WO2002002979A1

WO2002002979A1 - Method and apparatus for stabilising a submarine elongated structure

Info

Publication number: WO2002002979A1
Application number: PCT/AU2001/000803
Authority: WO
Inventors: Kenneth T. Kavanagh; Andrew T. Van Der Meer; James J. Van Der Meer; Jon C. Van Der Peyl
Original assignee: Neptune Technologies Pty Ltd
Priority date: 2000-07-04
Filing date: 2001-07-04
Publication date: 2002-01-10
Also published as: AUPQ854800A0

Abstract

A method of stabilising a submarine pipeline (10) on a seabed (12) includes delivering a settable cementitious material (16) from a marine vessel (18) through a conduit (28) to a former (30) located over the pipeline (10). The former (30) includes a reciprocating plate (44) for compacting the material (16) and extruding it from a nozzle (38) at the rear of the former (30). The action of the plate (44) in compressing the material (16) creates a reaction force to propel the former (30) in the direction D along the pipeline (10). The cover (14) is of a low profile generally convex transverse section.

Description

METHOD AND APPARATUS FOR STABILISING A SUBMARINE ELONGATED STRUCTURE

Field of the Invention

The present invention relates to the field of offshore pipeline stabilisation.

Background of the Invention

The laying of submarine pipelines, conduits and other elongated structures is now commonplace in many fields of human endeavour. For example, pipelines are used to link offshore oil and gas fields to each other and to land based process and distribution facilities. Telecommunication cables carrying both electrical and optical conductors are also laid in the oceans joining countries and continents. It is also known to lay power cables underwater. These structures are exposed to a variety of seabed conditions and are susceptible to damage arising from scour in the immediate vicinity of the structure and consequential lift and drag forces on the structure itself. As a result of this susceptibility to damage, offshore elongated structures are currently protected or stabilised by various known methods.

A common method of protection is trenching. This involves plowing or dredging the seabed to create a trench into which the structure is laid. Back-filling is not normally carried out as the structure is effectively placed in a negative protection situation with reduced environmental exposure as well as partial resistance to lateral movement. This process requires the trench to remain open until the structure is placed and requires high positional accuracy for placement of the structure. It also becomes increasingly expensive and difficult with depth, is susceptible to changes in, seabed material and, weather in the window between trenching and laying of the structure. The structure also remains vulnerable due to lack of discrete anchoring or cover armour. A further known method is burial, which is typically used in areas with significant turbulent or environmental effects or where there is a high risk of external damage due to anchors, fishing nets etc. This method consists of back-filling an elongated structure in a trench with dredge material to create an efficient final hydraulic profile. This method becomes very difficult in deep water and areas with highly variable seabed material. Alternately, the structure may be placed in a trench and covered with selected rock armour, mattresses or covers to create a hydro-dynamically efficient final profile. Unfortunately, this method becomes an expensive logistical exercise and requires high positional accuracy for the placement of the protection, particularly in deep water.

A further method of stabilisation used in areas with negligible turbulence or environmental effects, and where there is minimal risk of external damage is discrete anchoring. In this method, discrete anchors either in the form of intermittent rock dump zones or soil/rock anchors, are dumped at spaced locations on the structure. However this requires high positional accuracy and becomes significantly more difficult with increasing water depth. Further, between the anchor points, the structure is free spanning and exposed. In addition, the anchors often locally increase scour due to their inefficient hydraulic profile.

Summary of the Invention

It is an object of the present invention to provide an alternate method and apparatus for stabilisation of elongated submarine structure such as pipelines and cables.

According to a first aspect of the present invention there is provided a method of stabilising a submarine elongated structure laying on a submarine surface, said method including at least the step of: laying a cover of an underwater settable material over and along said elongated structure.

Preferably said underwater settable material is a cementitious material.

Preferably said cementitious material includes at least between 10% and 30%, by weight, cement. Preferably said cementitious material further includes at least between approximately 85% and 65%, by weight, sand.

Preferably said cementitous material further includes between 0.03% and 0.15%, by weight, of an anti-washout agent for substantially preventing said cement from dispersing from said cover.

Preferably said anti-washout agent includes methylcellulose or hydroxypropyl methylcellulose.

Preferably said cementitious material further includes plaster of paris in a proportion from 0% to 75% of the weight of said cement.

Preferably said plaster of paris is provided in a ratio of between 1 :3 to 2:3 to cement.

Most preferably, said plaster of paris is provided in a ratio of 1 :2 to cement.

Preferably said method includes the steps of providing a former for shaping said cover and laying said cover over said structure and moving said former along said structure.

Preferably said method further includes the steps of providing a marine vessel for supplying said settable material; and, transporting said settable material from said marine vessel to said former.

Preferably said transporting step includes providing a conduit from said marine vessel to said former through which said settable material is transported.

In a first embodiment, said transporting step includes gravity feeding said settable material through said conduit. However, in a second alternate embodiment, said transporting step includes pumping said settable material through said conduit. Preferably said method further includes the step of compacting said settable material as said settable material is laid over said structure by said former.

Preferably said compacting step includes providing a reciprocating plate supported on said former and operating said reciprocating plate to cyclically apply a compressive force on said settable material and generating a reaction force which acts to propel said former along said structure.

Preferably said method further includes the step of cleaning an outside surface of said elongated structure at a location in advance of said cover.

Preferably said cleaning step includes directing a spray of liquid onto the outside surface of said structure.

Preferably said cleaning step further includes providing a fluid operated scrubber for scrubbing said surface of said elongated structure, said scrubber being supported by said former and disposed at a leading end of said former.

Preferably said method further includes the step of manufacturing of said settable material on said marine vessel.

Preferably said method further includes the step of dredging sand from said marine surface and using said sand in the manufacture of said settable material.

According to a further aspect of the present invention there is provided an apparatus for use in laying a cover of a settable material over and along a submarine elongated structure, said apparatus including at least:

a former defining a cavity for location over said elongated structure and for receiving said settable material, said former including an opening through which said settable material can flow into said cavity. Preferably said apparatus further includes compacting means for compacting said settable material as said settable material flows into said cavity.

Preferably said compacting means includes a reciprocating plate for applying a cyclic compressive force on said settable material and providing a reaction force for propelling said apparatus along said elongated structure.

According to a further aspect of the present invention there is provided an underwater settable cementitious material including at least: between 10% and 30% by weight cement; between 0.03% and 0.15% by weight of an anti-washout agent; and the balance by weight, of a combination of sand and plaster of paris wherein said plaster of paris is provided in a range of 0% to 15% by weight.

Preferably said anti-washout agent includes methylcellulose and/or hydroxypropyl methylcellulose.

Brief Description of the Drawings

Figure 1 is a schematic representation of a system and apparatus used in the method for stabilising a submarine elongated conduit in accordance with an embodiment of the present invention;

Figure 2 is a schematic representation of an apparatus for laying a cover of a settable material over an elongated submarine structure in accordance with the method; and, Figure 3 is a section view of a cover applied to a submarine elongated structure by an embodiment of the present method and apparatus.

Detailed Description of the Preferred Embodiment

With reference to the accompanying drawings, an embodiment of the present method for stabilising a submarine elongated structure such as a pipeline 10 laying on a submarine surface, i.e. a seabed 12 includes laying a cover 14 of an underwater settable material 16 over and along the pipeline 10. The cover 14 is applied continuously over the pipeline 10.

The material 16 is typically a cementitious material including in combination cement, sand, and anti-washout agent, and optionally, plaster of paris (i.e. gypsum hemihydrate.). Depending on the required strength of the material 16 the material may include between 10% by weight (for low strength) and 30% by weight (for high strength) cement, such as Portland Cement. The anti-washout agent is provided in a proportion, by weight, of between 0.03% and 0.15%. The balance by weight of the material 16 is composed of sand and plaster of paris, with the plaster of paris being present, in terms of percentage of weight of cement of between 0% and 75%. However, most preferably, when the plaster of paris is incorporated, it is provided in a weight ratio of 1 :2 to cement.

The anti-washout agent may include methylcellulose or hydroxypropyl methylcellulose. A particular source of this is the product METHOCEL (™) manufactured by the Dow Chemical Company as a food gumming agent. An alternate anti-washout agent is RHEOMAC UW 450 (™) made by MBT (Australia) Pty Ltd.

Tables 1, 2 and 3 below provide typical compositions for the material 16.

Table 1:

Table 2:

Table 3:

Most conveniently, it is envisaged that performance of the method will involve the manfuacture/mixing of the material 16 on board a marine vessel 18. The vessel 18 includes a mother ship 20 and a barge 22. The barge 22 will carry a supply of one or more of the raw materials required to manufacture the settable material 16. For example the barge 22 may hold a supply of sand and/or cement and/or plaster of paris 24. The mother ship 20 may typically hold a supply of the anti- washout agent, as this is required in much smaller quantities. The mother ship 20 also includes conventional mixing apparatus and plant 26 for mixing the ingredients to form the settable material 16.

It is further envisaged that in some embodiments, rather than transporting the sand, which comprises the bulk of the settable material 16 on barge 22, the sand may be supplied by dredging the seabed.

An articulated conduit 28 is supported by the mother ship 20 and coupled at a downstream end with a former 30 which lays the material 16 over the pipeline 10. The mixed settable material 16 is transported through the conduit 28 to the former 30. This transport may be by way of gravity feed or, alternatively, by pumping.

Referring specifically to Figure 2, it can be seen that the former 30 includes a shroud 32 having a substantially convex profile which defines a cavity 34 for location over the pipeline 10 and for receiving the material 16. In this regard, the shroud 32 is provided with an opening 36 on its undersurface through which the conduit 28 feeds the material 16 via a coupling pipe 37 fixed to the shroud 32. A trailing end of the former 30 is formed with a reducing diameter extrusion nozzle 38 through which the material 16 is forced to create the cover 14. It is the shape and profile of the nozzle 38 that determines the shape of the cover 14.

A rubber skirt 40 extends laterally from the longitudinal edges of the shroud 32.

The former 32 also includes a compacting means 42 comprising a pusher plate 44 and a pair of jacks (only one shown) 46. The plate 44 is attached to the jacks 46 which in turn are supported by a support beam 48 at a leading end 50 of the former 30. The jacks 40 would typically be operated by a fluid, e.g. air or water and cyclically extend and retract to produce a reciprocating motion in the plate 44. The plate 44 is disposed in advance of the hole 36 and operates to cyclically compress the settable material 16 as it enters the cavity 30. As the plate 44 compresses and pushes against the material 16 it forces compacted material 16 from the nozzle 38 to form the cover 14 and generates a reaction force is created which advances the former 30 along the conduit 10, in direction D.

Cleaning means in the form of a water spray manifold 52 is further supported by the beam 48 about the exterior surface of the pipeline 10. The manifold 42 provides a high pressure spray of water onto the outer surface of the pipeline 10 to clean the surface prior to covering with the material 16. In addition, or as an alternate, scrubbers may also be incorporated to physically scrub the surface of the pipeline 10. The scrubbers may be in the form of scrubbing pads which are fixedly supported by the beam 48 or alternatively can be in the form of rotating brushes which are fluidly driven, i.e. by air or water, and supported by the beam 48.

A curved guide plate 54 depends from an underside of a forward end of the beam 48 and rolls along the pipeline 10. To this end, the plate 54 is provided with a plurality of rollers 56.

A transducer 58 is supported at a rear end of the former 30, while transducers 60 and 62 are supported at a leading end of the former 32 from the beam 48. The transducers 58, 60 and 62 are used to provide information regarding the position of the former 30, its speed of motion and optionally, physical conditions adjacent the pipeline 10 such as water current speed and water temperature.

It is further envisaged that the present method of stabilising the pipeline 10 can also include providing reinforcing within the cover 14. Reinforcing may take the form of, for example, wire strands which are laid continuously with the cover 14 through the former 30. Alternately, a mesh made from a plastics material can be embedded in the cover 14 as it is being laid. As a further alternative, the former 30 can be modified to insert arcuate staples over the pipeline 10 either solely within the material 16 being used to form the cover 14 or alternately also driven into the seabed 12.

As depicted in Figure 3, the cover 14 provided by the present method and apparatus is of a generally convex transverse section of low profile gently sloping from an apex, which is in substantial alignment with a central axis of the pipeline 10, to its opposite lateral edges. This profile has greater hydrodynamic efficiency than the exterior shape of the pipeline 10 and assists in reducing turbulence attributable to eddies that would otherwise be created by water currents flowing across an uncovered pipeline 10.

In use, the mother ship 20 and barge 22 are loaded with the materials required to make the settable material 16 and sealed along the length of pipeline 10. The material 16 are mixed on the mother ship 20 and delivered through the conduit 22 to the former 30. The former 30 may be either remotely located over the pipeline 10 jusing signals received from the transducers 58, 60 and 62 for positional purposes, or alternately physically initially placed over the pipeline 10 by a diver. As the material 16 is mixed it is delivered through the conduit 20 to the former 30. Once the material enters the cavity 34 it is compressed by the reciprocating plate 44. At initial startup, the plate 44 will reciprocate a number of times in order to fill the cavity 34 prior to extruding the cover 14 from the nozzle 38. Once the cavity 34 has been filled however additional material passing through the conduit 28 into the cavity 34 is compressed against the other material already in the cavity 34 resulting in a volume of that material adjacent the nozzle 38 being extruded from the nozzle 38 as part of the cover 14. The reaction force created by the reciprocating plate 44 as it compresses the material within the cavity 34 also acts to propel the former 30 along the pipe in direction D. When the vault 32 cleans the surface of the pipeline 10 in advance of the application of the cover 14. The mass of the former 30, conduit 38 and the guiding action of the guideplate 54 and manifold 52 maintain the former 30 substantially centrally over the pipeline 10.

In an alternate embodiment, above method and apparatus can also be applied directly to the seabed for seabed stabilisation. Such an embodiment would differ from that described above by (a) the absence of an elongated structure, and (b) the addition of steering equipment on the former for guidance purpose. The steering equipment could be in the form of conventional equipment used for steering small propelled underwater craft such as those used by scuba divers. Further, although the elongated structure is not required, such structure may be incorporated as a form of reinforcing. For example in place of the pipeline 10 elongated rods made from metal or plastics materials may be used as reinforcing (rather than as a means for transporting fluids). Also in this embodiment the former may be steered in any desired path to form a cover of say a matrix like configuration to stabilise the seabed near a groyne. All other aspects of the method and apparatus including the composition of the settable material remain the same as that described in relation to the stabilisation of the pipeline 10.

All modifications and variations of the present described embodiments that would be obvious to a person of ordinary skill in the relevant arts are deemed to be within the scope of the present invention the nature of which is to be determined from the above description and the appended claims.

Claims

The Claims Defining the Invention are as Follows:

1. A method of stabilising a submarine elongated structure laying on a submarine surface, said method including at least the step of: laying a cover of an underwater settable material over and along said elongated structure.

2. The method according to claim 1 further including the step of providing a former for shaping said cover and laying said cover over said structure and moving said former along said structure.

3. The method according to claim 2 further including the steps of providing a marine vessel for supplying said settable material; and, transporting said settable material from said marine vessel to said former.

4. The method according to claim 3 wherein said transporting step includes providing a conduit from said marine vessel to said former through which said settable material is transported.

5. The method according to claim 4 wherein said transporting step includes gravity feeding said settable material through said conduit.

6. The method according to claim 4 wherein said transporting step includes pumping said settable material through said conduit.

7. The method according to any one of claims 2 to 5 further including the step of compacting said settable material as said settable material is laid over said structure by said former.

8. The method according to claim 7 wherein said compacting step includes providing a reciprocating plate supported on said former and operating said reciprocating plate to cyclically apply a compressive force on said settable material and generating a reaction force which acts to propel said former along said structure.

9. The method according to any one of claims 1 to 8 further including the step of cleaning an outside surface of said elongated structure at a location in advance of said cover.

10. The method according to claim 9 wherein said cleaning step includes directing a spray of liquid onto the outside surface of said structure.

11. The method according to claim 9 or 10 wherein said cleaning step further includes providing a fluid operated scrubber for scrubbing said surface of said elongated structure, said scrubber being supported by said former and disposed at a leading end of said former.

12. The method according to any one of claims 1 to 11 wherein said underwater settable material is a cementitious material.

13. The method according to claim 12 wherein said cementitious material includes at least between 10% and 30%, by weight, cement.

14. The method according to claim 13 wherein said cementitious material further includes between 0.03% and 0.15%, by weight, of an anti-washout agent for substantially preventing said cement from dispersing from said cover.

15. The method according to claim 14 wherein said anti-washout agent includes methylcellulose or hydroxypropyl methylcellulose.

16. The method according to claim 14 or 15 wherein said cementitious material further includes at least between 85% and 65% by weight, sand.

17. The method according to claim 16 wherein said cementitious material further includes plaster of paris in a proportion from 0% to 75% of the weight of said cement.

18. The method according to claim 17 wherein said plaster of paris is provided in a ratio of between 1 :2 to 2:3 to cement.

19. The method according to claim 18 wherein said plaster of paris is provided in a ratio of 1 :2 to cement.

20. The method according to any one of claims 1 to 19 further including the step of manufacturing of said settable material on said marine vessel.

21. The method according to claim 20 further including the step of dredging sand from said marine surface and using said sand in the manfuacture of said settable material.

22. An apparatus for use in laying a cover of a settable material over and along a submarine elongated structure, said apparatus including at least:

a former defining a cavity for location over said elongated structure and for receiving said settable material, said former including an opening through which said settable material can flow into said cavity.

23. The apparatus according to claim 22 further including compacting means for compacting said settable material as said settable material flows into said cavity.

24. The apparatus according to claim 23 wherein said compacting means includes a reciprocating plate for applying a cyclic compressive force on said settable material and providing a reaction force for propelling said apparatus along said elongated structure.

25. An underwater settable cementitious material including at least: between 10% and 30% by weight cement; between 0.03% and 0.15% by weight of an anti-washout agent; and the balance by weight, of a combination of sand and plaster of paris wherein said plaster of paris is provided in a range of 0% to 15% by weight.

26. The underwater settable material according to claim 25 wherein said anti- washout agent includes methylcellulose and/or hydroxypropyl methylcellulose.

27. A method of stabilising a seabed including at least the step of:

laying a cover of an underwater settable material over said seabed.

28. A method according to claim 27 further including the steps of providing a former for shaping and laying said cover on said seabed; and, moving said former along said seabed.

29. A method according to claim 28 further including the steps of providing a marine vessel for supplying said settable material; and, transporting said settable material from said marine vessel to said former.

30. A method according to claim 29 wherein said transporting step includes providing a conduit from said marine vessel to said former through which said settable material is transported.

31. A method according to claim 30 further including the step of compacting said settable material as said settable material is layed on said seabed by saiud former.

32. A method according to claim 31 wherein said compacting step includes providing a reciprocating plate supported on said former and operating said reciprocating plate to cyclically apply a compressive force on said settable material and generating a reaction force which acts to propel said former along said seabed.