GB2242788A - System for laying cables or pipes on the sea bed - Google Patents

Abstract

A system for laying on the sea bed 18 a cable or pipe 12 stored in a vessel (Figs 1 & 2) which tows along the sea bed a sledge or trolley 16 for laying the cable 12, includes guide means 50 in the sledge for varying the tension in the cable. The guide means 50 may be rollers or a conveyor driven by a hydraulic motor 62 which is governed by a control system (Fig 4). The tension (TSM Fig 4) of the cable as it leaves the sledge is measured at 70, slack (BM) in the cable at 76, and the curvature of the cable as it enters the sledge at 78. These values are fed to a control system together with measurements of pressure supply (PM) to motor 62, tension (TN) as the cable leaves the vessel, depth (ZM) of sledge, the rotational speed (NM) of the driving mechanism 54, a desired tension value (TSC) and a control value (BC) set by the operator. The system may be used for optical or electric cables. <IMAGE>

Description

SYSTEM FOR LAYING A CABLE ON A SEA BED BY MEANS OF A CABLE LAYING VESSEL The invention relates to a system for laying on a sea bed a cable or flexible tube stored in a cable laying vessel which tows along the sea bed a trolley for laying the cable, through which trolley the cable passes and which comprises means for guiding the passage of the cable in the trolley.

The invention relates more particularly to the laying of submarine telecommunication cables.

In order to protect them against any damage which they might be caused by the anchors of vessels or trawl nets, submarine telecommunication cables are laid on the sea bed according to the technique called "embedding".

In order to bury the cable as it is unwound, the cable laying vessel tows a plough which slides over the sea bed and the share of which digs a trench approximately 1 metre deep. The telecommunication cable is unwound from the cable laying vessel as the latter advances and is guided along the axis of the plough before finally falling to the bottom of the trench, with the rear part of the plough possibly being provided with means for closing the trench over the cable which has thus been laid.

The risk of the cable being caught up by trawl nets or anchors makes it necessary to put this technique into practice up to depths of 1000 metres.

Although this technique substantially reduces the frequency with which submarine cables have to be repaired, it does have its drawbacks. The internal stress in the cable is as a rule reduced by attempting to unwind it such that it extends substantially at a tangent to the horizontal at the point at which it enters the plough. The forward speed of the cable laying vessel and the plough which it tows and the resulting hydrodynamic stresses applied to the cable are not very high and the telecommunication cable adopts the form of a "catenary" between the vessel and the plough.

The longitudinal stress in the cable may reach 40 kN on entering the plough and 60 kN on leaving the cable laying vessel, with the entire length of the cable which has been laid on and/or buried in the sea bed remaining subject to a stress of 40 kN.

The most recent telecommunication cables comprise optical fibres which permit a far higher data transfer rate. Because of these optical fibres, submarine telecommunication cables pose a serious challenge, in terms of performance and cost, to radio telecommunications relayed by satellite.

Cables which comprise optical fibres can withstand longitudinal tensile loads of up to 6 tonnes, although this is only on a temporary basis.

If optical fibres are subjected permanently to high loads, the cables lose their qualities through attenuation or suppression of the transmitted signals.

The internal longitudinal stress in telecommunication cables using optical fibres therefore has to be maintained at a low value, for example below 5 kN, once the cable has been laid on the sea bed.

It is known, moreover, that sea beds are not flat. A characteristic sea bed profile consists of undulations similar to a series of dunes at spacings of between 100 and 200 metres and with hollows of up to 30 metres. A cable which is laid on a bed having a profile of this kind and which is permanently subjected to a substantial longitudinal tensile stress tends to straighten out so as to approach straight lines at tangents to the peaks of two consecutive dunes. The cable then leaves the trench in which the plough had laid it beforehand and is therefore no longer protected against anchors and trawl nets.

For the various reasons which have just been explained, the internal longitudinal stress in the laid cable therefore has to be reduced as far as possible.

It must, however, be possible to adjust the stress in the cable, once it has been laid, to a given value of between 0 and 5 kN. The stress should not in fact be too low, as the cable, which was previously wound up in the manner of a helix on the cable laying vessel, is subjected to high stresses when passing from sea level and, once laid on the bed, would tend to form undulations and/or loops which would not be conducive to laying it correctly in the trench.

Various solutions have already been proposed to satisfy this requirement.

It has, for example, been proposed that the cable be prevented from forming into a "catenary", which is the cause of the internal stress in the laid cable.

For this purpose all that is required is to unwind the cable sufficiently quickly when the cable laying vessel advances. The cable then descends to the bottom of the water substantially vertically and its stress on the bed is zero. Its stress on leaving the vessel is equal to the weight in the water of the length of the cable, which is equal to the depth and, in the above-mentioned conditions, it is only 26 kN.

However the cable which is thus laid on the bed with a zero stress value then comprises undulations between the point at which it contacts the bed and the point at which it enters the laying plough, i.e.

over a distance of approximately 1500 metres in the preceding example. So, in order for the cable to enter the plough correctly, i.e. for it to be directed along the axis of the latter, the plough which is towed by the vessel must be guided longitudinally along the bed, an operation which necessitates a complex system which is not entirely satisfactory.

A second solution to prevent the cable from forming into a "catenary" has been proposed. This lies in guiding the cable along the tow line of the plough in a guide tube which is coupled to the tow line. The telecommunication cable thus enters the plough correctly and the stress in the cable on leaving the vessel is reduced by the effect of its friction in the guide tube.

However a guide tube of this kind is formed by tube sections of a plastic material which are approximately one metre long, i.e. more than 1700 tubes in the conditions of the previous example, which must be coupled to or uncoupled from the tow line according to whether the latter is unwound or taken up again when the depth varies. The latter operation is complicated and this second solution therefore also fails to give complete satisfaction.

The object of the present invention is to propose a system which enables the problems inherent in the laying techniques of the prior art to be solved in a simple and reliable manner.

With this object in view, the invention proposes a system for laying a submarine cable of the above-mentioned type which is characterised in that the trolley comprises means for varying the longitudinal stress in the cable during its passage in the trolley.

According to other characteristics of the invention: - the means for varying the stress in the cable apply to the latter a longitudinal tensile stress in the direction of displacement of the cable with respect to the trolley; - the means for applying the tensile stress to the cable comprise a non-slip friction drive apparatus which co-operates with the external surface of the cable; - the drive apparatus comprises at least two opposite wheels with parallel axles, at least one of which is a driving wheel and which grip the cable between them while applying to it a given transverse pressure;; - the drive apparatus comprises a belt or a caterpillar, which is supported by rollers and driven by the driving wheel, and a rectilinear track, with wheels or rollers, which is parallel to the plane of the driving belt and presses the cable against the opposite face of the driving belt so as to apply to it the given transverse pressure; - the system comprises a device for regulating the longitudinal tensile stress applied to the cable according to parameters which are representative of the state of the cable; - the regulating device comprises a sensor for the value of the stress in the cable at the exit of the drive apparatus, and the control of the drive motor of the driving wheel is subject to this stress value of the cable; - the drive motor of the driving wheel is, for example, a hydraulic motor; and - the regulating device comprises a sensor for the profile of the cable downstream of the drive apparatus; The following description of the accompanying drawings, which are given by way of non-limiting example, will provide a good understanding of the way in which the invention can be put into practice.

Figure 1 is a diagrammatic perspective view of the entire system for laying a submarine telecommunication cable.

Figure 2 is a part-sectional diagrammatic view of the bow of a cable laying vessel for laying a submarine cable.

Figure 3 is a part-sectional diagrammatic view of a plough, which is formed according to the teaching of the invention, for laying a submarine cable.

Figure 4 is a block diagram showing the cable drive device control system.

Figure 1 shows a cable laying vessel 10 for laying a submarine telecommunication cable 12.

Using a tow line 14, the vessel 10 tows a laying plough 16 which digs in the bed 18 of the sea 20 a trench 22 which is provided to receive the cable 12.

The vessel 10 is also connected to the plough 16 by a cable 24 which supplies the plough with electrical power and transmits various information and signals in both directions between the vessel 10 and the plough 16. In particular, a pressure sensor (not shown), which is mounted on the plough 16, indicates to the vessel 10 the submersion level ZM of the plough 16.

The considerable stress applied to the share of the plough which is necessary to dig the trench 22 in the bed 18 tightens the tow line 14. The length of the tow line 14 is regulated by a winch (not shown) which is mounted at the stern of the vessel.

For a given measured depth ZM, the length of the tow line 14 is determined such that the latter is slightly inclined with respect to the horizontal and does not tend to make the plough 16 move away from the bed 18.

The tow line 14 thus forms an angle of approximately 30 degrees with respect to the horizontal.

In an example of use in which the depth of the sea is 900 metres, the plough 16 lags behind the vessel 10 by approximately 1500 metres.

On the other hand, the telecommunication cable 12 is not taut. By increasing the length of the cable 12, in the section of water, as a result of unwinding it from the vessel 10, the internal longitudinal tensile stress in the cable is advantageously reduced at various points. The cable 12 is gradually lowered until is extends substantially at a tangent to the plough 16 at the point at which it enters the latter.

A telecommunication cable 12 of this kind, which is subjected to hydrodynamic stresses which are low in relation to its weight, more or less adopts the form of a catenary of a length, in the example considered, of 1800 metres.

For a submerged mass per unit length, i.e. taking account of the reduction due to buoyancy, of 3 kg/m, the internal longitudinal tensile stress in the cable reaches 41 kN applied horizontally at the level of the plough and 67 kN applied obliquely at the exit of the vessel.

Even if they are applied to a cable comprising optical fibres, stresses of this kind do not permanently affect the behaviour of the optical fibres, provided that they are only applied for the sufficiently brief period during which each finite section of the cable is laid, i.e. for half an hour if the vessel advances at a speed of 1 metre per second.

The telecommunication cable 12 is submerged in the water from the bow or the stern of the cable laying vessel 10 such that, due to its weight and its curvature, it is always disposed in front of and below the tow line 14.

The supply and remote control cable 24 is provided with floats 26 which are spaced at regular intervals over its entire length in order to increase its buoyancy and hydrodynamic drag so that it is always situated above and behind the tow line 14.

Figure 2 shows how the cable 12 is submerged at the bow 28 of the vessel 10 after passing over a grooved pulley 30.

The cable 12 comes from a chamber 32 which is disposed in the hold of the vessel 10 and in which it is stored wound up in the form of a cylinder, coiled with an angular twist of one lay per turn, so that it is unwound without twisting.

The cable 12 is driven in the longitudinal direction between the chamber 32 and the grooved pulley 30 by a so-called "tyred" drive machine.

The cable 12 is gripped between several pairs of tyred wheels in this machine 34. The wheels drive the cable by friction in a non-slip manner so as to pull and remove it from the chamber 32 and restrain it in the opposite direction when it is driven by its own weight to leave the vessel.

A sensor 36 measures the value of the longitudinal stress TN in the cable on leaving the vessel 10.

It was seen in the previous example that this stress, which is substantially proportional to the depth of the sea, is approximately 67 kN when the submersion level ZM of the plough is 900 metres, so that the telecommunication cable 12 enters the plough at a tangent to the latter in a substantially horizontal direction.

Upon reading the indication of the value TN delivered by the sensor 36, the task of the operator 38 supervising the unwinding of the cable 12 on the vessel is to maintain this stress at the indicated value of 67 kN. For this purpose the operator varies the supply pressure of the hydraulic motor of the tyred machine 34, the tensile stress applied to the cable being substantially proportional to the value of this pressure.

The plough 16 shown in Figure 3 is an embedding plough which is towed by the line 14.

The plough 16 is provided at the front with a runner 40 and at the back with two side runners 42, by means of which runners it slides over the bed 18 of the sea.

The share 44 of the plough 16 is disposed between the two back side runners 42 and penetrates the bed 18 to a depth of approximately 1 metre.

Due to the profile of the share 44 and the weight of the plough 16, the share 44 remains sunk in the bed 18, although the weight of the plough is decreased by the vertical component of the tensile stress applied to the plough 16 by the tow line 14. When the plough advances with the cable laying vessel 10, which unwinds the telecommunication cable 12, the latter enters the plough at the front, on the left- hand side of Figure 3, and passes through it to emerge from it again at the back.

At the back of the plough 16 an arm 46, which is provided with grooved rollers 48, for advancing the cable 12 forces the latter to enter and sink into the trench 22 dug by the share 44.

According to the invention, during its passage in the plough 16, the cable 12 passes through an apparatus 50, the object of which is to vary, by reduction, the internal longitudinal stress in the cable between its entrance E and exit S.

The device 50 is a non-slip friction drive apparatus for the cable 12 which comprises a belt in the form of a driving caterpillar 52 which is mounted on two wheels 54 and 56 and the upper run 58 of which is supported substantially horizontally by rollers 60.

The wheel 54 is a driving wheel which is rotated by a hydraulic motor 62.

The cable 12 passes over the upper plane face of the upper run 58 of the caterpillar 52, against which it is pressed by a track 64 which is provided with rollers.

The track 64 presses the cable 12 against the caterpillar 52 by means of a hydraulic jack 67.

The caterpillar 52 and the track 64 could of course be replaced by any other similar apparatus such as, for example, a tyred machine similar to the machine 34 with which the cable laying vessel 10 is provided, without departing from the scope of the invention.

The plough 16 is also provided with a sensor for the value of the measured longitudinal exit stress TSM of the cable after its passage in the drive apparatus 50.

The sensor is formed by a lever 68 which is hinged at one of its ends to the arm 46 and which bears the rollers 48. The cable 12 undergoes a change of direction of approximately 70 degrees during its passage over the rollers 48, the value of the internal longitudinal stress to which it is subjected being expressed by a force which is applied to the lever 68 and which is measured by a stress sensor 70.

The cable 12, which is highly inflexible, is substantially in the form of an "S" between leaving the apparatus 50 and its passage over the rollers 48.

The "S" profile of the cable 12 varies greatly with the internal stress in the cable.

The plough is also provided with a "slack" detector device which is mounted on the arm 46 and the function of which is to deliver a signal which is representative of the profile of the cable downstream of the exit "S" of the apparatus 50.

This sensor is formed by an arm 72 bearing a grooved pulley 74 which rests on the cable 12 and causes an angular displacement sensor 76 to rotate. The sensor 76 delivers a signal BM which, within a certain range, represents with a high degree of accuracy the value of the slack BM when the stress in the cable 12 is zero or negative.

In addition to the above-mentioned pressure sensor, the plough comprises an angle sensor 78, which enables the curvature of the cable to be measured upon entering the plough, and a rotational speed sensor (not shown), which measures the rotational speed NM of the driving wheel 54.

A hydro power plant which is not shown and comprises in particular an electric asynchronous motor, a variable capacity hydraulic pump and a proportional control pressure relief device is used to power the slow hydraulic motor 62 of the drive device 50.

The tensile stress exerted by the apparatus 50 on the cable 12 in the direction corresponding to its direction of displacement with respect to the plough 16 is substantially proportional to the hydraulic pressure PM supplied to the motor 62.

While operating, the drive device 50 thus applies to the cable 12 a tensile stress which opposes the internal longitudinal tensile stress applied to the cable before it enters the plough 16 and which is thus subtracted from this internal tensile stress.

The internal longitudinal tensile stress in the cable 12 at the exit "S" of the apparatus 50 is therefore equal to the difference between the value of the internal stress in the cable at the entrance "E" less the tensile stress which is applied to it by the apparatus 50.

The apparatus 50 thus acts as an internal stress reducer for the cable 12.

The sensor 70 for the measured exit stress TSM enables the internal stress in the telecommunication cable 12 to be measured at the very point at which it enters the trench 22, in which the desirable stress in the laid cable is zero or at least very low.

In order to control this stress, the hydraulic drive motor 62 of the apparatus 50 is controlled according to the control diagram shown in Figure 4.

The stress TSM in the cable laid on the bed is compared with the controlled exit stress TSC displayed by the operator in a comparator 80.

According to whether the difference between the values TSM and TSC is positive or negative, a regulator 82 for the control pressure of the drive motor 62 increases or reduces the value of the hydraulic pressure PCR supplied to the motor 62 and thus the value of the tensile stress applied to the cable 12 by the apparatus 50. The calculation step which enables the pressure PCR to be varied takes approximately several tenths of a second.

The variation in the S form or the "slack" of the cable which is measured by the angle sensor 76 enables the low values of the exit stress TSM to be accurately controlled. The measured output signal BM of this sensor 76 is thus compared with a control value BC, which is displayed by the operator, in a corrector 84 for correcting the cable slack downstream of the apparatus 50. The corrector 84 calculates a hydraulic pressure variation PC which increases or reduces the tensile force applied to the cable 12 by the apparatus 50 and thus contributes to the automatic control of the unit.

Finally, the delays inherent in the inertia and friction of all the components of the mechanical system have to be compensated by anticipating the various phenomena to give the control value to the motor 62.

This correction is carried out by means of a corrector 86 for the value of the tensile stress applied to the cable 12 on entering the plough 16, which corrector effects a weighting operation according to the rotational speed NM, the supply pressure PM of the motor 62, the internal stress TN in the cable 12 on leaving the cable laying vessel 10 and the submersion depth ZM measured by the pressure sensor of the plough 16.

Should the operation of the tyred machine 34 unexpectedly give rise to an increase in the stress TN as the cable leaves the vessel, or should the depth ZM increase rapidly, this would be expressed by an increase in the internal stress in the cable on leaving the plough, taking account of the mechanical inertia of the system. The corrector 36 makes it possible to anticipate this increase by immediately increasing the supply pressure PM of the motor 62 of the apparatus 50 on account of an algorithm which is brought into play by the corrector 86 and which enables an additional pressure value PA for the supply to the motor 62 to be determined.

Numerical switches IA, IR and IC enable the three automatic control loops for the motor 62 to be activated.

The block diagram of Figure 4 shows how the apparatus 50, which co-operates with the cable 12, subject on the one hand to its hydraulic supply pressure and on the other according to the stress in the cable on entering the plough 16, enables an exit stress TSM and slack BM to be obtained in the cable.* According to a further feature of the invention, all the calculations relating to the automatic control are carried out by a microcomputer which is installed on the cable laying vessel 10.

A control keyboard is used by the operator to input the data. The control instructions are then sent by the microcomputer or by the operator, according to whether the operating mode is automatic or manual, to the plough 16 via the cable 24.

The computer screen enables the operator to control his operations and to keep informed of the working state of the system, all the information and signals being transmitted in both directions via the telecommunication cable 24.

The system which has just been described applies both to the laying of transmission cables and to electricity supply cables or to flexible tubes carrying gas or other fluids, whether or not embedded.

Moreover, the system allows a submarine cable to be laid continuously and in a single pass while assuring the value of the residual stress in the cable at the end of the embedding operation.

Claims (12)

1. A System for laying on a sea bed a cable or flexible tube stored in a cable laying vessel which tows along the sea bed a trolley for laying the cable, through which trolley the cable passes and which comprises means for guiding the passage of the cable in the trolley, wherein the trolley comprises means for varying the longitudinal stress in the cable during its passage in the trolley.

2. A system as claimed in claim 1, wherein the means for varying the stress in the cable apply to the latter a longitudinal tensile stress in the direction of displacement of the cable with respect to the trolley.

3. A system as claimed in claim 2, wherein the means for applying a tensile stress to the cable comprise a non-slip friction drive apparatus which co-operates with the external surface of the cable.

4. A system as claimed in claim 3, wherein the drive apparatus comprises at least two opposite wheels with parallel axles, at least one of which is a driving wheel and which grip the cable between them while applying to it a given transverse pressure.

5. A system as claimed in claim 3, wherein the drive apparatus comprises a belt or a caterpillar, which is supported by rollers and driven by the driving wheel, and a rectilinear track, with wheels or rollers, which is parallel to the plane of the driving belt and presses the cable against the opposite face of the driving belt so as to apply to it the given transverse pressure.

6. A system as claimed in any of claims 3 to 5, wherein it comprises a device for regulating the longitudinal tensile stress applied to the cable according to parameters which are representative of the state of the cable.

7. A system as claimed in claim 6, wherein the regulating device comprises a sensor for the value of the stress in the cable at the exit of the drive apparatus.

8. A system as claimed in claim 7 in combination with claim 4 or claim 5, wherein the control of the drive motor of the driving wheel is subject to the said stress value of the cable.

9. A system as claimed in claim 8, wherein the drive motor is, for example, a hydraulic motor.

10. A system as claimed in any of claims 6 to 9, wherein the regulating device comprises a sensor for the profile of the cable downstream of the drive apparatus.

11. A system for laying cable on a sea bed substantially as hereinbefore described.

12. Apparatus for laying cable on a sea bed substantially as hereinbefore described with reference to and as illustrated in Figures 1 to 4 of the accompanying drawings.