AUV THE APPLICATION OF AUTONOMOUS UNDERWATER VEHICLES FOR CABLE ROUTE SURVEYS by Graham S. Evans
During recent years, there has been an increasing focus on burying submarine cables in deeper and deeper water. Ten years ago, burying cables to water depths of 200m was the norm, now; it is not uncommon for burial to be required to depths of 2000m or more. For the cable installer, burying cables in these depths presents a particular range of challenges. Not least of these challenges lies in the fact that burial tools, particularly ploughs, are operating close to the limits of stability and controllability, where even small seabed obstructions and have resulted in the loss of control of equipment which in turn can result in the failure to meet burial specifications, damage to the cable, or in extreme cases, the entire loss of the plough.
To mitigate exposure to such risks, and the resulting consequences of installation mishaps, it is important for a cable installer to fully understand the nature of the seabed surface and sub-seabed conditions along the path of the burial vehicle prior to commencing installation. Understanding the nature of the seabed requires that any potential hazard to burial operations lying in or close to the path of the burial vehicle is clearly identified and accurately mapped. The performance of conventional electronic survey tools operated from surface vessels is limited when reliably identifying and accurately mapping potential hazards in deep water. However, the application of new generation Autonomous Underwater Vehicles (AUVs) has clearly demonstrated that the 33
limitations of conventional survey methodology in high-resolution mapping in deep water can be overcome. The primary data sets typically collected during the electronic route survey comprises: l Multibeam bathymetric data providing
information on the topography of the seabed l Sonar imagery data which provides information on seabed surface features l Seismic profiling data which provides information on the sub surface soils and geological profile. To provide the installer with physical information on burial conditions, these are usually augmented by data provided by a burial assessment survey (BAS). In shallow water, the resolution of the various electronic data is usually adequate to identify and accurately geo-reference the spatial position of objects and seabed features likely to be hazardous to burial operations. As water depths increase, the laws of physics play an increasing role in degrading resolution with increasing distance between the transducers and sensors of the survey equipment being used and the seabed. To maintain optimum resolution in deep water, survey tools can be towed such that the distance (altitude) from the seabed