Observatory Cable Laying System

Abstract

Cabled ocean observatories use cable ships or other surface vessels to lay long heavy armoured cable on the seafloor which typically forms the backbone of the observatory network. It is not practical, and would be extremely difficult to lay shorter cables connecting two instruments or instruments to the nodes. Other Remotely Operated Vehicles (ROV's) have the technology to lay lighter weight or short cables where the users are fortunate to have either low power requirements, close proximity of instrument to node, and a benign environment for the cable to lay. There has been no solution to lay long extension cables that have armour packages, or large conductors required for high power or remote applications. Heavier cables also have better behaviour on the seafloor, resisting spanning, rock damage and movement in currents. NEPTUNE Canada, and other observatories have a requirement for numerous extension cables to be laid that are up to 10 km in length and have a cable weight in water of several thousand pounds. These cables are required for connecting instruments that are placed some distance from the node and main cable. The Canadian Scientific Submersible Facility (CSSF) has designed and built a Remotely Operated Cable Laying System (ROCLS) that works in conjunction with the ROPOS ROV. The ROPOS system was built with a through frame lift capability of 4000 lb. to a depth of 2500 metres. Thus the cable laying system can have an overall weight of 4000 lb. including cable. With this system the cable can be gently and precisely laid on the seafloor. With theROCLS attached to the ROV, we can visually watch cable touchdown, as well as using the obstacle avoidance sonar to identify obstacles. The ROV thrusters provide the force to manoeuvre around obstacles. The drum is hydraulically driven and can drive cable out, reel cable back in or provide back tension if required. Further capability is provided by a capacity to couple and decouple the ROV to the package while it is on the seafloor. This allows the vehicle to free swim and connect the cable ends to the instrument packages and to the node. The vehicle can also perform other routine ROV services. It can then re-couple to the cable laying system and continue laying cable, or recover the empty drum to the surface. Spare loaded cable drums can be carried and easily swapped out offshore, saving time and effort particularly when the offshore spread is on hire. This presentation describes the system design and operation, as well as the various cable types and weights that can be managed. CSSF has developed a system that meets a unique requirement for ocean observatories that cannot be otherwise easily achieved. The system is simple, robust, has a large payload and can be deployed with the existing ROPOS offshore spread. The ROPOS system, complete with the ROCLS, can be deployed from most scientific research vessels currently used for offshore oceanography in and around North America.