NZ755622B2 - Cable or flexible pipe with improved tensile elements - Google Patents
Cable or flexible pipe with improved tensile elements Download PDFInfo
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- NZ755622B2 NZ755622B2 NZ755622A NZ75562217A NZ755622B2 NZ 755622 B2 NZ755622 B2 NZ 755622B2 NZ 755622 A NZ755622 A NZ 755622A NZ 75562217 A NZ75562217 A NZ 75562217A NZ 755622 B2 NZ755622 B2 NZ 755622B2
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- Prior art keywords
- cable
- sheath
- tensile
- fibre reinforced
- elongated
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Abstract
The present invention solves the problem of providing a tensile member for a submarine cable configured for installations at high depths which could contribute to limit the elongation of the cable during installation, provide sufficient compression (crush) resistance, as required by the gripping equipment used for the cable deployment at the desired sea depth, and can be suitably plastically deformed for the connection to end fittings. In particular, the present invention relates to a cable comprising an elongated tensile element having a cross section area and comprising a fibre reinforced polymer composite core having an elastic modulus of at least 70 GPa and a sheath at least partially covering the composite core, the sheath being made of metal and being at least 30% of the cross section area of the tensile element. ipment used for the cable deployment at the desired sea depth, and can be suitably plastically deformed for the connection to end fittings. In particular, the present invention relates to a cable comprising an elongated tensile element having a cross section area and comprising a fibre reinforced polymer composite core having an elastic modulus of at least 70 GPa and a sheath at least partially covering the composite core, the sheath being made of metal and being at least 30% of the cross section area of the tensile element.
Description
CABLE OR FLEXIBLE PIPE WITH IMPROVED TENSILE ELEMENTS
BACKGROUND
The present invention relates to the field of cables and flexible pipes, in
particular for submarine applications and more in particular for submarine
applications in deep water. In particular the present invention relates to a cable
or a flexible pipe with improved tensile elements.
PRIOR ART
Optical cables, power cables, umbilical cables, flexible pipes for submarine
applications are well known in the art. In particular, flexible pipes comprise a
tube for safely transporting a fluid, such as crude oil or gas, in a confined
manner.
For the purpose of the present invention, unless otherwise specified, the
terms “submarine cable”, “cable for sub-sea applications” (or any similar
language) or simply “cable” will include an elongated flexible element configured
to transport power and/or telecommunication signals and/or one or more fluids.
For the purpose of the present invention, the term “cable” will include, for
instance, power cable, optical cable, flexible tube for transporting a fluid or any
combination thereof.
The present invention is not limited to cables for submarine application and
can be applied to other fields where tensile strength and lightweight are of
importance. For instance, lightweight is appealing in cables for elevators or the
like, especially when installed in very high buildings.
During installation and performance, cables and flexible pipes should sustain
high tensile loads. For example, a submarine cable or a pipe hangs off of the
installation vessel from the surface of the water to the floor of the sea with a
consequent substantial tensile stress.
It is known to provide tensile strength by the use of steel tensile members
placed axially or, more frequently, in a stranded arrangement around the cable
or pipe structure to form an armour, as shown for example, in
and .
Tensile strength elements made of steel represent an important portion of
the cable weight. The cable weight sets the limit for the deposition depth of a
submarine cable because the deeper the water, the longer is the cable span
(and, accordingly, the cable weight) that is “hanging” from the installation
vessel. A given cable weight can cause a substantial cable elongation (e.g.
greater than 0.5%) likely to damage to the cable itself. For current steel-
strengthened submarine cables, the deposition depth limit is around 2000
meters.
In addition, the payoff system of the installation vessel has to be
commensurate to the weight of the cable to be deployed. The heavier the cable
is, the stronger the gripping force of the payoff system needs to be. As the
gripping force increases, the compression resistance of the cable also has to
increase. Crush failure caused by gripping is a known mode of failure.
Tensile elements made of polymeric material, in particular of fibre composite
material (e.g. FRP), have been proposed.
For example, relates to a method of producing curved,
elongate fibre reinforced polymer element suitable for flexible pipe. The method
comprises impregnating a fibre bundle with a mouldable, curable impregnation
substance. The impregnated fibre bundle is further provided with a coating of
thermoplastic material.
Furugen M. et al., Furukawa Review No. 21, 2002 illustrates a submarine
cable with double armour and the compound armour consisted of high-density
polyethylene sheathed FRP (fiberglass-reinforced plastic) as the first layer.
US 3,980,808 relates to an electric cable having a fibre reinforced plastic
reinforcing member applied for imparting various mechanical strengths.
Applicant observed that tensile elements made of a fibre composite material
coated with a polymeric material cannot be plastically deformed without losing
or decreasing in functionality. They can only be substantially elastically
deformed.
The plastic deformation is especially sought when the tensile members are
plastically deformed in end fittings in order to obtain a strong bond between the
end fitting and the cable.
Methods for securing an end fitting to a cable tensile element are disclosed,
for example, in EP 1 867 906 and US 2004/066035 where the tensile elements
are curved to even very small bending diameter.
In addition, cable armour is generally made of elongated tensile elements
wound around the cable core with a certain lay length. If the elongated tensile
elements are wholly made of polymeric material, their substantial lack of plastic
deformation can also cause problems at the cable ends where, free from
constrain, the element can straighten and unwind, causing problems in the
cable handling.
SUMMARY OF THE INVENTION
The Applicant has tackled the problem of providing a submarine cable
configured for installations exerting a considerable tensile stress on the cable,
and, more in general, the problem of having a lighter cable which is felt in
vertical installations like those for elevator cables.
In the case of submarine cables, installations at high depths, for instance at
depths of about 2000 m or more are sought.
In particular, the Applicant has faced the problem of providing a tensile
member for a submarine cable configured for installations at high depths which
could contribute to limit the elongation of the cable during installation, provide
sufficient compression (crush) resistance, as required by the gripping
equipment used for the cable deployment at the desired sea depth, and can be
suitably plastically deformed for the connection to end fittings.
The Applicant has investigated tensile element designs made of composite
materials and has noted that the use of polymeric materials reduces weight
and/or increases the strength of a tensile element.
On the other side, metals are plastically deformable with little or no decrease
in functionality and, compared to composites, are in general more durable and
better withstand rougher handling, impacts and abrasions. They are more
manufacturing friendly.
The Applicant found that a tensile strength element suitable for coupling with
the end fittings and for high tensile applications of cables, umbilicals, flexible
pipes) can conveniently be made of a polymer/fibre composite partially or
entirely sheathed by a metal sleeve.
In particular, it has been found that a tensile strength element comprising a
predetermined amount of metal and a polymer/fibre composite having a given
elastic modulus is effective for providing a cable with suitable plastic
deformation and crush resistance for end fitting and suitable tensile strength for
the cable to be deployed at high depths (or otherwise bearing significant loads).
According to a first aspect, the present invention provides a cable comprising
an elongated tensile element having a cross section area and comprising a fibre
reinforced polymeric composite core having an elastic modulus of at least 70
GPa and a sheath at least partially covering the composite core, the sheath
being made of metal and being at least 30% of the cross section area of the
tensile element, wherein said elongated tensile element has a flat cross section.
A cable of the invention preferably comprises a plurality of elongated tensile
elements disposed in one or more armour layers to provide an armour. The
plurality of elongated tensile elements is advantageously wound around the
longitudinal cable axis. When the plurality of elongated tensile elements is
provided in more layers the winding direction of one layer is preferably opposite
to that of the adjacent layer.
The elongated tensile elements of the invention can have a cross-section of
any shape, for example round, square or flat (such as rectangular or oval)
having a minor and a major axis. A flat cross-section is preferable as an armour
made of flat elongated tensile elements (with the main axis of the cross-section
arranged circumferentially around the cable axis) is more compact and the
longitudinal axis of the single elements remains closer to the main axis of the
cable.
When the elongated tensile element of the cable of the invention has a flat
cross section, the fibre reinforced polymeric composite core has a thickness of
at most 80% of the thickness of the tensile element along its minor axis.
Advantageously, in an elongated tensile element having a flat cross section,
the fibre reinforced polymeric composite core is provided substantially centred
on the neutral bending axis of the tensile element. This positioning of the fibre
reinforced composite core causes less stress on the composite material when
the elongated tensile element is deformed for being connected to end-fittings.
The fibre reinforced composite polymer can be selected from carbon fibres,
aramid fibres or glass fibres or a combination thereof.
The fibres of the reinforced composite polymer are embedded in a polymeric
matrix, for example a thermoset matrix (e.g. an epoxy resin such as bisphenol
A epoxy vinylester; a polyester or a vinyl ester resin) or a thermoplastic matrix
(e.g. a polyester or a polyamide). While a fibre reinforced polymeric composite
with a thermoplastic matrix generally has lower strength to weight ratios than a
composite with thermoset matrix, a thermoplastic matrix material may offer
higher impact resistance, which is desirable against crush failure in some
circumstances.
Advantageously, the fibre reinforced polymer composite core has a
longitudinal axes and fibres embedded in a polymeric matrix with longitudinal
axes substantially parallel to the longitudinal axis of the fibre reinforced polymer
composite core.
When the fibre reinforced polymeric composite comprises a conductive
component (e.g. carbon fibres) a film of thermoset or thermoplastic coating can
be provided around the fibre reinforced composite polymer as electrically
insulating layer suitable for preventing any potential galvanic coupling between
the metallic sheath and a conductive component of the composite polymer.
Galvanic coupling could cause corrosion in the metallic sheath.
In the present description and claims, as “elastic modulus” it is meant a
measure of the stiffness of a solid material, in particular the ratio of the stress
(force per unit area) along an axis to the strain (ratio of deformation over initial
length). It is also known as Young’s modulus.
The elastic modulus of the fibre reinforced polymer composite for the
elongated tensile element of the invention is preferably of at least 100 GPa,
more preferably up to 300 GPa. Greater elastic modulus could result in an
elastic return or springback after deformation too strong to be restrained by the
metal sheath, especially when very small bending diameter (of 100 mm or less)
is required.
The sheath of the invention is preferably made of steel. Steel suitable for the
present invention has an elastic modulus of from 150 to 250 GPa.
The metal sheath advantageously extends substantially along the full length
of elongated tensile element.
Preferably, the metal sheath continuously covers the composite core all
along its length.
The metal sheath of the elongated tensile elements for the cable of the
invention represents at least 30% of the cross section area of the tensile
element. Such percentage may vary depending on the elastic modulus of the
fibre reinforced polymeric composite used in the core of the element and of the
bending diameter required for the elongated tensile element to be end-fitted.
The metal sheath of the elongated tensile elements for the cable of the
invention can represent up to 85% of the cross section area of the tensile
element. This upper limit is selected in view of the sought deployment depth of
the cable and, accordingly, of the overall cable weight (where the less amount
of metal in the elongated tensile element is, the better is) and of the plastic
deformation and crush resistance exerted at the end fitting (where the more
amount metal in the elongated tensile element is the better is).
The fibre reinforced polymeric composite core and a metal sheath are in
direct contact one another or, preferably, an adhesive layer is interposed in
between.
The sheath and the core are preferably bound together by an adhesive
and/or by a mechanical (pressure) bond.
The cable can be an optical cable, a power cable, an umbilical cable or a
flexible pipe. Preferably, the cable is for submarine application or for long
vertical deployment, like cables for elevators.
Advantageously, the metal sheath of the elongated tensile elements has a
longitudinal welded seam.
Preferably, the metal sheath is made of a sheet shaped to form an open
housing with edges welded together after the core has been inserted in the
housing.
BRIEF DESCRIPTION OF THE DRAWING
The present invention will become fully clear by reading the following detailed
description, to be read by referring to the accompanying drawings, wherein:
- Figure 1 is an axonometric view of a flexible pipe for transporting crude oil
comprising an armour structure comprising a plurality of elongated tensile
elements;
- Figure 2 is an axonometric view of an armoured three-core power cable;
- Figure 3 is a cross section of an umbilical cable
- Figures 4a to 4f are diagrammatic cross-sections of elongated tensile
elements according to some embodiments of the present invention; and
- Figure 5 shows an example of end-fitting known in the art.
DESCRIPTION OF EXAMPLES
In the present description and claims, unless otherwise specified, all the
numbers and values should be intended as preceded by the term “about”.
The present invention relates to an elongated tensile element for a cable and
a cable comprising such an elongated tensile member. More specifically, the
elongated tensile element is configured for making an armour of any of an
optical cable, a power cable, an umbilical cable, a flexible pipe or similar
elongated object requiring a tensile strength element, in particular, but not only,
for submarine applications. The present invention also includes an optical cable,
power cable, umbilical cable, flexible pipe (or the like) in particular, but not only,
for submarine applications comprising one or more elongated tensile elements.
The elongated tensile elements can form a cable armour.
A flexible pipe is shown in Figure 1. Figure 1 is an axonometric view of a
submarine flexible pipe 100 for transporting a fluid such as crude oil. The pipe
100 comprises, in a radially inner position thereof, a metal flexible carcass 10,
configured to operate in contact with the crude oil (or other fluid) to be
transported. The carcass 10 comprises a helical winding 11 of a stainless steel
elongated element.
The pipe 100 further comprises, in a radially outer position with respect to
the carcass 10, an inner polymeric liner 20 configured to prevent leakages of
the fluid out of the carcass 10. The polymeric material of the liner 20 is
preferably selected from the group comprising: polyamide, polyvinylidene
fluoride, polyethylene, cross-linked polyethylene.
The pipe 100 further comprises, in a radially outer position with respect to
the inner polymeric liner 20, a mechanical armour structure which is potentially
exposed to water contact. The mechanical armour structure comprises a
pressure resistant armour 30 configured to withstand to radial loads and a
tensile armour 40. The pressure resistant armour 30 is formed from one or more
short-pitch helical winding 31 of an interlocked profiled carbon steel elongated
element 32. A layer 50 of plastic material is arranged between the pressure
resistant armour 30 and the tensile armour 40.
As shown in Figure 1, the tensile armour 40 is arranged in a radially outer
position with respect to the pressure resistant armour 30. The tensile armour 40
is configured to withstand to the longitudinal tensile forces which the pipe 100
may be subjected to in operation. The tensile armour 40 comprises a first tensile
armour layer 41 and a second tensile armour layer 42 of long-pitch helical
windings of elongated tensile elements 400 extending parallel to each other in
a helix coaxial with the pipeline longitudinal axis X-X.
The helical windings of the second tensile armour layer 42 extend along a
winding direction which is opposite to the winding direction of the helical
windings of the first tensile armour layer 41 with respect to the longitudinal axis
X-X, so as to define a crossed configuration. Preferably, the helical windings of
the first and second tensile armour layers 41 and 42 are laid with substantially
the same pitch, in opposite directions, so as to prevent pipe rotation under axial
load. The elongated tensile elements 400 are laid, preferably, with winding pitch
comprised between 25 cm and 200 cm.
The elongated tensile elements 400 of the tensile armour 40 are arranged
side by side and have a substantially rectangular cross section, as it will be
detailed in any of the following Figures 4a-4f.
A layer 70 of polymeric material is arranged between the first tensile armour
layer 41 and the second tensile armour layer 42.
The pipe 100 further comprises, in a radially outer position with respect to
the radially second tensile armour layer 42, a protective polymeric outer sheath
80 aimed to be waterproof.
Figure 2 shows an armoured three-core power cable 200 suitable for
submarine deployment. Cable 200 comprises three stranded insulated
conductive cores 22 surrounded by a bedding/sheath system 21. A cushioning
layer 25 (made, for example, of polypropylene yarns) surrounds the
bedding/sheath system 21. Around the cushioning layer 25 a tensile armour 26,
comprising at least one layer of elongated tensile elements 400 according to
the invention, is provided. A protective polymeric outer sheath 23 surrounds the
armour 26. The tensile armour 26 is configured to withstand to the longitudinal
tensile forces which the power cable 200 may be subjected to in operation. The
elongated tensile elements 400 preferably extend parallel to each other in a
helix coaxial with the cable longitudinal axis X-X. While the tensile elements 400
of Figure 2 have a circular cross-section, they can also have a substantially
rectangular cross-section, as it will be detailed in any of the following Figures
4a-4f.
Figure 3 is a cross-section of an umbilical cable 300. This umbilical cable 300
comprises a central core 33. The central core 33 may be made of steel for
transporting a fluid. Disposed around the central core 33 are three steel tubes
34 for transporting a fluid; two optical fibre cables 35; two armoured electric
power cable 36; two thermoplastic fillers 37; and a sheath 38. Around the sheath
38 two layers of counter-helically wound elongated tensile elements 400
according to the invention are provided. The two layers of elongated tensile
elements 400 constitute the tensile armour 39 of the umbilical cable 300. An
outer sheath 3 surrounds the tensile armour 39. Outer sheath 3 may be made,
for example, of polymeric material. While the tensile elements 400 shown in
Figure 3 have a circular cross-section, they can also have a substantially
rectangular cross-section, as it will be detailed in any of the following Figures
4a-4f.
The elongated tensile element of the present invention is designed to replace
the known elongated elements made, for example, of carbon steel, in order to
provide improved installation performance while maintaining the beneficial
characteristics of carbon steel tensile members.
The elongated tensile elements of the present invention can be adopted in
the flexible pipe of Figure 1, in any of cables of Figures 2 and 3 or in any other
different design where tensile strength is an issue.
According to embodiments of the present invention, there is provided a cable
comprising a reinforcing structure with one or more elongated tensile elements,
wherein each elongated tensile element comprises a core and a metal sheath
at least partially sheathing said core, wherein the core comprises a fibre
reinforced polymeric composite material.
Figure 5 shows an example of end-fitting, in particular a portion of end fitting
is schematically illustrated. The end-fitting 500 comprises a cylindrical body 51
where a portion 52 - specifically the portion underlying the armour - of a cable
is inserted through a flange 54. Before the insertion, the elongated tensile
elements 53 of the cable armour are unwound, diverted from the underlying
cable portion 52 and passed through the holes 54a of the flange 54. Once
positioned within the cylindrical body 51, the terminal portion of the elongated
tensile elements 53 is suitably bent, and a thermoset resin 55 is injected to fill
the cylindrical body 51.
The possibility of plastically deforming the elongated tensile elements 53 is
advantageous both in the step of diverting them from the cable portion 52 and
in the step of injecting the thermoset resin 55 to embed the bent terminal portion
of the elongated tensile elements 53.
According to embodiments of the present invention, the elongated tensile
element has a core of composite material and is at least partially covered by a
metal sheath being at least 30% of the cross section area of the tensile element.
Figures 4a to 4f schematically show some different constructions of elongated
tensile elements of the invention through cross-sections. While the Figures only
show substantially flat tensile elements 10 having a substantially rectangular
cross-section, the present invention is not limited to such designs.
Figures 4a to 4c show elongated tensile elements 10 with the core of
composite material totally (Figure 4a) or partially (Figures 4b and 4c) covered
by a metal sheath. In particular, in Figure 4a the composite core 12 is completely
enclosed in a tubular metal sheath 14 covering both the major faces and the
minor faces of the rectangular core 12.
The closed tubular metal sheath 14 can be connected to the core 12 by any
known means, either mechanical or chemical (for instance, respectively, by
compression of the metal sheath against the composite core or by an adhesive
or the like interposed between metal sheath and composite core). Examples of
adhesives suitable for the present invention include solvent based adhesive,
hot-melt adhesive, cyanoacrylates, polyurethanes, epoxies, contact adhesives,
pressure sensitive adhesives. The adhesive connection is preferable also
because it can be implemented in a continuous process. In addition, the shear
forces created by an adhesive would better couple the two element components
(the metal layer and the composite core) together to provide reliable tensile
performance.
According to the embodiment of Figure 4b, the composite core 12 of the
tensile element 400 is sandwiched between a first metal layer 14a and a second
metal layer 14b substantially covering the major faces of the core 12.
The elongated tensile elements 400 can have a substantially circular or a flat
(rectangular or oval) cross-section. In the case of a flat cross-section, the cross-
section major axis is preferably oriented tangentially with respect to the cable
circumference. For example, the cross-section major axis can be from 5 mm to
mm long, and the cross-section minor axis can be from 2 mm to 5 mm long.
In the case of circular cross-section, the elongated tensile elements 400 can
have a diameter of from 2 to 20 mm.
The thickness of the sheath 14 is it least 30% of the cross section area of the
tensile element.
An elongated tensile element 400 according to Figure 4b might be formed by
roll forming or by any other continuous forming method. The first metal layer
14a and the second metal layer 14b can be made of carbon steel having an
elastic modulus of 200 GPa, and can have the thickness already mentioned for
sheath 14.
The first and second layers of steel 14a and 14b can be connected to the
core 12 by any known means, either mechanical or chemical, as already
mentioned above (for instance through an adhesive or the like). In addition, the
shear forces created by an adhesive would couple the two elements (the two
layers 14a and 14b of steel and the composite core 12) together to provide
reliable tensile performance.
According to the embodiment of Figure 4c, the composite core 12 is partially
covered by a “C” shaped sheath 14 of steel. Therefore, in addition to the major
faces of the core 12 which become covered by a layer of steel, also one of the
minor faces becomes steel sheathed. One side remains open. From such an
opening, it is possible to insert the composite core 12 either during or after the
forming step of the C-shaped sheath.
Figures 4d to 4f show some methods for manufacturing an elongated tensile
element 400 with the composite core 12 fully enclosed in a metal sheath 14,
according to figure 4a.
In one embodiment, the steel sheath 14 is formed by folding a foil (for
example by roll forming) to provide a tube with rectangular cross-section and
with edges substantially contacting along one of the minor sides or along the
border between a minor side and a major side. During manufacturing the sheath
edges are elastically parted; the composite core 12 is inserted within the parted
edges which are then put in contact one another again. A compression stage
follows, performed onto the steel sheath 14 and, accordingly, on the core 12 of
composite material enclosed within. The seam resulting from the contact
between the edges can be welded.
The manufacturing embodiment of Figure 4d can be used to make an
elongated tensile element as the one of Figure 4a. Half side segments 141 are
provided at each of the free ends of a “C” shaped sheath 14. In this way, after
the core 12 is inserted into the sheath 14 in its open configuration, the sheath
14 spring-closes and fully encloses the composite core 12. The two half side
segments 141 form a side of the steel sheath 14. Preferably, the seam of the
two half side segments 141 can be welded together.
The sheath 14 of Figure 4e comprises a foil configured to form a housing 143
for the composite core 12 and a spring loaded lid 144. During the composite
core insertion, the lid 144 is maintained open against the spring force tending
to close the housing 143. Once the core has been lodged into the housing 143,
the lid 144 is left free so that it elastically abuts against the composite core 12
so as to fully enclose it.
Preferably, the seam of the lid 144 is finally welded to the edge of the housing
143 to enclose the composite core 12 in a stable manner. An adhesive can be
employed for connecting one or more sides of the core 12 to one or more
corresponding sides of the sheath 14. As an alternative, mechanical means can
be used for providing a mechanical connection between the core 12 and the
sheath 14. For instance, such mechanical means can comprise small
protrusions in the steel sheath penetrating the composite core.
The manufacturing embodiment of Figure 4f is similar to the one shown in
Figure 4e. The difference is that in the embodiment of Figure 4e the side of the
housing 143 is flush with the core 12, while in the embodiment of Figure 4d, the
side of the housing 143 is projecting over the core 12. The length of the lid 144
accommodates the two embodiments, so that, the lid of Figure 4c is longer than
the lid of Figure 4d.
As for the elongated tensile element of Figure 4c, a suitable manufacturing
process analogous to that for the element of Figure 4a can be performed. For
example, a sheath 14 as from Figures 4d lacking of the half side segments 141
can be used.
With respect to known tensile elements made of metal only, the elongated
tensile element according to the present invention has reduced dimensions
and/or reduced weight and/or increased strength while retaining the advantages
of manufacturing friendliness and ease of end-fitting of a metal element.
EXAMPLE 1
An elongated tensile element (reference element) made of steel only has
been taken as comparative element. This tensile element has a substantially
rectangular cross section with dimensions of 2 mm x 7 mm, an elastic modulus
of 191 GPa and a density of 7.8 g/cm .
When a length of 1,430 m of this all steel elongated tensile element was
suspended vertically, its own weight (156 kg) caused an elongation of the steel
tensile element itself of 0.05 %.
A first elongated tensile element according to embodiments of the present
invention has been tested. This tensile element had a substantially rectangular
cross section and dimensions of 2 mm x 7 mm, a core made of epoxy resin
reinforced with carbon fibres having an elastic modulus of 150 GPa and a
density of 1.6 g/cm , and steel sheath having an elastic modulus of 191 GPa
and a density of 7.8 g/cm . The core had a substantially rectangular cross
section with dimensions of 0.75 mm x 6 mm, and it was totally covered by the
steel sleeve representing about 68% of the elongated tensile element cross-
section area.
When a length of 1,800 m of the first elongated tensile element according to
the invention was suspended vertically, its own weight (146 kg) caused an
elongation of the tensile element itself of 0.05 %.
A second elongated tensile element according to embodiments of the
present invention has been tested. This tensile element had a substantially
rectangular cross section and dimensions of 2 mm x 7 mm, a core made of a
thermoplastic matrix reinforced with aramid fibres having an elastic modulus of
100 GPa and a density of 1.45 g/cm , and steel sheath having an elastic
modulus of 191 GPa and a density of 7.8 g/cm . The core had a substantially
rectangular cross section with dimensions of 1 mm x 6 mm, and it was totally
covered by the steel sheath representing about 57% of the elongated tensile
element cross-section area.
When a length of 1,830 m of the second elongated tensile element according
to the invention was suspended vertically, its own weight (130 kg) caused an
elongation of the tensile element itself of 0.05 %.
A third elongated tensile element according to embodiments of the present
invention has been tested. This tensile element had a substantially rectangular
cross section and dimensions of 2 mm x 7 mm, a core made of a thermoset
matrix reinforced with glass fibres having an elastic modulus of 70 GPa and a
density of 2.4 g/cm , and steel sheath having an elastic modulus of 191 GPa
and a density of 7.8 g/cm . The core had a substantially rectangular cross
section with dimensions of 1 mm x 6 mm, and it was totally covered by the steel
sheath representing about 57% of the elongated tensile element cross-section
area.
When a length of 1570 m of the second elongated tensile element according
to the invention was suspended vertically, its own weight (121 kg) caused an
elongation of the tensile element itself of 0.05 %.
The above values are summarised in the below Table 1.
Table 1
Sample length
Elongated Elastic Weight of length
Density providing
tensile Modulus providing 0.05%
[g/cm ] 0.05%
element [GPa] elongation [kg]
elongation [m]
Reference 191 7.80 1430 156
1 element 150 1.6 1800 146
2 element 100 1.45 1830 130
3 element 70 2.4 1570 121
Thanks to the fibre reinforced polymeric composite, the elongated elements
of the invention reach a given percentage of elongation with a greater length
than a comparative elongated element made of metal (steel) only. Accordingly,
a cable comprising elongated elements of the invention can be deployed at
greater depth without experiencing elongation percentage possibly harming the
overall cable structure.
Also, a length of the elongated elements of the invention is lighter than the
same or even shorter length of a comparative elongated element made of metal
(steel). This has an advantageous impact, for example, when the cable is
deployed by suspension from a vessel.
EXAMPLE 2
Elongated tensile elements comprising a fibre reinforced polymeric
composite core, a metal sheath totally covering the composite core, a
substantially rectangular cross section and dimensions of 2 mm x 7 mm were
bent for obtaining a deformation with a final bending diameter of 60 mm at most.
The strain applied to each element took into account the elongation at break.
Such strain was significantly lower than the fibre elongation at break to preserve
the integrity of the elongated element core.
Table 2
No. Composite Core size Fibre elastic Steel Applied strain Final
fibres (mm) modulus % (bending bending
(GPa) radius) radius
1 Carbon 1.2% 27.1 mm
0.5x6 150 78.5
(20.8 mm)
2 Carbon 1.7% 29.6 mm
0.75x6 150 68
(22.1 mm)
3 Carbon 1.7% 45.1 mm
1x6 150 57
(29.4 mm)
4 Aramid 1.2% 27 mm
0.5x6 100 78.5
(20.8 mm)
Aramid 2.3% 29.4 mm
1x6 100 57
(21.7 mm)
6 Glass 2.3% 28.9 mm
1x6 70 57
(21.7 mm)
7 Glass 2.8% 42.7 mm
1.5x6 70 36
(26.8 mm)
Among the elongated tensile elements tested, No. 3 and No. 7 could not be
bent with a final bending diameter of 60 mm even by applying a strain near to
their elongation at break (1.8% for the carbon fibres of No. 3 and 3% for the
glass fibres of No. 7). This means that the elongated tensile elements No. 3 and
No. 7 can find an application when bending diameter greater than 60 mm is
required at the end-fitting. On the other side, the elongated tensile elements No.
3 and No. 7, when evaluated in the test of Example 1, reached a 0.05%
elongation at, respectively, 2,010 m and 1,680 m, lengths greater than, for
example, those of elongated tensile elements with a core made of the same
fibre reinforced polymeric composite, but comprising a greater amount of steel,
for example the elongated tensile elements No. 1 (0.05% elongation at 1,660
m) and No. 6 (0.05% elongation at 1,570 m), respectively.
The selection of the amount of metal (expressed as % of elongated tensile
elements cross-section area) within the limit set forth by the invention and the
elastic modulus of the fibre reinforced polymeric composite can be selected by
the skilled person in view of deployment requirements, such as deposition
depths and kind of end-fittings.
The above described tensile elements provide lower weight and better
mechanical characteristics than the known tensile members fully made of steel.
As a consequence, a submarine cable having an armour comprising tensile
elements according to the present invention can be installed in deeper water
than known cables with armour comprising elongated tensile members fully
made of steel, resulting in the same elongation. In addition, the lower weight of
the cable improves handling and transport thereof.
Remarkably, the metal sheath of the above described embodiments allows
maintaining use of known techniques and devices for connecting cables
together and/or for realizing splices or cable terminations. Such techniques and
devices substantially comprise subjecting the armour to a plastic deformation.
While a plastic deformation could not be obtained if a fully composite tensile
element were adopted, advantageously it can be accomplished with the present
invention having a steel sheath at least partially covering the core. This
behaviour is advantageous in that the metallic sheath of the tensile element can
be plastically deformed in end fittings in order to obtain a strong bond between
the end fitting and the cable.
Claims (13)
1. A cable comprising an elongated tensile element having a cross section area and comprising a fibre reinforced polymer composite core having an elastic modulus of at least 70 GPa and a sheath at least partially covering 5 the composite core, the sheath being made of metal and being at least 30% of the cross section area of the tensile element, wherein said elongated tensile element has a flat cross section.
2. The cable of claim 1, comprising a plurality of elongated tensile elements 10 disposed in one or more armour layers to provide an armour.
3. The cable of either claim 1 or 2, wherein the fibre reinforced polymeric composite core has a thickness of at most 80% the thickness of the elongated tensile element.
4. The cable of any one of claims 1 to 3, wherein the fibre reinforced polymeric composite core is provided substantially centred on a neutral bending axis of the elongated tensile element. 20
5. The cable of any one of claims 1 to 4, wherein the fibre reinforced polymer composite comprises carbon fibres, aramid fibres, glass fibres or a combination thereof.
6. The cable of any one of claims 1 to 5, wherein the fibre reinforced polymer 25 composite comprises a matrix selected from an epoxy matrix or a thermoplastic matrix.
7. The cable of any one of claims 1 to 6, wherein the fibre reinforced polymer composite core has a longitudinal axes and fibres embedded in a 30 polymeric matrix with longitudinal axes substantially parallel to the longitudinal axis of the fibre reinforced polymer composite core.
8. The cable of claim 5, wherein the elongated tensile element comprises a carbon fibre reinforced polymer composite core surrounded by an electrically insulating layer. 5
9. The cable of any one of claims 1 to 8, where the fibre reinforced polymer composite core has an elastic modulus of at least 100 GPa.
10. The cable of any one of claims 1 to 9, wherein the metal sheath is made of steel.
11. The cable of any one of claims 1 to 10, wherein the sheath and the core are bounded together by an adhesive and/or by a mechanical bond.
12. The cable of any one of claims 1 to 11, wherein the sheath is made of 15 metal and is at most 85% of the cross section area of the tensile element.
13. The cable of any one of claims 1 to 12 which is for submarine applications.
Applications Claiming Priority (1)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
PCT/EP2017/052711 WO2018145736A1 (en) | 2017-02-08 | 2017-02-08 | Cable or flexible pipe with improved tensile elements |
Publications (2)
Publication Number | Publication Date |
---|---|
NZ755622A NZ755622A (en) | 2021-10-29 |
NZ755622B2 true NZ755622B2 (en) | 2022-02-01 |
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