Persistent Unmanned Surface Vehicles for Subsea Support

Abstract

Abstract This paper discusses the role of unmanned systems in subsea support. Recent developments in mobile unmanned vehicle networks are reviewed, demonstrating the theoretical viability of the approach. The paper further describes the application of a wave energy harvesting unmanned surface vehicle (USV) as a platform for acoustic telemetry between the surface and subsea systems, including fixed installations and mobile assets (ROVs/AUVs). The paper reviews the concept of the USV, and presents field tests of the technology off Hawaii and California to demonstrate the USV's acoustic telemetry performance with commonly available acoustic systems. System tests off Hawaii demonstrated acceptable bit error rates and signal to noise ratios in water depths up to 1000 meters and at horizontal ranges up to 2.5 kilometers. Off California the system was used to augment the Deep-ocean Assessment and Reporting of Tsunamis (DART) system. In water depth of 3700 meters, using actual DART acoustic and satellite telemetry systems, the USV delivered identical telemetry to the conventional mooring. The results presented here lead to the conclusion that a persistent USV can provide effective, reliable telemetry to subsea systems in meaningful water depths. As subsea installations expand farther from manned platforms and undersea vehicles become more automated, acoustic telemetry and navigation become more critical to successful operations. Typically this requires fixed moorings, subsea transponders, drifting buoys, or expensive vessels. This paper presents an entirely new deployment approach. With persistent mobile unmanned platforms, mobile communications and navigation networks are feasible. Now that persistent USVs have come to market the economics of deploying acoustic networks above subsea systems have changed dramatically. New operational concepts are now possible. This paper will discuss the viability of subsea communications and navigation networks based entirely on unmanned vehicles. Improved operations at reduced cost can be expected as these concepts reach maturity. Introduction: Acoustic and Unmanned Systems in Offshore Support Offshore undersea operations have benefited from the development and deployment of acoustic and unmanned systems. Acoustic positioning tools such as Ultra-short baseline (USBL) and Long Baseline (LBL) systems are commonly used in a variety of applications. Acoustic telemetry of data is also employed where the bandwidth and range limitations are appropriate to the application. Likewise remotely operated vehicles (ROVs) have become a fundamental tool to underwater intervention and inspection. Autonomous underwater vehicles (AUVs) are now routinely used in subsea survey and are being promoted as a tool for inspection tasks. These developments have led to improvements in productivity and safety in offshore operations. Unfortunately all of these advances remain limited by a common factor: vessel support is needed throughout most phases of operation. A vessel can often represent cost and complexity that is poorly matched to the task at hand. In this case there is an economic inefficiency. In other cases vessels present risks to personnel or the environment that should be minimized. Unmanned systems have been proposed as a path to improving offshore productivity and safety for many years. As long ago as the early 1990s, technologists predicted a significant impact from unmanned systems in ocean operations [1].