WO2011030124A1

WO2011030124A1 - Method of laying a pipeline in a seabed

Info

Publication number: WO2011030124A1
Application number: PCT/GB2010/051437
Authority: WO
Inventors: Alasdair James Maconochie
Original assignee: Technip France
Priority date: 2009-09-11
Filing date: 2010-09-01
Publication date: 2011-03-17
Also published as: GB2473471A; GB0915945D0; GB2473471B

Abstract

A method of laying a pipeline in a seabed comprising at least the contemporaneous steps of : (a) dislocating material of the seabed to form a trench (18) and dislocated material (20); (b) locating the pipeline (12) in the trench; (c) allowing at least some of the dislocated material (20) to cover the pipeline; and (d) providing one or more additives (22, 24) to the dislocated material (20). In this way, the additive(s) reinforce the mechanically properties and/or the resettling of the dislocated material so that possible upheaval movement of the buried pipeline is minimised or eliminated in a much shorter time period, thus considerably reducing the time needed to fully install and use such a pipeline.

Description

Method of Laying a Pipeline in a Seabed The present invention relates to a method and apparatus for laying a pipeline in a seabed.

It is well known to cut trenches in seabeds and locate pipelines therein in order to avoid or protect such pipelines from any damage that may arise from a collision with other objects, such as ship anchors or fishing nets or boats. Burying pipelines below the seabed also helps insulate the pipeline in cold seas, particularly were the pipeline is intended to convey warmer substances such as hydrocarbons. Meanwhile, the temperature and pressure variations of substances such as hydrocarbons passing along such pipelines often lead to expansion of the pipeline. Where the pipeline ends are anchored, such as on or along the seabed, the compressive forces exerted on the pipeline by temperature and pressure variations can lead to the deformation of the pipeline, including in the lateral direction and the upward or 'top' direction. These are known as lateral buckling and upheaval buckling.

Where such pipelines are buried, generally in a formed trench in a seabed, lateral buckling is minimised because of the high degree of restraint provided by the trench walls. Furthermore, the weight of the soil or material 'backfilled' onto the pipeline (following the trench formation and pipeline laying) can resist some upheaval movement of the pipeline. However, the dislocation of material to form the trench will inevitably result in such backfilled soil or material being mechanically weaker than its pre- trench compacted formation, such that whilst such backfilled soil can provide some resistance to upheaval movement, upheaval movement can still occur.

Trenches in seabeds are generally formed by one or two methods, or a combination of same. One method is by ploughing, whereby a ploughed trench is formed, the pipeline is then located in the trench, and the pipeline cover with the excavated soil with a backfilled plough machine. This can lead to a lower disturbance of the backfilled material, but is a relatively slow process.

The other main trench forming method is termed jetting, wherein one or more high pressure water jets are directed to penetrate and destroy the structure of the seabed material to form the trench. This is a faster method than the ploughing method to create the trench, but creates more dislocated seabed material, such material generally becoming a form of 'liquefied soil' having a high degree of water. Thus, a much longer time is required to allow the backfilled material to resettle and consolidate again over the pipeline now buried in the trench, to hopefully provide resistance to upheaval movement. Indeed, the resettling of jetted backfilled soil can take several months.

It is an object of the present invention to provide an improved method of laying a pipeline in a seabed which reduces or eliminates the need for a consolidation period of the backfilled soil or material.

Thus, according to one aspect of the present invention, there is provided a method of laying a pipeline in a seabed comprising at least the contemporaneous steps of: (a) dislocating material of the seabed to form a trench and dislocated material;

(b) locating the pipeline in the trench;

(c) allowing at least some of the dislocated material to cover the pipeline; and

(d) providing one or more additives to the dislocated material.

In this way, the additive(s) reinforce the mechanically properties and/or the resettling of the dislocated material so that possible upheaval movement of the buried pipeline is minimised or eliminated in a much shorter time period, thus considerably reducing the time needed to fully install and use such a pipeline.

The term "laying" as used herein includes the term "burying", and relates to any type of locating a pipeline beneath the seabed, generally beneath the solid material of a seabed. Thus, the term "laying a pipeline in a seabed" includes "laying a pipeline beneath a seabed", or otherwise locating a pipeline at a level which is below the top surface of a seabed. The term "pipeline" as used herein includes any type, shape size or design of an elongated pipe, generally for the passage and/or conveying of one or more fluids, cables, fibres and the like, from one location to another location being onshore, offshore, or a combination of same. One example is one or more hydrocarbons including oil and gas, but the invention is not limited thereto. Types, sizes and designs of suitable pipelines are well known in the art, and can include single substance pipelines or a combination of one or more pipelines intended to be laid together, optionally conveying one or more different substances or materials. The term "seabed" as used herein relates to any surface, floor or bed covered by water, not limited to being a sea. Thus, the term "seabed" as used herein extends to the surface, floor or bed of any stretch of water, including rivers, lakes, lagoons, oceans, etc. beneath which it is possible to lay a pipeline.

The term "contemporaneous" as used herein relates to at least step (a) - (d) of the method of the present invention occurring in a co-timely manner, and within a relatively close or short period of time, without necessarily two or more of the steps being simultaneous or synchronous. Generally the steps of the method of the present invention occur as a series of continuously successive or overlapping steps.

Thus, the locating of the pipeline in the trench in step (b) cannot occur before the creation of the trench in step (a), but the present invention encompasses the location of the pipeline in the trench occurring as a direct or succeeding action to the creation of the trench. Thus, further trench can be being formed whilst pipeline is being located in a portion of the trench just formed.

Similarly, the one or more additives of step (d) cannot be provided to the dislocated material until the dislocated material is created in step (a). However, the present invention encompasses the provision of one or more additives to the dislocated material as a co-timely action with the relocation of at least some of the dislocated material over the pipeline located in the trench after step (b), so as to cover said pipeline in the trench.

It is particular feature of the present invention that steps (a) - (d) are not distinct or discrete, such that a distinct or non de minimus time period occurs between two or more of the steps; in particular where the structure of the trench requires support in order to maintain the existence of the trench prior to the location of the pipeline therein. This is in contrast to the method described in GB1163508, wherein a liquid, particularly described as a viscous liquid or gel liquid, is required to be located in a trench as a particular distinct step of the process in order to maintain the shape of a trench prior to laying a pipeline therein.

It is also a particular requirement of the invention of GB1163508 that the liquid supporting the existence of the trench is "gradually broken down" either by time and/or the action of salts in the sea water, so that the side walls of the trench will collapse over time, and so the pipeline is then buried below the sea-bottom by "actions of nature". This requires time. GB1163508 also describes that the trench may be filled with sand or other material as another distinct process step after the breaking up of the liquid in the trench.

Thus, the invention of GB1163508 requires a number of distinct and discrete process steps over time, not providing a contemporaneous method able to immediately reduce or minimise upheaval movement of a pipeline.

According to one embodiment of the present invention, step (d) is carried out simultaneously with step (c). In this way, agitation of the dislocated material caused by step (a) can assist in the admixing of the one or more additives with the dislocated material, as at least some of the dislocated material is then relocating to cover the pipeline now located in the trench. In particular, it is preferred that the additive(s) are dispersed in the dislocated material during step (c), such that the additive(s) are broadcast, spread or otherwise diffused with the dislocating material during the method of the present invention.

In another embodiment of the present invention, the trench of step (a) is at least partly created by jetting. Jetting to form a trench in a seabed is known in the art and not further described herein. Generally, the action of one or more high pressure water jets destroys the structure of the soils or other materials of a seabed, in particular soft soils such as clay or sand. According to another embodiment of the present invention, the trench of step (a) is at least partly created by ploughing. Ploughing of a trench in a seabed is also well known in the art, and not further discussed herein.

To assist the contemporaneous nature of the steps of the method of the present invention, the pipeline is preferably located on the seabed along the intended path of the trench prior to step (a), such that it is immediately available for locating in a trench as the trench is being formed. The pipeline may be located in the trench by gravity, optionally with guidance and/or assistance from a suitable apparatus.

According to another embodiment of the present invention, the one or more additives comprises one or more of the group comprising: calcium hydroxide, calcium hydroxide derivatives, potassium aluminate, potassium sulphate, and potassium sulphate derivatives. Preferably, at least one of the additives is a flocculent and/or a coagulant, and many suitable flocculent and coagulant materials able to assist the flocculation and/or coagulation of a solid dispersion in a liquid are well known in the art. The additive(s) could be added in the range of 5-10% of the trench volume, which may equal the volume of the dislocated material in step (c) depending on its dispersement. The additive(s) could be provided from one or more reservoirs on the seabed, such as one or more tanks or other containers able to provide at least one additive in conjunction with the apparatus, device or unit able to form the trench. Alternatively and/or additionally, one or more of the additives could be provided from a surface-floating vessel such as a control ship above the seabed, generally through one or more suitable hydraulic umbilicals between such a floating vessel and the apparatus, device or unit able to form the trench.

Apparatus, device or units able to form a trench in a seabed are well known in the art, and examples include those shown in

WO2008/093043A2 and US4714378, without being limited thereto. According to another embodiment of the present invention, one or more of the steps (a) - (d) could be provided by a trencher apparatus, preferably a remote operating vehicle (ROV) or based on a ROV.

Thus, according to another aspect of the present invention, there is provided a trencher apparatus for laying a pipeline in a seabed, the apparatus at least comprising:

(i) one or more seabed material dislocators able to dislocate material of the seabed to form a trench and dislocated material; and (ii) one or more additive outlets able to provide one or more additives to the dislocated material following location of the pipeline in the trench.

Preferably, the trencher apparatus comprises an ROV. The trencher apparatus may also comprise one or more tanks for one or more of the additives.

Embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings in which:

Figure 1 is a schematic side-cross sectional view of a method of laying a pipeline in a seabed according to one embodiment of the present invention; Figure 2 is a schematic side-cross sectional view of a diagrammatic trencher machine with high and low pressure jets creating a trench;

Figure 3 is a side view of a trencher apparatus according to a second embodiment of the present invention able to provide a method of laying a pipeline; and

Figure 4 is variation of the trencher apparatus in Figure 3.

Referring to the drawings, Figure 1 shows a seabed 1 , which may lie under any suitable area or passage of water, not limited to being a sea.

Located on the seabed is a pipeline 2, generally located along the path of an intended trench, particularly but not exclusively so as to be able to sink into a trench to be formed thereunder as hereinafter described. Many types and forms of pipelines are known in the art, and the present invention is not limited by the type, size, form or the design of the pipeline. Indeed, the pipeline may comprise a plurality of pipelines able to be acted on together. As described hereinafter with reference to Figure 2, a trencher machine (not shown in Figure 1) is able to form a trench 3 in the seabed 1 , generally in the direction of arrow (a1). The forming of the trench 3 creates dislocated material 4 in a manner known in the art. The dislocated material 4 shown in Figure 1 is in a form of a dispersed solid with the water which is now in the trench 3. The dislocated material 4 is now located rearwardly and possibly partially upwardly (shown by arrows a2), allowing the location of the pipeline 2 into the trench 3 as shown by arrow (b). The pipeline 2 now lies in the trench 3.

Contemporaneously, at least some of the dislocated material 4 falls back into position in the trench 3 (shown by arrows (c)) so as to cover the pipeline 2 located in the trench 3 in the manner of resettling and backfilling the trench 3 to form a refilled trench 5.

Contemporaneously, preferably simultaneously with the relocation of the dislocated material 4 into the trench 3, one or more additives are added to the dislocated material 4, represented in Figure 1 by arrows (d). The additive(s) are able to reinforce the mechanical properties and/or the settling of the dislocated material 4, such that following the relocation of at least some (usually the majority, all, or substantially all) of the dislocated material 4 to cover the pipeline 2 in the trench 3 as shown in Figure 1 , the enhanced mechanical properties of the backfilled dislocated material 4 are presently, possibly immediately, able to reduce, minimise or otherwise prevent the upheaval movement of pipeline 2 in its laid location in the refilled trench 5.

The present invention provides a contemporaneous method of laying a pipeline in a seabed which minimises the effect of the passage of any time or time period during which upheaval or buckling of the pipeline in the trench may still be a sufficient problem so as to reduce or prevent full use of the pipeline. Thus, the present invention provides a method of laying a pipeline following which the pipeline is immediately available for use and for continuous use, speeding up installation time and/or reducing pipeline downtime.

The action of suitable additives such as flocculents and/or coagulants to admix and act with various materials are known in the art. Many binders and other compounds, such as polymer complexes, are available to the skilled man to create a combined material. The present invention takes advantage of using this contemporaneously with subsea dislocated material to make a combined material immediately mechanically strong enough to wholly or substantially resist any upheaval or buckling of a newly laid pipeline in a re-filled trench, in particular in combination with a dedicated trenching machine.

Trenching apparatus and machines are well known in the art, and can form a trench either for a pipeline to be directly subsequently provided, or under an existingly placed pipeline. They generally form trenches by dislocating material of the seabed by the use of one or more jets and/or ploughs. Figure 2 shows a diagrammatic jet trenching machine 6, usually having one or two 'swords' 7 extending rearwardly and downwardly therefrom, each sword 7 having a series of forward/downward-facing high pressure jets 8 for forming the trench 3 as described above and as shown in Figure 1 , and at least one low pressure jet 9 to keep the dislocated material 4 formed by the high pressure jets 8 in suspension or dispersion with the seawater whilst the pipeline (not shown in Figure 2) is able to fall or sink to the base of the trench 3.

Figure 3 shows a first trenching apparatus 10a according to an

embodiment of the present invention for laying a pipeline 12 in a seabed 14. The apparatus can be based on or adapted from a remote operating vehicle (ROV), and is equipped with at least one jetting sword 16 extending downwardly therefrom, and having a number of high pressure water jets capable of pumping out high pressure water along the direction of arrow (e), thereby forming a trench 18 in the seabed 14 as the trencher apparatus 10a travels in the direction of arrow (f). The jetting sword 16 provides one or more seabed material dislocators able to dislocate material of the seabed 14 to form the trench 18 and dislocated material 20.

In addition, the trencher apparatus 10a includes at least one additive pipeline 22 having an additive outlet 24 at a distal end, said outlet 24 able to provide one or more additives to the dislocated material 20 following location of the pipeline 12 in the trench 18. The refilled trench 26 is created by the composition of the additive(s) provided by the additive outlet 24, and at least some of the dislocated material 20, which combined material now covers the pipeline 12. The additive(s) to be provided to the additive pipeline 22 can be provided from one or more tanks 26 on the first trencher apparatus 10a.

In a variation to Figure 3, Figure 4 shows a second trencher apparatus 10b having a supply umbilical 30 which could pass to one or more surface- floating vessels (not shown), able to provide one or more of the additives as a stream to the additive pipeline 22, as an alternative and/or addition to the additive tanks 28 shown in Figure 3.

Various modifications and variations to the described embodiments of the invention will be apparent to those skilled in the art without departing from the scope of the invention as defined herein. Although the invention has been described in connection with specific preferred embodiments it should be understood that the invention as defined herein should not be unduly limited to such specific embodiments.

Claims

1. A method of laying a pipeline in a seabed comprising at least the contemporaneous steps of:

(a) dislocating material of the seabed to form a trench and dislocated material;

(b) locating the pipeline in the trench;

(d) providing one or more additives to the dislocated material.

2. A method as claimed in claim 1 wherein step (d) is carried out simultaneously with step (c).

3. A method as claimed in claim 1 or claim 2 wherein the trench of step (a) is at least partly created by jetting.

4. A method as claimed in claim any one of the preceding claims wherein the trench of step (a) is at least partly created by ploughing.

5. A method as claimed in any one of the preceding claims wherein the pipeline is located on the seabed along the intended path of the trench prior to step (a).

6. A method as claimed in any one of the preceding claims wherein the pipeline is a hydrocarbon pipeline.

7. A method as claimed in any one of the preceding claims wherein the one or more additives comprise one or more of the group comprising: calcium hydroxide, calcium hydroxide derivatives, potassium aluminate, potassium sulphate, and potassium sulphate derivatives.

8. A method as claimed in any one of the preceding claims wherein at least one of the additives is a flocculant and/or a coagulant.

9. A method as claimed in any one of the preceding claims wherein the one or more additives are added in the range of 5- 0% volume of dislocated material in step (c).

10. A method as claimed in any one of the preceding claims wherein one or more of the additives are provided from a reservoir at the seabed.

11. A method as claimed in any one of 1 to 9 wherein one or more of the additives are provided from a surface-floating vessel above the seabed.

12. A method as claimed in one or more of the preceding claims wherein one or more of steps (a) - (d) are provided by a trencher apparatus, preferably an ROV.

13. A method as claimed in one or more of the preceding claims wherein one or more of the additives are dispersed into the dislocated material in step (d).

14. A trencher apparatus for laying a pipeline in a seabed, the apparatus at least comprising:

(i) one or more seabed material dislocators able to dislocate material of the seabed to form a trench and dislocated material; and (ii) one or more additive outlets able to provide one or more additives to the dislocated material following location of the pipeline in the trench. 5. A trencher apparatus as claimed in claim 14 comprising an ROV. 6. A trencher apparatus as claimed in claim 14 or claim 5 further comprising one or more tanks of one or more of the additives.