a cutting head  230  and a relevant motor drive unit  231 . 
         [0042]    Referring now to  FIGS. 7-10 , said anchoring frame  3  is substantially constituted of two mutually parallel and spaced spars  31 ,  31  which are transversally connected by several transversal members  32  and  33 . Each spar  31  is constituted of two parallel neighbouring bars  310  which are connected by several attachment plates  311  to define a guiding groove  312  extending substantially over the entire length of the bars. The anchoring frame  3  has mounted thereto securing means which are positioned at the transversal members  33  and connected thereto, said securing means comprising a pair of jaws  34 ,  34  each of which is hinged, at  340 , to a support  341  attached to the transversal member  33 . Each jaw  34  carries, at one end, a gripping roller  342  and, at the other end, an angled fin  343  which is hingedly engaged by a linear actuator  344  to actuate the jaw. Preferably, each pair of bars  310  has, at one end, two opposite angled guiding plates  345  outwardly diverging from each other. In the proximity of the opposite end of each pair of bars  310 , the frame  3  carries an extension  35  substantially triangular in shape and faced in the same direction as the jaws, substantially perpendicularly to the plane of the frame  3 . 
         [0043]    Referring now to  FIGS. 11-15 , said swinging frame  4  includes two half-frames  7  and  8  which are hinged to each other at one end in  9 . Particularly, the half-frame  7  is constituted of two mutually parallel and spaced side-members  71  which are transversally connected by a series of ribs  72 . Furthermore, each of the two side-members  71  is provided with a series of rollers  711  arranged along its length. Similarly, the half-frame  8  is constituted of two mutually parallel and spaced side-members  81  which are transversally connected by a series of ribs  82 . Each spar  81  is constituted of two parallel neighbouring bars  810  which are connected by at least one pair of attachment plates  811  to define a guiding groove  812  extending substantially over the entire length of the bars. The angular opening of two half-frames  7  and  8  is adjusted by a linear actuator  41  which is hingedly mounted between the two half-frames  7  and  8  in  410  and  411 , respectively. 
         [0044]    Alternatively, as it can be seen from  FIGS. 16 and 17 , the linear actuator  41  can be replaced with an opening-adjusting device  42 . Such a device is constituted of two levers  421 ,  421  which, at one end, are hinged to each other in  422 , and which, at the other end thereof, are pivoted to the two half-frames  7  and  8  in  423  and  424 , respectively. The pivot point  422  of the two levers  421  has swivel-mounted thereon an axial female threaded sleeve  425  with a threaded shaft  426  engaged therewith which is rotatably driven by a motor drive unit  427  articulated to the half-frame  7  in  428 . As it can be deduced comparing  FIGS. 16 and 17 , the rotation of the threaded shaft  426  results in an axial movement of the sleeve  425  which causes the two half frames  7  and  8  to move towards each other and away from each other. 
         [0045]    Referring now to  FIGS. 11 and 18 , there is shown the connection relationship between the penetrating/cutting frame  2  and the half-frame  8  of the swinging frame  4 . As it can be seen, the rollers  24  of each column  21  slidably engage the respective guiding groove  812  of the half-frame  8  in such a manner that the penetrating/cutting frame  2  can slide with respect to the half-frame  8 , as it will be clear from  FIG. 15 . This sliding movement is performed by a linear actuator  10  which is mounted between the penetrating/cutting frame  2  and the half-frame  8  of the swinging frame  4 . 
         [0046]    Similarly, referring to  FIGS. 7 and 18 , there is shown the connection relationship between the half-frame  7  of the swinging frame  4  and the anchoring frame  3 . In this case, as it can be observed, the rollers  711  of each of the side-members  71  of the half-frame  7  engage the respective guiding groove  312  of the anchoring frame  3  in order to secure the half-frame  7 , and therefore the swinging frame  4 , to the anchoring frame  3 . This engagement operation is assisted by the two guiding plates  345  of the anchoring frame  3 . 
         [0047]    Furthermore,  FIGS. 23 ,  24  and  25  show that the penetrating/cutting frame  2  of the cutting apparatus according to the present invention can be provided with auxiliary ejecting means  900  to assist the swinging chassis  4  in its curvilinear movement which drives the active portion of the penetrating/cutting frame  2  in the direction of the underwater structure to be cut. In this embodiment, such auxiliary ejecting means  900  are constituted of arrays of ejecting nozzles  901  arranged on the tubular columns  21 ,  21  and supplied with a pressurized fluid, typically pressurized water, from supply lines  902  which convey it towards the ejecting nozzles  901  through collecting conducts  903  mounted along the tubular columns  21 ,  21  and operatively connected to the arrays of ejecting nozzles  901 . These ejecting nozzles  901  are to provide both a feedback hydrodynamic thrust force and a disaggregation action of the soil of the seabed while it is perforated by the perforating devices  23 ,  23 . In the illustrated embodiment, the ejecting nozzles are arranged on the faces of the tubular columns  21 ,  21  which are directed towards the anchoring frame. 
         [0048]    The application and the operation of the preferred embodiment of the cutting apparatus according to the present invention will be described hereinbelow. As already stated above, the cutting apparatus  1  is used to cut an underwater structure, typically a support pier P of an offshore production platform, below the level of a seabed F, in order to avoid that a potentially dangerous stump of the pier can emerge. In a preparative step and specifically referring to  FIG. 19 , the anchoring frame  3  is lowered from a support boat into water up to the base of the pier P and, with the help of divers or with the use of remotely controlled submarine vehicles, it is abutted against the pier and secured thereto by actuating the jaws  34 ,  34  (see also  FIGS. 9 and 10 ). The operation of abutting the anchoring frame  3  against the pier P is facilitated by the presence of the fork, constituted of the extensions  35  of said frame, which can abut against possible transversal members T at the base of the pier P in order to confer an enhanced stability to the connection. 
         [0049]    At this point, turning to  FIG. 20 , the assembly comprising the penetrating/cutting frame  2  and the swinging frame  4  is lowered into water, the assembly operation being performed either in factory or extemporaneously on the support boat itself. When this assembly is in the proximity of the anchoring frame  3  and with the help of the guiding plates  345 , the rollers  711  of the half-frame  7  of the swinging frame  4  are fitted into the respective guiding grooves  312  of the anchoring frame  3 , and they are moved until both the frames are engaged with each other over their whole length. The initial angle between the two half-frames  7  and  8  of the swinging chassis  4 , and therefore between the penetrating/cutting frame  2  and the pier P, is a function of both the diameter of the pier and the desired type of cut. 
         [0050]    When the three frames of the cutting apparatus  1  according to this preferred embodiment of the present invention are positioned appropriately, the perforating devices  23  at the ends of the columns  21  of the penetrating/cutting frame  2  are activated, and the entire penetrating/cutting frame  2  is lowered into the seabed by switching on its linear actuator  10 . During this lowering step, the two columns  21 , due to their initial angle of inclination, partially fork the pier P, thus causing the cutting portion  51  to partially wind around the pier in a perpendicular direction to the same, and to take on the curved configuration as already previously described and illustrated in  FIG. 6 . During this step of lowering into the seabed F, the diamond-edged cable  5  is driven at a low speed, thus exerting a moderate shear and therefore facilitating its positioning at the desired level without exposing it to the risk of getting clogged with seabed debris. At this time, in order to compensate for the curved condition of the cutting portion  51 , the configuration of the circuit of the diamond-edged cable  5  is as illustrated in  FIG. 5 , with the compensating device  64  in a lowered position. 
         [0051]    At this point, the diamond-edged cable  5  is activated at the desired work speed and, simultaneously, the compensating device  64  is lifted ( FIG. 3 ) to cause the cutting portion  51  to take on the rectilinear configuration as shown in  FIG. 4 . This combined action allows to obtain a partial perpendicular cut T 1  of the pier P. Subsequently, the penetrating/cutting frame  2  is further tilted by extending the linear actuator  41 , until the cutting portion  51  is driven outside of the pier in order to complete the cutting of the pier with a curved cut T 2 . It has to be noted that, as best seen in  FIGS. 20 ,  21  and  22 , a major portion of the columns  21  of the penetrating/cutting frame  2  remains outside of the seabed and in the free water, so that the return length of the diamond-edged cable  5  can be cleared of debris possibly gathered during its cutting action into the seabed. 
         [0052]    As it can be inferred from the above description, the cutting apparatus according to the present invention can perform a substantially perpendicular cut of the underwater structure below the level of the seabed without removing a considerable amount of soil at the base of the structure, and without clogging the cutting means with debris which could compromise its action. Furthermore, if it is required to cut more structures in the same area, for example more piers of an offshore production platform, the modular nature of the present apparatus allows, for example, to install an anchoring chassis for every pier to be cut, and to re-use the same penetrating/cutting frame—swinging frame assembly by mounting and removing it to/from the respective anchoring frame every time. Alternatively, the operation of assembling the three frames can be performed before the whole apparatus is lowered into water, either in factory or extemporaneously on the support boat.