RU2849348C2 - Multisensor fiber optic cable - Google Patents

Abstract

FIELD: measuring supplies.

SUBSTANCE: an optical cable for distributed measurement comprises: a first metal tube (2) with at least two optical fibres (3) loosely arranged therein; a second metal tube (4) with at least two optical fibres (5) with a tight buffer, tightly arranged in the inner surface of the second metal tube (4); and a third metal tube (6) having an inner surface surrounding and in functional contact with both the first metal tube (2) and the second metal tube (4). The third metal tube is a metal foil formed to fit snugly and welded around the first metal tube (2) and the second metal tube (4). At least one of the first metal pipe (2) and the second metal pipe (4) is attached by means of an adhesive composition (6a) to the inner surface of the third metal pipe (6).

EFFECT: increased cable protection, reduced weight and dimensions.

14 cl, 1 dwg

Description

AREA OF TECHNOLOGY

The present disclosure relates to the field of multisensor fiber optic cables for distributed sensing. In particular, the present disclosure relates to a multisensor fiber optic cable suitable for underground, underwater, and/or submarine applications.

LEVEL OF TECHNOLOGY

Distributed sensing, such as distributed temperature sensing (DTS), distributed stress sensing (DSS), and distributed acoustic sensing (DAS), is a technology that enables continuous, real-time measurements along the entire length of a fiber optic cable by exploiting physical phenomena occurring in optical fibers (e.g., Raman scattering, Pleistocene scattering, or Brillouin scattering) to provide distributed measurements of temperature, vibration (for acoustic sensing), or mechanical stress. A distributed sensing system typically includes a cable containing one or more optical fibers suitable for distributed sensing and one or more devices for reading, recording, and/or processing the aforementioned physical phenomena.

Distributed sensing is increasingly in demand in various applications, such as monitoring oil and gas pipelines or water pipes, or civil engineering structures (monitoring bridges, tunnels or ground movements).

Cables containing one or more optical fibers suitable for distributed sensing may also contain optical fibers for optical communications.

As is known, an optical cable typically comprises an optical core, which includes one or more optical fibers, and one or more layers surrounding and protecting the optical core.

Optical fibers can be arranged in a cable's optical core in various ways. Specifically, one or more optical fibers (possibly grouped into different bundles) can be loosely arranged within a semi-rigid tube with loose fiber placement. Alternatively, each optical fiber can be tightly buffered in a corresponding buffer tube. While both loose and tightly buffered arrangements may be suitable for communications applications, depending on the environment for which the optical fiber is intended (outdoor, indoor, underwater, etc.), distributed sensing requires specific fiber placement. Specifically, while DTS sensing requires loosely arranged optical fibers, tightly buffered fibers may be preferable for DSS and DAS sensing, as this allows for mechanical alignment with the cable structure, thereby ensuring higher measurement accuracy.

Optical fibers for distributed sensing are known.

CN 201765352 discloses an optical cable for measuring seawater temperature along the depth axis. The cable contains two to four temperature-sensitive fibers, each covered by a stainless steel-coated sleeve; two pressure-sensitive fibers coated with a polyurethane layer; and an outer spring tube and stainless steel mesh. The measuring fibers are intertwined and then covered by a double stainless steel hose. The cable has an outer diameter of 10-12 mm.

CN 207424326 discloses a composite optical cable for connecting to a sensor, comprising a core containing communication optical fibers at the center and a plurality of tightly buffered measurement optical fibers uniformly intertwined at the periphery of the communication optical fibers. The tube containing the communication optical fibers is made of stainless steel. The tightly buffered optical fibers are coated with a reinforcing layer, preferably made of aramid. The protective coating comprises a tape coated with a glass layer, covered with a plastic-coated steel tape, and covered with an outer protective coating.

ESSENCE OF THE INVENTION

The applicant considers it necessary to provide an optical cable, in particular an optical cable for underground, underwater and/or submarine applications, which can combine all possible types of distributed sensing (DTS, DSS and DAS) and optical communication capabilities with a reduced cable size and reduced cable weight.

Water infiltration protection may be required, which is typically achieved by providing hermetically sealed metal tubes around the optical fibers. In the case of optical cables for subsea and submarine applications, metal reinforcement made of one or more layers of steel wires and an outer sheath made of a rigid polymer material (e.g., high-density polyethylene, HDPE) may be provided to withstand the high pressures (up to 300 bar) typical of subsea and submarine environments. This design results in cables with a large outer diameter and high mass, which can be critical, especially in subsea and submarine applications.

In addition, providing different types of distributed metering (DTS, DSS, and DAS) and optical communication capabilities in the same cable requires providing optical fibers with different arrangements and performance requirements in the same cable, which may increase the cable size.

The known distributed sensing cables described above exhibit some disadvantages in this regard.

In particular, the aforementioned cable according to CN 201765352 exhibits a large outer diameter (10-12 mm) compared to the limited number of fibers it contains (4+2). This is primarily due to the fact that each individual tight-buffered optical fiber is tightly buffered with a corresponding steel layer. The outer diameter of this layer—to ensure that the tight-buffered optical fibers are mechanically matched to the cable structure, as required for DSS and DAS measurements—matches the outer diameter of the steel sleeve housing the loose fibers. The diameter of the steel sleeve is selected to accommodate all the loose fibers, but this clearly results in excessive buffer thickness for the tight-buffered fibers. Furthermore, if there is a desire to increase the number of fibres by providing, for example, additional free fibres for DTS or optical communications, the diameter of all steel layers will increase accordingly, thus leading to a significant further increase in the outer diameter of the cable.

The applicant then faced the problem of providing an optical cable for the distributed metering system that overcomes the above mentioned shortcomings.

In particular, the applicant has solved the problem of providing an optical cable for a distributed sensing system, particularly suitable for underground, subsea and/or submarine applications, in which all optical fibres are adequately protected against water infiltration and, where necessary, against the high pressures (up to 300 bar) typical of the subsea and submarine environments, in which tightly buffered fibres are mechanically matched to the cable structure (as required for DAS and/or DSS sensing), and in which the size and weight of the cable are reduced.

According to embodiments of the present disclosure, the above-mentioned problem is solved by an optical cable for distributed sensing, which comprises a first metal tube loosely accommodating at least two optical fibers, and a second metal tube tightly accommodating at least two optical fibers with a tight buffer. The cable also comprises a third metal tube, which surrounds the first and second metal tubes and operatively contacts them. At least one of the metal tubes accommodating the optical fibers is attached to the inner surface of the third metal tube using an adhesive.

Using a single metal tube to contain all the tightly buffered fibers protects them from water infiltration while maintaining a smaller cable size than, for example, a cable in which multiple steel tubes are used to specifically protect each tightly buffered optical fiber. Furthermore, a third metal tube, enclosing both the first and second metal tubes, also effectively protects all the cable's optical fibers from the high pressures (up to 300 bar) typical of underwater and submarine environments. This is accomplished, for example, by providing reinforcement and a thick protective coating in the form of an outer sheath made of a hard polymer material (e.g., HDPE) outside the third metal tube.

Furthermore, the tight placement of tightly buffered optical fibers within the second metal tube, combined with an adhesive bonding at least one of the metal tubes housing the optical fibers to the inner surface of the third metal tube, ensures that the tightly buffered fibers are mechanically aligned with the cable structure. This ensures a cable with high measurement accuracy, particularly as it relates to DAS and DSS measurements.

Therefore, according to a first aspect, the present disclosure provides an optical cable for distributed measurement, comprising:

- a first metal tube with at least two optical fibers freely located therein;

- a second metal tube with at least two tightly buffered optical fibers tightly located within the inner surface of the second metal tube; and

- a third metal pipe having an internal surface that surrounds the first metal pipe and the second metal pipe and is in operative contact with them,

in which at least one of the first and second metal pipes is attached to the inner surface of the third metal pipe by means of an adhesive.

In an embodiment, the first metal pipe and the second metal pipe are made of stainless steel.

In an embodiment, the third metal pipe is made of stainless steel.

In an embodiment, the adhesive fills the gaps between the first and second metal pipes and the inner surface of the third pipe.

In an embodiment, the adhesive may be selected from hot melt adhesive, epoxy adhesive, silicone adhesive, and mixtures thereof.

In an embodiment, the adhesive (glue composition), immediately after hardening, has a tensile strength in the range of 1-10 N/mm ² , as measured according to ISO 37 (2017-11) for silicone (organosilicon) or according to ISO 527-2 (1996) for hot melt and epoxy adhesive.

In an embodiment, the internal adhesive composition substantially fills the voids and spaces between the tightly buffered optical fibers and the inner surface of the second tube.

In an embodiment, the first metal tube and the second metal tube may be placed in a third tube, nearly parallel to the longitudinal axis of the optical fiber, or twisted together.

In one embodiment, the optical cable comprises reinforcement in a radially external position relative to the third metal tube. The reinforcement may be made of at least one layer of wires. Alternatively, the reinforcement may be made of corrugated, optionally interlocking tape, helically wound around the third metal tube.

When the optical cable of the present disclosure comprises a reinforcement in a radially outer position relative to the third metal pipe, the optical cable may further comprise a bedding layer inserted between and in direct contact with the third metal pipe and the reinforcement.

In one embodiment, the optical cable comprises a protective coating positioned radially outward from the third metal tube, and, if present, with reinforcement. The protective coating, if present, constitutes the outermost layer of the cable as it opens.

In an embodiment, optical fibers freely located in the first metal tube are suitable for distributed temperature sensing (DTS) and for data transmission.

In an embodiment, at least two tight buffered optical fibers are suitable for distributed stress sensing (DSS) and distributed acoustic sensing (DAS).

According to a second aspect, the present disclosure relates to a method for producing an optical cable for distributed sensing, said method comprising the steps of:

i. providing a first metal tube with at least two optical fibers freely positioned therein;

ii. providing a second metal tube with at least two tightly buffered optical fibers tightly arranged therein;

iii. applying an adhesive composition to at least one of said first metal pipe and said second metal pipe;

iv. connecting the first metal pipe and the second metal pipe with a metal foil;

v. winding a metal foil around the first metal pipe and the second metal pipe; and

vi. welding a metal foil to provide a third metal pipe surrounding the first metal pipe and the second metal pipe together and in operative contact with them.

In an embodiment, at step iii, the adhesive composition is applied only to one of the first metal pipe and the second metal pipe.

For the purposes of this description and the appended claims, except where otherwise indicated, all values expressing amounts, quantities, percentages, and the like are to be understood as varying in all instances according to the term "about." Furthermore, all ranges include any combination of the disclosed maximum and minimum points and include any intermediate ranges therein, whether specifically designated or not designated herein.

The present disclosure, in at least one of the above aspects, may be implemented according to one or more of the following embodiments, optionally combined with each other.

For the purposes of this description and the appended claims, words denoting the singular shall be construed to include one or at least one, and a singular object also includes plural objects, unless it becomes apparent otherwise. This is provided for convenience only and to ensure the general meaning of the disclosure.

BRIEF DESCRIPTION OF DRAWINGS

The present disclosure should be further clarified by the following detailed description, given by way of example and not limitation, with reference to the accompanying drawing, in which:

Figure 1 schematically shows an optical cable according to an embodiment of the present disclosure.

DETAILED DESCRIPTION

Figure 1 shows an optical cable 1 according to an embodiment of the present invention. The optical cable 1 may be suitable for underwater applications or applications on a submarine.

The optical cable 1 comprises a first tube 2 in which at least two optical fibers 3 are freely arranged. The first tube 2 is also further referred to in this work as a “tube with free fiber placement.”

Loose-fiber tube 2 is made of metal, such as stainless steel or copper. In one embodiment, loose-fiber tube 2 is a hermetically sealed tube, such as a welded tube. Loose-fiber tube 2 may have an outer diameter of 0.9-3.5 mm. Loose-fiber tube 2 may have a thickness of 0.1-0.3 mm.

The loose fiber tube 2 may also contain (optionally) a lubricating gel to reduce friction between the optical fibers 3.

The optical fibers 3 may be suitable for distributed temperature sensing and/or optical communications. For example, the optical fibers 3 may be multimode optical fibers complying with ITU-T Recommendation G.651.1 (07/2007) or single-mode optical fibers complying with ITU-T Recommendation G.652 (11/2009) or G.657 (11/2009), with an outer diameter of 260 microns. The number of optical fibers 3 in the loose-fiber pipe 2 may be in the range of, for example, 2-12. As a non-limiting example, the number of optical fibers 3 in the optical cable 1 shown in Figure 1 is 4.

Optical cable 1 also comprises a second tube 4, in which at least two tight-buffered optical fibers 5 are tightly arranged. This second tube 4 is also referred to in this paper as a "tight-buffered fiber-laying tube."

The fiber-laid tube 4 with a tight buffer is made of metal, such as stainless steel or copper. In one embodiment, the fiber-laid tube 4 with a tight buffer is a hermetically sealed tube, such as a welded tube. In one embodiment, the fiber-laid tube 4 with a tight buffer has a diameter nearly equal to the diameter of the fiber-laid tube 2 with loose fiber placement. The fiber-laid tube 4 with a tight buffer may have an outer diameter of 0.9-3.5 mm. The fiber-laid tube 2 with loose fiber placement may have a thickness of 0.1-0.3 mm.

Optical fibers 5 with a tight buffer may be suitable for measuring mechanical stress and/or acoustic measurements. For example, optical fibers 5 may be multimode optical fibers complying with ITU-T Recommendation G.651.1 (07/2007) or single-mode optical fibers complying with ITU-T Recommendation G.652 or G.657 (11/2009). For example, the number of optical fibers 5 in a tube 4 with tight buffered fiber laying may be equal to 3.

Each optical fiber according to the present disclosure comprises a core, a cladding, and at least one polymer coating, such as an acrylate coating. In the case of tightly buffered optical fibers 5, each optical fiber further comprises a buffer layer 5' made of a polymer material (e.g., a copolymer of polyamide or polyethylene, optionally saturated with a halogen-free flame-retardant filler), which is in direct contact with the surface of the polymer coating of the fiber. The outer diameter of the tightly buffered optical fiber 5 (including its buffer layer 5') can be 500-1000 microns.

In one embodiment, the internal adhesive composition 5a fills the voids between the tightly buffered fibers 5 and the inner surface of the tightly buffered fiber tube 4. This can improve the mechanical adhesion of the tightly buffered fibers 5 to the tightly buffered fiber tube 4, and therefore to the entire cable structure. The internal adhesive composition 5a may comprise, for example, a hot-melt thermoplastic adhesive (e.g., a polyethylene copolymer, such as a low-density polyethylene copolymer), an epoxy adhesive, or an organosilicon adhesive.

Optical cable 1 also comprises a third tube 6, which surrounds tube 2 with loose fiber placement and tube 4 with tight-buffered fiber placement, and is in operative contact with both of them. The third tube 6 is also hereinafter referred to as the "outer tube."

The outer tube 6 is also made of metal, such as stainless steel. Examples of steels suitable for this cable include SAE 304, 316, and 316L. Alternatively, copper can be used. In one embodiment, the outer tube 6 is a hermetically sealed tube, such as a welded tube. The thickness of the outer tube 6 can be 0.1-0.3 mm.

The inner diameter of outer tube 6 may be nearly equal to the sum of the outer diameters of loose-fiber tube 2 and tight-buffered tube 4. Thus, the inner surface of outer tube 6 contacts both loose-fiber tube 2 and tight-buffered tube 4, so that tubes 2 and 4 fit tightly within outer tube 6. Thus, given the above thickness range (0.2-0.4 mm), the outer diameter of outer tube 6 may be 3.6-4.0 mm.

The adhesive composition 6a attaches at least one of the tube 2 with loose fiber placement and the tube 4 with tightly buffered fiber placement to the inner surface of the outer tube 6. In an embodiment, the adhesive composition 6a substantially fills the gaps between the tubes 2 and/or 4 and the inner surface of the outer tube 6. This can further improve the mechanical consistency of the tightly buffered fibers 5 with the entire cable structure. The adhesive composition 6a can be the same or different from the inner adhesive composition and can comprise, for example, a hot-melt adhesive, an epoxy adhesive, or an organosilicon adhesive.

In one embodiment, the adhesive composition is a hot-melt thermoplastic adhesive, such as a polyethylene copolymer, such as a low-density polyethylene copolymer. The polyethylene-based adhesive composition, immediately after curing, may have a tensile strength in the range of 3-10 N/mm ² , as measured according to ISO 527-2 (1996).

In another embodiment, the adhesive composition is an organosilicon adhesive. The organosilicon adhesive may have a tensile strength in the range of 1-4 N/ ^mm2 immediately after curing, as measured according to ISO 37 (2017-11). In another embodiment, the organosilicon adhesive is a room temperature vulcanizing adhesive.

In one embodiment, the pipe 2 with loose fiber placement, with loosely laid optical fibers 3 located in it, and the pipe 4 with tight-buffered fiber placement, with tight-buffered optical fibers 5 located in it, are laid in an outer pipe, which is almost parallel to the longitudinal axis A of the optical cable 1. In another embodiment, the pipe 2 with loose fiber placement, with loosely laid optical fibers 3 located in it, and the pipe 4 with tight-buffered fiber placement, with tight-buffered optical fibers 5 located in it, are laid in an outer pipe, being twisted with each other.

In the embodiment according to Figure 1, a reinforcement 7 is provided around the outer pipe 6. The reinforcement 7 comprises a plurality of wires 7', spirally wound around the outer surface of the outer pipe 6. The wires 7', in particular, can be spirally wound on the outer surface of the outer pipe 6.

The wires 7' of the reinforcement 7 may be metallic, such as optionally galvanized (GS) coated steel wires, stainless steel wires, and/or aluminum-coated wires. In another embodiment, the wires 7' may be made of aramid fibers, fiberglass reinforced plastic (GRP), or glass yarn. Reinforcement 7 may also be provided consisting of metallic and non-metallic wires 7'.

Each reinforcement wire can have a diameter of 0.5-3.6 mm. Wires can have low tensile strength (≤ 45 kg/ ^mm2 ), medium tensile strength (≤ 100 kg/ ^mm2 ), or high tensile strength (≤ 200 kg/ ^mm2 ).

In the optical cable 1 according to the first embodiment shown in Figure 1, the wires 7' of the armouring 7 are arranged in the form of a single peripheral (circumferential) layer, almost concentric with the outer pipe 6. Alternatively, the armouring 7 may comprise two or more peripheral (circumferential) layers of wires 7', almost concentric with the outer pipe 6.

In this embodiment, the spaces between the outer pipe 6 and the reinforcement 7, and between adjacent reinforcement wires 7', are filled with bitumen compound, gel, or adhesive. This provides protection for the 7' wire and prevents corrosion over the life of the optical cable.

In the embodiment shown in Figure 1, cable 1 also comprises a bedding layer 9 inserted between outer pipe 6 and reinforcement 7. The bedding layer may be made of a polymeric material such as, for example, polyethylene (PE), optionally low-smoke, halogen-free, or polyamide (PA). Its thickness may be 0.1-1 mm. The bedding layer acts as additional protection for outer pipe 6. Furthermore, by selecting a suitable bedding layer thickness, the reinforcement size can be adapted to any specific application.

In the embodiment according to Figure 1, the optical cable 1 comprises a protective coating 8 as an outer sheath. The protective coating 8 can be made of a polymer material, for example, high-density polyethylene (HDPE), PA, low-density polyethylene (LDPE), or linear low-density polyethylene (LLDPE). The thickness of the protective coating 8 can be 1.0-3.0 mm.

The outer diameter of the protective covering 8 (and therefore of the entire cable 1) can be in the range of 6-12 mm, depending on the number of fibers and the presence of reinforcement and bedding, as shown below.

In an embodiment not illustrated, the cable according to the present disclosure does not contain any armouring and no protective coating, with the outer tube 6 being the outermost layer of the cable.

The use of a single metal tube (second metal tube 2) to enclose all tightly buffered fibers 5 together allows for their protection from water infiltration while maintaining a cable size smaller than, for example, a cable in which multiple steel tubes are used to individually protect each tightly buffered optical fiber. Furthermore, the outer tube 6, which encloses both the loosely buffered tube 2 and the tightly buffered tube 4, can also effectively protect all optical fibers 3 and 5 from the high pressures (up to 300 bar) typical of underwater and submarine environments.

Furthermore, the tight placement of the tightly buffered optical fibers 5 within the tightly buffered fiber tube 4, in combination with the adhesive 6a that attaches the tightly buffered fiber tube 4 and/or the loosely buffered fiber tube 2 to the inner surface of the outer tube 6, ensures that the tightly buffered fibers 5 are mechanically aligned with the cable structure. This ensures a cable with high measurement accuracy, particularly for DAS and DSS measurements.

A method for manufacturing a distributed metering cable according to an embodiment of the present invention comprises the following steps:

i. providing a longitudinally bent and welded tube around the bundle of optical fibres, thereby providing a loose fibre tube with the optical fibres loosely arranged within it;

ii. providing a longitudinally bent and welded tube around a bundle of tight-buffered optical fibers (e.g. 3 tight-buffered optical fibers), optionally providing an internal adhesive to fill the voids and spaces between the tight-buffered optical fibers and the inner surface of the tight-buffered fiber tube, and then pulling the tube onto the fibers, thereby providing a tight-buffered fiber tube with the tight-buffered fibers tightly arranged within it;

iii. providing an adhesive composition on a pipe with loose fiber laying and/or on a pipe with tight buffer fiber laying;

iv. connections between fiber-laid pipe with tight buffer and loose fiber-laid pipe with metal foil;

v. ensuring that the metal foil adheres around the tightly buffered fiber laying tube and the loose fiber laying tube by folding and pulling the metal foil onto the loose fiber laying tube and the tightly buffered fiber laying tube;

vi. welding the metal foil, thus providing the outer tube with the loose fiber tube and the tight buffer fiber tube tightly positioned inside it;

vii. optionally, extruding a bedding layer onto the outer pipe;

viii. optionally, winding wire around the outer pipe (or on the bedding layer, if present), providing reinforcement; and

ix. optionally, providing a protective coating as the outermost layer.

In one embodiment, step iii of providing the adhesive composition is accomplished by extruding the adhesive composition. In this case, the adhesive composition may be a hot-melt thermoplastic adhesive.

In an embodiment, step iii of providing the adhesive composition is carried out by applying the adhesive composition around at least one of said first metal pipe and said second metal pipe, for example, around only one of said first metal pipe and said second metal pipe. In this case, the adhesive composition may be a silicone (organosilicon) adhesive.

In an embodiment, the adhesive composition of step iii is an organosilicon adhesive which, when applied before hardening, has a viscosity in the range of 10,000-30,000 Pa⋅s at 25°C.

It should be noted that in step iii, the adhesive composition need not be applied to both pipes. The applicant actually noted that it is sufficient to apply the adhesive composition to only one of these pipes. Then, during the next step v, the adhesive composition applied in step iii spreads and substantially fills all the voids and spaces between the first and second metal pipes and the inner surface of the third outer pipe, thereby ensuring a strong mutual attachment of the pipes, as well as their bonding to the inner surface of the outer pipe 6. Thus, the pipe 4 with the tightly buffered fiber arrangement is attached to the inner surface of the outer pipe 6 in a particularly strong manner. The tightly buffered fibers 5 then become mechanically matched to the cable structure, as required in DAS and DSS measurement applications.

Below in this paper, some exemplary cables according to the present invention obtained by the applicant are described.

A) The first example cable contains 4 loose fibers and 3 tight-buffered fibers, an armor made of 13 GS x 1.00 mm wires with a tensile strength of 170 kg/ ^mm2 , and an outer sheath of HDPE with a thickness of 1.5 mm. The cable's outer diameter was 8 mm, its weight was 0.13 kg/m, its maximum working tension was 5 kN, and its ultimate tensile strength was 15 kN.

B) The second example cable contained 8 loose fibers and 3 tightly buffered fibers, an armour made of 22 GS x 1.00 mm wires with a tensile strength of 170 kg/ ^mm2 , and an outer sheath of HDPE with a thickness of 1.5 mm. The cable's outer diameter was 6.8 mm, its mass was 0.10 kg/m, its maximum working tension was 3 kN, and its ultimate tensile strength was 10 kN.

Claims (24)

1. An optical cable (1) for distributed measurement, comprising: - a first metal tube (2) with at least two optical fibers (3) freely located in it; - a second metal tube (4) with at least two optical fibers (5) with a tight buffer, tightly located in the inner surface of the second metal tube (4); and - a third metal pipe (6) having an internal surface that surrounds together said first metal pipe (2) and said second metal pipe (4) and is in operative contact with them, wherein the third metal pipe is a metal foil formed adjacent to and welded around the first metal pipe (2) and the second metal pipe (4); and in which at least one of the first metal pipe (2) and the second metal pipe (4) is attached by means of an adhesive composition (6a) to the inner surface of the third metal pipe (6). 2. An optical cable (1) according to claim 1, in which the first metal pipe (2) and the second metal pipe (4) are made of stainless steel. 3. An optical cable (1) according to claim 1, in which the third metal pipe (6) is made of stainless steel. 4. An optical cable (1) according to claim 1, in which said adhesive composition (6a) fills the gaps between the first and second metal pipes (2, 4) and the inner surface of the third pipe (3). 5. An optical cable (1) according to claim 1, wherein said adhesive composition (6a) is selected from hot-melt adhesive, epoxy adhesive, silicone adhesive and mixtures thereof. 6. An optical cable (1) according to claim 5, wherein the adhesive composition (6a) immediately after curing has a tensile strength in the range of 1-10 N/ ^mm2 , as measured according to ISO 37 (2017-11) for silicone adhesive or according to ISO 527-2 (1996) for hot melt and epoxy adhesive. 7. An optical cable (1) according to claim 1, in which the internal adhesive composition (5a) fills the voids and spaces between the optical fibers (5) with a tight buffer and the inner surface of the second pipe (4). 8. An optical cable (1) according to claim 1, additionally containing reinforcement (7) in a radially outer position relative to the third metal pipe (3). 9. An optical cable (1) according to claim 1, additionally comprising an outer (8) sheath. 10. An optical cable (1) according to claim 8, additionally comprising a layer (9) of bedding inserted between the outer pipe (6) and in direct contact with it and the reinforcement (7). 11. An optical cable (1) according to claim 1, in which at least two optical fibers (3), freely located in the first metal pipe (2), are designed with the possibility of distributed temperature sensing (DTS) and data transmission. 12. An optical cable (1) according to claim 1, wherein at least two tightly buffered optical fibres (5) are suitable for distributed stress sensing (DSS) and distributed acoustic sensing (DAS). 13. A method for manufacturing an optical cable (1) for distributed measurement, said method comprising the steps of: i) providing a first metal tube (2) with at least two optical fibres (3) freely positioned therein; ii) providing a second metal tube (4) with at least two tightly buffered optical fibres (5) tightly located therein; iii) applying an adhesive composition (6a) to at least one of said first metal pipe (2) and said second metal pipe (4); iv) connecting the first metal pipe (2) and the second metal pipe (4) with a metal foil; v) ensuring that the metal foil adheres around the first metal pipe (2) and the second metal pipe (4); and vi) welding a metal foil to provide a third metal pipe (6) surrounding together said first metal pipe (2) and said second metal pipe (3) and operatively contacting them. 14. The method according to claim 13, wherein in step iii) the adhesive composition (6a) is applied only to one of the first metal pipe (2) and the second metal pipe (4).