GB2265685A - Laying pipe and cable under water - Google Patents

Description

2265685 METHOD AND APPARATUS FOR LAYING UNDERWATER PIPELINES The invention

relates to the laying of at least one underwater service line from an offshore laying vessel.

The term service line as used herein is intended to mean a small diameter pipe, preferably of metal, or an umbilical comprising a plurality of small diameter pipes, also preferably of metal, which are bound together, the pipes preferably being twisted helically around a central core.

In one particular application, the invention may be employed in the simultaneous laying of an electrical signalling and power supply cable and a number of steel pipes, either separate or in an umbilical, for hydraulic and other fluids for the remote control and operation of a sub-sea station of a gas or oil recovery well or manifold.

According to the invention there is provided a method of laying an offshore service line from a vessel in which the service line is unwound continuously from a storage reel which is mounted on the vessel and is driven to 2 - unwind the service line at the required rate, the unwound service line isa guided to form a substantially tensionless loop, and holding power is applied to the service line downstream of the loop to control the catenary of the service line between the vessel and the sea bed, the rate at which the storage reel is driven to unwind the service line being controlled by sensing any increase or decrease in the slack of the loop relative to a predetermined value.

The invention further provides a vessel for laying an offshore service line, comprising means for supporting a service line storage reel for rotation about its axis, driving means for rotating the reel to unwind the service line as required, a tensioner for acting on the service line so as to maintain the service line in tension as it is unwound, means for guiding the service line overboard into the water for laying on the sea bed, and means for applying holding power to the service line to control the catenary of the line between the vessel and the sea bed, the vessel including a curved roller guide path downstream of the tensioner for causing the service line to form a substantially tensionless loop, the curved roller guide path being provided with a loop position sensor which is responsive to any increase or decrease in the slack of the service line loop relative to a predetermined value and which is operative to control the driving means for the service line storage reel so as to restore the service line loop to the predetermined desired position on the guide path.

There may of course be more than one service line laid at the same time.

The service line pipe or pipes may be laid empty, but preferably will be laid filled with fluid at a certain overpres sure.

When the service line is an umbilical formed by a plurality of pipes twisted helically around a central core it is preferably unwound from a storage reel of a diameter such that substantially no permanent deformation is imparted to the umbilical This is not necessary when the service line is formed by an individual pipe, but if a curvature is imparted to the pipe as a result of being wound on its storage reel, it is preferable to remove the residual curvature by straightening the line after it leaves the reel.

To assist further understanding of the invention, one example of a laying vessel and a method of operation in accordance with the invention will now be described with 4 - reference to the accompanying drawings, in which:Figure 1 is a general arrangement deck plan of the laying vessel, which is designed for the laying of a service bundle consisting of a single cable and three service lines in the form of individual pipes; Figure 2 is a plan view showing part of Figure 1 to a larger scale and illustrating the deck region where the cable and service lines are brought together and strapped to each other to form the service bundle prior to being laid overboard; Figure 3 is an elevational view of the deck region of the vessel illustrated in Figure 2; Figure 4 is a diagrammatic side elevation of a straightener for removing the residual curvature from a service line after it has been unwound from its storage reel; Figure 5 is a diagrammatic end elevation of the straightener shown in Figure 4; Figure 6 is a plan view of the curved roller guide path section of the vessel in which the service lines are - formed into tensionless loops before being brought together and secured to the cable; Figure 7 is a plan view, to a larger scale, of the inlet end of the roller guide path showing more clearly the arrangement of three bending guide units for imparting a nominal loop curvature to the service lines entering the roller guide path; Figure 8 is a part sectional, detailed view showing part of one of the bending guide units of the roller guide path; Figure 9 is an elevational view of a part of the roller guide path showing the arrangement of horizontal support rollers for the service lines; Figure 10 is an elevational view of one of the service line loop position sensors of the roller guide path; Figure 11 is a view of part of the loop position sensor of Figure 6 shown to a larger scale; Figure 12 is a transverse section through the loop position sensor taken on the line XII-XII in Figure 11; 6 - Figure 13 is a vertical section taken on the line XIIIXIII in Figure 12;; Figure 14 is a detailed view of an emergency stop switch of the roller guide path.

Figure 15 is a front view of one of the tensioning units of the cable tensioning device; Figure 16 is a top plan view of the tensioning unit shown in Figure 15; Figure 17 is a vertical sectional view of the tensioning unit taken on the line XVII-XVII in Figure 15; Figure 18 is a scrap section through a part of the tensioning unit taken on the line XVIII-XVIII in Figure 15; Figure 19 is an elevational view of one of the friction shoes of a speed equalising tensioner located immediately upstream of the working station at which the cable and service lines are strapped together; Figure 20 is a cross-section through the strapped service bundle showing the spatial relationship of the cable and service lines in the bundle and the construction of the bundle strapping; Figure 21 is an elevational view of a strapping wagon of the working station, showing a clamp of the wagon in its open position; Figure 22 is an end view of the strapping wagon showing the clamp in its closed position; Figure 23 is a view similar to that of Figure 21, but showing an anode wagon of the working station; and Figure 24 is a view similar to Figure 20, but showing an alternative service bundle example consisting of an electrical cable and a service line umbilical.

The vessel shown is a self propelled, semi-submersible, multi purpose offshore support vessel 1 adapted for the continuous assembly and laying (and also retrieval) of an underwater service bundle 2 consisting of one multicore electrical cable 3 and three plastics coated stainless steel service pipelines 4 a,4 b and 4 c The outer diameter of the cable 3 is nominally 75 6 mm and the outer diameter of each coated service line 4 a,4 b,4 cis nominally 46 mm (stainless steel pipe 34 mm i d and 41 8 - mm o d,, and coating thickness 2 5 mm), although other cable and service line sizes can of course be handled.

In brief, the electrical cable 3 is unwound from a cable storage reel or carousel 5 which is mounted on the main deck 6 of the vessel 1 to rotate about a vertical axis, and is guided horizontally through a cable tensioning device 7 to a working area 8 extending in alignment with the tensioning device 7 towards the rear R of the vessel 1 In this description the terms horizontal and vertical are used with reference to the vessel floating levelly on a calm sea The three service lines 4 a,4 b and 4 c, which are preferably filled with a pressurised fluid, are unwound from separate storage reels 9 a,9 b and 9 c which are mounted on the deck 6 to rotate about horizontal axes, each service line being individually straightened by a straightening device l Oal Obl Oc under tension applied by a tensioner llallbllc downstream of the straightening device before being guided in a substantially tensionless loop around a curved roller guide path 12 which is positioned near the front of the deck and from which the service line 4 a,4 b,4 c leaves in a rearward direction towards the working area 8 The service lines 4 a,4 b and 4 c are vertically spaced from each other as they pass around the curved roller guide path 12, and between the downstream end of the guide path 9 - 12 and the working area 8 the service lines pass through a bundle assembly station 13 in which the axes of the service lines are brought into horizontal alignment with the axis of the electrical cable 3 prior to being strapped together with the cable 3 to form the service bundle 2 in the working area 8 The resulting service bundle 2 is guided overboard at the rear of the vessel by sliding over a fixed, substantially 900 stinger 14.

The cable carousel 5 is electro-hydraulically driven under manual control, and the required length of the electrical cable 3 to be laid will be prewound onto the carousel 5 from a cable transfer vessel by driving the carousel to rotate in the winding direction Associated with the carousel 5 is a conventional cable guide or spooling device 15 to facilitate the winding and unwinding of the cable During winding, a conventional tensioner (not shown) installed on the vessel I will pull the cable 3 from the cable transfer vessel.

During unwinding, the carousel 5 is driven to unwind the cable at a rate sufficient to maintain the cable in a slack condition between the spooling device 15 and an entry guide 17 of a guide channel 18 located at the front of the vessel and leading the cable into the straight line path through the main cable tensioning device 7, the bundle assembly station 13 and the working area 8.

- The cable tensioning device 7, which will be described later in more detail, is hydraulically driven under manual control and determines the feed rate of the cable 3 therethrough, and hence the rate at which the service bundle 2 is laid (or retrieved) For example, this rate may be varied between zero and about five metres/minute.

The tensioning device 7 also serves the important function of applying holding power to the service bundle, through the cable 3, for controlling the catenary of the service bundle between the stinger 14 and the sea bed.

The coated stainless steel service pipelines 4 a,4 b and 4 c are prewound onto the respective storage reels 9 a,9 b and 9 c onshore, and the wound reels lifted into position on support cradles ready for use on the laying vessel 1 by means of a crane 19 In operation, each reel is rotated independently by a hydraulically driven winch 20 a,20 b,20 c associated with the respective support cradle to rotate the reel as necessary to ensure the correct unwinding of the respective service line 4 a,4 b,4 c Each serviceline is unwound under a substantially constant tension applied by the respective tensioner llallb,llc, the service line passing from the reel through a conventional spooling guide 21 a,21 b, 21 c and a fixed entry guide 22 a,22 b,22 c to the straightener device l Oa,10 b,l Oc upstream of the tensioner.

I 1 - Each straightener l Oa,l Ob,l Oc is of the construction illustrated in Figures 4 and 5, and is designed to reverse the plastic bending imparted to the service line as a result of being wound on its storage reel The straightener device is of the five point type and comprises a fixed frame 23 on which five rollers 24,25, 26,27 and 28 are mounted to engage the upper and lower surfaces of the service line alternately as it is drawn through the straightener as shown in Figure 4 Each of the rollers is of steel and has a polyurethane coated peripheral groove having a profile corresponding to the radius of curvature of the service line circumference.

The upper rollers 24,26 and 28 are fixed in position, and the lower rollers 25 and 27 are each adjustable in position both vertically (i e perpendicularly to the direction of movement of the service line 4 through the straightener) and horizontally in a direction parallel to the direction of movement of the service line 4 For this purpose each roller 25,27 is individually mounted on a carriage 29,30 which is adjustable horizontally along the frame 23 and on which the roller can be adjusted vertically The rollers 25 and 27 are in fact mounted on hydraulic jacks 31,32 which are installed on the carriages 29,30 and are designed to move the lower rollers 25 and 27 quickly into and out of the operative pipe engaging position.

12 - The service line tensioners lla,llb,llc are of conventional back tensioner design, each comprising three successive pairs of grooved wheels or rollers between which the respective service line passes The wheels of each pair are hydraulically driven and are urged towards each other to grip the service line with a predetermined force by means of a hydraulic cylinder acting through a system of levers and adjustable spring means.

Downstream from the tensioners llallb,llc, the service lines 4 a,4 b,4 c possess very low residual tension and are guided into the curved roller guide path 12 so as to pass therearound in substantially horizontal planes at different heights from each other As each service line passes around the guide path 12, the degree of slack in the line 4 a,4 b,4 c is sensed by a position sensor 33 a,33 b, 33 c and used to control the corresponding laying winch a,20 b,20 c driving the respective service line storage reel so that the line is unwound at the required rate.

The curvature of the guide path 12 should be sufficiently large to avoid imparting a permanent bend to any of the service lines, and in the present example the radius of the path 12 is about 14 metres.

As shown in Figures 6 and 7, the curved roller guide path 12 comprises three inlet bending guide units 50 a,50 b,50 c 13 - for imparting a predetermined uniform bend to the service lines 4 a,4 b,4 c respectively which causes them to form loops La,Lb,Lc curving around the guide path 12, a substantially crescent shaped supporting structure 51 carrying the service line loop position sensors 33 a,33 b,33 c and three tiers 52 a,52 b,52 c of radially disposed, circumferentially spaced, horizontal support rollers for supporting the service line loops at intervals around the guide path, and three outlet bending guide units 53 a,53 b,53 c for straightening the service lines 4 a,4 b,4 c respectively as they leave the guide path 12 towards the bundle assembly station 13 by imparting a reverse bend to the service lines which is equal and opposite to that imparted by the inlet bending guide units.

In this example the inlet guide units 50 a,50 b,50 c are located at different circumferential positions relative to the start of the supporting structure 51, and each comprises a rigid support frame 54 a,54 b,54 c which is fixed to the deck 6, and a horizontal row of three pairs of opposed guide wheels 55 a,56 a; 55 b,56 b; 55 c,56 c mounted on the support frame at the level at which the respective service line enters the guide path 12 The wheels of each pair are rotatable about vertical axes and are arranged so that their peripheries engage radially 14 - opposite sides of the service line, and the three pairs of wheels of each inlet bending guide unit are arranged relative to each other so as to impart the desired bend to the service line as it passes through.

The construction and arrangement of one of the pairs of guide wheels 55 a,56 a of the inlet bending guide unit 50 a is shown in more detail in Figure 8, and it will be understood that the other pairs of guide wheels of the three units 50 a,50 b,50 c will be arranged in a similar manner Each of the guide wheels 55 a,56 a is made of a suitable plastics material, such as polyurethane, and is rotatably mounted on a vertical stub axle 57,58 through the intermediary of a bearing 59,60, e g of bronze, respectively The axle 58 of the radially inner wheel 56 a is secured to a horizontal slide plate 61 which is adjustable transversely with respect to the service line on horizontal support flanges 62 fixed to the support frame 54 a Similarly, the axle 57 of the outer guide wheel 55 a is secured to a second horizontal slide plate 63 which is adjustable transversely with respect to the service line on the first slide plate 61, the second slide plate 63 having an aperture 64 to accommodate the axle 58 of the inner wheel 56 a The positions of the two slide plates 61 and 63 are adjusted relative to each other and to the fixed support flanges 62 to set the - wheels 55 a and 56 a in the desired position with respect to each other and the passage of the service line 4 a therebetween, and are then suitably locked in position to maintain the setting of the wheels.

As shown, the peripheries of the wheels 55 a and 56 a are provided with a concave profile to ensure that the service line 4 a passes centrally between them As an additional safety precaution, however, the support frame 54 a of the bending guide unit is provided with fixed inlet and outlet guides each formed by a pair of rods 65,66 which are secured to the support frame 54 a and extend horizontally to positions above and below the desired path of the service line 4 a at positions spaced upstream and downstream from the first and third pairs of wheels respectively.

The three outer bending guide units 53 a,53 b,53 c are similar to the inlet bending guide units in that they each comprise a horizontal row of three pairs of opposed guide wheels 67,68 mounted on a support frame at the level at which the respective service line passes around the guide path 12 In the case of the outlet bending guide units, however, the three horizontal rows of opposed guide wheel pairs 67,68 are located one above the other on a common support frame 69 which is fixed to the 16 - deck 6 adjacent the outlet end of the supporting structure 51 The construction and arrangement of each pair of opposed guide wheels 67,68 of each outlet bending guide unit is similar to that of each pair of wheels a,56 a of the inlet bending guide units as described above with reference to Figure 8.

In addition to effecting the desired bending and straightening of the service lines at the entrance and exit of the curved roller guide path 12, it will be appreciated that the inlet and outlet bending guide units also act to support and restrain the service lines at predetermined positions at opposite ends of the loops La,Lb,Lc which curve around the guide path 12 Thus, any difference which occurs between the rate at which a service line 4 a,4 b,4 c enters the roller guide path 12 and the rate which it leaves the guide path will result in a change in the length of the service line forming the loop between the respective inlet and outlet bending guide units, and hence a change in the position of the loop relative to the fixed structure of the curved guide path 12 It is such changes in the position of each service line loop which are monitored by the respective loop position sensor 33 a,33 b,33 c, as described later in order to control the delivery rate of each service line.

17 - The cresent shaped supporting structure 51 of the curved roller guide path 12 comprises a series of vertically disposed, rigid rectangular support frames 70, for example comprising steel girders bolted or welded together, which are oriented radially with respect to the curved guide path 12 and are spaced at angular intervals around the path Each frame 70 is fixed firmly to the deck 6 of the vessel, and is also connected to its neighbouring frame or frames at the upper ends thereof by inner and outer rigid connecting members 71,72 respectively which are themselves rigidly connected by a diagonal bracing member 73.

As shown in Figure 9, each of the vertical rectangular support frames 70 supports one of the rollers of each of the three tiers 52 a,52 b,52 c of the horizontal loop support rollers, and in addition supports an uppermost horizontal retaining roller 74, each of the rollers having its axle secured at opposite ends to the radially inner and outer upright members 75 and 76 of the frame Radially inwardly of the outer frame member 76 and on the downstream side of the horizontal rollers, the upper and lower transverse members 77 and 78 of the frame support between them a vertical roller 79 which defines the outer limit of the guide path for the service line loops as indicated at La', Lb' and Lc' The 18 - radially inner limit of the guide path for the service line loops is indicated in Figure 9 at La", Lb" and Lc", and is defined by safety rails 80 a,80 b and 80 c which are fixed to the inner upright member 75 of the frame 70 between the horizontal rollers 74, 52 a,52 b and 52 c The safety rails 80 a,80 b,80 c extend continuously from the respective inlet bending guide unit 50 a,50 b,50 c to the respective outlet bending guide unit 53 a,53 b,53 c along a curved path which has a radius sufficient to avoid damage to the service lines by overbending, and in this example is about 14 metres.

The three loop position sensors 33 a,33 b, and 33 c of this example are located near the apex of the curved roller guide path 12 at positions circumferentially spaced apart downstream, intermediate and upstream of the two central support frames 70 of the guide path supporting structure 51 as indicated in Figure 6 and approximately level with the respective loop support rollers 52 a,52 b,52 c.

Essentially, each loop position sensor 33 a,33 b,33 c comprises a carriage 81 which is freely movable along a track 82 extending transversely across the curved roller guide path 12, means 83 on the carriage for coupling the carriage to the respective service line loop La,Lb,Lc in a manner which allows the service line 4 a,4 b,4 c to travel 19 - freely around the curved guide path 12 but which causes the carriage to move along the track in response to inward or outward movement of the loop relative to the curved path, and detection means 84 which is responsive to movement of the carriage to provide a signal corresponding to the position of the carriage on the track and hence to the position of the loop on the guide path.

Figures 10 to 13 illustrate the construction of the loop position sensor 33 b of this example in detail, and it will be understood that the other position sensors 33 a and 33 c are constructed in a similar manner.

As shown, the track 82 comprises a single upright rail extending radially across the guide pathl between a position inward of the safety rail 80 b to a position outward of the vertical roller 79 The track rail 82 is laterally spaced from and securely fixed to a rigid support member 85 which extends across the guide path parallel to the track rail and is suspended at opposite ends from the inner and outer rigid connecting members 71,72 of the supporting structure 51 by rigid vertical members (not shown).

The carriage 81 comprises an upright plate 86 disposed - laterally of the track rail 82, and two spaced pairs of upper and lower wheels 87,88 which are mounted on the plate 86 so as to engage and run smoothly on the upper and lower edges respectively of the rail 82, each of the wheels 87,88 having a peripheral groove 87 a,88 a which receives the respective edge of the rail The means 83 for coupling the carriage 81 to the respective service line loop Lb comprises a yoke which receives the loop and is formed by a pair of upright posts 89 which are fixed to the plate 86 and project upwardly therefrom at its radially inner and outer ends to define a gap between the posts 89 As can be seen from Figure 10, the carriage plate 86 is located below the level of the upper surface of the horizontal loop support rollers 52 b on which the service line 4 b travels, and the posts 89 project upwardly above the rollers 52 b to bracket the service line.

The carriage position detection means 84 comprises a continuous chain 90 to which the carriage plate 86 is fixed and which extends around a pair of sprocket wheels 91 secured on axles 92 which are rotatably mounted on the support member 85 near opposite ends thereof The chain extends parallel to the track rail 82 on the opposite side of the carriage plate 86 from the rail 82, and is supported along most of its length between the sprocket 21 - wheels 91 by upper and lower fixed chain guide bars 93 and 94 carried by a support assembly 95 fixed to and extending from the main support member 85 The chain 90 has a bracket 96 fixed to a part of its upper run, and the bracket in turn is bolted to the side of the carriage plate 86 near its upper edge so that the carriage 81 and the chain 90 are firmly connected to each other With this arrangement, any radial movement of the service line loop Lb passing between the posts 89 of the carriage 81 is transmitted to the carriage, causing it to move correspondingly along the rail 82 This movement is transmitted, in turn, by the bracket 96 to the chain 90, which causes the sprocket wheels 91 to rotate proportionally to the distance and direction moved by the carriage The carriage position detection means 84 further comprises a detector 97 which is mounted on the support member 85 and is arranged to respond to rotation of one of the sprocket wheel axles 92 to generate an output signal proportional thereto, and hence proportional to the radial movement of the carriage 81 and the service line loop Lb coupled therewith.

The output signal from the detector 97 thus enables the position of the service line loop to be monitored, and is supplied to a controller (not shown) which is operative to increase or decrease the speed of the winch 20 b 22 - driving the service line storage reel 9 b in a manner such as to increase or-decrease the rate at which the service line 4 b is unwound as necessary to restore the service line loop Lb to a predetermined desired radial position on the guide path 12.

In the event of the service line position regulating system failing to prevent any of the service line loops moving radially inwards on the guide path 12 until it reaches the respective safety rail 80 a,80 b,80 c this rail acts to prevent further inward movement of the loop in order to avoid damage to the service line as a result of overbending The guide path 12 is in fact fitted with emergency stop switches 98 a,98 b,98 c mounted on the safety rails 80 a,80 b,80 c respectively, each of which is arranged to be activated to shut down the operation of the laying apparatus when the respective service line 4 a,4 b,4 c moves into engagement with the safety rail 80 a,80 b,80 c The guide path 12 is further provided with similar emergency stop switches 99 a,99 b,99 c positioned near the outer limit of the guide path determined by the vertical rollers 79, each of these outer switches 99 a,99 b,99 c also being arranged to shut down the operation of the laying apparatus if engaged and activated by the respective service line loop La,Lb,Lc.

23 - Each of the inner and outer emergency stop switches 98,99 is of similar construction, and for the purposes of illustration the construction of one of the outer stops 99 is shown in Figure 14 The switch comprises a channel shaped support 100 carrying a switch contact housing 101 and a pivotally mounted actuator 102 which is biassed by a spring 103 to an inactive, rest position (shown in full lines) in which a sensor portion 104 of the actuator projects through an aperture 105 in the base of the channel shaped support The actuator 102 comprises a U-shaped member 106 which is pivotally mounted at 107 near the free ends of its legs, on the opposite side walls of the channel shaped support 100 so that the legs of the U-shaped member 106 lie inwardly of and parallel to the side walls of the channel shaped support Fixed to the cross bar 108 of the U-shaped member 106 is a plate 109 which projects towards the aperture 105 and which carries on its free edge the sensor portion 104 in the form of a cylindrical rod The cross bar 108 of the U-shaped member 106 further carries a central tab 110 projecting in the opposite direction from the plate 109, and the biassing spring 103 is provided by a coiled tensionspring connected between an upper attachment 111 fixed to the tab 110 and a lower attachment 112 fixed to the base of the channel shaped support 100.

24 - In use, the switch will be mounted by connecting the channel shaped support 100 in a suitable manner to the supporting structure 51 of the guide path so that the sensor portion 104, in its rest position, lies in a position where it will be engaged by the respective service line loop in the event the loop moves too close to the corresponding outer (or inner) limit of the guide path On engagement of the sensor portion 104 by the service line loop, further movement of the loop in the same direction will push the sensor portion of the actuator 102 back through the aperture 105 against the action of the biassing spring 103, bringing a cam member 113 mounted on the plate 109 into engagement with a switch contact closure member 114 of the switch contact housing 101 to actuate the switch.

If at any time the automatic control of any of the three service line laying winches 20 a,20 b,20 c in response to the respective loop position sensor 33 a,33 b,33 c fails to operate satisfactorily and cannot readily be corrected, the automatic control of that winch can be disconnected and the necessary control exercised manually instead.

Turning now to the construction of the cable tensioning device 7 of this example, the device consists of two identical tensioning units 7 ' which are mounted one after - the other on the deck 6 of the vessel to act in series on the electrical cable 3 Each tensioning unit 7 ' comprises a pair of driven, continuous caterpillars 115,116 which are disposed parallel to and opposite each other, and which are preferably adjustable relatively towards and away from each other, to exert a predetermined clamping force on the cable as it is advanced between them In this way a tension is applied to the cable 3, and hence to the service bundle 2 which is laid from the vessel.

As shown in Figures 15 to 18, the caterpillars 115,116 of each unit are mounted one above the other in upper and lower horizontal housings 117,118 respectively which support and guide the caterpillars, and which are themselves mounted on a structural support frame 119.

The lower caterpillar housing 118 is fixed relative to the frame 119, whereas the upper caterpillar housing 117 is secured to a carriage 120 which is arranged to be movable upwards and downwards along vertical guide tracks 121 on the support frame 119 by means of a pair of hydraulic jacks 122 acting between the carriage 120 and the base 123 of the frame 119.

Each caterpillar 115,116 comprises a continuous chain 124 which extends around a pair of sprockets 125,126 mounted 26 - at opposite ends of the respective housing 117,118 and which is supported between the sprockets in upper and lower chain guide chambers 127,128 The links of each chain 124 are fitted on each side with roller or slide bearings 129 which engage and run on a support surface of the respective guide chamber 127,128 In addition, each link of each chain has a cable engaging steel shoe 131 fixed to it on the outer side so that the shoe projects outwards from the guide chamber, each shoe having a suitably lined concave outer face 132 As shown in Figures 13 and 14, the shoes 131 of the portion of the upper caterpillar 115 running in the lower chain guide chamber 128 of its housing 117 face the shoes 131 of the portion of the lower caterpillar 116 running in the upper chain guide chamber 127 of the housing 118, and define a passage for receiving the electrical cable 3 in frictional engagement with the lined concave faces 132 of the shoes.

The forward sprocket 125 of each caterpillar 115,116 (i.e at the inlet end of the tensioning unit) is arranged to be driven by a reversible hydraulic motor 133 (or other suitable drive means) mounted on the respective housing 117,118 The rear sprocket 126 is an idle sprocket and is carried by an adjustable chain tensioning slide 134 mounted in the housing 117,118.

27 - In operation, the upper caterpillar housing 117 of each tensioning unit 7 ' is raised to allow the electrical cable 3 to be placed in the channel formed by the concave faces of the shoes on the upper run of the lower caterpillar 116 The upper housing 117 is then lowered by means of the hydraulic jacks 122 to bring the shoes 131 on the lower run of the upper caterpillar 115 into engagement with the upper surface of the cable 3 in order to grip the cable firmly between the shoes of the two caterpillars 115 and 116 with a predetermined clamping force The sprocket drive motors 133 of the caterpillars 115,116 of each tensioning unit 7 ' are then driven synchronously in order to feed the cable through the tensioning units 7 ' in the desired direction and at the desired rate As mentioned earlier, the motors 133 are reversible so that the tensioning device 7 can be used not only for paying out and tensioning the cable and service line bundle during a laying operation, but also for retrieving the service bundle should this become necessary.

The hydraulic motors 133 and the hydraulic jacks 122 of the two tensioning units 7 ' of the device 7 will be driven by a common hydraulic power unit operated from a control centre enabling control of the upward and downward movement of the upper caterpillar housings 117, 28 - and control of the direction and speed of movement of the cable through the device 7 as necessary Furthermore, either or both of the tensioning units 7 ' may be operated as desired.

Downstream of the roller guide path 12 and the cable tensioner 7 the three service lines 4 a,4 b and 4 c are guided into the bundle assembly station 13 spaced one above the other directly below the electrical cable 3 as shown in Figure 3 The assembly station 13 comprises a pair of successive gutter guide assemblies 34235 by which the service lines 4 a,4 b and 4 c are gradually brought into horizontal alignment with the electrical cable 3 (i e.

with their axes parallel and in a common plane), and a track tensioner device 36 which operates to equalise the speeds of the service lines in relation to the cable before they enter the working area 8.

The construction of the speed equalising tensioner 36 is essentially the same as that of one of the cable tensioning units 7 ' described above with reference to Figures 15 to 18, except in respect of the friction shoes 131 carried by the caterpillars 115 and 116, and in respect of the hydraulic motors 133 for driving the caterpillars, which in the speed equalising tensioner are arranged so that the caterpillars can be driven or 29 allowed to free-run as desired.

As shown in Figure 19, the friction shoe 135 which is mounted on each link of each caterpillar chain (not shown) of the speed equalising tensioner comprises a body 136 of firm yet resilient material, such as nitrile rubber having a Shore hardness value of from 70 to 75, having its upper (i e outwardly facing) portion formed with four parallel ribs 137, 138, 139 and 140 which are separated from each other by grooves 141 The upper face of each rib is concave and defines a channel 142, 143, 144, 145 having an arcuate profile The two outermost channels 142, 145 and one of the intermediate channels 143 have a radius of curvature corresponding to that of the service lines 4 a,4 b and 4 c, and the other intermediate channel 144 has a radius of curvature corresponding to that of the electrical cable 3, the channels being arranged so that their centres of curvature all lie in a common plane indicated by the dotted line 146.

The body 136 is bonded to a metal backing plate 147 to which is welded or otherwise fixed a mounting plate 148 having a pair of pins 149 by which the shoe 135 is fixed to its chain link.

- As will be appreciated, the shoes 135 will be fitted to the chain links so that their channels 142 to 145 lie parallel to the longitudinal axis of the chain and so that corresponding channels of successive shoes are aligned with each other and lie directly opposite the corresponding channels of the shoes of the opposing caterpillar.

Thus, the caterpillars of the speed equalising tensioner are arranged so that, in use, the electrical cable 3 and the service lines 4 a, 4 b and 4 c will be received in the respective channels of the caterpillar shoes and will be engaged frictionally and evenly thereby in dependence upon the clamping force exerted by the hydraulic jacks acting on the upper caterpillar housing In operation the caterpillars will move with the travel of the cable and the service lines through the tensioner 36, but because the cable and service lines are all gripped firmly between the caterpillars, relative movement between them is prevented and any difference between their speeds upstream of the speed equalising tensioner 36 will be eliminated immediately on entering the tensioner.

The operation of the speed equalising tensioner 36 is essential to the correct operation of the roller guide 31 - path 12 when laying or retrieving that length of the service bundle (usually about 500 meters) which will be left unbanded (termed the separation zone), the tensioner 36 serving to ensure that equal lengths of the cable and the service lines are laid or retrieved.

The electrical cable and the service lines leave the speed equalising tensioner 36 of the bundle assembly station 13 with their axes parallel and lying in a common horizontal plane, and pass in this condition to the working area 8 In the working area 8 the surface condition of the electrical cable 3 and service lines 4 a,4 b and 4 c is checked and made good as necessary, and the cable and service lines are secured together by strappings 37 applied at intervals (for example, approximately every 6 metres) along their length as shown in Figure 20 Periodically, for example at intervals of about 30 metres, a sacrificial anode is fixed to the service bundle by means of similar strappings 37, the anode being electrically conductively connected to one of the service lines to provide it with cathodic protection against corrosion Accordingly, each service line will have such an anode connected to it at about 90 metre intervals.

Each strapping 37 is formed by upper and lower strapping 32 - members 38 and 39 comprising stainless steel shells 40,41 containing moulded rubber inserts 42,43 defining seatings for the cable and service lines The rubber insert 42 of the upper strapping member 38 is formed in one piece, whereas the insert of the lower strapping member 39 is formed by two separate pieces 43 a and 43 b which are spaced from each other at 44 adjacent the electrical cable 3 The strapping members 38 and 39 are secured together by bolts 45 which extend through the upper member 38 into threaded sockets 46 embedded in the lower member 392 the bolts passing between the cable and service lines as shown.

Although bolted strappings 37 are used to secure the cable and service lines together in the present example, other suitable securing means may be used if preferred.

The working area 8 is provided with an overhead rail 150 forming a loop 151 extending along substantially the whole length of the working area and having a portion 152 which extends along and above the path of the service bundle 2 through the working area A plurality of strapping wagons 153 and anode wagons 154 are suspended from the overhead rail 150 for facilitating the application of the strappings and the sacrificial anodes respectively to the cable 3 and service lines 4 a,4 b,4 c as 33 - they pass through the working area The wagons 153 and 154 are freely movable along the rail 150 around the loop 151, and each has clamping means for temporarily fastening it to the cable and service lines to support and hold them in the required spaced relationship while they move through the working area and are strapped together.

As shown in Figures 21 and 22, each of the strapping wagons 153 comprises a carriage 155 having two twin wheel pair sets 156,157 mounted to run on the overhead rail 150, and a subframe 158 suspended from the carriage 155 so that the subframe is movable vertically relative to the carriage between raised and lowered positions For this purpose the carriage 155 has a depending guide post 159, and the subframe 158 has a sleeve 160 which is slidably mounted on the guide post 159 and is movable thereon by means of a manually operated winch device 161.

The subframe 158 has a table portion 162 which is disposed below the cable and service lines when the wagon is suspended from the loop portion 152, and the table 162 carries a lower clamp part 163,164 at each end Each lower clamp part is constructed in a manner similar to a lower strapping member 39 so that when the subframe 158 is raised, the lower clamp parts 163,164 are brought into 34 - supporting contact with the cable and service lines, these being received in corresponding channels of the lower clamp parts.

The subframe 158 also carries an upper clamp part 165 at a position above the lower clamp part 163 The upper clamp part 165 is constructed in a manner similar to an upper strapping part 38, and is mounted on a mechanism 166 by which the upper clamp part can be moved between a raised, open position (shown in Figure 21) and a lowered, closed position in which, when the subframe is in its raised position, the upper clamp part engages the cable and service lines directly opposite the lower clamp part to fasten the wagon 153 temporarily to the cable and service lines as shown in Figure 22.

In operation, empty strapping wagons 153 are loaded, in turn, with upper and lower strapping parts ready for application to the cable and service lines The loading takes place in a loading area 167 which is served by a branch rail 168 connected to the return run of the overhead rail loop 151 The lower strapping part 39 is mounted on the table 162 in a predetermined position substantially midway between the lower clamp -parts 163 and 164, and with the seatings of the lower strapping part 39 in alignment with the corresponding seatings of - the lower clamping parts The upper strapping part 38 is mounted in a temporary storage position on the subframe 158, such as indicated in Figure 21 When a strapping wagon 153 has been loaded, it is moved from the loading area 167 back into the loop 151, and then into a storage position 169 near the inlet end of the working area 8.

In this position the subframe 158 of the strapping wagon 153 is in its lowered position.

When required, the loaded strapping wagon 153 is moved into the portion 152 of the rail loop 151, and when the length of cable and service line between it and the previous wagon is about 6 metres the subframe 158 is raised to bring the lower clamp parts 163 and 164 and the lower strapping part 39 into engagement with the cable and service lines The upper clamp part 165 is then lowered by means of the clamping mechanism 166, and the wagon 153 is then fastened to the cable and service lines so that it will be pulled thereby along the overhead rail loop portion 152 During this travel, the upper strapping part 38 is removed from the subframe 158 and installed in position on the cable and service lines directly opposite the lower strapping part 39 The bolts carried by the upper strapping part 38 are engaged with the respective sockets of the lower strapping part 39 and are tightened simultaneously by means of a pneumatically 36 - driven nut runner machine which is temporarily fitted onto the wagon by the strapping operators For this purpose, the wagon 153 is provided with a hook 170 and a vertical guide 171 to support the nut runner position in the correct position When all the bolts have been fully tightened the nut runner machine is removed from the wagon 153, the upper clamping part 165 is raised by the mechanism 166, and the subframe 158 is lowered to disconnect the wagon from the cable and service lines, leaving these securely strapped together by the assembled and bolted strapping 37 The disconnected strapping wagon 153 is then moved around the remainder of the loop 151 to the loading area 167 for reloading.

As shown in Figure 23, each of the anode wagons 154 is similar in construction to the strapping wagons in comprising a carriage 172 having two twin wheel pair sets 173,174 mounted to run on the overhead rail 150, and a subframe 175 suspended from the carriage so that it is movable vertically relative to the carriage between raised and lowered positions by means of a manually operated winch device 176 The subframe 175 also has a table portion 177 which is disposed below the cable and service lines when the wagon is in the loop portion 152 above the path of the cable and service lines, the table carrying a lower clamp part 178,179 at each end identical 37 - to the lower clamp parts of the strapping wagons Thus, when the subframe:175 is raised, the lower clamp parts 178,179 will be brought into supporting contact with the underneath of the cable and service lines, these being received in corresponding seatings of the clamp parts.

In contrast to the strapping wagons, however, the sub- frame of each anode wagon is provided with an upper clamp part 180,181 above each of the lower clamp parts 178, 179 Each upper clamping part is identical to that of the strapping wagons and is mounted in a similar manner on a mechanism 182,183 by which the respective upper clamping part can be moved between a raised, open position (shown in Figure 23) and a lowered, closed position in which, when the subframe 175 is in its raised position, the upper clamping part engages the cable and service lines directly opposite the corresponding lower clamping part to fasten the wagon temporarily to the cable and service lines.

In addition, the subframe 175 of the anode wagon 154 is larger than that of the strapping wagons, and is provided with a central support post 184 on which an anode carrier is mounted for vertical movement up and down.

In operation, the anode wagons 154 are loaded, in turn, 38 - in the loading area 167 Two lower strapping parts 39 are mounted in predetermined positions on the support table 177 with their cable and service line seatings in alignment with the corresponding seatings of the lower clamp parts 178 and 179 In addition, an anode 186 is mounted on the anode carrier 185, the anode being fixed at opposite ends to a pair of upper strapping parts 38 so that they are disposed directly above the lower strapping parts 39 on the table 177 The anode 186 has a pair of sheathed electrical conductors 187,188 attached to it as shown in Figure 23 The loaded anode wagon 154, with the subframe 175 lowered and the anode carrier 185 raised, is then moved back onto the rail loop 151 to a storage position 189 at the inlet end of the working area 8.

As mentioned earlier, anodes will be attached to the service bundle at approximately 30 metre intervals, and the service lines 4 a,4 b,4 c are accordingly each provided with suitable anode connections at approximately 90 metre intervals, the connections of each service line being offset by 30 metres with respect to the connections of each of the other lines Figure 23 shows one such attachment 190 on the service line 4 c, the attachment comprising a pair of sheathed electrical conductors which are electrically connected to the service line on opposite sides of a weld joint in the service line under 39 - a heat shrunk insulating sleeve 191 so that only the free ends 192,193 of the two conductors are exposed.

When an anode is to be attached to the service bundle, the next loaded anode wagon 154 is pulled from the storage area 189 onto the loop portion 152 and positioned substantially symetrically about the sleeve 191 on one of the service lines, and the subframe 175 is raised to bring the lower clamp parts 178,179 and the lower strapping parts 39 into engagement with the underside of the cable and service lines The upper clamp parts 180,181 are then closed onto the cable and service lines by the clamping mechanisms 182,183 so as to secure the wagon 154 to the cable and service lines The wagon will then be drawn along the loop portion 152 by the cable and service lines, and during this travel the anode 186 is attached to the service bundle Firstly the free ends of the anode conductors 187,188 are connected to the free ends of the service line conductors 192,193 respectively by crimping sleeves and the connections protected by heat shrinking insulating sleeves thereover The anode carrier 185 is then lowered to just above the service bundle, and the connected conductors folded neatly into position beneath the anode The a-node carrier 185 is then further lowered to bring the upper strapping parts 38 into engagement with the cable and service lines - directly opposite the lower strapping parts 39 The positioning of the anode and the strapping parts is checked and the strapping bolts located in the respective sockets of the lower strapping parts An automatic nut running machine is then located on one of two guides 194,195 on the sub frame 175 for tightening all six bolts of one of the strappings simultaneously to the required torque The nut running machine is then transferred to the other guide and used to tighten the six bolts of the other strapping The nut running machine is then removed from the wagon 154, the anode carrier 185 is disconnected from the anode and moved to its raised position, the clamping mechanisms 182,183 are operated to open the upper clamping parts 180,181, and the subframe 175 is lowered to disengage the lower clamp parts 178,179 from the service bundle, leaving the anode 186 secured to the service bundle by the bolted strappings 37 at each end of the anode The empty wagon 154 is then returned around the remainder of the loop 151 to the loading station 167 for reloading.

As can be seen from Figure 2, the overhead rail loop 151 also includes a branch loop 196 for the storage of replacement strapping and anode wagons in the event that any of the wagons temporarily has to be taken out of service for repair The working area 8 also includes 41 - further overhead rails 197,198 extending along most of the length of the working area 8 for supporting the automatic nut running machines and their associated air supply hoses for movement back and forth along the working area as required.

At the downstream end of the working area 8 the completed service bundle 2 moves onto the stinger 14 by which the bundle 2 is guided gradually from its horizontal position to a downwardly directed position for entry into the water The stinger 14 comprises a stiff tubular steel framework supporting rollers providing a fixed curvature guide path (in the present case having a radius of 12 5 metres) for the service bundle The stinger 14 comprises a fixed upper portion 47 on the deck 6, and a movable lower portion 48 hinged thereto at deck level, as indicated at 49, so that the lower portion 48 may be raised to an inoperative position during transit of the vessel to and from the laying site During laying operations, however, the lower portion 48 will be fixed in its operative position as shown in Figures 1 to 3.

The top catenary angle (i e the angle to the vertical that the service bundle enters the water) will be monitored constantly to ensure that it remains within a predetermined permissible range for the water depth existing at the time If the angle approaches either the 42 - upper or lower limit, the speed of the vessel may be increased or decreased as appropriate to correct the situation.

Possible problems arising from the cable 3 and the service lines 4 a,4 b,4 c being of different diameters and being paid out over the curved stinger 14 are avoided by the clamping of the cable and service lines together so that their axes lie in a common plane as shown in Figure During laying (and, if necessary, retrieval) operations, the functioning of the various different sections of the vessel, i e the service line section (including the storage reels 9, straighteners 10, back tensioners 11, loop guide path 12, and bundle assembly station 13), the cable carousel section, the cable tensioner section, the working area 8, and the stinger section, are all monitored and controlled by separate operators or controllers reporting to a deck leader who in turn reports to an overall controller on the bridge of the vessel Most of the sections will also be monitored by means of suitably placed video cameras Each section is controlled in accordance with the selected operating speed which is relayed to each section operator or controller As mentioned earlier, the nominal laying 43 - speed will be about 5 metres per minute and the nominal retrieval speed will be about 3 metres per minute, but in the event of any problem arising in any of the different sections, the selected speed may be suitably reduced or even stopped by the respective operator or controller, and the other sections will follow accordingly.

As already explained, during laying the laying winches a,20 b,20 c driving the storage reels 9 a,9 b,9 c to unwind the service lines are controlled automatically in response to the service line loop position sensors 33 a, 33 b,33 c, the back tensioners lla,llb,llc being operative to maintain the service lines upstream thereof in tension as they are unwound and passed through the spooling devices 21 a,21 b,21 c and straighteners l Oa,l Ob and 10 c.

The spooling devices are manually controlled so as to avoid any permanent lateral bend being imparted to the service lines as they are unwound from the storage reels The loop position sensors, although operating automatically, are also monitored via video camera by the cable tensioner operator so that the tensioner 7 can be slowed in order to reduce the laying speed if it is noticed that any of the position sensors is approaching close to the inner limit, thereby possibly avoiding actuation of the inner emergency stop switch which would stop the laying operation altogether Either one or both 44 - of the cable tensioner units 7 ' may be used during the laying operation as desired The speed equalising tensioner 36 of the bundle assembly station 13 will be operative with only a very low pressure, except when a length of bundle is to be laid without the cable and service lines being strapped together During this part of the operation the speed equalising tensioner 36 will be set to apply a relatively high clamping pressure on the cable and service lines in order to take the weight of the laid service lines until the strappings are again applied to the bundle.

During a retrieval operation, both units 7 ' of -the cable tensioner 7 will be driven in the opposite direction in order to retrieve the bundle, the strappings and sacrificial anodes being removed from the bundle in the working area 8 following the opposite procedure to their application during the laying operation The speed equalising tensioner 36 will be set to exert a medium pressure on the cable and service lines and serves to push the separated service lines 4 a,4 b,4 c back through the assembly station 3 and the loop guide path 12 for rewinding onto their respective storage reels 9 a,9 b,9 c.

These reels will again be driven by their respective winches in response to the loop position sensors 33 a,33 b,33 c, and the back tensioners lla,llb,llc will be - operative to hold the service lines in tension as they are wound on the -reels The straighteners l Oa,l Ob and l Oc will of course not be operative during retrieval.

Figure 24 illustrates the service bundle construction in an alternative embodiment of the invention in which a multicore electrical cable 203 is laid simultaneously together with a service line 204 in the form of an umbilical comprising a plurality of steel hydraulic pipes 205 twisted helically around the outside of a continuous central core 206 formed, for example, by an extruded polypropylene rope The umbilical service line 204 also includes flexible binding means 207, such as glass fibre reinforced nylon tape, which is wrapped periodically or continuously around the periphery of the umbilical.

In the typical example illustrated, there are ten steel pipes (steel quality: Duplex UNS-532750) having an internal diameter of 12 7 mm and a wall thickness of 1 3 mm, and the central core has a diameter ofapproximately mm, giving the umbilical an overall diameter of about 66 mm The cable 203 has a diameter of about 74 mm The twist of the pipes 205 around the core 206 (i e the helical pitch) is such that each pipe executes a full 3600 turn around the core in not less than 4 4 metres axial length of the umbilical This gives a twisting 46 - ratio of 4:1 related to the minimum bending diameter of the umbilical.

The cable 203 and the umbilical service line 204 are strapped together at intervals along their length to form the service bundle by means of strappings 208 which are somewhat similar in construction to the strappings 37 described earlier As shown in Figure 24, each strapping 208 comprises upper and lower strapping members 209,210 consisting of steel shells 211,212 containing moulded rubber inserts 213,214 defining seatings for the cable 203 and service line umbilical 204 such that the axes of the cable and service line will lie in a common plane coincident with the median plane of the strapping The upper and lower strapping members 209,210 are secured together by bolts 215 (for example two at each side of the strapping) which extend through the upper member 209 into threaded sockets or nuts 216 embedded in the lower member 210 The steel shell 211 of the upper strapping member 209 is provided with an anode 217.

The bundle is assembled and laid in much the same way as the bundle 2 of the first embodiment, the cable 203 being delivered from the storage carousel 5 and passing through the cable tensioner 7, the bundle assembly station 13 and the working area 8 as previously described Similarly, 47 the service line umbilical 204 is unwound under tension from a storage reel which is driven under 'the control of a loop position sensor responsive to the position of the service line umbilical as it passes in a substantially tensionless loop around the roller guide path 12 to the bundle assembly station 13 and working area 8 In this case, however, the storage reel for the service line umbilical will have a diameter such that substantially no permanent deformation is imparted to the service line as a result of being wound on the reel As a result of this, the service line umbilical comes off the storage reel in a substantially straight condition when it is unwound, and there is no need for a straightening device as in the first embodiment.

As will be appreciated, when the service line umbilical is wound on the storage reel, the curvature caused by conforming to the reel diameter causes the helical lay angle of the individual pipes 205 to decrease on the outer side of the umbilical and to increase on the inner side Provided these deformations are small, they are desirable since they relieve some of the internal stresses that would have otherwise developed in the pipes if they were fully restrained Accordingly, in order to ensure that these small deformations are possible, it is preferable that the diameter of the central core 206 is 48 - made slightly oversize, i e such that there will be a small gap between adjacent pipes 205 when these are disposed evenly around the core and in contact therewith In the example illustrated, it will be sufficient to adjust a core diameter of 35 36 mm, which will result in an average gap between the pipes 205 of about 0 35 mm.

During tests using this umbilical configuration and a pitch angle for the pipes 205 such that each pipe extends completely around the core three or four times during one complete Lurn of the umbilical around the storage reel, which had a minimum radius of about 4 metres, the service line umbilical behaved very coherently during both winding on and winding off the reel During unwinding the umbilical came off the reel in a straight line, without any signs of permanent deformation and with the individual pipes 205 remaining in their original helical arrangement and in close contact with the core 206.

49 -

Claims (18)

1 A method of laying an offshore service line from a vessel in which the service line is unwound continuously from a storage reel which is mounted on the vessel and is driven to unwind the service line at the required rate, the unwound service line is guided to form a substantially tensionless loop, and holding power is applied to the service line downstream of the loop to control the catenary of the service line between the vessel and the sea bed, the rate at which the storage reel is driven to unwind the service line being controlled by sensing any increase or decrease in the slack of the loop relative to a predetermined value.

2 A method according to claim 1, in which the service line comprises a single pipe and is straightened after leaving its storage reel in order to remove the residual curvature attained by having been wound on the storage reel, and tension is applied to the service line downstream of the straightener to maintain the line in tension as it is unwound and straightened.

3 A method according to claim 2, in which a plurality of service lines are laid at the same time, each line being unwound continuously from a separate storage reel and being straightened before being guided to form a substantially tensionless loop.

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4 A method according to claim 3, in which the service lines are secured firmly together side by side downstream of the loops to form a service bundle before being guided overboard into the water to be laid on the sea bed.

A method according to claim 4, in which the loops formed by the separate service lines are disposed in substantially horizontal planes at different heights from each other, and the lines are guided into a common horizontal plane downstream from the loops before being secured together to form the service bundle.

6 A method according to claim 1, in which the service line comprises an umbilical formed by a plurality of pipes twisted helically around a central core, the umbilical being unwound from a storage reel of a diameter such that substantially no permanent deformation is imparted to the umbilical.

7 A method according to any one of claims 1 to 6, in which a cable is laid simultaneously with the service line or lines, the cable being unwound continuously from a separate storage reel mounted on the vessel and being secured firmly to the service line or lines downstream from the service line loop or loops to form a service bundle which is guided overboard into the water for laying on the sea bed.

51 -

8 A vessel for laying an offshore service line, comprising means for supporting a service line storage reel for rotation about its axis, driving means for rotating the reel to unwind the service line as required, a tensioner for acting on the service line so as to maintain the service line in tension as it is unwound, means for guiding the service line overboard into the water for laying on the sea bed, and means for applying holding power to the service line to control the catenary of the line between the vessel and the sea bed, the vessel including a curved roller guide path downstream of the tensioner for causing the service line to form a substantially tensionless loop, the curved roller guide path being provided with a loop position sensor which is responsive to any increase or decrease in the slack of the service line loop relative to a predetermined value and which is operative to control the driving means for the service line storage reel so as to restore the service line loop to the predetermined desired position on the guide path.

9 A vessel according to claim 8, in which the curved roller guide path comprises an inlet bending guide unit for imparting a predetermined uniform bend to the service line which causes it to form a loop curving around the guide path, a substantially crescent shaped supporting 52 structure carrying the service line loop position sensor and radially disposed, circumferentially spaced, horizontal support rollers for supporting the service line loop at intervals around the guide path, and an outlet bending guide unit for straightening the service line as it leaves the guide path by imparting a reverse bend to the service line which is equal and opposite to that imparted by the inlet bending guide unit.

A vessel according to claim 9, in which the loop position sensor is located at or near the apex of the curved roller guide path and comprises a carriage which is freely movable along a track extending transversely across the guide path, means on the carriage for coupling the carriage to the service line loop in a manner which allows the service line to travel freely around the curved guide path but which causes the carriage to move along the track in response to inward or outward movement of the loop relative to the curved path, and detection means which is responsive to movement of the carriage to provide a signal corresponding to the position of the carriage on the track and hence to the position of the loop on the guide path.

11 A vessel according to claim 10, in which the means for coupling the carriage to the service line loop 53 - comprises a yoke formed by a pair of upright posts which are fixed to the carriage and project upwardly therefrom to define a gap therebetween for receiving the service line.

12 A vessel according to claim 10 or claim 11, in which the track comprises a single upright rail extending radially across the guide path, and the carriage is provided with two spaced pairs of upper and lower wheels mounted so as to engage and run smoothly on the upper and lower edges respectively of the rail.

13 A vessel according to any one of claims 10 to 12, in which the ca Xriage position detection means comprises a continuous chain to which the carriage is fixed and which extends around a pair of rotatably mounted sprocket wheels so that any movement of the carriage along the track is transmitted to the chain to cause the sprocket wheels to rotate proportionally to the distance and direction moved by the carriage, the detection means further comprising a detector which is arranged to respond to rotation of one of the sprocket wheels to generate an output signal proportional thereto, and hence proportional to the radial movement of the carriage and the service line coupled therewith.

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14 A vessel according to any one of claims 9 to 13, in which the curved roller guide path includes means defining radially inner and outer limits to the curved path for the service line loop, and emergency stop switches which are mounted adjacent the inner and outer limit means for stopping the laying operation when engaged and actuated by the service line loop as it nears the inner or outer limit means.

A vessel according to claim 14, in which the inner limit means of the roller guide path comprises a curved safety rail which extends continuously from the inlet bending guide unit to the outlet bending guide unit.

16 A vessel according to any one of claims 8 to 15, in which the means for guiding the service line overboard comprises a stinger.

17 A vessel according to any one of claims 8 to 16, in which the vessel is arranged to support more than one service line storage reel so that a plurality of service lines can be laid simultaneously, each service line being treated in a similar manner but independently of each other.

18 A vessel according to claim 17, in which the curved - roller guide paths for the different service lines are disposed substantially one above the other so that the loops formed by the service lines are disposed in substantially horizontal planes at different heights from each other.

0790 U