Underwater cable burying device and underwater cable burying method

Abstract

To provide an underwater cable burying device and an underwater cable burying method capable of efficiently and surely burying an underwater cable in a water bottom ground.SOLUTION: An underwater cable burying device includes: a cylindrical injection pipe 6 externally mounted on a cable 3 so as to be movable to a position above a water bottom ground 1; forward injection nozzles 8, 8 directed toward a water bottom ground side of the cylindrical injection pipe 6; rearward injection nozzles 9, 9 directed toward a water surface side of the injection pipe 6; and injection control means for controlling a fluid injection amount of the forward injection nozzles 8, 8 and the rearward injection nozzles 9, 9. While controlling a posture of the injection pipe 6, the underwater cable burying device allows a portion 3b that is positioned ahead of a water bottom ground side end face of the ejection pipe 6 of the cable 3 to sink by its own weight, the water bottom ground 1 being fluidized by a fluid 7 injected from the forward injection nozzles 8, 8, thereby burying the cable 3 in the water bottom ground 1.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to an underwater cable burying device and an underwater cable burying method for burying cables such as communication cables on the seabed or the like. [Background technology]

[0002] Generally, underwater cables such as submarine cables are installed by moving a dedicated cable-laying ship loaded with the cable on the surface of the water, letting out the cable into the water, and then sinking the cable to the seabed.

[0003] However, if this type of underwater cable is laid exposed on the seabed, it may be damaged by natural forces such as waves, ship anchors, bottom trawls, etc. Therefore, it is desirable to bury it in the seabed, especially in shallow waters down to the continental shelf.

[0004] Therefore, conventionally, a dedicated excavator or the like is used to excavate the bottom of the water to form a trench for burying the cable, and the underwater cable is then buried in the trench (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-112136 Summary of the Invention [Problem to be solved by the invention]

[0006] However, with the conventional technology described above, there was a problem that, depending on the location and time of year when waves and currents are constantly acting, even if the ground is excavated to bury the cable, it is immediately backfilled, making it difficult to bury the underwater cable in the ground at the bottom of the water.

[0007] In view of the above-mentioned conventional problems, the present invention has been made with the object of providing an underwater cable burying device and an underwater cable burying method that can efficiently and reliably bury an underwater cable in the bottom ground. [Means for solving the problem]

[0008] The invention described in claim 1, which solves the above-mentioned problems of the prior art, is characterized in that the underwater cable burying device pays out a cable toward the waterbed from a cable payout means of a moving work barge and buries the cable in the waterbed. The device comprises a cylindrical jet pipe movably fitted to a position above the waterbed surface of the cable, a front jet nozzle that sprays fluid toward the waterbed side of the jet pipe, a rear jet nozzle that sprays fluid from the other end face of the jet pipe facing the water surface, and jet control means that controls the amount of fluid sprayed from the front jet nozzle and the rear jet nozzle.The position and attitude of the jet pipe are controlled by the front jet nozzle and the rear jet nozzle, and the part of the cable paid out by the cable payout means that is located beyond the waterbed side end face of the jet pipe is allowed to sink under its own weight into the waterbed that has been fluidized by the fluid sprayed from the front jet nozzle, and the cable is buried in the waterbed.

[0009] The invention described in claim 2 is characterized in that, in addition to the configuration of claim 1, the injection pipe is provided with a pore water pressure gauge, and the depth of the injection pipe from the water surface is detected by the water pressure measured by the pore water pressure gauge.

[0010] The invention described in claim 3 is characterized in that, in addition to the configuration of claim 1 or 2, the injection pipe is provided with an earth pressure gauge, and the depth of the injection pipe from the bottom of the water is detected by the earth pressure measured by the earth pressure gauge.

[0011] The invention as set forth in claim 4 is characterized in that, in addition to the configuration of claim 1, a cleaning nozzle for spraying a fluid onto the inner periphery of the spray pipe is provided.

[0012] The invention as set forth in claim 5 is characterized in that, in addition to the configuration of claim 1, a recovery wire is provided which is attached to the ejection pipe.

[0013] The invention described in claim 6 is characterized in that, in a method for burying an underwater cable in which a cable is let out from a moving work barge toward the waterbed and then buried in the waterbed, the method comprises the steps of: a waterbed fluidization step in which a fluid is sprayed toward the water surface from a spray pipe movably fitted onto the part of the cable above the surface of the waterbed, and the fluid is sprayed from the spray pipe toward the waterbed to fluidize the waterbed; a cable subsidence step in which the part of the cable that has been let out into the fluidized waterbed that is located beyond the waterbed end face of the spray pipe sinks under its own weight; and a position return step in which the spray pipe is retracted along the cable, exposing the part of the cable that is to be newly submerged beyond the waterbed end face of the spray pipe, thereby burying the cable in the waterbed by repeating the following steps.

[0014] The invention described in claim 7 is characterized in that, in addition to the configuration of claim 6, the position return process stops the injection of fluid toward the water surface and retracts the injection pipe along the cable by injecting fluid toward the bottom ground.

[0015] The invention described in claim 8 is characterized in that, in addition to the configuration of claim 6 or 7, the depth of the injection pipe from the water surface is calculated based on the water pressure measured by a pore water pressure gauge attached to the injection pipe, and when the injection pipe reaches a predetermined depth, the injection pipe is retracted along the cable.

[0016] The invention described in claim 9 is characterized in that, in addition to the configuration of claim 6 or 7, the depth to which the injection pipe is buried in the bottom ground is calculated based on the earth pressure measured by an earth pressure meter attached to the injection pipe, and when the injection pipe reaches a predetermined depth, the injection pipe is retracted along the cable. [Effects of the Invention]

[0017] The underwater cable burying device according to the present invention is provided with the configuration described in claim 1, and can therefore bury underwater cables in the waterbed efficiently and reliably.

[0018] Furthermore, in the present invention, by providing the configurations described in claims 2 and 3, the position of the injection pipe and the buried depth of the cable can be accurately grasped and controlled appropriately.

[0019] Furthermore, in the present invention, by providing the configuration described in claim 4, it is possible to loosen the soil (sediment) that has firmly entered between the injection pipe and the underwater cable so that the movement of the injection pipe is not hindered by the soil (sediment). It is possible.

[0020] Furthermore, in the present invention, by providing the configuration described in claim 5, the ejection pipe can be pulled onto the ship by the recovery wire and recovered.

[0021] Furthermore, by employing the configuration of claim 6, the underwater cable burying method according to the present invention can bury the underwater cable in the bottom ground efficiently and reliably.

[0022] Furthermore, in the present invention, by providing the configuration described in claim 7, the ejection pipe can be suitably retracted.

[0023] Furthermore, in the present invention, by providing the configurations set forth in claims 8 and 9, the position of the injection pipe can be accurately grasped and controlled appropriately. [Brief explanation of the drawings]

[0024] [Figure 1] 1 is a schematic cross-sectional view showing an example of an underwater cable burying device according to the present invention. [Figure 2] FIG. 2 is a perspective view showing an example of the injection pipe of the same. [Figure 3] FIG. 2 is a longitudinal cross-sectional view showing the outline of the structure of the injection pipe of the same. [Figure 4]1 is a schematic cross-sectional view showing an example of the work of burying the burial start portion of a cable in the underwater cable burying method according to the present invention. FIG. [Figure 5] These are diagrams showing the steps of the cable burying work, where (a) is a schematic cross-sectional view showing the state of the water bottom fluidization process, (b) is a schematic cross-sectional view showing the state of the cable sinking process, and (c) is a schematic cross-sectional view showing the state of the position return process. [Figure 6] 4 is a schematic diagram for explaining a force acting on the injection pipe of the embodiment; FIG. DETAILED DESCRIPTION OF THE INVENTION

[0025] Next, an embodiment of the underwater cable burying device according to the present invention will be described based on the examples shown in Figures 1 to 3. In the figures, reference numeral 1 denotes the bottom ground, reference numeral 2 denotes the water surface, and reference numeral 3 denotes a cable such as a communication cable or a power cable.

[0026] The device for burying this cable 3 in the underwater ground 1 such as the seabed (hereinafter referred to as the underwater cable burying device) comprises a work barge 4 that moves on the water and a cable payout means 5 mounted on the work barge 4, and while the work barge 4 is moving, the cable 3 is paid out from the cable payout means 5 toward the underwater ground 1, and the cable 3 is successively buried in the underwater ground 1.

[0027] The underwater cable burying device also includes a cylindrical injection pipe 6 movably fitted over the cable 3, forward injection nozzles 8, 8... that inject a fluid 7 such as water toward the bottom ground 1 side of the injection pipe 6, rearward injection nozzles 9, 9... that inject the fluid 7 from the other end face of the injection pipe 6 facing the water surface 2 side, and an injection control means (not shown) that controls the amount of fluid injected from the forward injection nozzles 8, 8... and the rearward injection nozzles 9, 9...

[0028] The work barge 4 can move on the water under its own power or by being towed, and can also travel at a constant speed.

[0029] In addition, the work barge 4 is equipped with a cable reeling means 5 having a drum 5a on which the cable 3 is wound, and by rotating the drum 5a at a constant speed, the cable 3 can be reeled out into the water at a constant speed in synchronization with the navigation speed of the work barge 4.

[0030] As shown in Figures 2 and 3, the injection pipe 6 is formed in a cylindrical shape with an inner diameter slightly larger than the diameter of the cable 3 to be buried, is movably fitted onto the outer periphery of the cable 3, and is positioned near the bottom ground 1 along the cable 3.

[0031] The injection pipe 6 may be split into a pair of semi-cylindrical halves 6a, 6a so that it can be easily fitted onto the cable 3. The injection pipe 6 is not limited to being split into halves, and may be a cylinder split into multiple pieces.

[0032] In addition, one end of a recovery wire 11 made of steel wire or the like is attached to the spray pipe 6 and is reeled out from a winch 10 installed on the work barge 4, and the spray pipe 6 can be pulled up onto the work barge 4 by winding up the recovery wire 11 with the winch 10.

[0033] Furthermore, the injection pipe 6 is provided with front injection nozzles 8, 8 . . . and rear injection nozzles 9, 9 . . . at both axial ends thereof, so that a fluid 7 such as water can be injected forward and backward in the axial direction of the cylinder.

[0034] The forward injection nozzles 8, 8... are arranged at intervals in the circumferential direction at one end of the injection pipe 6, and a fluid 7 such as water is injected forward in the axial direction from each forward injection nozzle 8, 8... on one end face of each injection pipe 6, so that the fluid 7 is spread over a certain range forward from one end of the injection pipe 6.

[0035] The rear injection nozzles 9, 9... are arranged at intervals circumferentially at the other end of the injection pipe 6 (the end opposite the front injection nozzles 8, 8...), and a fluid 7 such as water is injected rearward in the axial direction from each rear injection nozzle 9, 9... on the other end face of the injection pipe 6, so that the fluid 7 is spread over a certain range rearward from the other end of the injection pipe 6.

[0036] In this embodiment, the front injection nozzles 8, 8... and the rear injection nozzles 9, 9... are arranged parallel to the cylindrical axis of the injection pipe 6, but if necessary, the front injection nozzles 8, 8... and the rear injection nozzles 9, 9... may be installed so as to form a predetermined angle with respect to the cylindrical axis of the injection pipe 6.

[0037] Furthermore, it is preferable that the inner periphery of the injection pipe 6 is provided with one or more cleaning nozzles 12, 12 with their nozzles facing inward or diagonally inward, and that by injecting a fluid 7 such as water from the cleaning nozzles 12, 12, the bottom sediment that has entered the gap between the inner periphery of the injection pipe 6 and the outer periphery of the cable 3 is washed away, so that the relative movement between the injection pipe 6 and the cable 3 is not hindered.

[0038] In addition, a pore water pressure gauge (not shown) may be fixed to the injection pipe 6 at a predetermined position on the outer periphery, particularly at the front in the axial direction of the tube (the side that penetrates into the waterbed ground 1), and the pore water pressure data measured at any time may be transmitted from the pore water pressure gauge to a terminal on the work barge 4 via wire or wirelessly.

[0039] The depth of the injection pipe 6 from the water surface 2 is detected based on the water pressure measured by this pore water pressure gauge, making it possible to accurately grasp the position of the injection pipe 6 and the buried depth of the cable 3.

[0040] In addition, an earth pressure gauge (not shown) may be provided on the injection pipe 6 at a predetermined position on the outer periphery, particularly at the front in the axial direction of the tube (the side that penetrates into the bottom ground 1), so that earth pressure data measured at any time can be transmitted from the earth pressure gauge to a terminal on the work barge 4 via wire or wirelessly.

[0041] Based on the earth pressure measured by this earth pressure meter, the depth from the bottom of the water of the cable 3 buried in the bottom ground 1 by the injection pipe 6 is detected, and the buried depth of the injection pipe 6 and the cable 3 can be accurately determined.

[0042] Furthermore, by using the above-mentioned pore water pressure gauge and earth pressure gauge together and comparing the measured data, it is possible to detect the position of the injection pipe 6 and the buried depth of the cable 3 more accurately.

[0043] Although not specifically shown, the injection control means comprises independent pumps and supply pipes 13, 13... that compress and supply fluid 7 such as water from each pump to the front injection nozzles 8, 8..., rear injection nozzles 9, 9... and cleaning nozzles 12, 12, respectively, and is configured so that the control device can control the injection of fluid 7 such as water from the front injection nozzles 8, 8..., rear injection nozzles 9, 9... and cleaning nozzles 12, 12 at the desired water volume, water pressure and injection time (stop and start), respectively.

[0044] In this underwater cable burying device, the attitude of the injection pipe 6 is controlled by injecting fluid 7 from the front injection nozzles 8, 8... and the rear injection nozzles 9, 9..., and the cable 3 is buried in the waterbed ground 1 by causing the portion 3b of the cable 3 that is located beyond the end face of the injection pipe 6 on the waterbed ground 1 side, which is unwound from the cable unwinding means 5, to sink under its own weight into the waterbed ground 1 that has been fluidized by the fluid 7 injected from the front injection nozzles 8, 8...

[0045] Next, a method for burying an underwater cable using the above-mentioned underwater cable burying device will be described with reference to the drawings. Note that the same components as those in the above-mentioned embodiment will be given the same reference numerals and their description will be omitted.

[0046] First, although not specifically shown, one end of the cable 3 is let out from the work barge 4, and with the cable 3 supported by a balloon buoy, the other end is landed at a landing station, after which the balloon buoy is detached and the starting part of the cable 3 is lowered onto the bottom ground 1.

[0047] The portion of the cable 3 that begins to be buried may be started by first digging a trench in the waterbed 1, lowering the cable 3 into the trench, and then backfilling the trench. Alternatively, as shown in Figure 4, a cylindrical end-buried injection pipe 15 equipped with injection nozzles 14, 14 on its outer periphery may be fitted onto the portion of the cable 3 that has sunk into the waterbed 1, and water or other fluid 7 may be injected from the injection nozzles 14, 14 to excavate the waterbed 1, causing the end-buried injection pipe 15 to sink and be buried in the waterbed 1.

[0048] Next, the jet pipe 6 is fitted onto the cable 3 on the work barge 4, and as shown in FIG. 5, the jet pipe 6 is moved toward the bottom ground 1 while being guided by the cable 3 by its own weight.

[0049] At this time, the cable 3 reeled out from the cable reeling means 5 is in an oblique state in the water, bent near the bottom ground 1, and the burial start part is buried or laid in the bottom ground 1 via the bent part 3a, so the front end of the injection pipe 6 that sinks along the cable 3 comes into contact with the bent part 3a and stops at that position (this position is the original position).

[0050] One end of a recovery wire 11 is attached to the spray tube 6, and the recovery wire 11 is paid out as the spray tube 6 sinks, and after the front end of the spray tube 6 comes into contact with the bent portion 3a and stops, the recovery wire 11 is further paid out and left slack. The length of the recovery wire 11 to be slackened is set to a length that does not cause tension in the recovery wire 11 and does not hinder the operation of the spray tube 6, even when the spray tube 6 operates as described below.

[0051] Next, as shown in the figure, the supply of fluid 7 such as water to the front jet nozzles 8, 8... and the rear jet nozzles 9, 9... is started, and while the attitude of the jet pipe 6 is controlled by the front jet nozzles 8, 8... and the rear jet nozzles 9, 9..., the cable 3 is buried in the waterbed ground 1 (hereinafter referred to as the fluidized area 16) fluidized by the fluid 7 jetted from the front jet nozzles 8, 8... by allowing the portion 3b of the cable 3, which is located beyond the end face of the jet pipe 6 on the waterbed ground 1 side, of the cable 3 unwound from the cable unwinding means 5 to sink under its own weight into the waterbed ground 1 (hereinafter referred to as the fluidized area 16).

[0052] At this time, it is preferable to also start supplying fluid 7 to the cleaning nozzles 12, 12 and constantly or intermittently spray fluid 7 between the inner surface of the spray pipe 6 and the outer surface of the cable 3, thereby preventing soil (bottom sediment) from the bottom of the water 1 from being drawn into and clogging the gap between the inner surface of the spray pipe 6 and the outer surface of the cable 3 as the spray pipe 6 operates, thereby preventing the relative movement between the spray pipe 6 and the cable 3 from being obstructed.

[0053] Specifically, while the work barge 4 is sailing at a constant speed, the cable 3 is paid out from the cable payout means 5 in accordance with the sailing speed of the work barge 4, and as shown in the figure, the rear jet nozzles 9, 9... are used to suppress the retreat of the jet pipe 6, while the front jet nozzles 8, 8... are used to fluidize the soil and sand on the waterbed 1 (waterbed fluidization process).

[0054] At this time, the injection control means controls the injection amounts of the front injection nozzles 8, 8... and the rear injection nozzles 9, 9... in the horizontal direction (hereinafter referred to as the x direction) and the vertical direction (hereinafter referred to as the y direction) so as to satisfy the following relationship: (x direction) F w +F jt1_x +R x ≧F jt2_x ···(1) (y direction) F jt2_y +g1+g2>F jt1_y +R y ···(2)

[0055] Furthermore, the code F w is the horizontal force that the jet pipe 6 receives from the wire as the work barge 4 moves, and F jt1_x is the thrust F due to the jets from the forward jet nozzles 8, 8... jt1 x component of σ, F jt1_y is the same y component, F jt2_x is the thrust F due to the jets from the rear jet nozzles 9, 9... jt2 x component of F jt2_y is the same y component, R x is the x component of the reaction force that the injection pipe 6 receives from the bent portion 3a, R yis the y component, g1 is the gravity acting on the jet pipe 6, and g2 is the gravity acting on the jet pipe 6 due to the weight of the cable 3 itself.

[0056] In addition, the reaction force from the bottom ground 1 also affects the direction of laying due to the fluidizing effect caused by the injection from the front injection nozzles 8, 8..., and the resistance that the injection pipe 6 receives from the bottom ground 1 is so small that it can be ignored.

[0057] In this way, while the work barge 4 is sailing at a constant speed, the cable 3 is paid out from the cable payout means 5 in accordance with the sailing speed of the work barge 4, and the spray pipe 6 is controlled as described above, so that, as shown in Figure 6, the fluid 7 is sprayed from the front spray nozzles 8, 8..., causing the part 3b of the spray pipe 6 fitted around the cable 3 paid out into the fluidized waterbed 1, which is located beyond the end face of the waterbed 1 side, to sink due to the weight of the cable 3 and the spray pipe 6 (cable sinking process).

[0058] During the cable sinking process, measurement data is sent from the pore water pressure meter installed in the injection pipe 6 to the injection control means at any time, and based on the water pressure measurement data from the pore water pressure meter, the depth to which the injection pipe 6 is buried in the waterbed 1 is calculated, and when the injection pipe 6 reaches a predetermined depth, the injection pipe 6 is retracted (moved) along the cable 3.

[0059] In addition, the buried depth of the cable 3 from the bottom surface of the water can be calculated based on the earth pressure measured by an earth pressure gauge attached to the injection pipe 6, and by comparing this with the measurement data of the pore water pressure, the position of the injection pipe 6 and the buried depth of the cable 3 can be determined more accurately.

[0060] Next, when the front end of the jet pipe 6 reaches a predetermined buried depth of the cable 3, the jetting of the fluid 7 from the rear jet nozzles 9, 9... toward the water surface 2 is stopped, and the jet pipe 6 is retracted (moved) along the cable 3 by jetting the fluid 7 from the front jet nozzles 8, 8... toward the bottom ground 1, as shown in the figure (position return process).

[0061] At this time, the injection control means controls the injection amounts of the front injection nozzles 8, 8 . . . and the rear injection nozzles 9, 9 . (x direction) F x +F jt2_x ≧0 (3) (y direction) g1+g2 <F jt1_x ···(4)

[0062] By controlling the jet pipe 6 in this manner, the jet pipe 6 retreats along the cable 3, exposing the newly submerged portion of the cable 3 beyond the end face of the jet pipe 6 on the side of the bottom ground 1 as a new bent portion 3a, and moves in the x direction while returning to its original position in the y direction.

[0063] Then, by sequentially repeating the above-mentioned bottom fluidization process, cable sinking process, and position return process, the fluidization area 16 is moved sequentially in the laying direction of the cable 3 by the injection from the forward injection nozzles 8, 8..., and the cable 3 exposed in the front part of the injection pipe 6 gradually sinks there, and when the fluidization converges, the sunken cable 3 is fixed to the soil and sand. [Explanation of symbols]

[0064] 1 Underwater ground 2 water surface 3 Cable 4 Work Barges 5 Cable payout means 6 Injection tube 7 fluid 8 forward injection nozzle 9 Rear injection nozzle 10 winch 11 Recovery wire 12 Cleaning nozzle 13 Supply pipe 14 spray nozzle 15 End buried injection pipe 16 Fluidization Area

Claims

1. An underwater cable burying device that pays out a cable toward the bottom of the water from a cable payout means of a moving work barge and buries the cable in the bottom of the water, a cylindrical jet pipe movably fitted onto the cable at a position above the water bottom ground surface; a front jet nozzle for jetting fluid toward the water bottom side of the jet pipe; a rear jet nozzle for jetting fluid from the other end face of the jet pipe facing the water surface; and jet control means for controlling the amount of fluid jetted from the front jet nozzle and the rear jet nozzle, An underwater cable burying device characterized in that the position and attitude of the jet pipe are controlled by the front jet nozzle and the rear jet nozzle, and the part of the cable that is reeled out from the cable reeling means and that is located beyond the end face of the jet pipe on the bottom side of the water is allowed to sink under its own weight into the water bottom ground that has been fluidized by the fluid sprayed from the front jet nozzle, thereby burying the cable in the water bottom ground.

2. 2. The underwater cable burying device according to claim 1, wherein the injection pipe is provided with a pore water pressure gauge, and the depth of the injection pipe from the water surface is detected by the water pressure measured by the pore water pressure gauge.

3. 3. The underwater cable burying device according to claim 1, wherein the injection pipe is provided with an earth pressure gauge, and the depth of the injection pipe from the bottom of the water is detected by the earth pressure measured by the earth pressure gauge.

4. 2. The underwater cable burying device according to claim 1, further comprising a cleaning nozzle for spraying a fluid onto the inner periphery of the spray pipe.

5. 2. The underwater cable burying device according to claim 1, further comprising a recovery wire attached to said injection pipe.

6. 1. An underwater cable burying method for burying a cable in the water bottom while paying out the cable from a moving work barge toward the water bottom, comprising: a water bottom fluidization step in which a fluid is sprayed from a spray pipe movably fitted onto a portion of the cable above the water bottom surface toward the water surface while spraying the fluid from the spray pipe toward the water bottom to fluidize the water bottom; a cable sinking step in which a portion of the cable that has been paid out onto the fluidized water bottom ground and that is located beyond the end face of the jet pipe on the water bottom ground side is allowed to sink by its own weight; a position return step in which the jet pipe is retracted along the cable, and a newly submerged portion of the cable is exposed beyond the end face of the jet pipe on the water bottom ground side; and A method for burying an underwater cable, comprising burying the cable in the waterbed.

7. The underwater cable burying method according to claim 6, wherein the position returning step stops the injection of fluid toward the water surface side and retracts the injection pipe along the cable by injecting fluid toward the bottom ground side.

8. 8. A method for burying an underwater cable according to claim 6 or 7, wherein the depth of the jet pipe from the water surface is calculated based on the water pressure measured by a pore water pressure gauge attached to the jet pipe, and when the jet pipe reaches a predetermined depth, the jet pipe is retracted along the cable.

9. 8. The underwater cable burying method according to claim 6 or 7, wherein the depth to which the jet pipe is buried in the bottom ground is calculated based on the earth pressure measured by an earth pressure meter attached to the jet pipe, and when the jet pipe reaches a predetermined depth, the jet pipe is retracted along the cable.

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