CA1063816A - Laying of underwater pipelines - Google Patents

Abstract

Abstract of the Disclosure Method and apparatus are described for laying a pipeline on the sea bed to interconnect two spaced underwater installations. The pipeline is towed by a tug exerting a traction effort at a point spaced to the rear of the front end of the pipeline, while another tug exerts a holding effort at a point spaced in front of the rear end, the front and rear ends being maintained above the sea bed by a system of floats and trail-ropes and the intermediate portion of the pipeline section being ballasted so as to be supported on the sea bed. The two end portions of the pipeline are subsequently inserted into the respective installations, and are weighted so that they rest on the sea bed.

Description

This invention relates to the laying of a pipeline on the bed of the sea or of a body of water for such uses as an oil duct or a gas duct.
It is sometimes necessary to position a length of pipeline between two fixed locations on the bed of the sea or a lake. The pipeline is built into the required length in -; some suitable manner, for instance by progressively adding : pipeline units to one another on shore while drawing the growing pipeline into the water. As each unit is added ; 10 the pipeline is drawn further into the water. The pipeline - -; ~ units themselves consist of a certain number o~ pipes assembled together end-to-end and capable of being of considerable length of the order of 1 km or even more. The assembly of the tubes to each other and the assembly of the lS elements to the rear end of the pipeline can easily be effected by welding, provided this is carried out on land.
`~ It has been found that the pipeline is more readily manoeuvred into position if the ends of the pipeline are made more bouyant than the rest of the pipeline. At . ..
least some of the towing effort is transmitted to the pipeline downstream from the leading end of the pipeline and the ends of the pipeline are attached to the underwater fixed locations -after first positioning the pipeline between those locations. ;
Accordingly in one of its aspects the invention j 25 provides a method of laying a pipeline on the bed of a body of water to interconnect two spaced underwater installations.
The method comprises the steps of towing the pipeline to .
; bring it between said installations or sites, while allowing its front and rear end portions to float in the midst of water at a certain distance above the bed and while ~` ~ 2 ~''- .. .

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maintaining the rest of the pipeline on the bed or adjacentthereto, then inserting the end portions into the respective installations or sites and weighting the end portions so that they rest on the bed.
According to another of its aspects, the invention provides apparatus for positioning a pipeline on the bed of a body of water to interconnect two spaced underwater installations located on said bed, the apparatus comprising:
towline means adapted for use with respective traction and holding means to exert pulling and holding forces to submergible pipeline, at least a portion of those forces being applied to the pipeline at points situated respectively at selected distances from the front and rear ends of the pipeline being towed; and means adapted to be coupled to portions of the pipeline between the respective ends and said points to give these end portions a positive buoyancy so that the end portions tend towaras positions above the remainder of the pipeline.
Reference is now made to the drawings in which:

2 a Fig . 1 iS a schematic side view of a pipeline connected to two underwater installations, namely an underwater petroleum borehole and the base of the underwater structure ~; of a platform;
Fig. 2 is a sectional top view on a line A-A of Fig. l;
Fig. 3 is a diagrammatic side view of a pipeline section in the course of being towed to the underwater installations;
Fig. 4 and 5 are views similar to Fig. 1, showing -; 30 on a larger scale the front and rear portions, respectively, - of the pipeline section;

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Fig. 6 is a partial side view, again on a larger scale, illustrating the action of the floats;
Fig. 7 is a section on the line B-B of Fig. 6;
FigO 8 is a view-similar to Fig. 4, showing how the ends of a pipeline section can be located to resist I a storm;
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Fig. 9 is a view similar to Fig. 4, showing another embodiment;
I Fig. 10 is a view similar to Fig. 4 showing yet -~ 10 another embodiment;
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Fig. 11 (adjacent to Fig. 2) is a schematic side ~~ view of a pipeline being towed towards a location between two spaced underwater installations; and ; Fig. 12 (adjacent to Fig. 8) is a view similar to Figs. 4 and 5 showing the junction of two pipeline sections where . .~ .
there is a need to use two sections which are attached to one , another after being located relative to the installations.
Reference is made first to Figs. 1 and 2. A
pipeline 301 has been towed to a position in which it is located parallel to a line joining the centre of an underwater ~-structure 41 containing the irruption head (or Christmas tree~ of a petroleum borehole to that of the base 42 of an I underwater structure 42a supporting a platform 42b while keeping clear of these elements 41 and 42. The end portions ~;~ 25 316 and 334 of the pipeline are maintained in equilibrium in ; , the midst of water by the trail-ropes during towing and are ~ . , .
relatively flexible and easy to manipulate. This facilitates engaging them respectively in the structure 41 and in the base 42 and connecting them to the pipework ~not shown) that '.
they contain. The portions 316 and 334 of the pipeline are -- weighted to rest on the bed.

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;Reference is next made to Figs. 3 to 5 to describe a method and apparatus incorporating the invention for transporting and locating a pipeline prior to attaching the pipeline to underwater installations such as base 42 and structure 41 (Figs. 1 and 2).
A pipeline section 1 is being towed in the sea (or other body of water) in the direction of the arrow F, by means of a towing cable 2 drawn by a tug 3 and fixed to a traction head 4 formed by a collar tightened by bolts 5 around the pipeline section a certain distance from its front end 6. Another collar 7 grips the pipeline section in similar fashion at a certain distance from ltS
rear end 8 and is fixed to one end of a holding cable 9 of which the other end is attached to a small tug 10 which exerts a certain pulling force thereon such that the pipeline section 1 is maintained lightly in tension and does not risk forming S-bends or buckles on the sea bed. The holding cable 9 is furthe~rmore provided, to the rear of the end 8 of the pipe-- line section, with a heavy block 11 forming a drag which rubs on the sea bed 12. The principal portion 13 of the pipeline section, which is comprised between the collars

4 and 7, is equipped in known manner with a system of floats 14 which give it a slightly positive buoyancy and with , . .
chains or cables 15 which drag on the bottom 12 and act as -~ 25 aerostatic trail-ropes. The principal portion 13 of the pipe-line section is thus maintained slightly above the bed 12~
~The front end portion 16 of the pipeline section, ; i.e. the portion situated in ront of the traction head 4, is provided with a plurality of floats 17 distributed along its length and each connected by a suspender 18 to the towing cable 2. The arrangement of one of these floats and its ~ 5 ~
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suspension is shown in more detail in Figs. 6 and 7: The float 17 shown in these figures consists of a drum having a diameter greater than that of the pipeline section and held against the upper surface thereof by hoops 19 consisting of ` 5 metal bands and provided with pyrotechnic rupture devices : .
20. To one of these hoops is attached one end of a suspender . 18 the other end of which is fixed to the towing cable 2.

. The float is provided on its upper part with a tube 21 normally closed by a cock 22 and on its lower part with a tube 23 ~ ~

normally closed by a cock 24. This arrangement enables the :.

buoyancy of the float to be regulated as required; to weight it, it suffices to open temporarily the cock 24 and possibly also the cock 22 in order to allow a certain quantity of water .

to enter the float, and to lighten it, it suffices to connect the 15 tube 21 to a source of compressed air and open temporarily :` the cocks 22 and 24 in order to discharge a certain quantity .. of water from the float~ These operations can easily be carried out by divers provided with bottles of compressed air.

The suspender 18 is provided at its ends with . : ~
.20 welded buckles 25 and 26 which engage respectively in eyelet . ~. 27 welded to one of the hoops 19 and an eyelet 28 carried by ; a sleeve 29 fitted on the cable 2. ThP floats 17 are controlled . . ~ .
so that the assembly of floats 17, the front end portion 16, ~ the suspenders 18 and the portion 30 of the towing cable .: 25 situated facing, has a slightly negative buoyancy, the portion 16 of the pipeline floating between two waters and ballasting being assured by the weight of the cable 2 and . the suspenders 18, the latter being stretched and the cable . 2 resting on the bed 12 in its parts situated between them and forming a sort of festoon.
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The towing cable 2 should be sufficiently strong to pull the pipeline section the length of which could be as much as several kilometres. The cable 2 is thus very heavy and drags on the bed 12 in front of its portion 30.

- 5 In the towing position shown in Fig. 3 and in full lines in Fig. 4, i.e. when the tug 3 pulls on the cable 2, the traction force applied to the front end of the portion 30 of this cable is thus practically horizontal and tends to straighten the portion 30 (i.e. to give it a rectilinear configuration) against the action of the suspenders 18 which pull the peaks 31 of che festoon upwards so that the latter is little accentuated. The lengths of the suspenders are chosen such that the pipeline section 16 forms, in the ~owing position, a curve such as that shownj placing the end 6 several metres (for example 7 to 8 metres) below the surface 32 of the sea, in order to protect this end against the effect of swell during towing, with radii of curvature large enough not to impose excessive flexing strains on the pipeline. The :; . .
- suspenders 18 form with the part 30 o~ the cable a system of 2a trail-ropes which co-operates with the floats 17 to absorb the towing shocks and to maintain the pipeline section 16 in stable equilibrium.
~; As soon as the tug 3 stops pulling on the towing cable 2, the stretching forcer which opposes the action of the suspenders 18 drawing the peaks 31 of the festoon upwards, is no longer generated except by rubbing of the ~' ; cable 2 on the bed 12. In the embodiment shown, it is assumed that the weight of the cable is not too great; so - that its rubbing on the bed does not prevent sufficient ~ -3Q sliding to allow the floats 17 to lift the pipeline section 16 to a rest position shown in broken lines in Fig. 4, in ~;
which the end 6 is not disposed more than four or five metres , :~

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below the surface 32. From this rest position, it is possible to lighten all or some of the floats 17, for example in the manner indicated above in connection with Figs. 6 and 7, so as to bring the pipeline section 16 into a position in which the front end 6 is located practically on the surface. This is advantageous if the end is to be welded to the rear end of a further pipeline section as will be described hereinaftex with reference to Fig. 12. ~ -In the case of bad weather or if a storm is fore-cast, the pipeline laying operations are stopped, and to : . .
protect the end portion 16 against the effects of the sea, the floats 17 may be weighted, for example in the manner indicated above with regard to Figs. 6 and 7, in order to bring the section 16 into the folded position shown in Fig. 8.
In this position, the end 6 is located at a sufficient depth, for example of ~ifteen metres or more, below the surface 32, as not to risk being damaged by the swell. One can also, when the bad weather forecast: is not very severe, simply pull on the towing cable 2 by means of the tug 3 in order to bring the pipeline section back into the towing position (Fig. 3), or even attach to the end 6 a cable 33 - and pull thereon by means of the tug, then fix the cable 2 or ;" ~
~- 33 to a dead weight not shown, until it is possible to resume ~;~ the pipeline laying operations. It is also possible to use ; these means together, i.e., to weight the floats and pull on the cable or cables. -- It has been stated above that the suspenders 18 ;~ and the portion of cable 30 produce a trail-rope effect in the towing position (Fig. 3). This effect is still more apparent in the rest positions (shown in broken lines in Fig. 4) and the folded position (Fig. 8). The weights of the suspenders and of the portion of cable 30 are selected as a function of j the buoyancy conferred by the floats 17 on the pipeline .

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section 1~, so as to maintain the latter in stable equilibrium in the various positions shown, in order that there is no risk of it deteriorating due to excessive flexing. In the embodiment shown, the suspenders 18 are metal cables as thP
- 5 tow~line 2 and the portion 30 thereof but, according to other embodiments, the suspenders and/or the portion 30 of the tow-line could consist of chains, articulated metal rods or bars, or any other equivalent arrangement.
The rear end portion 34 of the pipeline section 1 - 10 (Figs. 3 to 5) is equipped with floats 35 and suspenders 36 similar respectively to the floats 17 and suspenders 18 of the front end portion 16. The suspenders 36 are attached to the portion 37 of the holding cable 9 and form therewith a system of trail-ropes which co-operates with the floats in order to maintain the portion 34 of the pipeline in stable ; equilibrium. The latter can be brought from the towing position (shown in Fig. 3 and in full lines in Fig. 5) into the rest position (shown in broken lines in Fig. 5), into a joining position (Fig. 12~ or into a folded position by means ~ 20 sLmilar to those described above for the portion 16. It may ; be pointed out, however, that the holding cable 9 is subjected to an effort much smaller than the towing cable 2 and can be much lighter than the latter. It is therefore necessary to keep the portion of cable 37 on the bed, by means of the drag 11, so that it can function as a trail~
rope. However, this drag should not be too heavy so that its rubbing on the bottom does not risk preventing the portion 34 Ofthe pipeline from rising to the rest position (shown in broken lines in Fig. 5~ as soon as one ceases applying a holding force to the cable 9, and then rising into ~he joining position (Fig. 12) when the floats are lightened.

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Referring again to Fig. 4, it may be noted that a relatively snort sliding of the cable 2 on the bed allows a relatively large ascent of the end 6 of the pipeline section. In effect, the height of ascent of each suspender 18 can be assimilated to the radial 38 of an arc 39 formed by the cable 2 between this suspender and the adjacent suspender in the position shown in broken lines, while the sliding of the cable produced by the curvature of this arc 39 is equal only to the difference between the initial distance d between these two suspenders and the length of the cord 40 of this arc, a difference which is very small for arcs of slight curvature. It can also be seen from Fig. 4 that the distance of sliding o the cable 2 on the bed is only 60 to 70% of the ascent of the end 6 of t.he pipeline section 1.
Normally, the rubbing of the cable on the bed should not therefore prevent the ascent of the end 6.
However, it can happen, especially if the trail- `
ropes 15 are very heavy in order to prevent the pipeline ~ section deviating under the action of strong transverse :~ ~0 currents, that one is obliged to use an equally robust ,.; . :
~ towing cable 2 which is so heavy that its rubbing on the bed .
; would prevent the end portion of the pipeline section, connected to the cable by the suspenders, from rising towards .~ .
` the surface. One could then use the arrangement shown in Fig. 9 in which the members playing the same part as in the previous Figs. aredesignated by the same references plus ` 100. In Fig. 9 the suspenders 18 are replaced by cables 118 which are not connected to the towing cable 102 and the ends of which trail on the bed 112, thus forming trail-ropes, and the end 106 of the pipeline section 101 is fixed to a cable 133 which is connected to the tug 103 in parallel with `
the towing cable 102. There is no risk of these two cables ~-' `' ~

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becoming entangled'if one takes the precaution of stretching them properly. In the embodiment shown, the floats 117 are moreover of the "crayon" type, i.e. in the form of cylinders connected by lines 117a to the portion 116 of the pipeline and maintained near to the surface 132 or extending through " said surface. This arrangement makes it possible to lighten and weight the floats as required from the surface of the sea, for example by means of a boat carrying bottles of compressed air. The portion 116 of the pipeline is provided in various places with collars 117b to which are fixed the lines 117a and the trail-ropes 118. The pulling cable 133 - is fixed to the'collar 117b nearest to the front end 106 of the pipeline section.
~ The rear end portion (not shown) of the pipeline -' 15 section 101 can be equipped either in the manner shown in '~
~ig. 5 or with floats and trail-ropes similar to the trail-~' ropes 118. ' ' Towing and joining o~ the pipeline section 101 are ~
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; effected as described with reference to the preceding figures. ~
2a The depth of immersion of the end 106 can be regulated by ~' varying the tension of the pulling cable 133 and/or by '' ,. ....................................................................... .
' varying the buoyancy of some at least of the floats 117, for example with the aid of the boat referred to above. Here again, one can also bring the end portion 116 into the ~oining position or into a folded position if a storm is ' forecast. In the folded position, the end portion of the pipeline will advantageously be immersed at a greater depth than in the preceding figures in order to bring the floats 117 to a depth of calm water.
3Q However, it goes without saying that the trail-ropes 118 could equally well be associated with floats of a ~'' type different from the floats 117, for example with floats - 1 1 - , ' . ' ' ' ~ , ' - .: .

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similar to those described with reference to Figs. 4 and 5;
it is then obviously sufficient to immerse the section 116 of the pipeline, in the folded position, at a depth of calm water. Whatever type of floats used, the trail-ropes 118, or - 5 at least one or more thereof that are nearest to the end 106, are preferably less heavy in their lower end portions than in those which serve to maintain the system in stable equilibrium in the ~owing and folded positions, so that the floats do not need to be lightened too much for raising the end 106 of the pipeline to the surface. In other words, the weight per metre ,! length of the lower end portions of the trail-ropes 118 should preferably be smaller than the weight per metre length of the portions which alternately touch and leave the bed in the towing and folded positions of the pipeline section 116.
Fig. 10 in which the elements playing the same part as in Figs. 3 to 8 are designated by the same reference `~ numerals with the addition of 200, illustrates an embodiment ~ in which the towing cable 202, instead of hauling the pipeline .. ' :
section 201 by pulling on a traction head, extends right along g ~ .: .,, ` ; 20 the pipeline section and is connected to the principal portion 213 thereof at a plurality of points b~ means of suspenders 215. ~he pulling effort applied to the pipeline section is thus distributed among the suspenders 215 which reduces considerably the pulling strains to which they are subjected. In the . ~. . .
i~ 25 embodiment shown, the suspenders 215 are fixed, not directly to the pipeline section, but to hoops 219 of the floats 214 by meàns similar to those shown in Figs. 6 and 7. In the rear part (not shown) of the pipeline section, the cable 202 extends rearwards as far as a drag similar to the block 11 of Figs. 3 to 5 and as far as a holding device similar to the tug 10. The suspenders 215 and the portions of the cable 202 to which they are attached form a system of trail-ropes similar to that described above with reference to Fig 4, and . .
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co-operate with the floats 214 to maintain the principal section 213 of the pipeline section in stable equilibrium above the bed 212.
The front end portion 216 of the pipeline section is equipped in the manner described above with reference to Figs. 3 to 8 and the rear end portion (not shown) is advantageously equipped in the same manner. Towing and joining of the pipeline section 201 are effected in the manner described with reference to Figs. 3 to 8.
Fig. 11 in which the elements playing the same .:
; part as in the preceding figures are designated by the same reference numbers with the addition of 300, show the laying of a submerged pipeline between an underwater petroleum ~ ~
borehole and the underwater structuxe of a platform. ;
As seen in Fig. 11, a pipeline 301 is in the course of being towed in the sea, in the direction of the arrow F, ~ -by means of a towing cable 302 drawn by a tug 303 and fixed to a traction head 4 formed by a collar secured by bolts around the pipeline at a certain distance from its front end 306.
Another collar 307, secured in a similar manner to the pipeline section at a certain distance from its rear end 308, is fixed -to an end of a holding cable 309 the other end of which is secured to a small tug 310 which exerts on it a certain pulling ~ force so that the pipeline section is kept slightly tensioned - 2~ and does not risk forming S-bends or kinks on the sea bed.
The retaining cable 309 is furthermore provided, to the rear of the end 308 of the pipeline section, with a heavy block 311 forming a drag which rubs on the sea bed 312.
The intermediate portion 313 of the pipeline, which is comprised between the collars 304 and 307, has a negative buoyancy either per se lbeing for example filled with water) . . .

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or in association with ballasting means not shown, so that it rests on the bed.
The front end portion 316 of the pipeline section, that is to say the portion situated in front of the traction head 304, is provided with a plurality of floats 317 . -distributed over its length and each connected by a suspender 318 to the tow~ng cable 302. The length of the suspenders 318 is chosen so that the portion 316 of the pipeline floats "between two waters" at a certain distance above the cable ." . .
. 10 302 resting on the bed, but far below the surface of the body : . -.
of water. The floats 317 and the suspenders 318 are similar -:: to those described hereinbefore, in which means are parti~

. cularly disclosed enabling the floats to be weighted as desired, during towing, the ~loats ~eing regulated so that the .

assembly of the floats 317, the front end portion 316 of :.

~ the pipeline, the suspenders 318 and the portion 330 of the .: : towing cable situated facing, has a slightly negative buoyancy, the pipeline portion 316 :~loating "between two im waters" and ballasting being assured by the weights of the cable 302 and the suspenders 313, the latter being taut and ~ the portion 330 of the cable resting on the bed 312 in the : parts between them thus forming a kind of festoon. As is `

explained hereinbefore, the suspenders 318 and the portion ~ :

330 of the cable produce a trail-rope effect during towing . 25 and even more so when the tug 303 ceases to pull on the : cable 302.

. The rear end portion 334 o~ the pipeline 301 is ` also equipped with floats 335 connected by suspenders 336 to the portion 337 of the retaining cable 309 located opposite `

-: 30 and, still as hereinbefore, the suspenders 336 and the portion . 337 of the cable produce a trail-rope effect during towing and even more so when the tug 310 ceases to pull on the .: .

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retaining cable 309.
Reference is next made to Fig. 12 which illustrates the joining of two pipeline sections at sea in the general position they would occupy on the bed. Pipeline section 1 has been brought by tugs 3 and 10, into a position in which its rear end portion 34 in which it is approximately an ex-tension of the front end portion-16a of a similar previously laid pipeline section 16a (Fig. 12~, and in which the rear end 8 of the pipeline section 1 is adjacent the front end 6a of the pipeline section la. The floats 35 and 17a are then ~-.i - .
lightened in the manner already described so as to bring the ends 8 and 6a to or very near the surface, and the two ends welded together. The floats are then progressively weighted by the means described with reference to Figs. 6 and 7 so ~ 15 as to cause the joined ends 34 and 16a to sink until they i rest lightly on the bed. Finally, the pyrotechnic rupturing devices 20 (Figs. 5 and 7) are actuated so as to free the hoops 19. The pipeline being thus freed from floats and suspenders, it is sufficient to send down divers to free the 0 collars 4 and 7 in order to recover the cables, the suspenders ~-~
and the floats 17 and 35. ~ne can c~use the principal ` ; portion 13 of the pipeline to stay resting on the bed by freeing, by similar means, the floats 14 and recovering them together with the trail-ropes 15.
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~ 25 - It goes without saying that the embodiment ; described is only an example and could be modified, notably by the substitution of technical equivalents, without ~- departing from the scope of the invention as defined in the appended claims. In particular, one could utilise any of the devices described with reference to Figs. 3 to 10 for towing ~i . ~ 15 ~ :.
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the pipeline described with reference to Figs. 1 and 2.
The front and/or rear end portion of the pipeline section .
;~ could be equipped both with the suspenders 18 and the trail- ~
ropes 118. s One would also not depart from the scope of the ~ :
invention by replacing the tugs by other haulin~ devices. t .
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Claims (18)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of connecting two underwater installations, comprising the steps of towing a pipeline assembly to bring the same to a position between said installations, while making front and rear end portions of the pipeline assembly buoyant to maintain said end portions in the midst of water and while giving a length of the pipeline assembly intermediate said end portions a negative buoyancy to maintain the same on the sea bed to trail thereon, thereby to prevent said intermediate length of the pipeline assembly from deviating, engaging the rear and fore ends of the pipeline assembly in and connecting them to said underwater installations respectively, and there-after ballasting said fore and rear end portions of the pipe-line assembly to rest at least part thereof on the bed.

2. A method according to claim 1, in which the towing step includes exerting a pulling force at a point spaced from the front end of the pipeline assembly, and exerting a holding force at a point spaced from the rear end.

3. Apparatus when used for carrying out the method of claim 1, comprising:
(a) towing means, including a traction machine coupled during towing of said pipeline assembly for applying a traction effort at a traction point on the pipeline assembly to draw the same forward, said traction point being spaced from the front end of said pipeline assembly;
(b) holding means, including a holding machine connect-ed during towing of said assembly for applying a holding effort at a holding point on the pipeline assembly to maintain the same in tension, said holding point being spaced from the rear end of said pipeline assembly;

(c) traction head means about the pipeline assembly connecting a front end portion of said pipeline assembly to the traction machine, said front end portion being situated in front of said traction point;
(d) ballasting means arranged along an intermediate portion of said pipeline assembly, located between said traction point and said holding point, for supporting said intermediate portion of said pipeline assembly on the bed of the body of water during towing, thereby to prevent said intermediate portion from deviating, and (e) buoyancy means conferring during towing of said assembly a positive buoyancy to said front end portion and to a rear end portion situated to the rear of said holding point, in order to maintain said front and rear end portions of the pipeline assembly in the midst of water, thereby to make easier respective connections of said front and rear ends to said underwater installations, respectively.

4. Apparatus according to claim 3, in which said ballasting means incorporate a plurality of short trail-ropes spaced along said intermediate portion of the pipeline assembly.

5. Apparatus according to claim 3, comprising a system of trail-ropes for maintaining said front and rear end portions of the pipeline assembly above the bed of the body of water.

6. Apparatus according to claim 3, claim 4 or claim 5 in which said towing means and said holding means comprise respectively a towing tow-line and a holding tow-line connecting respectively the traction machine to the traction point and the holding machine to the holding point.

7. Apparatus according to claim 3, claim 4 or claim 5 in which said towing means and said holding means comprise a traction distribution cable connected to said traction machine and to said holding machine, and means connecting said distribution cable to said traction point, to said holding point, and to a plurality of points on said intermediate portion.

8. Apparatus according to claim 3, claim 4 or claim 5, in which said buoyancy means comprise a plurality of floats and means for regulating the buoyancy of some at least of said floats.

9. Apparatus according to claim 3, in which said means connecting said front end portion to the traction machine comprise a plurality of suspenders connecting a plurality of points on said front end portion to a tow-line connecting said traction point to said traction machine.

10. Apparatus according to claim 9, in which said suspenders are connected at spaced points to said tow-line to form therewith a system of trail-ropes.

11. Apparatus according to claim 3, claim 4 or claim 5 in which said means connecting said front end portion to the traction machine comprises a cable connecting said front end to the traction machine.

12. Apparatus according to claim 3, comprising a plurality of trail-ropes fixed to a plurality of points on at least one of said front and rear end portions and trailing on the bed of the body of water.

13. Apparatus according to claim 12, in which at least some of the trail-ropes comprise a lower end portion less heavy than the rest thereof.

14. Apparatus according to claim 3, comprising traction coupling means for connecting said rear end portion of said pipeline to the holding machine.

15. Apparatus according to claim 14, in which said coupling means connecting said rear end portion to the holding machine comprise a plurality of suspenders connecting a plurality of points on said rear end portion to a holding tow-line connecting said holding point to said holding machine.

16. Apparatus according to claim 15, in which said suspenders are connected at spaced points to said tow-line to form therewith a system of trail-ropes.

17. Apparatus according to claim 14, claim 15 or claim 16, in which said coupling means connecting said rear end portion to the holding machine comprise a cable connecting said rear end to said holding machine.

18. Apparatus according to claim 3, claim 4 or claim 5, in which said holding means comprise a cable connected to said holding point and to the holding machine, and a heavy block connected to the cable to the rear of said holding point to trail on the bed.