Optical waveguides – Optical transmission cable – Loose tube type
Reexamination Certificate
2002-01-28
2004-01-20
Patel, Tulsidas (Department: 2839)
Optical waveguides
Optical transmission cable
Loose tube type
Reexamination Certificate
active
06681070
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a submarine optical cable resistant to a longitudinal flow of water accidentally penetrated inside it and to a method for controlling said longitudinal flow of water inside said cable.
Submarine optical cables are subjected, in case of accidental rupture thereof, to a sudden ingress of a sea-water flow at high pressure (e.g. 100 bar, when the cable is at 1000 m below the sea level). Such high pressure water flow may propagate for a relevant length inside the cable if suitable water blocking means are not provided in the cable, thus damaging a remarkable portion of said cable which has then to be replaced.
2. Description of the Related Art
A number of cables designed for submarine installation are known in the art.
For instance, U.S. Pat. No. 5,125,062 discloses an undersea cable comprising a central metallic tube, filled with a sealing compound, e.g. silica gel, and containing optical fibers embedded therein, said tube being surrounded by a helical lay of metallic (preferably steel) wires. Interstices between wires and between the helical lay and the central tube are filled with a sealing material (preferably polyurethane resin) which opposes longitudinal propagation of water along the cable. Alternatively, the central tube can be made of plastic and in this case the helical lay also presents the characteristics of an arch for withstanding pressure.
U.S. Pat. No. 4,684,213 relates to a submarine cable comprising a pressure resistant steel tube containing optical fibers, surrounded by two layers of steel wires and by an outer metal tube made of copper or aluminum. Dams of a sticky compound and/or of a jelly of plastic material are disposed at regular intervals inside the central tube and in the gaps between the lay of wires disposed between the central tube and the outer tube.
U.S. Pat. No. 5,463,711 discloses an underwater cable for shallow-water, comprising a central tube made of plastic or preferably of metal, optical fibers arranged within said tube and surrounded by a water blocking material and six steel wires wound in a helical lay around the central tube.
UK patent application no. 2,253,717 describes a method for manufacturing an optical cable in which a plurality of optical fibers are enclosed as they are fed in a longitudinal direction by a continuously fed metal strip folded around the fibers; the edges of said metal strip are welded together to form a tubular moisture barrier, the diameter of which, at the welding stage, provides sufficient clearance between the fibers and the heat affected zone of the welded tube. The diameter of the welded tube is then reduced by passages through the one or more sets of reducing rollers, the tube then closely surrounding the optical fire package. The diameter reduction increases the tensile strength of the metal tube.
In general, for submarine cables apt to operate at deep depths (e.g. below 500 meters) it is required that the water penetration length along the cable is less than 1000 meters after 2 weeks.
SUMMARY OF THE INVENTION
Applicant has now observed that when a tube (in particular a buffer tube containing optical fibers) is of relatively small internal diameter (e.g. below about 7 mm), it is rather difficult to achieve a complete filling of said tube with a suitable water-blocking gel. As a matter of fact, because of the relatively small diameter of buffer tubes specifically adapted for submarine cables (typically of about 2-5 mm) and of the relatively high viscosity of the filling jelly compound (generally from about 50 Pa·s to about 150 Pa·s), the filling of the tube can only be completed for about 80-95% of the internal volume of the tube. In addition, an incomplete filling of the buffer tube may be desirable in some instances, also for buffer tubes of larger dimensions. For instance, as disclosed in European Patent Application Publ. No. EP 883007, if optical fibers are exposed to hydrogen gas, the transmission properties thereof are altered, the higher being the partial pressure of hydrogen the more relevant said alterations. EP 883007 thus suggests filling only partially the buffer tube with a filling material (maximum 95%), in order to leave an expansion volume inside the tube so that the partial pressure of hydrogen is kept relatively low.
Thus, for different reasons, from about 5 to about 20% of the internal volume of the buffer tubes can be left free from water blocking gel material.
The applicant has now observed that, while this incomplete filling can be considered of relatively low importance for terrestrial cables (where longitudinal water penetration tests are conducted under a water head of 1 m), it becomes much more important for underwater cables. As a matter of fact, Applicant has observed that the voids due to the incomplete filling tend to dispose longitudinally along the whole length of the buffer tube, thus creating a preferential path along which a high pressure water flow (e.g. 100 bars if the cable is laid at a depth of 1000 m) can flow with relatively low head losses for a relevant length of the cable before being blocked.
In addition, if water has a preferential flow path inside the buffer tubes, the respective hydrostatic pressure may act on the inner surface of the plastic tube, thus increasing the diameter of the same; as a consequence, the flow path is increased in dimension and the water flow increases the speed and the penetration length inside the buffer tube.
Applicant has now found that by providing a suitably dimensioned longitudinal cavity (or “gap”) defined along the outer surface of a buffer tube containing optical fibers, said buffer tube being made of a deformable material (e.g. polymeric material), it is possible to substantially reduce the longitudinal flow of water inside said buffer tube. For instance said buffer tube can be surrounded by a second tube made of a substantially non-deformable material (e.g. metal) having a larger inner diameter with respect to the outer diameter of the buffer tube, so to create the desired gap around the buffer tube. In particular, the Applicant has observed that the dimensions of the longitudinal space surrounding the buffer tube should be selected in order to provide along said space a preferential flow path for the water accidentally penetrated inside the cable, with respect to the flow path inside the buffer tube, thus preventing water from flowing inside said buffer tube. In other terms, the head losses of the water flowing along said longitudinal space surrounding the buffer tube should be lower than the head losses of the water flowing inside said buffer tube.
One aspect of the present invention thus relates to a method for controlling a longitudinal flow of water accidentally penetrated inside the structure of a submarine cable, said cable comprising a deformable elongated hollow body comprising at least one optical fiber loosely housed therein, wherein said method comprises causing the water to flow inside the cable, but outside said deformable elongated hollow body, for a distance longer than the distance of the water flowing inside said deformable elongated hollow body.
In particular, the water flow through said deformable elongated hollow body has a first head loss, while the water flow inside the cable, but outside said deformable elongated hollow body, has a second head loss, said second head loss being lower than said first head loss.
Preferably, said method comprises the steps of:
disposing within said cable structure a first elongated hollow body defining an inner surface;
disposing within the inner surface of said first elongated hollow body an optical core comprising at least a second deformable elongated hollow body comprising at least one optical fiber loosely housed therein;
defining a longitudinal cavity inside said first elongated hollow body and outside said deformable elongated hollow body;
disposing a water blocking element within said space
causing the water to flow with a lower head loss through said cavity than through said deform
Cecchi Feliciano
Ginocchio Alessandro
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Patel Tulsidas
Pirelli Cavi e Sistemi S.p.A.
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