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Profiling From 6,000 Meters With the APEX-Deep Float

By Ernest Petzrick • James Truman • Hugh Fargher

APEX-Deep float deployed near Puerto Rico.
The oceanographic community has a new tool that promises to mitigate the significant challenges of collecting data in the deep oceans, the APEX-Deep—a 6,000-meter-rated profiling float. Motivated by research, Teledyne Webb Research (TWR), based in East Falmouth, Massachusetts, recognized they were uniquely qualified to take on the challenge of designing an affordable, low-logistics, profiling float capable of repeated cycles to 6,000 meters. After an intensive design and testing effort, TWR conducted multiple profiles to more than 6,000 meters in the Puerto Rico Trench in February and March 2013.

The Main Elements
The main elements of the APEX-Deep float consist of a controller, high-pressure hydraulic pump for the buoyancy engine and a pressure housing. The patent-pending housing, which incorporates entry ports for sensors, is also protected inside a custom hard hat.

The pressure housing was achieved using a patent-pending glass sphere with holes. This represented the most cost-effective solution for deep profiling hulls and avoided the use of more expensive materials, such as titanium. The housing was also small and light enough to be carried by two people. Once populated with the controller and hydraulic pump, the whole system was placed in a custom protective hard hat, resulting in a float that could be deployed without damage.

The hydraulic pump and bladder system used on existing APEX floats (rated for 2,000 meters) would not function on APEX-Deep floats. A larger oil displacement was required for the float to descend to 6,000 meters, while being able to then reach the surface again. After extensive design studies and testing of prototype pump and bladder combinations, a fixed displacement pump was chosen, with oil pumped into rubber tubes wrapped in a patent-pending configuration around the housing to change the overall float volume. In June 2012, the pump and bladder passed bench testing to 150 simulated profiles (at pressure) without failure.

Similar to existing APEX profiling floats, a pneumatic system was used to optimize satellite communications when the profiler surfaced by ensuring that the float antenna was positioned well above the surface. Air bladders wrapped around the housing (similar to the rubber tubes used to contain pumped oil) were used, being inflated with air from the partially evacuated housing. This provided a more efficient mechanism (in terms of energy consumption) than pumping extra oil once the float was at the surface.

TWR's new APF-11 controller was used for APEX-Deep to take advantage of its new capabilities and long life expectancy. A robust code base, the APF-11 was easily modified to command the new pump and valve system. TWR software engineers also invented unique testing environments to refine the APF-11 code branch and optimize deep performance.

Once assembled, the profiler hydraulic system was bench-tested at full pressure using a copper ocean assembly, with the CTD pressure port attached to a tube containing water at high pressure. This was particularly useful for ensuring that the APEX-Deep float completed the required park/profile/ascend/telemetry phases, while being able to closely monitor each step through a connected laptop. Also, an external power supply was attached to the float during these tests to avoid draining the batteries.

Another set of tests concentrated on proving that the glass pressure housing could undergo repeated cycling to the required depths. Although glass spheres have routinely been deployed to 6,000 meters, they are usually not cycled multiple times. In order to understand the viability of glass spheres as multiuse vehicle hulls, Teledyne conducted cycle tests on representative glass spheres through 2011 and 2012. First, flotation type spheres with one hole were repeatedly cycled to 10,000 psi (representing around 6,900 meters depth). After surviving 600 cycles, it was determined that glass spheres could survive repeated cycling. Next, holes of various sizes were drilled to determine the maximum diameter feed-through and configuration. These spheres were also tested to 10,000 psi for multiple cycles. During these tests it was discovered that spalling occurred around the edges of most holes, and fine glass dust would accumulate in the hole enclosed within the feed-through fixture device. Further testing revealed that the spalling occurred during the early cycles, and the quantity of glass dust decreased with subsequent cycles. It was determined that spalling resulted from stress relief as the fixtures seated, and movement of the fixtures in the holes was the source of the glass dust.

Various techniques were explored to reduce glass dust generation, resulting in a glass housing that could survive several hundred cycles to 10,000 psi. As a result of lessons learned from the deployment, minor changes to the hole sizes and through glass connectors were made and tested to 600 cycles. To continue this article please click here.

Ernest Petzrick is a senior product line manager of APEX profiling floats at Teledyne Webb Research. He is a graduate of Suffolk University and the Naval Postgraduate School, United States Naval Academy.

James Truman is a hardware engineering manager at Teledyne Webb Research.

Hugh Fargher is an APEX applications engineer at Teledyne Webb Research.

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