Measuring Subsea Cable Depth of Burial with Passive Magnetics from an Autonomous Vehicle.
Robert Nunn, Terry Slater, Lachlan Filshie
Innovatum Ltd, Bury St Edmunds, Suffolk, UK
To date the current measurement methodologies used to aid AUV’s in locating, tracking and (where possible) measuring the depth of burial of subsea cables, has involved an active element, i.e. there is some type of electromagnetic or acoustic transmission. The need for an active transmitting element adds a degree of complexity and, potentially, cost to the tracking process.
This paper describes the AUV tracking trials performed, in 2016 to track a small pre-magnetised fibre optic cable. Neither the cable tracking technology nor the process of magnetising the cable, used to improve the equipment’s tracking performance, are new to the industry. However, the implementation of the cable tracking system in a self-contained form with, on board processing and data quality control, to guide the AUV could potentially offer a step change in subsea cable survey. Using industry standard components, a hybrid ROV/AUV and a normally ROV-mounted cable-tracker, development time was spent both on and offshore, creating a software interface that will allow an industry standard vehicle to autonomously track buried subsea cables.
The paper additionally provides an overview of the fundamental principles of the cable tracking system, the system architecture of the combined AUV and cable tracker and the management of the combined data.
At the outset of the trial, an 80m length of magnetised (15mm diameter) armoured fibre optic cable was laid on the seabed, at a depth of 15m, to create a subsea proving environment for the AUV. During the first phase of the trial the AUV was operated in a tethered hybrid mode to prove the performance of the cable tracker, whilst mounted on the AUV and to optimise the interfacing between the two systems. Following completion of the modifications to the AUV guidance system, further trials are planned in early 2017. This next phase of the trials will test the capability of the AUV to operate in a fully autonomous mode using the cable tracker as a positional reference system.
The trials in 2016 where used to prove the concept that the passive magnetic cable tracker could be mounted onto an AUV and track a cable. The tracking system was successfully modified to allow it to operate in a standalone capacity. Furthermore, whilst operating the AUV in tethered mode it was possible to fly along the cable confirming the accuracy and functionality of the cable tracker. The AUV command and control software is currently being modified to enable an autonomous mode of operation.
Following the successful completion of the trials it is expected that this cable tracking technology will offer Offshore Transmission Owners (OFTO’s) the potential for cost effective cable surveying. The cable tracker will have a standard protocol to interface to the AUV guidance system, requires relatively low power and being of a modest size enables this technology to be fitted to many off the shelf AUV systems.
An introduction to the use of commercially available equipment to autonomously track subsea cables, commonly used to transmit power and telecommunications commonly used within energy and telecommunications industries.
The principles of electromagnetic cable tracking and the benefits of magnetising the cable.
The opportunity for low cost cable tracking and the potential benefits to OFTO’s.