Cable Laying Techniques
Four different installation techniques may be used for different segments of a cable route.
1) At the shoreline, directional drilling is often used to install cable conduits passing under the beach and any nearshore reef, to minimize impacts on them.
2) In shallow waters particularly in areas where fishing is prevalent it is becoming increasingly desirable to bury the cable so that it does not become damaged. Trawler equipment particularly large beam trawls can completely sever an underwater cable. When crossing soft bottom areas that are potentially subject to ship anchoring and trawling or other bottom-fishing techniques, the cable typically is buried, to protect the cable from the fishing gear. This is typically done by the cable vessel pulling an underwater plow that continuously cuts a furrow and places the cable into the furrow. Before long, the furrow smoothes out due to natural forces. Burying submarine cables is expensive, requiring as it does a dedicated ship having the necessary equipment, particularly for burying the cable in the seabed. Such equipment conventionally comprises a plough device which requires substantial amounts of power from the ship in order to operate and tow it. For example the plough has an attitude control, controls to lift and lower the plough, television cameras to check if the cable is entering the plough correctly, magnetometer to check if the cable has been properly buried behind the plough, and a depresser arm which lifts and closes when a repeater of the cable link passes through the plough. Such a plough has three lines extending from the ship down to the plough, namely an umbilical cable for the various controls, a tow wire to tow the plough, and it has the cable itself which is to be buried.
3) When crossing hard bottom areas where burial is infeasible and anchoring or bottom-fishing gear is expected, typically "armored" cable is used. It has a diameter no more than a soft drink can (i.e., up to 2.5 inches). The evidence shows that such cables do not move laterally once placed. Old cables are found encrusted with corals and other sea life.
4) When crossing the deep ocean where no anchoring or bottom-fishing gear is expected, the cable typically is just laid flat on the ocean bottom. It has no known adverse effects.
Installation can be a low impact process, especially when compared to other commercial activities currently allowed in the marine environment (cables are small in diameter, the plow cuts a narrow trench, cable is buried to one meter, etc.). Little data exists on the cumulative environmental impacts associated with the installation, maintenance, operation, and repair of submarine cables. Additional information is needed on the immediate and long-term impacts of fiber optic systems. Reassurance is needed to demonstrate that impacts are indeed low, as industry claims, and that submarine cables are and will remain buried. A more succinct and clear policy for submarine cables would alleviate the current confusion over the approval of such projects.
The prior art of laying cables on the sea floor in deep water has concentrated primarily on long lines such as intercontinental telecommunications lines. All cables known to have been laid on the sea floor in deep water are long lines and/or are suitable for fabrication with a strength member, or have in some way compromised the need for cost effective, highly reliable installation. Conventional cable laying vessels are not well suited for handling short cable lengths. Furthermore, since laying short cables does not economically justify employment of a purpose built cable lay vessel, there is a need for a cable laying technique that is compatible with vessels of opportunity.
The method by which short cables in deep water are connected to a surface facility also differs significantly from prior art in that a vertical "riser" section must be installed from the sea floor to the surface. Seismic cables are highly sensitive cables comprising jacketed electrical conductors and hydrophones or geophones. The requirements of seismic cables are such that the design of the riser section of the cable must be significantly different from the part that lays on the sea floor, although it is necessary to maintain electrical continuity of multiple internal conductors. It is also desirable to minimize the number of electrical connections.
Seismic cable cannot be subjected to tensile forces or compressive loads, which would damages the geophones or break the fine electrical conductors in the cable. Laying seismic cable by hanging it from the deck of a vessel to the sea floor would result in high load forces in the cable due to the weight of suspended cable, vessel heave, water currents, and other perturbations. Lashing the cable to a strength member, such as a wire rope, is a hazardous activity for deck crews, and it extends the vessel time required to lay the cable. Such a requirement also adds expense for the strength member, hold back equipment and lashings. Long lengths of suspended electrical cable are prone to conductor damage due to tension in the cable. Relatively high tensile loads are encountered when cables are deployed to the seafloor from surface vessels in deep water. Strength members are commonly used to take the tension and thereby protect the wires. Strength members must be either inside or outside of the cable. An external strength member is not suitable for use on a seismic cable because it would isolate the geophones and hydrophones from the vibrations they are meant to detect. An internal strength member is not suitable because the device which supports the weight of the suspended cable, be it a winch drum or a tensioning device would impart high compression loads to the geophones and hydrophones. It is desirable to have a means to deploy such cables in deep water in a cost efficient manner without imparting high loads to the electrical conductors or sensors.
Except in the deepest waters, submarine cables normally need to be buried in order to avoid the risk of damage due to engagement with trawling gear and/or abrasion arising from tidal movements. Perhaps the ideal method is to excavate a trench first and lay and bury the cable in a separate subsequent operation, but this requires extremely precise navigation and is feasible only for short routes in very shallow water. Another method used is to lay the cable first and then bury it as a second operation. The main problems with this are (a) that the burial operation is relatively slow and expensive, (b) the cable is left exposed between the times of laying and burial, and (c) if the cable is laid over an obstruction it is frequently impossible to alter its position on the sea bed to enable it to be buried.
The preferred conventional method in most cases is therefore to form a trench and lay the cable in it at the same time, using submerged trenching and positioning apparatus known as a "plough". This method works well under good conditions, but if an obstruction (for example a rocky area or a sunken vessel or other wreckage) is encountered it is difficult to divert the cable route around it and there is a risk that the cable will be damaged or even broken in attempting to do so.
A trench is formed in the bed of the sea or other body of water and there is laid in it a line having a greater strength and/or a lower cost than the cable or other flexible member to be laid; and then in a separate second operation the flexible member is laid from a cable-laying ship in substantially the same trench by submerged apparatus caused to follow the path of the line.
Two or more lines may be laid simultaneously, if desired: for example to permit a corresponding number of cables to be laid in succession close together but without risk of fouling one another. Alternatively by using an appropriately equipped vessel, two or more cables or other flexible members may be laid simultaneously following a single line.
The Line will usually be a steel hawser sufficiently protected against corrosion. By using a line of high strength, the risk of damage is greatly reduced, and a line of low value can be abandoned in case of extreme difficulty. If an obstruction is encountered in laying the line, a length of line may be temporarily recovered and re-laid on a new route diverting to avoid the obstruction; or alternatively the laying operation can be suspended and the line held whilst steps are taken to remove the obstruction.
The trench formed in the line-laying operation will often be filled in by natural processes before the laying process proper takes place, and it may be deliberately filled if there is risk of the line being displaced. In both cases the submerged apparatus will take the form of a plough, fitted with suitable guidance instruments, re-forming a trench on substantially the same route (but not necessarily of the same cross-section).
The line may be left in position (in which case it may be used, if required, to lay a further cable (or other flexible member) along the same route. Alternatively it may be removed in the second laying operation, in which case the length of line removed gives an accurate and valuable measure of the length of flexible member laid, enabling the paying-out operation on board ship to be reliably regulated: in some cases the removed section of line may be usable as a towing cable for the submerged apparatus.
If the line is of sufficient strength, it may guide the plough or other submerged apparatus purely by direct physical engagement with it, but normally power-assisted guidance is to be preferred: this may be controlled by feelers engaging the line, or contactless position detectors may be used, for example electromagnetic detectors responsive to the ferromagnetic property of a steel line or to a current passed through an insulated element of the line. When the removed section of line is used as a towing cable, the horizontal angle of deflection may be measured and used to guide the movement of the laying vessel: a very simple and robust instrument of a known kind may be used for this purpose.
Especially for the second laying operation, a plough is preferably of the kind using high-velocity water streams to form the trench; preferably high-velocity water streams are also used to clear debris from the line, especially if its position is to be detected by feelers.
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