EP0042398A1

EP0042398A1 - A method for laying a flexible pipeline and pipeline to carry out this method

Info

Publication number: EP0042398A1
Application number: EP19810900134
Authority: EP
Inventors: Tjerk Hans Duyster
Original assignee: DUYSTER Hoite Cornelis; Wavin BV
Current assignee: DUYSTER Hoite Cornelis; Wavin BV
Priority date: 1979-12-18
Filing date: 1980-12-17
Publication date: 1981-12-30
Also published as: WO1981001735A1; NL7909107A

Abstract

Procede de pause d'un pipeline flexible consistant en un tuyau externe et un ou plusieurs tuyaux internes (6), l'espace defini entre eux etant rempli d'un fluide durcissable. Le pipeline est pourvu d'elements pouvant supporter des forces de traction considerables dans le sens longitudinal, lesquels elements, ne reduisent cependant presque pas la flexibilite dans un plan. Ces elements peuvent consister en un cable (20) positionne au centre du pipeline (6) a l'aide d'organes d'espacement extensibles (25), un ou plusieurs cables ayant la forme d'une spirale de grand developpement ou un plusieurs cables se trouvant dans un meme plan que l'axe du pipeline. Un tel pipeline flexible peut egalement etre utilise pour coupler deux troncons de pipeline ou pour monter un pipeline sur un corps.Method for pausing a flexible pipeline consisting of an external pipe and one or more internal pipes (6), the space defined between them being filled with a hardenable fluid. The pipeline is provided with elements capable of supporting considerable tensile forces in the longitudinal direction, which elements, however, do not almost reduce the flexibility in a plane. These elements can consist of a cable (20) positioned in the center of the pipeline (6) using extensible spacers (25), one or more cables having the shape of a spiral of great development or one or more cables lying in the same plane as the axis of the pipeline. Such a flexible pipeline can also be used to couple two pipeline sections or to mount a pipeline on a body.

Description

A method for laying a flexible pipeline and pipeline to carry out this method.

The invention relates to a method for laying a flexible pipeline comprising a flexible pipe within which one or more flexible pipes are contained, whereby the spaces between the outer pipe and the inner pipe(s) are filled with a hardable or hardening fluid. Purthexmore, the invention relates to a flexible pipeline to carry out this method.

Laying a pipeline does not only mean bringing (a segment of) a pipeline at the desired situs, but also coupling the pipe segments, connecting a pipeline with various apparatus and the like, in short all actions required to bring a pipeline into a position in which it can function.

By a pipeline is also understood a number of pipes or pipelines which are combined to a bundle, and possibly in one casing. By a flexible pipeline is understood a pipeline which can be bent with a radius smaller than 40 times the diameter of the pipeline. This is a radius which is more than five times smaller than the radius allowed when laying normal steel pipes. As in case of laying a pipeline this is as a rule only bent substantially in one (vertical) plane, it is sufficient for the pipeline to be flexible in one plane only.

When laying rigid pipelines expensive devices are required to keep the bending of the pipeline within acceptable limits. When these devices do not function well, the risk of damaging the pipeline is big. Furthermore, rigid pipelines cannot be laid in rough weather. Lastly, coupling of segments of a rigid pipeline is sometimes a difficult job.

It is known to use flexible pipelines in order to eliminateat least partly - the problems arising during laying rigid pipelines. A disadvantage of such flexible pipelines, generally made of plastic material, is their small strength as compared with pipelines made of steel.

Therefore, pipelines known a.o. under the trade name "COPLEXIP" are used, consisting of a pipe of plastic material, enwrapped by a small-pitched steel coil, which coil in its turn is enwrapped by large-pitched steel coils, the latter coils being encased in a second plastic pipe to protect them against corrosion. The disadvantage of this type of pipeline is that the coil has to surround the inner plastic pipe tightly as otherwise this pipe is insufficiently supported in case of high inner pressure. In consequence thereof the flexibility of the pipeline is reduced, while furthermore the method of fabrication is expensive. As a result, flexible pipe lines of a big diameter (e.g. bigger than 500 mm), resistant to high pressures (e.g. over 5 N/mm²) are not commercially available. Prench patent specification 1,269,182 proposes to use a flexible pipeline which, after laying, is reinforced and made rigid by means of concrete mortar. Thus, during the laying operation, the advantages offered by the flexibility can be utilized, while after this pipeline has been laid and reinforceed, its strength increases considerably, and can be compared with that of a steel pipeline. The relatively low strength of this type of flexible pipeline during laying - as compared with steel pipelines - however, remains a disadvantage. Por, during the laying operation big tensions can occur in the pipeline in longitudinal direction, and this type of flexible pipeline sometimes cannot stand such tensions, or it shows an inadmissible elongation due to the occurrence of such tensions.

Also Prench patent application 76,37344 proposes to use a flexible pipeline. During the laying operation this flexible pipeline is surrounded by a casing of fluid concrete mortar which hardens after laying, while also one or more cable are wound around the pipeline in the shape of a coil in order to prevent the movement of the fluid mortar in longitudinal direction of the pipeline. These cables are not suitable to withstand tensions in the longitudinal direction of the pipeline, so that also this pipeline has a relatively low strength during laying.

An article entitled "Met plastic omhulde gewapend betonnen pijpleiding" (Plastic-encased concrete pipeline), published in "Polytechnisch Tijdschrift", Nr. 22 of October 27, 1975, describes a method for laying a flexible pipeline, whereby the occurrence in the pipeline of big tensions in longitudinal direction is prevented by fixing cables at regular distances to the pipeline to be laid and by exercising a pulling force to all these cables simultaneously when moving the pipeline. Needless to say that this method is far from being ideal. The elongation of the cables is limited, so that when the pipeline is bent, the outer cable withstands the full pulling force, whilst the cables positioned nearer the centre of the bend are relieved. The flexible pipeline is thereby loaded by shearing forces which can be of the same order of magnitude as the full tension applied to the cables. As the longitudinal axis of the pipeline is located at some distance from the outer - tense - cable, this axis will get shorter when the pipeline is bent, due to which the walls of the line can kink. The risk of damaging the pipeline is increased still when between the inner and the outer pipe there is substantially incompressible concrete mortar which - when the pipeline is shortened - is moved sideways, thus exercising a pressure on the outer and the inner pipes. Similar difficulties also occur in case only one excentric cable is used. Furthermore, the pipeline is locally stressed very strongly, not only in longitudinal direction where the cables are fixed to the pipeline, which may cause kinking.

The object of the invention is to provide a method for laying a flexible pipeline whereby on the one side the pipeline to be laid is not subjected to big tensions in longitudinal direction and on the other side the flexibility of the pipeline is substantially preserved. This object is achieved by the above-mentioned method for laying a flexible pipeline, which method according to the invention is characterized in that the pipeline is provided with elements substantiallyin longitudinal direction to withstand pulling forces, and that these elements are, furthermore, provided in such a way the flexibility of the pipeline in at least one plane through the axis of the pipeline is hardly reduced if at all, and that when the pipeline is bent, the length of the axis is hardly changed if at all.

The elements to withstand pulling forces may consist of one or more cables provided in or around the flexible pipeline in the form of large-pitch coils, which cables are supported in radial direction by a body which is rigid in this direction.

It is also possible to provide the pipeline with one or more cables in longitudinal direction, all positioned in one plane in which plane also the axis is positioned. Lastly, it is also possible that before the flexibe pipeline is laid a cable is put inside with expandable spacers around this cable, that subsequently these spacers are expanded by means of a pressurized fluid, due to which the cable is fixed along the axis of the pipeline and integrates mechanically with the pipeline and that after the pipeline is laid on the desired situs the pressure in the expanded spacers is released and the cable, together with these spacers is removed from the pipeline.

The meτhod according to the present invention can also be used for coupling two segments of a pipeline under water, whereby the two ends of the pipe segments to be coupled are lifted above the water surface, kept at some distance from each other by means of a flexible element which can withstand pulling forces and are subsequently coupled above water to a segment of a flexible pipeline, after which the two coupled ends with the segment of the flexible pipeline are laid on the bottom. During laying onto the bottom the segment of the flexible pipeline can be stretched by moving the free end of the coupled line into a direction away from the coupling. The flexible element used in the coupling operation can comprise a number of hydraulic jacks with a common fluid line. The method according to the present invention can also be used for connecting a pipeline e.g. to an artificial island. It is known to lead lines through a stand pipe which near the bottom is provided with a bent terminal. Naturally, flexible pipelines are more suitable in this case than rigid lines. However, when these lines are drawn up (or down) considerable contact pressure and friction occur between the pipeline and the bent terminal of the stand pipe, due to which the pipeline can be damaged. When a flexible pipeline is used, provided with cables in longitudinal direction, all in one plane with the axis of the pipeline, and furthermore these cables are led through leads connected to an artificial island, it is achievedthat the contact pressure and the friction are exercised directly on these cables and no longer on the pipeline. As cables can well stand such forces, damage of the pipeline is prevented in this way.

The invention will now be explained with reference to the drawing in which: fig. 1 shows an inner pulling element in a flexible pipeline; fig. 2a is a side view of a flexible device to withstand pulling forces; fig. 2b is a top plan view of a flexible device to withstand pulling forces; fig. 3 shows a schematic way of the fixture of the flexible pipeline to the flexible device to withstand pulling forces; fig. 4a shows, the lifting of a pipeline; fig. 4b shows the veering of a pipeline coupled above water; figs.5, 6, 7 are cross sections of flexible pipelines provided with cables; fig. 8 shows the installation of a stand pipe in side view, and fig. 9 shows some large-pitched coil-shaped pulling cables.

Figure 1 shows an inner pulling element. As already stated, a flexible pipeline is mechanically weak during the laying operation. Further, it is known that during the laying operation are considerable pulling forces often applied to the pipeline. These pulling forces can be withstood by an interior element provided in the flexible pipeline, which interior element comprises a cable

(20) in or near the axis of this element, an expandable pipe (24) and flexible sausage-like elements (25). The pipe (24) is expandable in radial direction, at least as far as this pipe is positioned in the flexible sausage-like elements (25). When air or liquid is fed via the line (21) and the valve (22), the pipe (24) expands till the sausage-like element (25) presses forcefully against the inner pipe (6) of the flexible pipeline. The cable (20) is then fixed along the axis of the flexible pipeline and forms a mechanical unity with the pipeline.

As the cable (20) is positioned about centrally within the temporarily flexible pipeline, the flexibility of the unity is not impeded. The inner pulling element can be extended at the place of the joint. (26). The terminal plug (27) seals the channel (24). After laying the pressure in the channel (24) is reduced, so that the inner pulling element can be removed from the flexible pipe. The device described above can be further perfected by shunts and valves (not shown), due to which the inner channels (24)can be pressurized selectively and allowing a flexible pipeline segment to be shifted into a tensionless state over a sausage-like and flexible body (25). which is advantageous e.g. in composing a pipeline aboard a laying craft.

Figures 2a and 2b show clamps (30) with which a pipeline (not shown) can be clamped; 31 are hinges with a substantially vertical axes; 32 are legs; 33 are hinges with substantially horizontal swivel axis; 34 a set of coupling bars and 35 lifting cables. By connecting a flexible line with the clamps (30) the flexible pipeline will continue to be tensionless also when the connecting pipeline segments are exposed to tension or pressure, while the flexibility of the line is maintained. Transversal forces can be exercised on the flexible device for opposing pulling forces, due to which the flexible pipeline will bend without showing sharp kinks (which would be the case when these transversal forces are applied directly to the flexible pipeline) and due to which it is possible to lift or veer a flexible pipeline or a segment of it from a surface craft. It is clear that the form of a flexible device to withstand pulling forces can differ from case to case; e.g. cardanic hinges can be used instead of one-leg hinges shown in the drawing, or cables instead of the legs shown in the drawing, etcetera. Figure 3 shows the bending line (40) of the flexible pipe and the bending line (41 ) if the flexible device for withstanding pulling forces, both in the bent state. From figure 3 and from a simple goniometric calculation it follovs that, provided the rat between the length of the legs (32) and that of the coupling bars (34) (vide figure 2) is selected correctly, the change in length of this pipeline when bending same according to line 40 can be neglected, while also the average distance between the lines (40) and (41) is about nil. The ratio between the length of coupling bars (34) and that of the legs (32) (figure 2), or between the distance 33-33 and the distance 30-33 (figure 3) must preferably be between 3-25 and 4.25.

This can be used for fixing a flexible pipeline to a flexible device for withdrawing pulling forces. Such a fixturg is desirable e.g. for withdrawing the weight of the flexible pipeline itself. The flexible pipeline is connected via jacks (43 a, b) with points 42 which are connected with the flexible device for withdrawing pulling forces. By using a common line (not shown) for the jack (43a, 43b), it is achieved that by bending the unity the jacks (43a) get shorter and corresponding jack (43b) longer, due to which the flexible pipeline is supported without bending of the unity.

Figure 4a shows the lifting of a pipeline (50a) lying on the bottom (55) of the sea (58) from a craft (54) at the water surface (56), which craft is equipped with a lifting device (57) with a cable (53). By means of a - preferably elastic - cable (51) which e.g. is connected with an anchor of a tug (not shown) a. sufficiently strong pulling force is applied to the pipeline (50a). Figure 4b shows the veering of a pipeline consisting of the segments (50a, 50b) and the flexible pipeline segment (60) from the surface craft (54). Beforehand the pipeline segments (50a, 50b) are lifted, e.g. in the way shown in figure 4a, connected mechanically above water, with the flexible device for withdrawing pulling forces (59) and coupled liquid-tight and gas-tight to the flexible pipeline portion (60).

During veering the flexible pipeline segment (60) is stretched by moving the pipeline segment (50a) in the direction of the arrow (61), e.g. by means of a tug (not shown) fixed to the free end of this pipeline portion whereby during the operation a sufficiently strong pulling force is maintained in the pipelines (50a, 50b) in order to prevent sharp bending.

It is known that in many cases a pipeline is bent during laying substantially in one, mostly the vertical, direction, as appears from figures 4a and 4b. Therefore, it is possible, as shown in figure 5, to connect a flexible pipeline with two cables

(70) which together with the axis of the flexible one plane (80). It is true, this reduces the flexibility of the unity in this plane (80) - which in many cases is not important - but the flexibility perpendicular to this plane is maintained.

As figure 6 shows, the cables (70) can be kept at the desired mutual distance, not only by a flexible pipeline, but also by snacers (72). Figure 7 shows that it is possible to use one cable (70) in combination with a bundle of flexible pipes.

Figure 8 shows the installation of a stand pipe near an artificial island (76). To this end cables (70) are led through conduit constructions (73) (vide also figures 5, 6 and 7) which are connected with the artificial island (76). The flexible pipeline (60), connected with the cables (70), can be coupled, if desired above water to a pipeline (50a), e.g. according to a method described on the basis of figure 4b. When veering the pipe line (50a/60), the flexible pipeline is drawn, by means of the winch (75) along the conduit constructions (73) to a level above the water surface (56), whereby the segment of the flexible pipe line (60) which is bent on the site of the flexible device for withdrawing pulling forces (59), is fully or portially stretched. As is apparent from figure 8, the lower conduit construction (73) has a bent form, due to which during pulling a contact pressure occurs between the cables (70) and this bent conduit construction (73), and consequently also friction forces. It is remarked that the flexible pipeline itself is not subject to these contact and friction forces, which is an important advantage over the conventional way of pulling a pipeline into a stand pipe, whereby the line itself is pulled and consequently is subject to contact and friction forces.

Figure 9 shows a pipeline according to the invention, whereby two cables (81, 82) are wound helically at a large pitch in opposed directions, around a pipe. When a pulling force is exercised on these cables, they will bring radially directed forces to bear on pipe 80. The pipe must be sufficiently rigid to withdraw these forces.

Claims

C L A I M S

1. A method for laying a flexible pipeline comprising a flexible pipe within which one or more flexible pipes are contained, whereby the space between the outer pipe and the inner pipe(s) are filled with a hardable or hardening fluid, c h a r a c t e r i z e d i n that the pipeline is provided with elements substantially in longitudinal direction to withstand pulling forces, and that these elements are, furthermore provided in such a way that the flexibility of the pipeline in at least one plane through the axis of the pipeline is hardly reduced if at all, and that when the pipeline is bent, the length of the axis is hardly changed if at all.

2. A method according to claim 1, c h a r a c t e r i z e d i n that in or around the flexible pipeline one or more cables are provided in the form of a large-pitch coil and that these cables are supported in radial direction by a body which is rigid in this direction.

3. A method according to claim 1, c h a r a c t e r i z e d i n that the pipeline is provided with one or more cables in longitudinal direction, all in one plane which also contains the axis of the pipeline.

4. A method according to claim 3, c h a r a c t e r i z e d i n that before laying the flexible pipeline one cable is installed centrally, with expandable spacers around this cable, that subsequently the spacers are expanded by means of a pressurized fluid by which the cable is fixedly positioned along the axis of the pipeline forming a mechanical unity with the pipeline, and that after the pipeline has been positioned at the desired site the pressure in the expanded spacers is taken off and the cable together with the spacers is removed from the pipeline.

5. A method according to claim 1, 2, 3 or 4 for coupling two under-water segments of a pipeline, whereby the two ends of the pipe segments to be coupled are lifted above the water surface, then coupled and subsequently lowered onto the bottom, c h a r a c t e r i z e d i n that the two ends of the pipe segments to be coupled are kept at some distance from each other by means of a flexible device which can withstand pulling forces, that the two ends are then coupled above water to a segment of a flexible pipeline and that the two coupled ends together with the segment of the flexible pipeline are positioned onto the bottom.

6. A method according to claim 5, c h a r a c t e r i z e e i n that during positioning of the coupling onto the bottom the segment of the flexible pipeline is stretched by moving the free end of the coupled line in the direction away from the coupling.

7. A method according to claim 5 or 6, c h a r a c t e ri z e d i n that the flexible element comprises a number of hydraulic jacks with a common liquid or gas line, which hydraulic jacks screws support the flexible pipeline during positioning onto the bottom.

8. A method according to claim 3 for mounting a flexible pipeline to a body, c h a r a c t e r i z e d i n that the pipeline is provided with cables on the outside of the pipeline, that the body is provided with leads corresponding to the mutual position and the size of these cables, and that mounting of the pipeline is effected by pulling the cables in logitudinal direction through the leads.

9. A flexible pipeline to carry out the method according to claim 2, c h a r a c t e r i z e d i n that in or around the flexible pipeline one or more cables are installed in the form of a lare-pitch coil, and that these cables are supported in radial direction by a body rigid in this direction.

10. A flexible pipeline to carry out the method according to claim 3, c h a r a c t e r i z e d i n that the pipeline is provided with one or more cables in longitudinal direction, all in one plane which also comprises the axis of the pipeline.

11. A flexible pipeline to carry out the method according to claim 4, c h a r a c t e r i z e d i n that in this pipeline there is a cable around which expandable spacers are provided, that the spacers can be expanded by means of a pressurized fluid and that elements are provided to bring the spacers at will into the expanded state or not.

12. A flexible device to carry out the method according to claim 5, c h a r a c t e r i z e d i n that the flexible device consists of one or more rigidly connected parallel coupling bars of equal length and two equally long legs each of which is pivotable connected with one end to an end of the coupling bars, the other end being provided with elements for fixing a pipeline and that the ratio between the length of the coupling bars and that of each of the legs is between 3.25 and 4.25.