SPAWAR considers current design solutions cost inefficient and size inefficient. Vessels can cost upwards of tens of thousands of dollars and weigh dozens of pounds. They employ expensive and heavy materials and utilize obsolete and bulky electronic components. It is desirable to create a new generation of submersible vessels that are much smaller in size and cheaper to produce, allowing for a more expansive ocean monitoring network at the same or lesser cost.
Our project called for a pressure vessel that is submersible up to 100 meters which corresponds to a pressure of about 1 MPa It should be able to operate in a corrosive saltwater environment (30-45 parts of salt per thousand parts of water) at ocean temperatures between 0-30 degrees Celsius. It should integrate a variable ballast system, Conductivity, Thermal, Depth (CTD) sensor, and an Android smartphone. Above all, the vessel should be able to be produced easily and inexpensively (<$2000).
Our prototype design solution consists of a PVC cylinder of 15.24 centimeters nominal diameter and 0.508 meters in length. Two delrin bulkheads fitted with dual redundant O-rings plug both ends of the cylinder. An electronics tray is bolted to the inward face of the top bulkhead, and houses all of the vessel’s electronics. An internal housing is bolted to the inward face of the bottom bulkhead, and houses the vessel’s variable ballast system and an array of forty rechargeable 1.5 volt batteries. A delrin collar attaches to the bottom bulkhead to accommodate the anodized aluminum external bladder. The CTD sensor is mounted to the top bulkhead wall.
For operation, the cylinder is placed in the water such that its axis is perpendicular to the sea floor. When activated via Android or computer terminal, the ballast system pumps fluorinert fluid between the internal reservoir and external bladder to vary the system’s overall volume. This increases or decreases the buoyancy force of the vessel, allowing it to rise or sink respectively.
A finite element analysis as well as a physical pressure chamber test determined that our vessel is able to function at the required depth. An underwater pool test verified the vessel’s ability to maintain an upright posture and confirmed the vessel’s predicted vertical speed of no less than 0.15 meters per second. At sea level, the vessel is able to move all of its fluorinert to and from the external bladder in less than 30 seconds. Excluding the CTD sensor but including the Android smartphone, the cost of the buoy is predicted at 900 dollars per unit.