Defining the hydrodynamic properties of an underwater glider is a critical operation that should be properly optimized since it has a great impact on the vehicle’s efficiency. It also defines the allocated budget for the Variable Buoyancy System (VBS) of the glider. This paper describes the work performed to optimize the hydrodynamic forces, represented in the lift and drag forces of the vehicle, as well as optimizing the angle of attack that the vehicle should perform its cycles at. The approach used in the analysis is Computational Fluid Dynamics (CFD) by importing the 3D model of the glider to Ansys software. The best angle of attack is chosen based on the highest efficiency characterized by the lift-to-drag ratio. The polar charts of the vehicle are also drawn based on the vertical and horizontal components of the vehicle’s speed. The results are implemented in the calculations of the VBS of the vehicle, and the definitions of the general operation states of the vehicle.

Underwater gliders are power-efficient autonomous vehicles that can gather ocean data for extended periods. Small-sized underwater gliders may be more suitable for operation, deployment, and recovery using unmanned platforms in coastal areas. Designing a light compact underwater glider with a relatively compact battery and confined space makes the electronics hardware system design a significant challenge regarding compactness and power efficiency. This work presents an overview of the electronics system designed to fulfill these requirements.

This paper outlines the design and testing process of the hull of a deep small Autonomous Underwater Vehicle (AUV), rated at 2000m depth. Many existing AUV pressure housings use aluminum or other isotropic traditional metals, instead of composites due to the complexities of the design of composites at such big load. The research at hand explains the process of design starting from setting the geometrical constraints for the design to mass production. To the best of the authors’ knowledge, none of the previous studies has presented such detailed description of the work. Carbon fiber reinforced epoxy material was chosen thanks to its high strength-to-weight ratio and similarity of its compressibility to sea water. Material characterization was performed to obtain the material properties under loading conditions using a modified method of the Combined Loading Compression testing technique. A specific fixture was designed and manufactured to test filament-wound tubes. An analytical model was developed using MATLAB, a finite element model was created using ABAQUS, and the results of the two models were compared. A set of recommendations was introduced for the stacking sequence to provide the lowest possible stresses, regardless on the diving depth of the vehicle. Afterwards, a quality control set of tests was conducted, including seawater absorption under high pressure and void analysis using destructive and non-destructive tests. Pilot samples were manufactured and tested in a pressure vessel, where it was cycle-tested and inspected using visual and ultrasonic testing. Other samples were fail-tested and showed a failure at ∼93% of the expected failure load. Such range can be considered good to provide safe operation for the vehicle at the designated depth, given that the factor of safety included covers more than 7% of the failure load. The proposed design methodology has shown that CFRE can be safely used even at such high depths.

The hull of an Autonomous Underwater Vehicle (AUV) is one of the main factors that determine its overall compressibility and drag. The hull as well, acting as a pressure vessel, is the most essential part that will allow the vehicle to accomplish deep diving. In order to minimize the total weight and volume of the vehicle, many modern vehicles benefit from composite materials rather than conventional materials. Moisture absorption, known as the hygral effect, has a significant impact on the mechanical properties of the material. Not to mention it also has an effect on the buoyancy of the vehicle, since the overall weight changes. This paper characterizes the seawater absorption and diffusivity of Carbon Fiber Reinforced Epoxy (CFRE) pressure hull samples in ambient conditions at sea level and at 1000m depth. The tests were performed using seawater taken from Oslo fjord in Norway and using a pressure vessel. Twelve specimens, all manufactured from CFRE using filament winding technique, were tested in both conditions, and the moisture absorption curve is compared. Periodic gravimetric measurements were taken, where the equilibrium state was reached after approximately 65 days at sea level and after 35 days at 1000m depth. The results showed that moisture diffusivity for composites used in underwater applications should be defined by both pressure and temperature since it changes with respect to submersion depth.