An underwater towed vehicle to monitor the Sicily-Malta channel

Abstract

The problem of monitoring pollution coming from oil spills assumes wide importance for the highly congested Sicily-Malta channel. Hydrocarbons, as well as other polluting substances, have a huge influence on the health status of the sea. In this paper we present the preliminary design of an underwater towed vehicle (UTV) to monitor the Sicily-Malta channel. The design of this towfish incorporates ideas for a camera, lens system and stroboscope illumination system that can be used to take images of phytoplankton and zooplankton having a size range of 100 microns up to 1 centimeter. The underwater platform includes a high definition (HD) camera for monitoring jellyfish population at different sea depths. Unlike the autonomous underwater vehicles (AUVs), an UTV is not independent and must be towed by a surface boat. This disadvantage is balanced by having a simpler design and control system and an increased payload for instruments, sensors and cameras due to the absence of heavy battery systems. In order to increase maneuverability, stability and depth control, actuated hydroplanes are used to vary the angle of attack and to change the total downward force exerted on the moving towfish. The depth of dive of the towfish is automatically controlled to a set value. Automatic control is preferred so as to reduce the work and human concentration necessary during a monitoring mission. The hydroplanes are used to control rolling and pitching of the towfish. This kind of corrective action and a means of knowing the inclination of the towfish are deemed to be necessary because of the effect that underwater currents may have on the dynamics of the towfish. In addition to active control against the rolling action, the main hydroplanes (wings) of the towfish are at a small anhedral angle in order to create a passive anti roll action by creating a corrective moment acting about the main longitudinal axis of the towfish. The stern of the towfish also carries a rudder. The rudder would mainly be used when turning and to steer the towfish away from the surface boat wake when taking surface or close to surface measurements. The towfish is towed via an umbilical cord which carries all the power supply and signal lines necessary for towfish control and data acquisition. The umbilical cord is mechanically strong enough in order to tow the underwater towfish which is subjected to hydrodynamic drag. For proper logging and mapping of pollutants and camera images it is required to know the exact position and positional depth of the towfish during a mission. The positional depth of the towfish is recorded by means of a depth sensor. The position of the towfish is found by having a Global Positioning System (GPS) on the surface boat coupled with a commercially available sonar based instrument that can be used to calculate the relative position between the surface boat and the towfish.