Large numbers of pipes have been laid on the seabed in recent years in connection with the extraction of oil and gas at sea.

The pipes are either laid freely on the seabed, possibly with fixing points arranged at intervals from each other, or are buried in the seabed and covered.

The prior art describes solutions in which floating elements are used in connection with the laying of a pipeline. However, these floating elements do not serve as buoyancy elements for a floating, permanently anchored pipeline. They are removed after the pipeline has been positioned on the seabed.

In areas in which the seabed is very uneven with high peaks and deep, wide depressions (valleys) at great depths, the existing pipe-laying methods cannot be used.

The present invention concerns a submerged pipeline which is characterised in that it is arranged in floating fashion for all or at least part of its length, that the buoyancy for the pipeline is provided by floating elements and/or floating material arranged at intervals and in a mainly uniform layer around the pipeline, possibly in combination with weights or sinker material, and that the pipeline is anchored to the seabed by stays or anchor lines arranged at intervals.

Dependent claims 2-5 define advantageous features of the present invention.

The present invention will be described in further detail in the following using examples and with reference to the attached drawings, where: Fig. 1 shows a drawing of a part of a submerged floating pipeline with anchor lines arranged at intervals and in a V shape and with buoyancy elements arranged for each anchor point.

Fig. 2 shows the same as in Fig. 1. However, here the buoyancy elements are evenly distributed along the entire pipeline.

Fig. 3 shows a submerged floating pipeline with only one vertical anchor line for each anchor point.

Fig. 4 shows a submerged floating pipeline where the pipeline is arranged in arc form between each anchor point.

Figs. 5 and 6 show two different buoyancy/weight-loading situations for an anchored pipe.

Fig. 7 shows a submerged floating pipe bundle solution with diagonal anchor lines.

Fig. 8 shows, in larger scale, a cross-section of the pipe bundle solution shown in Fig. 5.

Unlike conventional pipe-laying methods for submerged pipelines in which the pipes 1 are laid on or buried in the seabed along their entire length, the present invention is characterised in that the individual pipelines float for all or part of their length and are anchored to the seabed at a distance from it. The pipe anchors can be in the form of anchor lines or stays arranged in a V shape 2 (diagonal), as shown in Figs. 1 and 2, or as a single vertical line or stay 3, as shown in Fig. 3. Alternatively, the pipeline can be arranged in an arc and be fixed, at its ends, directly to the anchor points 4 on the seabed without anchor lines, as shown in Fig. 4.

In order to keep the pipeline floating, buoyancy elements are used, for example one buoyancy element 5 for each support point, as shown in Fig. 1, buoyancy elements 6 evenly distributed along the entire length of the pipeline, as shown in Figs. 2,3 and 4, a floating layer of uniform thickness arranged around the pipeline along its entire length (not shown), or a combination of these solutions. The buoyancy elements can be in the form of dense, solid elements of steel or other metallic material or they can be made of foamed plastic material, for example polypropylene or PVC.

Apart from the lines/stays mentioned above, the anchors also comprise an anchor point 7 in the seabed and pipe clips (not shown) for fixing the line/stay to the pipe.

The anchor point 7 in the seabed may be gravitation anchor, pillar anchor, suction anchor, plate anchor or penetration anchor.

As shown in the figures, the pipeline can have one or two, or possibly also more, anchor lines for each anchor point. Calculations show, however, that, for reasons of lateral rigidity and dynamics, it will be most expedient to use two anchor lines arranged in a V shape. The diagonal position (angle) of the lines/stays will be determined by, among other things, the tension in both lines/stays under the actual load conditions for the pipeline.

Moreover, the lines/stays can be in the form of fibre rope of aramid, polyester, etc., steel wires, steel chains or rigid stays of steel, titanium, composites, etc.

Oscillations (vibrations) induced by the local water currents re-present a major problem which must be solved for floating pipelines. Over time, the oscillations can lead to fatigue and in certain situations to uncontrolled excess loads in connection with natural oscillations, which can result in fracture in the worst case scenario.

One way of controlling the oscillations is by axial tension to the pipeline. When the pipeline is laid, filled with water, it is most expedient for it to be in a straight line. When the pipeline is emptied, the axial tension will, as a result of the net buoyancy of the pipeline itself and the buoyancy elements, produce the most favourable oscillating frequency situation, i. e. in connection with high natural oscillations.

Another way of controlling the oscillations is by varying the distance between the support points (pipe sections/pipe spans). These distances must be chosen so that the natural oscillations of the pipe sections will be higher than the threshold value of the excitation frequency generated by the surrounding water current.

The underlying theory shows that pipe sections of different lengths have different natural oscillations. As the pipe sections will"prefer"to oscillate at their own natural oscillations, neighbouring sections of different lengths will contribute to damping each other's oscillating amplitudes.

In areas with varying water current speed along the pipe, the length of the sections can be adapted to the local current speed and thus increase the pipe's fatigue life.

The tension in the pipeline and the distance between the support points should preferably be co-ordinated and optimised in order to achieve minimal stress in the pipeline.

The total pipe system rigidity is represented by a combination of tension (cable power) and bending power. As the distance between the support points increases, the cable power will be dominant. The maximum static sag/deflection as a consequence of buoyancy/weight for an anchored pipe can be calculated purely from the point of view of the cable using the expression: <BR> <BR> qL2<BR> W-8. N e. f. f' where (see also Figs. 5 and 6): w deflection between the anchor points q = distributed, net buoyancy/weight L length between the anchor points Ne = effective tension the actual tension in the pipe, corrected for external and internal pressure. The oscillation frequency (vibration frequency) of a pipe can consequently be determined using: (purely the cable) where: fn natural frequency (Hz) for oscillation form number n M distributed mass (incl. co-oscillating water mass) El bending rigidity of the pipe.

It can easily be seen from the above formulae that the natural frequencies of a floating, anchored pipe can be changed by, among other things, changing the effective tension Ne and the length L between the anchor points.

On the other hand, it is necessary also to take deformation (bending outwards) and the bending moment of the pipe into consideration. Figs. 5 and 6 show examples of two different load situations for an anchored pipe which produce different bending moment and deformation results.

Fig. 5 shows a pipe for which only buoyancy is used and where the buoyancy is evenly distributed along the pipe. The deformation W of the pipe takes place in this example only in an upward direction. As can be seen in the bottom diagram in Fig. 5, the maximum bending moment here will occur at the individual anchor points of the pipe. This load situation is less favourable than, for example, the load situation shown in Fig. 6, where evenly distributed buoyancy and evenly distributed weight are used alternately between each pair of anchor points along the pipe. In such a load situation, the minimum moment will be at the anchor points, as indicated in the bottom diagram in the figure.

Even though the load pattern is the most favourable in relation to the bending moment in the pipe, a pipe with such a load pattern will be harder to lay than a pipe which has evenly distributed buoyancy as mentioned above. For floating pipes, therefore, it is necessary to take into consideration not only factors associated with the ideal/optimal situation for the pipe when it has been laid but also factors associated with the laying process itself.

The present invention, as it is defined in the attached claims, is not limited to a single pipeline. It applies also to two or more pipelines arranged, for example, in a bundle.

Figs. 7 and 8 show an example of a pipe bundle which is arranged inside a surrounding pipe 8 and which is anchored with anchor lines 2 via a pipe elevator 9.

Here the surrounding pipe will be able to create the buoyancy for the"overall" pipeline/bundle.