Multisensing Optical Fiber Cable

Abstract

Disclosed is an optical cable for distributed sensing. The optical cable comprises a first metal tube with at least two optical fibers loosely arranged therein and a second metal tube with at least two tight buffered optical fibers tightly arranged within an inner surface of the second metal tube. A third metal tube having an inner surface collectively surrounds and operatively contacts the first metal tube and said second metal tube. At least one of the first metal tube and the second metal tube is fixed by means of an adhesive compound to the inner surface of the third metal tube.

Claims

1. An optical cable (1) for distributed sensing comprising: a first metal tube (2) with at least two optical fibers (3) loosely arranged therein; a second metal tube (4) with at least two tight buffered optical fibers (5) tightly arranged within an inner surface of the second metal tube (4); and a third metal tube (6) having an inner surface and collectively surrounding and operatively contacting said first metal tube (2) and said second metal tube (4), wherein at least one of the first metal tube (2) and the second metal tube (4) is fixed by means of an adhesive compound (6a) to the inner surface of the third metal tube (6).

2. The optical cable (1) of claim 1, wherein the first metal tube (2) and second metal tube (4) are made of stainless steel.

3. The optical cable (1) according to claim 1, wherein the third metal tube (6) is made of stainless steel.

4. The optical cable (1) according to according to claim 1, wherein said adhesive compound (6a) fills interstices among the first and second metal tubes (2, 4) and the inner surface of the third tube (3).

5. The optical cable (1) according to claim 1, wherein said adhesive compound (6a) is selected from a holt melt adhesive, an epoxy adhesive, a silicone adhesive and mixtures thereof.

6. The optical cable (1) according to claim 5, wherein the adhesive compound (6a), once hardened, has a tensile strength ranging from 1 to 10 N/mm.sup.2 as measured according to ISO 37 (2017-11) for silicone or according to ISO 527-2 (1996) for hot melt and epoxy adhesive.

7. The optical cable (1) according to claim 1, wherein an inner adhesive compound (5a) fills voids and interstices between the tight buffered optical fibers (5) and the inner surface of the second tube (4).

8. The optical cable (1) according to claim 1, further comprising an armor (7) in radially outer position with respect to the third metal tube (3).

9. The optical cable (1) according to claim 1, further comprising an outer sheath (8).

10. The optical cable (1) according to claim 8, further comprising a bedding layer (9) interposed between the outer tube (6) and in direct contact thereto, and the armor (7).

11. Optical cable (1) according to claim 1, wherein the at least two optical fibers (3) loosely arranged in the first metal tube (2) are capable of distributed temperature sensing (DTS) and of transmitting data.

12. Optical cable (1) according to claim 1, wherein the at least two tight buffered optical fibers (5) are capable of distributed strain sensing (DSS) and distributed acoustic sensing (DAS).

13. A process for manufacturing an optical cable (1) for distributed sensing, said process comprising the step of: i. providing a first metal tube (2) with at least two optical fibers (3) loosely arranged therein; ii. providing a second metal tube (4) with at least two tight buffered optical fibers (5) tightly arranged therein; iii. applying an adhesive compound (6a) on at least one of said first metal tube (2) and said second metal tube (4); iv. joining the first metal tube (2) and second metal tube (4) to a metal foil; v. conforming the metal foil around the first metal tube (2) and second metal tube (4); and vi. welding the metal foil to provide a third metal tube (6) collectively surrounding and operatively contacting said first metal tube (2) and said second metal tube (3).

14. Process according to claim 13, wherein at step iii the adhesive compound (6a) is applied on one of the first metal tube (2) or second metal tube (4), only.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0049] The present disclosure will be further clarified by the following detailed description, given by way of example and not of limitation, with reference to the attached drawing wherein:

[0050] FIG. 1 schematically shows an optical cable according to an embodiment of the present disclosure.

DETAILED DESCRIPTION

[0051] FIG. 1 shows an optical cable 1 according to an embodiment of the present disclosure. The optical cable 1 can be suitable for underwater applications or submarine applications.

[0052] The optical cable 1 comprises a first tube 2 wherein at least two optical fibers 3 are loosely arranged. The first tube 2 is also termed herein after as “loose tube”.

[0053] The loose tube 2 is made of a metal, for example stainless steel or copper. In an embodiment, the loose tube 2 is a hermetically sealed tube, for example a welded tube. The loose tube 2 can have an outer diameter comprised between 0.9 mm and 3.5 mm. The loose tube 2 can have a thickness comprised between 0.1 mm and 0.3 mm.

[0054] Optionally, the loose tube 2 may also contain a lubricating gel for reducing frictions amongst the optical fibers 3.

[0055] The optical fibers 3 can be suitable for distributed temperature sensing and/or optical communications. For example, the optical fibers 3 may be multimode optical fibers compliant with ITU-T Recommendation G.651.1 (07/2007) or single mode optical fibers compliant with ITU-T Recommendation G.652 (11/2009) or G.657 (11/2009) with an outer diameter of 260 microns. The fiber count of optical fibers 3 within the loose tube 2 may range, for example, from 2 to 12. By way of non limiting example, the fiber count of the optical fibers 3 in the cable 1 depicted in FIG. 1 is 4.

[0056] The optical cable 1 also comprises a second tube 4 wherein at least two tight-buffered optical fibers 5 are tightly arranged. The second tube 4 is also termed herein after as “tight buffer tube”.

[0057] The tight buffer tube 4 is made of a metal, for example stainless steel or copper. In an embodiment, the tight buffer tube 4 is a hermetically sealed tube, for example a welded tube. In an embodiment, the tight buffer tube 4 has a diameter substantially equal to the diameter of the loose tube 2. The tight buffer tube 4 can have an outer diameter comprised between 0.9 mm and 3.5 mm. The loose tube 2 can have a thickness comprised between 0.1 mm and 0.3 mm.

[0058] The tight buffered optical fibers 5 can be suitable for strain sensing and/or acoustic sensing. For example, the optical fibers 5 may be multimode optical fibers compliant with ITU-T Recommendation G. 651.1 (07/2007) or single mode optical fibers compliant with ITU-T Recommendation G.652 or G.657 (11/2009). For example, the fiber count of optical fibers 5 within the tight buffer tube 4 may be equal to 3.

[0059] Each optical fiber of the present disclosure comprises a core, a cladding and at least one polymeric coating, for example an acrylate coating. In the case of the tight buffered optical fibers 5, each optical fiber further comprises a buffer layer 5′ of polymer material (for example, polyamide or a polyethylene copolymer, optionally charged with a zero halogen flame retardant filler) in direct contact with the surface of the polymeric coating of the fiber. The outer diameter of the tight buffered optical fiber 5 (comprehensive of its buffer layer 5′) can be comprised between 500 microns and 1000 microns.

[0060] In an embodiment, an inner adhesive compound 5a fills the voids between the tight buffered fibers 5 and the inner surface of the tight buffer tube 4. This could improve the mechanical congruency of the tight buffered fibers 5 to the tight buffer tube 4 and, then, to the whole cable structure. The inner adhesive compound 5a may comprise for example a holt melt adhesive (e.g. a polyethylene polymer, for example a low density one), an epoxy adhesive or a silicone adhesive.

[0061] The optical cable 1 also comprises a third tube 6 collectively surrounding the loose tube 2 and the tight buffer tube 4, and operatively contacting both of them. The third tube 6 is also termed herein after as “outer tube”.

[0062] The outer tube 6 is also made of metal, for example stainless steel. Examples of steel suitable for the present cable is a SAE 304, 316, 316L grade. Alternatively, copper may be used. In an embodiment, the outer tube 6 is a hermetically sealed tube, for example a welded tube.

[0063] The thickness of the outer tube 6 can be comprised between 0.1 mm and 0.3 mm.

[0064] The inner diameter of the outer tube 6 can be substantially equal to the sum of the outer diameters of the loose tube 2 and the tight buffer tube 4. This way, the inner surface of the outer tube 6 contacts both the loose tube 2 and the tight buffer tube 4, so that the tubes 2, 4 are tightly accommodated within the outer tube 6. Hence, taking into account the above disclosed thickness range (0.2 mm-0.4 mm), the outer diameter of the outer tube 6 can be comprised between 3.6 mm and 4.0 mm.

[0065] An adhesive compound 6a fixes at least one of the loose tube 2 and the tight buffer tube 4 to the inner surface of the outer tube 6. In an embodiment, the adhesive compound 6a substantially fills the interstices among the tubes 2 and/or 4 and the inner surface of the outer tube 6. This could further improve the mechanical congruency of the tight buffered fibers 5 to the whole cable structure. The adhesive compound 6a may be the same or different from the inner adhesive compound and may comprise for example a holt melt adhesive, an epoxy adhesive or a silicone adhesive.

[0066] In an embodiment, the adhesive compound is a hot melt adhesive, e.g. a polyethylene polymer, for example a low density one. The polyethylene adhesive compound, once hardened, can have a tensile strength ranging from 3 to 10 N/mm.sup.2 as measured according to ISO 527-2 (1996).

[0067] In another embodiment, the adhesive compound is a silicone adhesive. The silicone adhesive, once hardened, can have a tensile strength ranging from 1 to 4 N/mm.sup.2 as measured according to ISO 37 (2017-11). In an embodiment, the silicone adhesive is a room temperature vulcanizing adhesive.

[0068] In an embodiment, the loose tube 2 with the loose optical fibers 3 arranged therein and the tight buffer tube 4 with the tight buffered optical fibers 5 arranged therein are laid within the outer tube 6 substantially parallel to the longitudinal axis A of the optical cable 1. In another embodiment, the loose tube 2 with the loose optical fibers 3 arranged therein and the tight buffer tube 4 with the tight buffered optical fibers 5 arranged therein are laid within the outer tube 6 stranded one with the other.

[0069] In the embodiment of FIG. 1, an armor 7 is provided around the outer tube 6. The armor 7 comprises a plurality of wires 7′ helically wound around the outer surface of the outer tube 6. The wires 7′ in particular may be wound in close helix on the outer surface of the outer tube 6.

[0070] The wires 7′ of the armor 7 can be of metal, for example steel wires optionally galvanized (GS), stainless and/or aluminum cladded. In a further embodiment, the wires 7′ can be made of aramid fibers, GRP (glass reinforced plastic) or glass yarns. An armor 7 made of metallic and non-metallic wires 7′ can be also envisaged.

[0071] Each armor wire may have a diameter comprised between 0.5 mm and 3.6 mm. The wires may have a low tensile strength 45 kg/mm.sup.2), a medium tensile strength 100 kg/mm.sup.2) or a high tensile strength 200 kg/mm.sup.2).

[0072] In the optical cable 1 according to the first embodiment depicted in FIG. 1, the wires 7′ of the armor 7 are arranged in a single circumferential layer substantially concentric with the outer tube 6. Alternatively, the armor 7 may comprise two or more circumferential layers of wires 7′ substantially concentric with the outer tube 6.

[0073] In an embodiment, a bituminous compound, a jelly or adhesive fills the interstices between outer tube 6 and armor 7 and between adjacent wires 7′ of the armor. This provides protection to the wires 7′ and also avoids corrosion during the lifetime of the optical cable.

[0074] In the embodiment of FIG. 1, the cable 1 also comprises a bedding layer 9 interposed between the outer tube 6 and the armor 7. The bedding layer can be made of polymeric material such as, e.g., polyethylene (PE), optionally low smoke zero halogen, or polyamide (PA). Its thickness can be comprised between 0.1 mm and 1 mm. The bedding layer acts as an additional protection of the outer tube 6. Also, by selecting a suitable thickness of the bedding layer, the armor size can be tailored to any specific application.

[0075] In the embodiment of FIG. 1, the optical cable 1 comprises a jacket 8 as outer sheath. The jacket 8 can be made of a polymeric material, for example high density polyethylene (HDPE), PA, low density polyethylene (LDPE) or linear low density polyethylene (LLDPE). The thickness of the jacket 8 can be comprised between 1.0 mm and 3.0 mm.

[0076] The outer diameter of the jacket 8 (and then of the whole cable 1) can range from 6 mm to 12 mm, depending on the fiber count and on the presence of armor and bedding, as shown in the following.

[0077] In an embodiment, not illustrated, the cable according to the present disclosure does not comprise any armor and any jacket, the outer tube 6 being the outermost layer of the cable.

[0078] Using a single metal tube (second metal tube 4) to collectively enclose all the tight buffered fibers 5 allows protecting them against water seepage while keeping the cable size smaller than e.g. that of a cable wherein multiple steel tubes are used for individually protecting each tight-buffered optical fiber. Further, the outer tube 6 collectively surrounding both the loose tube 2 and tight buffer tube 4 also can allow effectively protecting all the optical fibers 3, 5 against the high pressures (up to 300 bar) typical of underwater and submarine environments.

[0079] Further, the tight accommodation of the tight-buffered optical fibers 5 within the tight buffer tube 4, in combination with the adhesive compound 6a fixing the tight buffer tube 4 and/or the loose tube 2 to the inner surface of the outer tube 6, guarantees that the tight buffered fibers 5 are mechanically congruent with the cable structure. This provides a cable with a high sensing accuracy, in particular as far as DAS and DSS sensing is concerned.

[0080] The process for manufacturing a distributed sensing cable according to embodiments of the present invention can comprise the following steps: [0081] i. providing a longitudinally folded and welded tube about a bundle of optical fibers, thereby providing the loose tube with the optical fibers loosely arranged within it; [0082] ii. providing a longitudinally folded and welded tube about a bundle of tight buffered optical fibers (e.g. 3 tight buffered optical fibers), optionally providing an inner adhesive to fill voids and interstices between the tight buffered optical fibers and the inner surface of the tight buffer tube, and then drawing the tube down onto the fibers, thereby providing the tight buffer tube with the tight buffered fibers tightly arranged within it; [0083] iii. providing an adhesive compound on the loose tube and/or the tight buffer tube; [0084] iv. joining the tight buffer tube and loose tube to a metal foil; [0085] v. conforming the metal foil around the tight buffer tube and loose tube by folding and drawing the metal foil down onto the loose tube and the tight buffer tube; [0086] vi. welding the metal foil thereby providing an outer tube with the loose tube and the tight buffer tube tightly arranged within it; [0087] vii. optionally, extruding a bedding layer onto the outer tube; [0088] viii. optionally, winding wires around the outer tube (or on the bedding layer, if present) to provide an armor; and [0089] ix. optionally, providing a jacket as outermost layer.

[0090] In an embodiment, the step iii of providing the adhesive compound is carried out by extruding the adhesive compound. In this case, the adhesive compound can be a hot melt adhesive.

[0091] In an embodiment, the step iii of providing the adhesive compound is carried out by spreading the adhesive compound around at least one of said first metal tube and said second metal tube, for example around only one of said first metal tube and said second metal tube. In this case, the adhesive compound can be a silicone adhesive.

[0092] In an embodiment, the adhesive compound of step iii is a silicone adhesive which at the application, before hardening, has a viscosity ranging from 10,000 to 30,000 Pa.Math.s at 25° C.

[0093] It shall be noticed that, at step iii, the adhesive compound is not necessarily provided on both the tubes. The Applicant has indeed noticed that it is sufficient to provide an adhesive compound on one of these tubes only. Then, during the next step v, the adhesive compound provided in step iii flows and substantially fills all the voids and interstices between the first and second metal tubes and the inner surface of the outer third tube, thereby providing a firm reciprocal fixing of the tubes and also their fixing to the inner surface of the outer tube 6. The tight buffer tube 4 is thus fixed to the inner surface of the outer tube 6 in a particularly stable way. The tight buffered fibers 5 are then mechanically congruent with the cable structure, as required by DAS and DSS sensing applications.

[0094] Herein below, some exemplary cables according to the present disclosure obtained by the Applicant are described. [0095] A) a first exemplary cable comprised 4 loose fibers and 3 tight buffered fibers, an armor made of 13 wires GS×1.00 mm wires with tensile strength of 170 Kg/mm.sup.2 and an HDPE outer sheath with a thickness of 1.5 mm. The outer diameter of the cable was 8 mm, its weight was 0.13 Kg/m, its maximum operational tension was 5 kN, and its ultimate tensile strength was 15 kN. [0096] B) a second exemplary cable comprised 8 loose fibers and 3 tight buffered fibers, an armor made of 22 wires GS×1.00 mm with tensile strength of 170 Kg/mm.sup.2 and an HDPE outer sheath with a thickness of 1.5 mm. The outer diameter of the cable was 6.8 mm, its weight was 0.10 Kg/m, its maximum operational tension was 3 kN, and its ultimate tensile strength was 10 kN.