WHOI engineer Rod Catanach assists in the installation of the upper forebody block of syntactic foam around the personnel sphere on Alvin during the project to upgrade the human-occupied deep-sea submersible. (Tom Kleindinst, Woods Hole Oceanographic Institution) [ Hide caption ]
From the beginning of 2011 to May 2013, Alvin, the U.S. science community’s only human-occupied submersible dedicated to deep-sea research, underwent a thorough overhaul and upgrade to greatly enhance its capabilities. In the end, the iconic sub was redesigned and rebuilt, piece by piece, by a one-of-a-kind team of engineers, technicians, and pilots at WHOI.
Engineer Rod Catanach describes the fabrication, testing, and installation of the syntactic foam that provides buoyancy for Alvin.
We use syntactic foam on many of our deep-sea vehicles. It’s the white material on the outside of Alvin. The titanium sphere and frame are Alvin’s structural backbone. The foam isn’t structural; it’s for flotation. It’s made of tiny, hollow, glass microspheres mixed with epoxy to make hard blocks. The microspheres are so small they look like talcum powder. When they are packed tightly together with minimum amounts of epoxy, the foam is strong enough to withstand high compression but still be buoyant in water. The mixture is then put into molds, like a big bread pan, and mixed and allowed to cure, to make blocks of foam.
The new sub is bigger and has a heavier sphere, and it is designed to eventually dive to 6,500 meters. Because this is a manned submersible, we use a greater safety factor and needed the foam to go 1.5 times the maximum depth of 6,500 meters. So we had to get stronger foam that could go deeper, but we didn’t want it to be heavier.
We worked with two different companies because the development and production were critical, and we could not afford to have one company fail to produce. They extensively tested their foam, and then we did additional testing here.
Instead of the usual procedure of testing small samples from random blocks, we tested all of the foam in a pressure test chamber to a minimum of 12,100 psi, which is 1.25 times 9,677 psi. We used transducers that were built to monitor microcracking on structures like bridges to ‘listen’ for small implosions or microcracking in the foam that would indicate that it was failing. As long as we didn’t hear it start cracking, we knew it was strong enough. One hundred percent of our foam was tested, and we have great confidence in it.
We also tested the foam to ensure that it did not soak up water over time. We weighed 20 percent of the blocks, put them through a 24-hour soak under pressure. Lastly, we tested one block from each batch to destruction to make sure that it crushed only at a depth greater than14,515 psi [pounds per square inch]. That is 1.5 times the pressure at 6,500 meters, or 9,677 psi.
These blocks were then bonded together into larger shapes. They were machined flat at joining surfaces, so that only a minimum amount of adhesive was needed to bond them. The more adhesive, the heavier the blocks get, and we also wanted to eliminate any air to get a good bond joint. Prior to this, we did bonding tests to prove both adhesive and technique. We did tension tests, where we tried to pull two blocks apart, and shear tests, where we pushed sideways on them.
The bonded blocks were then machined to make the shapes we needed. And finally, in certain areas, we added on a protective layer of fiberglass and paint. Here we are installing the upper forebody blocks of syntactic foam. The inside edges of these blocks have been shaped with a spherical curve to fit around the personnel sphere. The foam is buoyant and light in water, but in air, these pieces are 1,390 pounds each.
Alvin is owned by the U.S. Navy and operated by WHOI. The current upgrade was funded largely by the National Science Foundation, with support from private donations to WHOI.