The invention also relates to a transport arrangement comprising a pipeline structure between two points at sea, which pipeline structure hangs between two anchored points of support at such a depth that wave stresses are eliminated or reduced to an acceptable level.

The invention has been especially developed in connection with the transfer or transport of hydrocarbons from a floating production unit to a loading buoy and then to a tankship. In connection with such transfer or transport, it is usual to use pipelines that are laid on the seabed, when the offshore installations are in shallow waters, or to use pipelines held just above the seabed by means of supports resting on the seabed.

Greater water depths make it increasingly difficult to lay pipelines on the bed, and moreover, the seabed is often irregular and thus quite unsuitable the laying of pipelines.

It is known to use submerged pipelines, by using either buoyant pipes that are weight- loaded and anchored, or pipes having negative buoyancy and supported by anchored floats.

The use of expensive flexible pipelines, provided with buoyancy members, floating relatively freely in the water, is also known. These are sensitive to density variations ; transported fluid and have the drawback of being difficult to install and represent a costly solution.

To prevent varying external forces, originating, for instance, from ocean currents, from having an excessive impact on a pipeline and inducing local stresses great enough to damage the pipeline, complex and costly anchoring means are required. The use of a flexible pipeline that is suspended in the sea, at a depth where the flexible pipeline will be protected from underwater currents, waves etc., is also known. A known solution of

this kind is found in US 4,263,004. In this document a flexible pipeline is suspended from buoys that are anchored to the seabed by vertical lines. This known solution requires the use of flexible pipelines.

The object of the present invention is to provide a transport arrangement for fluid, as for instance hydrocarbons, between two points at sea, which transport arrangement can be positioned in a relatively simple manner with the aid of a surface vessel and involves the use of rigid pipelines, i. e., pipelines that are not of flexible construction, i. e., that ordinary and relatively inexpensive steel pipes can be used.

One of the objects of the invention is thus to establish a sort of subsea suspension bridge, comprising rigid transport pipelines, essentially below the so-called wave zone, so that there are no pipelines on the surface, but where they are at a depth at which they are relatively undisturbed by waves and do not obstruct the movement of any vessels on the surface.

In this free pipe bridge, the pipeline will form a definite arc-shaped curve, and a particular object of the invention is to be able to optimise the shape and curvature of this arc-shaped curve during the planning stage and obtain a desired shape, curvature and depth in the sea.

A particular object of the invention is to permit the suspended pipeline to be in the form of a single-curved arc, so as to avoid varying torque tensions in the pipeline.

Therefore, in its basic form, the inventive idea is to establish a sort of underwater suspension bridge comprising one or more tension-loaded, rigid pipelines that are suspended under the so-called wave zone, and which in terms of load are uncoupled from the floating units where one sends and the other receives the transported fluid.

Environment-induced motions of these floating units are not transferred to the"bridge" so that dynamic loads are eliminated or reduced considerably.

According to the invention, a method is therefore proposed for providing a pipeline connection between two spaced-apart points at sea at such a depth that wave stresses are eliminated or will be reduced to an acceptable level, the pipeline structure being suspended between two points of support at said depth, and what characterises the method according to the invention is that the pipeline structure is suspended from the

two points of support by means of a torque-free connection to the points of support, and via these is subjected to an external tension load.

The method according to the invention can be carried out in a particularly advantageous manner by providing as points of support two submerged buoys that are anchored so that they will exert the said external tension load on the pipeline structure.

A particularly favourable method according to the invention is one where the individual buoy is anchored with a weight hanging from a cable between an adjacent one of the said points and the buoy.

Yet another favourable method according to the invention is one where each individual buoy is anchored with a cable that is so heavy that it assumes a catenary shape, with a sag that is sufficiently great to allow a non-coupled motion of the buoy/pipe bridge relative to the respective point in the sea to be obtained.

Furthermore, according to an advantageous embodiment of the method, one end of a pipeline structure floating in the sea can be connected to one of the buoys on the surface, whereupon the pipeline structure is drawn towards the second buoy and connected thereto at the surface, the buoys being connected to a respective one of the two points with a respective cable, that is weight-loaded so that the buoys sink in the sea, whereupon the pipeline structure is ballasted so that it is caused to sink and remain hanging, tensioned-loaded, between the buoys.

According to the invention, a particularly advantageous way of accomplishing said ballasting is to admit water ballast at one of the ends of the pipeline structure whilst allowing air or gas to escape at the other end of the pipeline structure. A particular advantage of a method of this kind is that a gentle and balanced transfer of the pipeline structure to the desired tension-loaded state in the sea and at the desired depth under the wave zone is achieved.

It is a particular advantage if the pipeline structure includes one or more ballastable supporting pipes, by means of which the pipeline structure is suspended in a torque-free manner from the two points of support.

According to the invention there is also proposed a transport arrangement, comprising a pipeline structure between two points at sea, which pipeline structure hangs between

two anchored points of support at such depth that wave stresses are eliminated or reduced to an acceptable level, and what characterises the transport arrangement according to the invention is that the pipeline structure is suspended by means of a torque-free connection from the two points of support, and that these are so anchored that they exert a tension load on the pipeline structure.

According to the invention, it may be particularly advantageous if the two points of support are a respective, submerged buoy that is anchored with a weight hanging from a cable between an adjacent one of the said points and the buoy, or the cables may be very heavy themselves.

According to the invention, the two said points may be respectively a hydrocarbon production unit and a loading buoy, a flexible pipeline or hose connecting the pipeline to the hydrocarbon production unit and the loading buoy respectively.

According to the invention, it is especially advantageous to allow the pipeline structure may include one or more ballastable supporting pipes by means of which the pipeline structure is suspended in a torque-free manner from the two points of support.

The invention will now be described in more detail with reference to the drawings, wherein: Figure 1 is a side view of a transport arrangement according to the invention; Figure 2 is a top view of the transport arrangement in Figure 1; Figure 3 is a cross-sectional view of an embodiment of a pipeline structure in the form of a pipeline bundle, where Figure 3a shows the pipeline bundle whilst in tow, floating on the surface, Figure 3b shows the pipeline bundle submerged after completed installation, and Figure 3c shows the pipeline bundle submerged and in operation.

Figure 4 is a cross-sectional view of an alternative embodiment of the pipeline bundle in Figure 3, Figures 4a, 4b and 4c showing the same phases as Figures 3a, 3b and 3c.

Figure 5 shows a second alternative embodiment of a pipeline structure, where Figures 5a, 5b and 5c show the same phases as Figures 3a, 3b and 3c respectively.

Figure 6 is a side view of a transport arrangement according to the invention, and shows in particular an anchoring arrangement for buoys and a production vessel or loading buoy.

Figure 7 is a top view of the embodiment in Figure 6.

Figure 8 is a side view of a particularly favourable transport arrangement according to the invention.

Figure 8B is a side view of a modified embodiment of the transport arrangement shown in Figure 8.

Figure 9 shows pre-installed anchor lines, where the left hand line is connected to a support buoy and the right-hand line is temporarily connected to a marker buoy (which in the figure is hidden behind the pipeline), and that the pipeline structure is being towed towards the right-hand buoy for connection thereto.

Figure 10 shows the pipeline structure connected to the permanently anchored buoy on the left.

Figure 11 shows a situation where one of the support vessels has fished up the anchor line in preparation for connection.

Figure 12 shows the connection of the pipeline structure with attached buoy to the right- hand mooring line, and connection with the mooring lines.

Figure 13 shows the situation after the pipeline structure has been connected to the two anchored buoys; and Figure 14 shows the ballasting and positioning of the pipeline structure between the two buoys anchored to the seabed so as to initially obtain the position illustrated in Figure 1.

Figures 15-19 show in a schematic manner yet another possible positioning for providing the pipeline structure in the sea, where: Fig. 15 shows the pipeline structure floating in the sea, between two towing/installation vessels ;

Fig. 16 shows the pipeline structure connected respectively to a hydrocarbon production unit and a loading buoy ; Fig. 17 shows an installation phase where the two buoys at the ends of the pipeline structure are in the process of being submerged; Figure 18 shows the two buoys once submerged ; and Figure 19 shows ballasting and positioning of the pipeline structure between the two anchored, submerged buoys, with the pipeline structure connected to the hydrocarbon production unit and the loading buoy respectively by means of flexible hoses, so that the arrangement is ready for use.

Figures 1 and 2 show a pipeline structure two points at sea. A pipeline structure 1 is suspended between two spaced-apart points of support 3,4 that are submerged in the water 2. These points of support in Figures 1 and 2 are in the form of two buoys 3,4 anchored to the seabed 5. The necessary mooring lines are indicated by means of the reference numerals 6,7,8 and 9. The pipeline structure 1, more precisely a pipeline bundle, is suspended from the two buoys 3,4 by means of a torque-free connection (not shown). The torque-free connection may, for example, be effected using short lengths of chain.

In its free span, the pipeline bundle 1 forms a definite arc-shaped curve whose shape and curvature may be optimised during the planning phase and obtained by maintaining the pipeline bundle 1 under specific tension load by means of the anchor lines 6 and 9.

If a pipeline bundle 1 of this kind is exposed to water currents of varying directions and intensity, the pipeline bundle 1 will be able to yield and constantly maintain the desired single-curved arc shape, thereby avoiding varying torque tensions in the pipeline bundle 1. The broken line in Figures 1 and 2 indicates a position the pipeline bundle 1 may take up if exposed to a sideways current.

A hose 10 runs from the end of the pipeline bundle at the buoy 3 to a production vessel 11. A hose 12 runs from the end of the pipeline bundle at the buoy 4 to a loading buoy 13. These two transfer hoses 10 and 12 are not shown in the top view in Figure 2, and the production vessel 11 and the loading buoy 13 have also been omitted.

An advantageous embodiment is shown in Figure 3, where Figure 3a shows the pipeline bundle floating on the surface whilst being towed, Figure 3b shows the pipeline bundle submerged after completed installation, and Figure 3c shows the pipeline bundle submerged and in operation, i. e., that hydrocarbons are being transported.

As can be seen from Figure 3, the pipeline bundle 1 consists of three pipelines, namely a supporting pipe 14 and two transport pipes 15 and 16. These three pipes are gathered together as a pipe bundle in a conventional, known manner with the aid of suitable means, here merely symbolised by the encircling line 17. The two transport pipes 15, 16 are equipped at their respective ends with connectors for connection to the flexible transport hoses 10 and 12 (see Fig. 1). It is possible to use, for example, two flexible transport hoses 10 and two transport hoses 12, or just one transport hose 10 and 12 respectively, which are connected in a suitable fashion to the two transport pipes 15,16, in a manner that will be well-known to the skilled person. The flexible transport hoses 10,12 are yielding to waves and can easily be changed as required. The buoys 3,4 (Fig. 1) are placed so deep that the pipeline bundle 1 will essentially be located below the so-called wave zone.

The supporting pipe 14 is fastened between the buoys 3,4 by means of the torque-free connections, for example, with the aid of lengths of chain.

The pipeline bundle 1 is put in place by being towed to the installation site whilst floating on the surface. The supporting pipe 14 is kept full of air. The buoyancy can be adjusted by ballasting the supporting pipe 14 when required by filling it partially with water. This is accomplished, for example, by dividing the supporting pipe 14 into chambers and perhaps also by its being built up of a plurality pipes which may be of different diameters, in order to allow control and adjustment of the desired buoyancy.

In a preferred embodiment an externally positioned pipe can be used.

Figure 4 shows an alternative pipeline bundle 1', consisting of one supporting pipe 14 and one transport pipe 15. The various phases of the installation are shown in Figures 4a, 4b and 4c, and these phases are the same as those in Figures 3a, 3b and 3c.

Figure 5 shows another alternative embodiment where the pipe bundle 1"consists of two transport pipes 15 with no a supporting pipe, but with buoyancy-providing elements 23 mounted on the outside of the pipes. Whilst in tow, the transport pipes 15 are filled with ballast liquid. The weight is counterbalanced by the temporary buoyancy-

providing elements 23, in this case shown as single floats. Furthermore, a reinforcing member 24 is included, e. g., a wire or stay, which transmits the weight of the structure and other stresses thereon to the points of support 3,4 (Fig. 1). The buoyancy- providing elements 23 are connected in such manner that they can easily be removed successively from one end of the pipeline bundle, whereby the same effect and distribution are achieved as if ballast liquid had been used as in the two embodiments described above. A buoyancy material 25 is also arranged around the outside of the transport pipes 15 for necessary buoyancy under operating conditions.

Figures 6 and 7 show an embodiment of the invention which may be regarded as a variant of that shown in Figures 1 and 2. The difference between the two arrangements in Figures 1,2 and Figures 6,7 is found primarily in the anchoring systems for the loading buoy 13 and for the two submerged buoys 3,4, the buoys 3,4 being anchored by means of two respective diverging anchor lines 18,19. The loading buoy 13 is anchored in a conventional known manner with the aid of a plurality of anchor lines 20.

The pipeline bundle 1 is tension-loaded by the anchor lines 18,19.

Figure 8 shows an arrangement including submerged buoys 3,4 as in Figures 1,2 and 6, 7, but unlike the arrangements described above, the pipeline bundle 1 in this case is suspended from the production vessel 11 and the loading buoy 12 respectively in that each individual buoy 3,4 is anchored by means of a respective cable 30, 31 that is weight-loaded with weights 32.33. This special suspension provides the desired tension load of the pipeline 1, whilst the suspension is such that it attenuates most of the motions/loads applied by the floating production unit 11 on the one end and by the loading buoy 13 on the other end. Thus, the pipeline is in turn spared from dynamic stresses that have an impact on the lifetime of the structure. Stresses that cause material fatigue are reduced. As mentioned, the suspension at each end comprises a cable or line 30,31 that is made to form a respective V-shape with the aid of a weight. The weight 32,33 may be a lump weight or a hanging chain and the like as shown in Fig. 8b. These embodiments prevent/reduce the formation of fluctuations of load that are due to movement of the extreme point of the suspension cables, i. e., the respective connection points of the production unit 11 and/or the loading buoy 13. The weight of the lump weight or the cable is transferred both to the production unit or the loading buoy and to the end of the pipeline bundle 1, where a buoy 3,4 or buoyancy tank is located. The depth of the buoys or the buoyancy tanks and the pipeline bundle is selected so that stresses from waves are below an acceptable level and the danger of damage from boat traffic on the water surface is eliminated. In Figure 8b, as an alternative, the weight of

the weights 32 and 33 in Fig. 8 is distributed along a certain length of the cable 30,31.

Typically: heavy chain, several pieces in parallel.

The installation shown in Fig. 8 or 8b is very installation-friendly. It permits the pipeline to be brought to the surface in a simple way for repair, inspection, cleaning/scraping off of fouling and the like. Reference is made to a more detailed discussion of the installation method in connection with Figures 15-19.

First, an installation method in connection with the embodiments in Figures 1 and 2 will be described in more detail, with particular reference to Figures 9-14.

Figure 9 shows pre-installed anchor lines, where one of the anchor lines 18 is connected to a buoy 3 and the other anchor line 19 is temporarily connected to a marker buoy 26 (hidden behind the pipeline 1 in Figure 9). The pipeline structure 1 is being towed towards the buoy 3 for connection thereto. The buoys 3,4 and 26 are all floating on the surface of the sea.

Figure 10 shows the pipeline bundle 1 connected to the permanently anchored buoy 3.

The marker buoy 26 is shown in this figure.

Figure 11 shows a situation where one of the support vessels has fished up the mooring line 19 in preparation for connection. The marker buoy 26 is disconnected from the mooring line 19 once this has been fished up.

Figure 12 shows the connection of the pipeline bundle 1 with attached buoy 4 to the mooring line 19. The pipeline bundle 1 and the buoys 3,4 are under tension from the support vessel, which is shown on the right in Figure 12, so that the free end of the mooring line 19 reaches far enough to allow the connection to the buoy 4 to be carried out on the surface of the sea. During this operation the buoy 3 will submerge and tauten the anchor line 18.

Figure 13 shows the situation once the pipeline bundle 1 has been connected to the two anchored buoys 3,4. The buoys 3,4 are now both below the surface of the sea, and the mooring lines 18,19 are under tension.

On the surface the pipeline bundle 1 is connected to the buoy 4, and the tow is released.

The pipeline bundle 1 will then take up the position in the water 2 as shown in Figure

13. One of the ends of the air-filled pipeline is then opened, at the end that is at the buoy 3, and it ensured that air can escape from here without water entering. Water is supplied in the other end of the pipeline, so that the initially empty transport pipe is filled with water. The pipeline bundle 1 will then move as indicated by means of the successive broken lines and finally take up the position indicated by the reference numeral la, a position that corresponds to the position shown in Figures 1 and 6. The general method described here ensures a gentle and balanced transfer of the pipeline bundle 1 from the floating position on the surface, with the two submerged buoys 3,4, to the desired suspended position in the sea, essentially below the wave zone.

The pipeline connection provided according to the invention is such that the arrangement, i. e., the pipeline bundle 1, is essentially only subjected to tensile loads : Such loads are easy to take up in the structure. This advantageous state is achieved by suspending the pipeline bundle 1 between two points of support 3,4 by means of a torque-free connection and under tension, the whole assembly being submerged to a depth such that wave stresses are eliminated or reduced to an acceptable level.

The buoys 3,4 take up the submerged weight of the pipeline bundle and generate the necessary tension. The transport arrangement is terminated with components (loading hoses 10,12) that are flexible and thus also yielding to motions.

As an alternative to the preferred method described above, the buoy 4 may be preinstalled on the mooring line 19, whilst the other buoy 3 is secured to the pipeline structure 1. This alternative method will require the buoy 4 to be constructed in such a way that the free end of the pipeline structure 1 can be pulled into the connection point on the buoy 4 without the pull wire subjecting the buoy to undesirable stresses. The buoy 4 can advantageously be made of two interconnected, spaced-apart cylindrical elements (not shown) which have a passageway for the pull wire therebetween.

Optionally the buoy may be made in the form of a single element (not shown) with a through hole for the pull wire in the centre thereof.

A preferred installation of the transport arrangement that is shown in Fig. 8 will now be explained in more detail with reference to Figures 15-19.

Figure 15 shows a pipeline structure or bundle 1 being towed to an installation site, with associated buoys 3,4.

In Figure 16 it is shown that suspension lines or mooring cables 30,31 are connected between the production unit 11 and the buoy 3 and between the loading buoy 13 and the buoy 4 respectively.

In Figure 17 it is shown how weights, for the anchor cable 30 a lump weight 32 and for the anchor cable 31 and chain weight 33, are used for submerging the buoys 3,4 in the sea.

Figure 18 shows the situation after the weights have forced the buoys 3,4 down. The pipeline structure 1 is still floating on the surface of the water or has been submerged with the sags close to the surface, but of course follows the buoys 3,4, as shown in Fig.

18.

Figure 19 shows lowering phases of the pipeline structure 1, carried out in the same way as described above in connection with Figure 14.

The pipeline structure 1 can be transferred into a floating position on the surface by carrying out the installation steps mentioned above, but in the reverse order.

Needless to say, the invention can be carried out using buoyancy elements other than those shown and described herein. For instance, the buoyancy may be in the form of elements of materials that are lighter than water, i. e., that there is a"permanent" buoyancy which cannot be changed. All the pipelines may also be used in temporary phases for the active adjustment of the buoyancy by, for example, filling with water. A plurality of pipelines and/or combination of transport pipes and supporting pipes could also be used as shown, or in other combinations.