Lessening biofouling on long-duration AUV flights: Behavior modifications and lessons learned

Abstract

With recent developments in battery technology and ocean energy harvesting systems, biological fouling, or biofouling, a process referring to the gradual accumulation of organisms on underwater surfaces, has gained a foothold as the primary adversary in long-duration autonomous underwater vehicle (AUV) flights of the Challenger glider mission. Limiting biofouling on long-duration AUVs is essential to the success of the flight. Inverse relationships and correlations were drawn between biofouling, vertical velocity of the AUV, and in turn, steering capability. As organisms settle and grow on the AUV, the hydrodynamics of the vessel changed, resulting in larger volume and more drag, adding buoyancy discrepancies as well. The increased drag results in a lower vertical velocity, and therefore less water flow over the rudder, or fin, directly causing the reduction in steering capability. Additionally, the organisms were not evenly distributed about the AUV, causing an imbalance in the drag. The fin then needed to maintain an offset to counteract this imbalance, resulting in less overall range in fin movement, further reducing the ability to steer. Analysis of the data from four separate legs of ocean basin crossings has shown that as the AUV begins to foul, it needs to maintain a vertical velocity of greater than 12 cm/s to maintain viable steering. Overall the fin will move more as it attempts to compensate for biofouling, which will use additional power throughout the duration of the flight, bringing power budgets back into the equation. Although the biofouling issues facing long-duration AUVs are subject to the same settling processes as boats and ships, porting commercially available antifoulant technologies from larger, faster vessels to the AUVs has proven challenging. Non-like metals combined with biofilms can result in increased galvanic corrosion, so copper coating compounds on steel components and aluminum AUV hull are not ideal, particularly with limited space and weight for sacrificial anodes. Ablative paints, by design, can wear away, causing possible ballast issues while failing to prevent fouling. Biocides, for obvious reasons, have not been in the list of candidates for consideration in the ongoing battle against biological growth. We introduced some behavioral modifications in the flight characteristics of the AUV that provided some assistance. For example, we avoided the majority of the warmer and illuminated euphotic zone, where primary production occurred, or zones above a certain temperature range that could hamper organism growth, even if it was not possible to prevent settling. This paper will explore the differences in biofouling, flight environment, preventative measures, and lessons learned on the four legs of ocean basin crossings completed by two separate Slocum electric gliders, part of the Rutgers AUV fleet.