referring now to the present invention in greater detail , fig1 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 fig2 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 . also referring to fig3 - 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 . for reasons that will appear clear hereinafter , said cutting portion 51 can take on two alternate configurations , i . e . a rectilinear configuration ( fig3 and 4 ) and a curved configuration ( fig5 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 . 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 . referring now to fig7 - 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 . referring now to fig1 - 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 . alternatively , as it can be seen from fig1 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 fig1 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 . referring now to fig1 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 fig1 . 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 . similarly , referring to fig7 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 . furthermore , fig2 , 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 . 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 fig1 , 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 fig9 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 . at this point , turning to fig2 , 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 . 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 fig6 . 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 fig5 , with the compensating device 64 in a lowered position . at this point , the diamond - edged cable 5 is activated at the desired work speed and , simultaneously , the compensating device 64 is lifted ( fig3 ) to cause the cutting portion 51 to take on the rectilinear configuration as shown in fig4 . 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 fig2 , 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 . 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 .