WO2000075546A1

WO2000075546A1 - Buoyant composite material

Info

Publication number: WO2000075546A1
Application number: PCT/GB2000/002130
Authority: WO
Inventors: Andrew Charles Ratcliffe Tyrer
Original assignee: Crp Group Limited
Priority date: 1999-06-08
Filing date: 2000-06-02
Publication date: 2000-12-14
Also published as: AU5232300A

Abstract

A moulded composite material is disclosed which comprises a set of elongate tubular members 22 arranged substantially parallel to each other within a surrounding matrix 20 which comprises plastics, and most preferably syntactic foam. The matrix material is excluded from the interior of the tubular members, e.g. by provision of end caps 34 on the tubular member ends, and the resulting material is buoyant. The tubular members can be particularly closely packed to minimise density of the material and contribute strength to the material. Applications of the material include buoyancy modules, particularly for pipes and risers used subsea.

Description

DESCRIPTION

BUOYANT C0MPOSTE MATERIAL

The present invention relates to composite materials which are moulded and

buoyant, and also to buoyancy modules and insulating jackets comprising such

materials.

Fig. 1 illustrates the structure of a composite material well known for certain

maritime applications and comprising a large number of macroscopically sized

spherical beads 2, known to those skilled in the art as macrospheres, set in a moulded

matrix 4 of syntactic foam. The macrospheres 2 are of low density, being formed

with an expanded polystyrene core 6 surrounded by a harder shell 8 formed of

thermosetting plastics - typically polyester or epoxy. The macrospheres are of a

generally regular size in the illustrated material although in the cross section seen in

FigJ the exposed parts of the macrospheres vary in diameter due to the random bead

positioning. The syntactic foam matrix itself comprises thermosetting plastics with

an admixture of microspheres - hollow glass spheres too small to be seen in the

drawing.

The macrospheres can be manufactured by tumbling the expanded

polystyrene spheres 6 in a thermosetting plastics resin to coat them and so form the

shell 8, but this proves a troublesome process in practice. Alternative known

macrospheres are formed by ultrasonically welding together two injection moulded

plastics hemispheres or by epoxy bonding two forged aluminium hemispheres.

Diameters can be in excess of 30 centimetres. It is also known to use macrospheres of mixed diameter to improve their packing factor.

The material has proved highly effective in use. While the thermosetting

plastics is relatively dense, the inclusion of macrospheres and microspheres results

in a low overall density and hence good buoyancy. The material is thus suitable for

buoyancy elements. One example of the material's use is in subsea oil extraction.

Risers through which oil is conducted upward from a subsea well head to a rig

require support along their length and this is typically provided by buoyancy modules

formed by two generally "C" sectioned members formed of the material in question

and positioned to surround the riser. Means such as a tension band around the "C"

members secure them to the riser and a clamp engaging with the riser itself may be

provided to prevent motion of the buoyancy modules along the riser. The risers in

question may be of flexible type or may be of more rigid metal construction.

Despite the success of the above described material there are commercial and

technical incentives to improve upon it.

It is desirable, particularly where the material is used in a buoyancy element,

to minimise density and so maximise buoyancy for a given volume. Clearly it is

therefore desirable to increase the volume of the low density macroscopic elements -

in the known material, the macrospheres 2 - relative to that of the more dense matrix

material 4. Such decrease in the relative proportion of the matrix material is also

desirable in order to reduce costs (the matrix material is typically relatively

expensive) and to reduce the amount of heat given off during the exothermic curing

of the thermosetting plastics of the matrix, since excess heat can create problems during the moulding process and can damage the macrospheres. This heating can

undesirably limit the dimensions of items to be moulded, there being in effect a

critical size above which heating effects become unacceptable.

Unfortunately an upper limit on the proportion of the material which can be

taken up by the macrospheres of the known material is imposed by their mode of

packing.

Furthermore, any decrease in material density must be achieved while

maintaining acceptable mechanical properties - resistance to crushing, creep,

breakage etc. Indeed it is desirable to improve upon the resistance to breakage of the

known composite material since existing buoyancy modules made from the material

can on occasion break if mishandled.

It is an object of the present invention to provide an improved buoyant

composite material. Additionally or alternatively it is desired to provide a buoyant

composite material which can achieve lower density than the known composite

materials while maintaining required material properties such as resistance to

structural failure.

A further object of the present invention is to provide an improved buoyancy

module. Additionally or alternatively it is desired to provide a buoyancy module of

increased buoyancy (for a given volume) which nonetheless has required material

properties such as resistance to structural failure.

In accordance with a first aspect of the present invention, there is a moulded

composite material having positive buoyancy in water and comprising a matrix comprising plastics within which is disposed a plurality of elongate, substantially

mutually parallel, tubular members, the matrix material being excluded from the

interiors of the tubular members.

The plastics material of the matrix is preferably a thermosetting plastics.

Mouldable resins are particularly preferred. Polyester or epoxy are suitable, as are

vinyl ester (in particular epoxy vinyl ester) and phenolic. Polyurethane may be used.

Thermoplastics may alternatively be used.

The material's properties, and in particular its capacity to withstand

hydrostatic pressure without structural damage, result from a combination of the

properties of the tubular members and the matrix. Structural damage could in

particular occur if excessive pressure is placed on the tubular members, causing one

or more to be crushed. It is especially preferred that the matrix should be such as to

contribute resistance to crashing of the tubular members by hydrostatic pressure. In

paraticular it is preferred that the matrix should be stiff enough to serve this function.

Thus the entire compressive force due to immersion need not be borne by the tubular

members.

The stiffness of the matrix can be specified in terms of its Uniaxial

Compressive Modulus (UCM). It is preferred that the matrix should comprise

material having UCM not less than 150 Mpa. Still more preferably the material

should have UCM not less than 300 Mpa. UCMs as high as 2500MPa can be

achieved using plastics materials and may prove useful for deep water applications,

although materials with UCM in the region of 1200 Mpa are considered suitable for systems usable at 6000m depth.

The matrix material may itself be less dense than water. Currently preferred

materials have densities in the range 0.3 to 0.7gm./cc.

Whereas the known macrospheres, even when closely packed, can take up

approximately 55-60% of the volume of the known material previously described, the

elongate tubular members utilised in the material according to the present invention

can be packed to take up 70% volume or more. This allows a considerable reduction

in the relative volume of the matrix material and so in density, cost and in heat given

off on curing of thermosetting matrix material.

These advantages can be achieved while maintaining acceptable mechanical

properties of the material, since the tubular members can reinforce the material in a

manner not possible with macrospheres. Resistance of the material to breakage can

be particularly great along the tubular member's axes. Hence for example elongate

buoyancy modules, as used on sub-sea risers, can contain tubular members oriented

along their length, resulting in a module which is highly resistant to breakage.

The tubular members may be even more closely packed if, in accordance with

a preferred aspect of the invention, the material comprises first tubular members

having a first lateral dimension and second tubular members having a second lateral

dimension smaller than the first, the second tubular members being disposed in

interstices between the first tubular members.

The packing of the tubular members can be still further increased by forming

them to have a polygonal cross section, eg quadrilateral (which may be square or rectangular), triangular or hexagonal, such that the members can tessellate.

A further advantage of the material according to the invention is that the

tubular members can straightforwardly be made resistant to heat effects which can

damage macrospheres during moulding of an item.

The material may further comprise macrospheres. These can be disposed in

spaces around the tubular members and/or can be disposed in other re-entrant features

of the moulding, further reducing density.

Lateral dimensions of the tubular members are preferably macroscopic.

Lateral dimensions of substantially 5 mm and greater are preferred, and lateral

dimensions as large as 100mm. are considered suitable, but the currently favoured

range of lateral dimensions is from 20 to 60 mm.

Walls of the tubular members are typically in the range 1 - 5 mm. in

thickness.

Ends of the tubular members are preferably provided with closure means for

excluding matrix material from the tubular member interiors during the moulding

process. The end caps also help to withstand hydrostatic pressure in use. The closure

means may comprise caps shaped to mate with respective tubular member ends.

The tubular members may comprise metal walls. For example, aluminium

or titanium tubing may be used.

However in the preferred embodiment, composite material forms the tubular

member walls. Fibre reinforced plastic is a suitable material. A pultruded or

extruded fibre reinforced plastic tube is especially suitable. The currently preferred embodiments utilise filament wound tubes. It is particularly preferred that the walls

comprise helically or circumferentially wound fibres to improve crush resistance.

Glass fibres may be used but carbon fibres are particularly preferred.

Mechanical properties of the material may be improved by providing the

exteriors of the tubular members with shaped features which securely couple the

tubular members to the surrounding matrix material. For example a

circumferentially ribbed exterior prevents motion of the tubular member through the

matrix. Alternatively or additionally the tubular member exterior may be adapted to

bond to the matrix material. For example aluminium, or other, tubes may be first blasted clean and then externally primed to form the bond.

The tubular members are preferably shaped to improve their crush resistance.

In particular it is preferred that the tubular members' cross section should vary along

their lengths such as to increase the members' crush resistance. This may be by

ribbing or convolution of the walls of the members.

It is especially preferred that the matrix material comprises syntactic foam.

The tubular members may be filled with gas - typically air. However

according to a preferred embodiment of the present invention, the tubular members

are at least partly evacuated. In this way a material with low thermal conductivity

can be produced which is suitable for items such as insulating jackets for use subsea.

To further improve insulation properties, interior surfaces of the tubular members

could be given an insulative coating, eg a reflective coating. Aluminium paint

could be used for this purpose. It is especially preferred that the tubular members be closely packed. They

may be disposed one in contact with another or may be slightly mutually separated.

According to a second aspect of the present invention there is a buoyancy

module or float comprising material according to the first aspect of the present

invention.

The float may be for attachment to a riser. Alternatively it may be a float for

use in a submersible vehicle, for example a Remotely Operated Vehicle (ROV). The

float may alternatively take the form of an underwater buoy.

According to a third aspect of the present invention there is a thermal

insulation module or jacket comprising material according to the first aspect of the

present invention.

Such a jacket may be shaped and adapted to be arranged around a subsea oil

conduit and so to reduce loss of heat to surrounding water. Such jackets, formed

from other materials, are well known and help to prevent oil, following extraction,

from being cooled by the surrounding water to a temperature at which hydrate

formation - build up of waxy deposits - takes place causing the conduit to become

partially or wholly blocked.

Known insulating jackets comprising syntactic foam are subject to

deformation due to temperature variations, particularly due to the difference between

the relatively warm interior and the cooler exterior. The present invention serves to

reduce bending or other deformation since the tubular members resist bending

moments. Hence the module is maintained in more intimate contact with the pipeline, improving its performance.

In items formed from the material according to the present invention, the

tubular members are preferably aligned with a longitudinal axis of the item. The

tubular members preferably each extend along substantially the full length of the

item. Alternatively the tubular members may extend along only part of the length of

the item and be of various lengths such that tubular member ends are misaligned

with each other. In this way it is possible to avoid creating zones of weakness in the

material.

Specific embodiments of the present invention will now be described, by way

of example only, with reference to the accompanying drawings, in which: -

Fig. 1 illustrates the structure of a known buoyancy composite material in

perspective;

Fig. 2 illustrates the structure of a material according to the present invention

in perspective;

Fig. 3 is a side view of a tube used in an embodiment of the present invention;

and

Fig. 4 is a perspective view of a riser buoyancy module according to the

present invention.

The material illustrated in Fig. 2 comprises a matrix 20 of syntactic foam in

which are set circular tubes 22, formed in the Fig. 2 embodiment of aluminium. The

tubes serve as density reducing elements. Axes of the tubes are mutually parallel.

The lateral positioning of the tubes is not, in the illustrated embodiment, according to a uniform pattern. The tubes are straight. The tubes are also hollow.

Ends of the tubes are closed by shaped caps (not seen in Fig. 2) inserted into

the tube ends prior to the moulding process. In this way syntactic foam is excluded

from the tube interiors during moulding and the tubes are thus empty - that is to say

they contain only air. In other embodiments, the tubes may be at least partially

evacuated. The tubes are, in the completed moulding, sealed against ingress of water

which could otherwise enter the material due to hydrostatic pressure on immersion.

While the tubes of the illustrated embodiment are all of the same diameter and

construction, the material may comprise tubes of two or more types. For instance,

by utilising tubes of two different diameters the packing factor of the tubes may be

increased. It may be arranged that the tubes lying toward the surface of an item are

more robust that those lying toward the item's centre, making possible a reduction

in density without unacceptable impairment of the item's strength, crush resistance,

etc.

In the preferred method of moulding items from the material in question, the

tubes 22 with their ends closed are placed in a mould with their longitudinal axes

parallel (or at least substantially parallel). Syntactic foam resin (or other

thermosetting plastics resin) is then introduced to the mould and allowed to set.

In an alternative mode of construction, it is arranged that the tubes are slightly

mutually separated so that the resin can enter space between the tubes to improve the

strength of the material. For example, small spacers could be placed around the

tubes, at intervals along their length, to slightly separate the tubes during moulding. Fig. 3 illustrates in isolation an alternative form of tube 30 to be used in the

composite material according to the present invention, the tube being of filament

wound type. Manufacture of this tube involves in a known manner coating of glass

fibres or other filamentary material with plastics resin followed by winding of the

fibres onto a mandrel, the mandrel being rotated and the fibre being made to traverse

the mandrel to provide helical winding. After setting of the plastics resin, the

resulting tube can be removed from the mandrel and is both light and stiff. The

process allows properties of the tube - such as wall thickness- to be adjusted straight

forwardly in order to tailor the properties of the composite material to a particular

application. While the wall may be of uniform thickness, the process makes it

straight forward to create a tube wall whose thickness varies along the length of the

tube. The exterior of the illustrated tube can be seen at 32 to undulate along the tube

length due to the arrangement of its fibre windings. While dimensions are adjustable

to suit the particular application the wall thickness typically varies between 1 and 5

mm. It has been found that a tube having wall thickness which varies along the tube

length can have a greater crush resistance for a given weight than an equivalent tube

having uniform wall thickness. Relatively thick annuli along the tube length greatly

increase crush resistance while relatively thin intervening portions of the tube

minimise weight.

The desired variation in wall thickness could be achieved by using fibres of

two different thicknesses in fabrication of the tube, eg by winding relatively thick

fibres - say 2 mm in fhickness-onto the mandrel, then winding over relatively thin fibres - say 20 or 30 microns in thickness. Alternatively a tape, e.g. a metal tape

could be wound onto the mandrel to provide a desired contour for the tube exterior.

The tape could again be overwound with fibres.

Alternatively the necessary shape may be imprinted on the tube exterior, eg

by winding tape on the exterior before the resin of the tube has set.

An advantage of tubes whose exterior profile is shaped along the tube length

is that during the process of moulding the composite material, plastics resin can

penetrate spaces left between adjacent tubes, thereby minimising "dry" portions of

the moulding which the resin does not reach.

Resistance to crushing of the tube is of particular importance for items to be

used at depth, since hydrostatic pressure tends to crush such items and could cause

tubes of inadequate strength to collapse, with the danger of serious structural damage

to the item.

Seen in Fig. 3 is an end cap 34 which closes the end of the tube 30 during the

process of moulding the composite material, the end cap being shaped at 36 to

engage with the tube end and having a domed outer portion 38 which can rest against

the interior wall of the mould during moulding, ensuring by virtue of its domed shape

that plastics resin can flow into the region around the tube end.

Illustrated in Fig. 4 is a buoyancy module for attachment to a riser of the type

used in conducting crude oil away from a sub-sea wellhead, the module comprising

two elongate, generally "C" shaped buoyancy elements 40 (formed from composite

material according to the present invention) which can be secured together by tension straps 42 to form an internal cylindrical cavity 44 to receive the riser.

As can be seen in cutaway portion 46, the tubes of the composite material

extend axially of the elements thereby maximising their strength and resistance to

breaking.

While the tubes in the illustrated buoyancy module are all parallel, differently

shaped mouldings may have tubes oriented in two or more different directions in

different regions of the moulding, as necessary to accommodate features of the

moulding or to maximise strength.

The above described embodiments are, as will be apparent to the skilled

reader, merely illustrative of the range of possibilities arising from the present

invention as defined in the appended claims.

Claims

1. A moulded composite material having positive buoyancy in water and

comprising a matrix which comprises plastics and within which is disposed a

plurality of elongate, substantially mutually parallel tubular members, the matrix

material being excluded from the interiors of the tubular members.

2. A moulded composite material as claimed in claim 1 wherein the matrix

is such as to contribute resistance to crushing of the tubular members by hydrostatic

pressure.

3. A moulded composite material as claimed in claim 1 or claim 2 wherein

the matrix comprises plastics material having Uniaxial Compressive Modulus not

less than 150 Mpa.

4. A moulded composite material as claimed in any preceding claim wherein

the tubular members are closely packed.

5. A moulded composite material as claimed in any preceding claim

comprising tubular members having a lateral dimension which is substantially 5 mm

or greater.

6. A moulded composite material as claimed in any preceding claim

comprising tubular members whose lateral dimensions are between substantially 20

mm and substantially 60 mm.

7. A moulded composite material as claimed in any preceding claim wherein

the matrix comprises thermosetting plastics.

8. A moulded composite material as claimed in any preceding claim wherein

the matrix comprises syntactic foam.

9. A moulded composite material as claimed in any preceding claim wherein

macrospheres are incorporated within the plastics material of the matrix.

10. A moulded composite material as claimed in any preceding claim

wherein the tubular members are circular in cross section.

11. A moulded composite material as claimed in any of claims 1 to 9 wherein

the cross sections of the tubular members are polygonal and are such that the tubular

members can tessellate, the tubular members being arranged such that they tessellate.

12. A moulded composite material as claimed in any preceding claim

wherein the tubular members comprise walls formed of composite material.

13. A moulded composite material as claimed in any preceding claim

wherein the tubular members comprise walls formed of fibre reinforced plastics

material.

14. A moulded composite material as claimed in any preceding claim

wherein the tubular members comprise pultruded or extruded tubes.

15. A moulded composite material as claimed in any of claims 12 to 15

wherein the tubular members have walls comprising generally helically or

circumferentially wound reinforcing fibres.

16. A moulded composite material as claimed in any preceding claim

comprising first tubular members having a first lateral dimension and second tubular

members having a lateral dimension smaller than the first such that the second tubular members are disposed in interstices between the first tubular members.

17. A moulded composite material as claimed in any preceding claim in

which the tubular members are provided with closure means for excluding matrix

material from the tubular member interiors during the moulding process.

18. A moulded composite material as claimed in claim 17 wherein the

closure means are formed as end caps shaped to mate with respective tubular member

ends.

19. A moulded composite material as claimed in any preceding claim

wherein exteriors of the tubular members are provided with shaped features which

securely couple the tubular members to the surrounding matrix.

20. A moulded composite material as claimed in claim 19 wherein the shaped

features comprise helical or circumferential ribbing.

21. A moulded composite material as claimed in claim 18 wherein the tubular

member exteriors are adapted to bond to the matrix.

22. A moulded composite material as claimed in claim 21 wherein the tubular

member exteriors are textured to aid bonding to the matrix.

23. A moulded composite material as claimed in any preceding claim

wherein the tubular members are provided with respective spacers projecting from

the tubular members radially and thereby separating each of the tubular members

from its neighbours.

24. A buoyancy module, float or underwater buoy moulded from composite

material as claimed in any preceding claim.

25. A buoyancy module for coupling to a pipe or riser, the module being

moulded from composite material as claimed in any of claims 1 to 23.

26. A buoyancy module as claimed in claim 25 comprising a pair of

buoyancy bodies which are generally "C" shaped in cross section and adapted to be

secured around the pipe or riser .

27. A thermal insulation module or jacket comprising material as claimed in

any of claims 1 to 23.

28. A thermal insulation module or jacket as claimed in claim 37, shaped and

adapted to be arranged around a subsea oil conduit and so to reduce loss of heat to

surrounding water.

29. A moulded composite material having positive buoyancy in water

substantially as herein described with reference to, and as illustrated in,

accompanying Figs. 2 to 4.

30. A buoyancy module substantially as herein described with reference to,

and as illustrated in, accompanying Fig. 4.