GB2435083A

GB2435083A - Buoyancy apparatus and method of use

Info

Publication number: GB2435083A
Application number: GB0602628A
Authority: GB
Inventors: Jean-Baptiste Pose
Original assignee: Individual
Current assignee: Individual
Priority date: 2006-02-10
Filing date: 2006-02-10
Publication date: 2007-08-15
Also published as: GB0602628D0

Abstract

An assembly of underwater buoyant elements 2A, 2B connected to a submarine article such as a pipeline 1 allows it to float with appropriate reserve of floatability but significantly below the surface away from the zone A disturbed by waves, surface currents and weather. The apparatus consists in splitting the required amount of buoyant elements 2A,2B between different water depth levels. Such a split limits stress and fatigue damage induced to the submarine article 1 to a minimum, hence allowing for long distance and less weather sensitive towing operations in open seas. It is possible to further enhance the apparatus by introducing dampening devices 6 within the surface buoys connecting links.

Description

1 2435083 1 Improved buoyancy apparatus and method of use

4 Field of the invention

6 The present invention relates to an improved buoyancy apparatus and method of use, 7 and in particular an improved apparatus and method for support and transport of a 8 subsea article. In one of its aspects, the invention relates to a pipeline transportation 9 method and apparatus, and to a method for transporting pipelines to an offshore pipeline installation site.

12 In an embodiment, the invention provides a safe and efficient method to transport pipeline 13 sections from construction to installation site and pipeline sections may be towed over 14 large distances across seas and oceans.

16 Background to the invention

18 In the field of transportation of pipelines it is desirable for offshore contractors to tow 19 pipelines that are buoyant, quasi buoyant or non-buoyant to a pipeline installation site.

This helps to reduce transportation and installation costs as the overall requirements on 21 marine effort are reduced by making significant lengths of preassembled pipe sections 22 available at the site.

24 Various towing methods have been used and may be classified in two categories: 26 a) Buoyant tows -: 28 These consist of towing an over buoyant assembly (including the pipeline) that 29 naturally floats towards the water surface as depicted in Figures 1-A and 1-B.

Figure 1-A represents a pipeline (1) being supported by two continuous sections 31 of polymer pipelines (HDPE) (2). Figure 1-B represents a pipeline (1) being 32 supported by conventional buoys (2).

34 In these cases, the connection between the floating system and the pipeline is made with soft slings or similar rigging.

37 b) Non Buoyant tows 1 These consist of towing an under buoyant assembly (including the pipeline under 2 transport) that naturally sinks towards the seabed as depicted in Figures 2-A to 2-3 C. Figure 2-A represents a bottom tow where the pipeUne (1) is connected to a 6 towing skid (4) which is being pulled by a vessel. Figure 2-C represents a 7 different kind of bottom tow where chains (5) are fitted onto the pipeline (1) and 8 help to regulate the pipeline altitude above the seabed. The overall assembly 9 chains, pipeline and buoys (2) is under-buoyant, while the sub-assembly comprising pipeline and floats is over buoyant. Figure 2-B shows a mid-depth 11 tow, where the pipeline (1) is suspended at both extremities from surface vessels 12 (6).

14 The prior art tow types (a) and (b) described above have a number of limitations and drawbacks: which are being highlighted below.

17 In the case of towing an over-buoyant assembly, the pipeline (1) is close to the surface.

18 The presence of surface waves acting on the assembly causes pipeline fatigue and 19 damage. Fatigue is directly generated through the accelerations of the surrounding liquid impacting on the pipeline walls and indirectly through the attachment points of the 21 buoyancy assembly (2).

1)l) .. e * S S.

23 The amount of fatigue generated within the pipeline while under tow is a critical 24 parameter affecting operational performance of the pipeline. Fatigue damage incurred over surface tows using currently available towing arrangements is such that it impacts 26 on the life expectancy of the pipeline. This transportation method is not viable if lengthy 27 operations are expected on open seas with weather forecast availability not exceeding 28 the expected duration of the operation itself. These tows are limited to favourable 29 weather conditions.

31 In the case of towing an under-buoyant assembly, there are problems associated with 32 towing in regions anomalous seafloor topography. In particular, boulders, debris or other 33 hazards on the seafloor can cause pipeline to be damaged if the assembly is towed over 34 these features.

36 Accordingly, to avoid damage the seafloor where the tow is to take place needs to be fully 37 surveyed and mapped. This is costly and time-consuming if seafloor data is not 38 available. In particular, if the pipeline needs to be towed long distances data is typically 39 not available. In general therefore, it is not suitable to tow the assembly long distances.

1 It is an aim of the invention to obviate or at least mitigate the drawbacks and 2 disadvantages associated with prior art apparatus and methods.

4 Other aims and objects of the invention will become apparent from the description below.

6 Summary of the invention

8 According to a first aspect of the invention there is provided apparatus for supporting a 9 subsea article, the apparatus comprising a buoyancy assembly having at least one surface buoy and at least one submerged buoy spatially separated from but coupled to 11 the surface buoy.

13 Preferably, the submerged buoy is arranged to be located at a depth beneath a disturbed 14 region of water. Preferably, the submerged buoy is arranged to be located at a depth greater than 20m. More preferably, the submerged buoy is arranged to be located at a 16 depth in the range of 20m to lOOm. The submerged buoy may be arranged to be located 17 at a depth in the range 30m to 50m.

19 Preferably, the apparatus comprises a plurality of surface buoys.

21 Preferably, the submerged buoy is adapted to be coupled to an article to be supported in 22 use.

24 Preferably, the submerged buoy is coupled to the article. More preferably, the surface buoys are coupled to the submerged buoy by a buoy tether.

27 Preferably, the surface buoys are connected to the submerged buoy via a damped 28 coupling.

Preferably, the shape of the surface buoy is selected to minimise tension transmitted 31 through the buoy coupling during vertical movement of the buoy. Preferably, the surface 32 buoy is elongate, having a longitudinal axis arranged substantially vertically in use.

34 Optionally, the shape of the submerged buoy is selected to minimise drag forces during lateral movement of the assembly. Preferably, the submerged buoy is elongate, having a 36 longitudinal axis arranged substantially vertically in use.

38 Preferably, the submerged buoy provides the majority of the total buoyancy of the 39 buoyancy apparatus, but not enough to float the subsea article. The addition of surface buoys makes the system fully over-buoyant.

2 Preferably, the submerged buoy provides 50% to 98% of the total buoyancy of the 3 buoyancy apparatus. In this case, the one or more surface buoys wiU provide 50% to 4 10% of the total buoyancy.

6 In one embodiment, the submerged buoy provides 60% to 80% of the total buoyancy of 7 the buoyancy apparatus. In this case, the one or more surface buoys will provide 40% to 8 20% of the total buoyancy.

In a further embodiment, the submerged buoy provides approximately 70 % of the total 11 buoyancy of the buoyancy apparatus.

13 Preferably, the article is a subsea pipeline.

According to a second aspect of the invention there is provided an assembly comprising 16 buoyancy apparatus and a subsea article supported by the buoyancy apparatus, wherein 17 the buoyancy apparatus includes at least one surface buoy and at least submerged buoy.

19 Preferably, the net buoyancy of the buoyancy assembly is greater than that required to *.:.

supportthearticle.

22 Preferably, the buoyancy of the submerged buoy is less than that required to support the 23 article. Preferably, the buoyancy of the at least one surface buoy is less than that 24 required to support the article. Thus the buoyancy apparatus provides an over buoyant: system comprising two under buoyant subsystems. e.. I *

27 Preferably, the submerged buoy is coupled to the article. More preferably, the surface 28 buoys are coupled to the submerged buoy by a buoy tether.

Preferably, the surface buoys are connected to the submerged buoy via a damped 31 coupling.

33 Preferably, the shape of the surface buoy is selected to minimise tension transmitted 34 through the buoy coupling during vertical movement of the buoy. Preferably, the surface buoy is elongate, having a longitudinal axis arranged substantially vertically in use.

37 Optionally, the shape of the submerged buoy is selected to minimise drag forces during 38 lateral movement of the assembly. Preferably, the submerged buoy is elongate, having a 39 longitudinal axis arranged substantially vertically in use.

1 Preferably, the submerged buoy provides 50% to 90% of the total buoyancy of the 2 buoyancy apparatus. In this case, the one or more surface buoys will provide 50% to 3 10% of the total buoyancy.

In one embodiment, the submerged buoy provides 60% to 80% of the total buoyancy of 6 the buoyancy apparatus. In this case, the one or more surface buoys will provide 40% to 7 20% of the total buoyancy.

9 In a further embodiment, the submerged buoy provides approximately 70 % of the total buoyancy of the buoyancy apparatus.

12 Preferably, the submerged buoy is arranged to be located at a depth beneath a disturbed 13 region of water. Preferably, the submerged buoy is arranged to be located at a depth 14 greater than 20m. More preferably, the submerged buoy is arranged to be located at a depth in the range of 20m to lOOm. The submerged buoy may be arranged to be located 16 at a depth in the range 30m to 50m.

18 Preferably, the article is a subsea pipeline.

According to a third aspect of the invention there is provided a method of transporting a 21 subsea article, the method comprising the steps of: 23 -providing a buoyancy apparatus to support a subsea article, the buoyancy; *. :* 24 apparatus comprising an arrangement of surface buoys and at least one submerged buoy; S...

26 -Supporting the subsea article beneath a disturbed region at the sea surface, and; . . 27 -Transporting the subsea article. .. : 29 Preferably, the buoyancy apparatus is as defined by the first aspect of the invention.

31 According to a fourth aspect of the invention there is provided a method of transporting a 32 subsea pipeline, the method comprising the steps of: 34 -providing a buoyancy apparatus to support a subsea pipeline, the buoyancy apparatus comprising an arrangement of surface buoys and at least one 36 submerged buoy; 37 -Supporting the subsea pipeline beneath a disturbed region at the sea surface, 38 and; 39 -Transporting the subsea pipeline.

1 Preferably, the buoyancy apparatus is as defined by the first aspect of the invention.

3 According to a fifth aspect of the invention there is provided a vessel for laying subsea 4 pipelines, the vessel comprising means for receiving a length of pipeline at an offshore location.

7 Preferably, the vessel is adapted to receive a pipeline supported by the apparatus of the 8 first aspect of the invention.

Preferably, the vessel is provided with a reel for storing the pipeline received from the 11 buoyancy apparatus.

13 According to a sixth aspect of the invention there is provided a method of installing a 14 pipeline, the method comprising the steps of: 16 -Supporting a pipeline by a buoyancy apparatus at an offshore location; 17 -receiving onto a pipe laying vessel; 18 -apparatus at an offshore location, and; 19 -installing the pipeline from the pipe laying vessel.

21 Preferably, the pipeline is supported by the pipeline apparatus of the first aspect of the 22 invention.

24 The method may include the additional step of transporting the pipeline to the offshore: location using the method of the fourth aspect of the invention. *S..

27 The method may include the steps of receiving the pipeline onto a storage reel while a 28 second pipeline is installed from the pipe laying vessel.

Brief description of the drawings

32 There will now be described, by way of example only, embodiments of the invention with 33 reference to the following drawings, in which: Figure 1-A is a schematic representation of a prior art surface-buoyant pipeline towing 36 assembly; 38 Figure 1-B is a schematic representation of a further prior art surface-buoyant pipeline 39 towing assembly; 1 Figure 2-A is a schematic representation of a prior art under-buoyant pipeline towing 2 assembly; 4 Figure 2-B is a schematic representation of an alternative prior art under-buoyant pipeline towing assembly; 7 Figure 2-C is a schematic representation of further alternative prior art under-buoyant 8 pipeline towing assembly; Figure 3 is a schematic representation of a pipeline towing assembly in accordance with 11 an embodiment of the invention; 13 Figure 4 is a schematic representation of behaviour of different buoys for the pipeline 14 towing assembly of Figure 3; 16 Figure 5 is a schematic representation of an alternative buoy according to an 17 embodiment of the invention; and 19 Figure 6 is a schematic representation of a pipeline towing assembly in use together with a pipe laying vessel according to an embodiment of the invention.

22 Detailed description of the invention

24 With reference firstly to Figure 3, there is shown a method of towing a pipeline (1). The pipeline (1) is towed below the zone A typically affected by surface disturbances, i.e. 26 about 40 meters below the surface.

28 Buoyant material (2B) is attached to the pipeline (1) also within the disturbed zone. The 29 shape of those submerged buoyant material 2B is optimised to reduce the drag forces.

31 A number of buoys (2A) are attached in turn to the buoyant material 2B to provide further 32 buoyancy to the pipeline and turns the entire assembly into a fully over buoyant 33 assembly. Surface floats directly connected to the mid depth buoyant material 2B limits 34 the stress transmitted to the pipeline.

36 The buoys 2A are linked (via the intermediate buoyant material 2B) to the pipeline 37 through damping tethers (6). The buoys 2A have shapes that are optimised to minimise 38 both the accelerations transmitted through the float attachments and the drag forces 39 while under tow.

1 This arrangement allows the pipeline to be towed at depth, while transmission of surface 2 effects on the floats or buoys at the surface to the pipeline at depth is minimised through 3 the damping tethers (6). Damping may be achieved through elastic properties or could 4 be for example a shock absorber.

6 The buoyancy of the tow assembly, which is shared between surface buoys 2A and mid- 7 water buoyant material 2B is determined as follows: 9 X is the net buoyancy required to zero the pipeline gravity. A Safety Factor SF1 is required to be greater than 1 and is chosen this way to guarantee that the pipeline 11 will always remain buoyant regardless of expected sea water densities and allows for 12 accidental loss of some floats. For example, SF1 could be chosen as equal to 1.3.

14 On this basis, the total net buoyancy to be installed is X*SF1 tons.

16 Note: submerged pipeline own buoyancy is taken into consideration in all 1 7 calculations. *

19 Further, the pipeline is set below the disturbed area of the ocean and to avoid it: * coming accidentally to the surface, the pipeline 1 and 2B floats must be slightly 21 heavy. Accordingly, the amount of net buoyancy provided by 2B must be below X, 22 say (SF21)*X. * * * S.

24 In order to prevent the pipeline breaking through the surface, SF2 is to be chosen between 1 and 2 providing a safety margin. *.* S...

27 The amount of buoyancy is split as follows: .. * : 28 2A net buoyancy = (SF1 -SF2+1)*X.

29 2B net buoyancy = (SF2-i) *X.

31 Various safety factors shall take into account buoyancy compressibility when 32 subject to increasing external pressure.

34 In Figure 4, the effect of buoy shape on the tension transmitted to the pipeline via the tether (6) is illustrated. Buoy 1 (Bi) is a classical spherical buoy. Buoy 2 (B2) is a 36 cylindrical buoy providing same uplift as Bi but with a high height to radius ratio.

38 The graph F(force) vs t(time) illustrates tension profiles through time and illustrates the 39 amount of tension transmitted through the buoy tether when the buoy is hit by a wave.

1 Due to the shape of the buoy itself, the tension profile is smoothened and as a 2 consequence the transmitted fatigue minimised.

4 In order to reduce further the drag forces, in the embodiment of Figure 5, assembling together the vertical cylinders to generate a long vertically orientated parallelogram with 6 profiled extremities (refer to figure 5) may also be an option for buoy design.

8 The buoyant material 2B may be a continuous horizontal foam cylinder or an HDPE 9 pipeline resisting to the crushing pressure.

11 An underwater shock absorber is inserted in each of 2A type buoyancy tethers. In other 12 embodiments, this device might not be required depending upon the pipeline

13 specification.

In Figure 6, there is shown a method of using the pipe towing apparatus as described 16 above together with a conventional pipe laying ship fitted with a reel and clamp. In this 17 arrangement the towed floating pipeline is fed onto the pipe laying ship which undertakes 18 S-Lay or J-Lay pipe laying. In this case, the ship would be modified to welcome the near * * 19 surface floating string. Using this method, the pipe laying vessel would not to have to travel back and forth to a spoolbase as pipeline sections would be brought to the vessel 21 by the assisting tug.

23 The ship would first reel the floating string onto its reel and then lay it. On a ship 24 equipped with 2 reels, it would be possible to feed a reel at the same time the other one is being laid. In this way, pipe sections can be stored to then lay it at a later time. * 27 Various modifications and improvements may be made to the invention herein described ** 28 without departing from the scope of the invention. (0

Claims

Claims Improved Buoyancy Apparatus & Method of Use: What is claimed is:

1. An apparatus to provide buoyancy to a submarine article in order to make it float with appropriate reserve of floatability but significantly below the surface away from the zone disturbed by waves, currents and weather. The apparatus consists in splitting the required amount of buoyant elements attached to the submarine article at different water depth levels; * some buoyant element(s) would be totally submerged close to the submarine article itself below the disturbed zone; and, * others would be at the surface within the disturbed zone.

2. The apparatus of claim I wherein the majority of the required buoyancy is provided by the submerged buoys whereas the remaining required buoyancy (including contingency guaranteeing the entire assembly will remain over buoyant despite some buoyancy loss or environmental changes or potential incidents) is provided through the surface buoys.

3. The apparatus of claim I wherein the surface buoys are optimised in shape -such as a vertically elongated volume -to provide as progressive as possible vertical forces variations when subject to waves.

4. The apparatus of claim 1 wherein the submerged buoys are optimised in shape -such as horizontally elongated volume -to minimise generated drag forces when under tow or subject to sub surface currents.

5. The apparatus of claim I wherein the link between surface buoys and submarine structure or submerged buoys includes a dampening mechanism such as a shock absorber or an elastic link.

6. A towing method allowing to move a submarine article and consisting in connecting tow means including one or more towing vessels to the subsea article and apparatus as described in claim I and possibly including any of the features described in any of the claims 2 to 5.

7. The towing method of claim 6 wherein the subsea article is a pipeline or section of pipeline.

8. A wet storage method consisting in connecting anchor means to the subsea article and apparatus as described in claim 1 and possibly including any of the features described in any of the claims 2 to 5. The anchor would be connected to the seabed or to any fix point.

9. The storage method of claim 8 wherein the subsea article is a pipeline to be kept in storage for an indefinite duration without suffering from swell, tidal currents or contact with the seabed.

10. A pipelay method wherein a pipelay vessel equipped with the appropriate equipment known in the industry -i.e. allowing a controlled lowering of a section of pipeline to the seabed -would be suitably equipped to recover a pipeline or a pipeline section having been previously brought to said vessel using the method of claim 7 and subsequently lay it to the seabed.

11. A pipeline loading method wherein a vessel would be suitably equipped to receive and load a pipeline or a pipeline section having been previously stored using the method of claim 9.