One of the critical areas for underwater munition detection and remediation is very shallow water less than 5 meters (m) deep. Very shallow water is emphasized since munitions are most likely to be encountered by the general public in depths that are suitable for wading, swimming and scuba diving, with potential encounters more likely in the shallowest water. In many environments there will be occasional to frequent breaking waves in depths from 0.5 m to 3 m, and there are very few platforms that have been optimized to work in this region. The current most used approaches consist of specialized large amphibious vessels or small personal watercraft (jet-skis) that are not optimized for survey work. The objective of this project was to develop and test a range of unmanned semi-autonomous surface vessels (USV) for use in shallow water and surf zone conditions.
In the past five years, the laboratory at Woods Hole Oceanographic Institution has developed two shallow water USVs. The Jetyak is a 3.5 m long, 1 m wide gas powered, 150 kg, jet drive vessel capable of carrying payloads of up to 100 kg with moderate hydrodynamic drag, moderate endurance (~ 8 hours), moderate speed (~ 4 to 6 m/s maximum), and relatively poor surf zone performance due to the air intake of the gas engine. The small surf vessel (SSV) is a 1.8 m long, 50 cm wide, 10 kg electric motor/battery powered, jet drive vessel capable of carrying small, low drag payloads of up to 5 kg, with low endurance (~ 1.5 hours), high speed (~ 10 m/s) and good surf zone performance due to the semi-submersible hull and drive system and self-righting design. Neither vessel is optimized for munitions response work in the surf zone as the Jetyak cannot handle breaking waves reliably, and the SSV is too small to carry sensors for munition detection. The goal of this project was to develop and test USVs with lengths of approximately 3 m and weight less than 100 kg, with wave piercing and self-righting hulls that can carry acoustic sensors capable of detecting proud unexploded ordnance (UXO) in and outside the surf-zone. Based on previous experience, extensive in situ testing is required to determine the performance envelope of these vehicles with respect to wave height, breaking frequency, and mean currents and to optimize maneuverability characteristics which are essential for navigation in the surf zone with endurance and speed. The in situ testing will be combined with numerical modelling analysis using commercial off the shelf software to optimize performance.
The work completed in this project indicates that USVs are capable of launch, recovery and navigating in surf-zone conditions through a combination of direct remote control by the operators in the swash and autonomous waypoint following modes in regions of intermittent breaking. Smaller single person portable USVs (1.7 m long, 10 kg, SSV) have been proven to take high quality single beam echo sounder data with vertical accuracies of under 10 cm. A larger (3 m long, 55kg) USV was developed that is capable of carrying a bathymetric sidescan sonar such as the PingDSP 3DSS to detect proud UXO under certain conditions.
The USVs developed in this project will benefit both munition response work and near-shore research in general. Unmanned semi-autonomous surface vessels that can carry acoustic munition detection and swath bathymetry sensors while operating in and outside the surf zone in nearshore water depths offer many cost and performance advantages over the manned systems that are currently used. In addition to allowing more efficient surveys of munitions, the bathymetric survey capability of these systems will improve the ability to understand coastal erosion processes, which will become increasingly important in the next century with anticipated rising sea levels.