GB2325507A

GB2325507A - Tubular structure comprising inner and outer tubular members

Info

Publication number: GB2325507A
Application number: GB9710758A
Authority: GB
Inventors: Terence Jeffrey Corbishley; Jonathan Guy Corbishley
Original assignee: T J CORBISHLEY
Current assignee: T J CORBISHLEY
Priority date: 1997-05-23
Filing date: 1997-05-23
Publication date: 1998-11-25
Anticipated expiration: 2017-05-23
Also published as: GB2325507B; WO1998053179A1; AU7664098A; GB9710758D0

Abstract

The structure comprises an inner tubular member 1 or parallel members located within a co-extending outer tubular member 3, comprising a double skinned sealed unit defining an annular space within, either voided or containing a liquid, gas, thermal insulant, lightweight material or ballast, supports 4 being provided to hold the outer tubular member spaced apart from the inner tubular member to thereby define an annulus between the inner and outer members, either voided or containing a gas, a liquid, a thermal insulant, a lightweight material, ballast, a settable liquid or flowable material. The structures are joined by welding between the respective inner members and outer members. Further supports 11 are provided at the junctions between the structures.

Description

IMPROVEMENTS IN TUBULAR MEMBERS The invention relates to elongate tubular members which have co-extending elongate members there within and a method of connecting together such tubular members. The invention will be described with reference to its use in pipeline and tubular structural members such as those typically utilised in the offshore oil and gas industry.

With the growth of the offshore oil and gas industry, it is necessary to install prefabricated pipelines between offshore oil and gas fields and onshore processing facilities and between different offshore locations. These may be for processing purposes or for loading the oil or gas into shuttle tankers from offshore loading buoys.

To meet the need to install pipelines with a high degree of reliability, it is possible to use barges and vessels with several aligned welding, inspection and coating stations that can fabricate a pipeline from a large number of short lengths of coated pipe, typically 12 metres long. The welded joints of the pipeline have then to be inspected and coated before the pipeline is lowered to the sea bed down a long curved stinger or ramp.

A further alternative is to fabricate, at a suitable shore site, a bundled assembly formed of a pipeline or group of pipelines housed within a large diameter outer steel pipeline, known as a carrier pipe. The void space within the carrier pipe and around the pipelines housed within it is sealed to prevent the ingress of water.

A particular trend is to install pipelines which are to operate at high temperatures and/or pressures, which give rise to significant integral forces when the pipeline is prevented from expanding or allowed only partial expansion. There is also a trend to install pipelines in deep water which are prone to collapse due to external water pressure In use, submarine pipelines are often required to maintain certain fluid temperatures, preventing adverse affect such as hydrate formation or wax deposition. Thermal insulation of the pipelines helps to avoid this and is currently achieved by the application to their external surface a suitable insulation material. The materials are presently applied by various processes including extrusion, impingement, wrapping and casting.

For high fluid temperatures and pressures provision of thermal insulation is achieved by housing the fluid carrying pipeline or flowline, as it is commonly termed, within an outer and additional pipeline. This is known as a "pipe-in-pipe" system.

The annulus between the inner flowline and outer pipeline may be filled with thermal insulant having low order thermal conductivity, such as polyurethane foams, mineral wool or ceramic microspheres.

Alternatively, the annulus gap may be a vacuum or full of gas.

Also a particular trend is to install pipelines which are to operate at high pressures and high temperatures which, if the pipeline is prevented from expanding or is allowed only limited expansion, would lead to the pipeline experiencing induced strains beyond the elastic limit of the material from which it is made.

To form elongate tubular structural members such as the tethers on Tension Leg Platforms, short lengths of itube-in-tube" structural members are adjoined end to end by either butt welding, internal or external collars secured by fillet welds or mechanical means such as screwed ends or splined connectors.

Where pipe-in-pipe pipelines or tube-in-tube structural members are constructed, it is necessary to complete the connection together of two inner pipes or two inner structural members prior to connecting together the two outer pipes or two outer structural members. Where the annulus between the inner pipe or tube and the outer pipe or tube contains materials or mechanical apparatus which prevent relative longitudinal movement of the two co-extending pipes or structural members, there is a technical problem faced in closing the gap between the ends of the outer pipes or structural member which is required for access to connect the inner pipes or structural members.

One solution has been proposed in our co-pending patent application GB 9623131.1. This method of joining pipelines utilises an intermediate member which is screwed on a thread relative to a first pipe section in order to bring the intermediate member into abutment with the second pipe section. Once the intermediate and second pipe members are brought into abutment with each other, they can be welded together.

This method enables the assembler to access the interior flowline before fixing together the external tubular members, whilst eliminating the need for fillet welds which are prone to failure and which are hard to x-ray.

A further solution to the problem of joining pipe-in-pipe or tube-in-tube members is proposed in our co-pending application GB 9703218.9 for situations where the inner and outer pipe or tube members are able to move relative to each other in the longitudinal direction.

An object of the present invention is to provide a pipe-in-pipe type of structure which can be used to provide buoyancy and/or thermal insulation and/or strength to the inner pipeline, but which does not inhibit the thermal expansion or movement thereof.

A further object of the present invention is to provide a convenient method of constructing such a structure.

According to the invention there is therefore provided a tubular structure comprising an inner tubular member located within a coextending outer tubular member, the outer tubular member being formed from coextending inner and outer tubular elements, the ends of which elements are joined to form a double skinned sealed unit defining an annular void within, said outer tubular member being held spaced apart from the inner member by means of supports, to thereby define an annulus between the inner and outer members.

A preferred embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: Fig. 1 shows a side sectional elevation of a section of a tubular structure according to the present invention in which the outer members are abutted; Figs. 2 and 3 show side sectional elevations of another embodiment of a tubular structure according to the invention in which the outer members are not abutted; and Fig.4 shows a schematic illustration of a flowline according to the invention during installation on the seabed.

Referring to Fig. 1, there is shown a section of a tubular structure. The structure comprises inner tubular members la, lb,lc, joined end-to-end by a welded connections 2, to form a flowline, and coaxial outer tubular members 3a, 3b, 3c within which the inner members la, lb are located. The outer tubular members 3a, 3b, 3c comprise double skinned sealed direct shells preferably of an impermeable material.

The members 3 may contain a thermal insulating material, such a ceramic microspheres, mineral wool or polyurethane foam. Alternatively, according to the required use of the tubular structure, the shells may contain a gas such as nitrogen or carbon dioxide, ballast material or be empty or evacuated. Each of the outer members 3a,3b,3c terminates short of each end of the respective inner members la, lb, lc.

Each outer member 3 is constructed from two coaxial tubular elements the ends of which are welded together by means of annular connectors 8,9 to form a sealed unit. In the embodiment of Figure 1 the outer element is set back relative to the inner element and the front and rear connectors are shaped to allow the rear end 10 of the outer element to project from the member 3. Each outer member 3 is slid along the inner members 1 until its front end abuts the rear end connector of the previously positioned member 3 and is welded thereto. In this embodiment of the invention the front end of each outer member 3 is located under the rearwardly projecting end 10 of the outer element of the preceding outer member 3. The end 10 is joined by means of weld 6 to the outer element of the adjacent outer member 3.

Supporting the outer members 3 on the inner members 1 are a pair of supports 4, 5. The supports 4,5 can act as spacers or bulkheads and are substantially annular disks of rubber/steel or another suitable material which is capable of being slid along the inner member 1. The supports 4, 5 fix the position of the outer members 3 relative to the inner members 1. The supports 4,5 may be positioned around the inner members 3 before the outer members are placed around them Alternatively, the may be positioned afterwards, in which case they need to slide inside the outer members also.

In preferred embodiments of the invention the annular spaces defined between the co-axial inner and outer members 1,3 between the spacers 4,5 are evacuated, or empty or are filled with an insulating material or a gas, such as nitrogen or carbon dioxide.

Additional fluid may be passed through the spaces to provide a means of heating or cooling the inner members or for balancing pressures acting on the external surface of the outer member and that between he inner and outer member. Alternatively, the spaces may be filled with a liquid or flowable mixture which subsequently sets to form a solid matrix.

Although for convenience only a single flowline made up of inner pipes la, lb, lc is illustrated, a plurality of parallel pipes or other elongate members maybe housed within the outer pipes. Furthermore, whilst the tubular members described in the foregoing description are cylindrical pipes for a flowline, the tubular members could have any chosen cross section.

For high temperatures and/or pressures the gap between the inner and outer members may be sufficient to allow expansion or partial expansion of the inner pipe, laterally, in a spiral formulation or longitudinally beyond the ends of the pipeline, or a combination of all three.

Referring to figure 2, there is shown an alternative tubular structure of the present invention. In this embodiment the individual outer double skinned members 3 do not wholly abut each other. The facilitates the making of welded connection 6. In this embodiment each end of the outer elements of the outer members 3 projects beyond the inner elements and these are welded to the outer elements of adjacent outer members. It should be noted that Figure 2 only shows the use of a single annular spacer 5 per outer member 3. As many or as few can be used as required for each application.

In Figure 3, a further embodiment is illustrated in which the outer members 3 do not wholly abut each other. However in addition to supports 4 further supports 8 are used which fill the gap between the front and rear ends of adjacent outer members 3 as well as supporting them. This structure helps to spread the external loads applied to the outer members 3 to the flowline 1. The basic shape of the outer members 3 is very similar to that shown in Figure 1.

A plurality of the tubular structures described above can be joined together to form an elongate tubular structure or flowline using the method described in our co-pending application GB 9703218.9 the contents of which are incorporated herein by reference. Essentially adjacent ends of two inner members are first joined together. The outer tubular member of the second structure is displaced along the second outer tubular member in an axial direction until it at least partially overlies the first inner tubular member and at least partially abuts the adjacent outer tubular member. The adjacent outer members are then joined together. The joins are made preferably by welding.

Although the foregoing describes the joining of the outer members by welding, they may alternatively be joined by mechanical connectors, or other suitable means.

Figure 4 shows an elongate tubular structure or flowline 20, constructed from a plurality of tubular structures, being installed on the seabed 21. During installation the structure extends from the seabed 21 to the surface of the sea 22 with a liquid or a flowable mixture 23 introduced during the installation of the structure 20 to exert pressure on the outer member 3 from within the annulus between the outer and inner members 3,1 to oppose the pressure acting on the external surface of the outer member 3 by the surrounding sea water. The free surface of the liquid 23 in the annulus is determined so that the pressure exerted on the internal surface of the outer members 3 resists or opposes the hydrostatic collapse of the outer member 3 due to the pressure exerted on it by the seawater external to the outer members 3. A liquid or flowable mixture may be selected such that it sets either during or following installation onto the seabed.

Claims

CLAIMS:

1. A tubular structure comprising an inner tubular member located within a coextending outer tubular member, the outer tubular member being formed from coextending inner and outer tubular elements, the ends of which elements are joined to form a double skinned sealed unit defining an annular void within, said outer tubular member being held spaced apart from the inner member by means of supports, to thereby define an annulus between the inner and outer members.

2. A tubular structure as claimed in claim 1 in which the void within the outer tubular member is left empty or evacuated.

3. A tubular structure as claimed in claim 1 in which the void is filled with a gas, a liquid, a thermal insulant, a lightweight material or ballast.

4. A tubular structure as claimed in any one of the preceding claims in which the annulus between the inner and outer members is left empty or evacuated.

5. A tubular structure as claimed in any one of claims 1 to 3 in which the annulus is filled with a gas, a liquid, a thermal insulant, a lightweight material, ballast or a settable liquid or flowable material.

6. A tubular structure as claimed in any one of the preceding claims in which the outer member is made of an impermeable material.

7. A tubular structure as claimed in any one of the preceding claims in which there are a plurality of parallel inner members.

8. An elongate flowline comprising a plurality of the tubular structures as claimed in any one of the preceding claims in which the inner tubular members are joined end to end with inner tubular members in adjacent structures and the outer tubular members are joined end to end with the outer tubular members in adjacent structures.

9. An elongate flowline as claimed in claim 8 in which the inner members of the structures are joined together by welding.

10. An elongate flowline as claimed in claim 8 or claim 9 in which additional supports are located at least partially between adjacent ends of the outer tubular members to hold the outer members spaced form the inner members.

11. An elongate flowline as claimed in any one of claims 8 to 10 in which there are a plurality of parallel inner members each of which is joined to adjacent ends of inner tubular members in adjoining tubular structures.

12. An elongate tubular structure formed by a method as claimed in any of the preceding claims.

12. A method of connecting together end-to-end first and second tubular structures as claimed in any one of claims 1 to 7 comprising the steps of joining together adjacent ends of the inner tubular members of adjacent structures, displacing the second outer tubular member axially along the second inner tubular member until it at least partially overlies the first inner tubular member and abuts the first outer tubular member, and joining the first and second outer tubular members.

13. A method of installing on the sea bed a flowline as claimed in any one of claims 8 to 11 whereby a liquid or flowable mixture is introduced in the annulus between the inner and outer tubular members to an extent where during installation the flowline extends from the seabed to a surface of the sea, the liquid or flowable mixture subjects internal surfaces of the outer members to a pressure sufficient to oppose the pressure exerted on an external surface of the outer member in order to prevent or resist collapse of the outer member.

14. A tubular structure substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

15. An elongate flowline substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

16. A method of connecting end-to-end tubular structures substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

17. A method of installing an elongate flowline substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

Amendments to the claims have been filed as follows 1. A method of forming an elongate tubular structure by connecting together end to end first and second tubular structures, in which each tubular structure comprises an inner tubular member located within a coextending outer tubular member, comprising the steps of forming the outer tubular members by connecting together the ends of coaxial tubular elements to form a double skinned sealed unit defining an annular void within; joining together adjacent ends of the inner tubular members of the first and second tubular structures; providing supports to hold the outer tubular members spaced apart from the inner tubular members to thereby define an annulus between the inner and outer members; displacing the outer tubular member of the second structure axially along the inner tubular member of the second structure, until it at least partially overlies the inner tubular member of the first tubular structure and at least partially abuts the outer tubular member of the first structure; and joining together the abutting outer tubular members of the first and second tubular structures.

2. A method as claimed in claim 1 in which the first tubular structure is an elongate tubular structure formed from a plurality of tubular structures connected end to end.

3. A method as claimed in claim 1 or claim 2 in which the inner tubular members of the tubular structures are joined together by welding.

4. A method as claimed in any one of the preceding claims further comprising the step of providing additional supports located at least partially between adjacent ends of the outer tubular members to hold the outer tubular members spaced from the inner tubular members.

5. A method as claimed in any one of the preceding claims in which each tubular structure comprises a plurality of parallel inner tubular members each of which parallel inner tubular members is joined to a corresponding inner tubular member in an adjoining tubular structure.

6. A method as claimed in any one of the preceding claims further comprising the step of introducing a liquid or flowable mixture into the annulus between the inner and outer tubular members to an extent whereby, during installation of the elongate structure on a sea bed when the elongate structure extends upwards from the sea bed towards or to the surface of the sea, the liquid or flowable mixture subjects internal surfaces of the outer members to a pressure sufficient to oppose the pressure exerted on the external surface of the outer member in order to prevent or resist collapse of the outer members.

7. A method as claimed in any one of the preceding claims further comprising the step of evacuating the outer tubular member.

8. A method as claimed in any one of claims 1 to 6 further comprising the step of filling the void within the outer tubular member with a gas or a liquid or a thermal insulant or a lightweight material or ballast.

9. A method as claimed in any one of the preceding claims in which the annulus between the inner and outer members is left empty or evacuated.

10. A method as claimed in any one of claims 1 to 8 in which the annulus between the inner and outer members is filled with a gas or a liquid or a thermal insulant, or a liquid or flowable material which is settable.

11. A method as claimed in any one of the preceding claims in which the outer member is made of an impermeable material.