Crush Depth: Adventures with Scientific Submersibles

Tissot in Submersible 2003

Me entering the Delta off Santa Cruz Island, 2005. Photo: Keri York.

“What’s your depth?” echoed down from the surface radio, unanswered for the third time in a row and sounding increasingly desperate. It was 2002 and I was riding inside the scientific submersible Delta, heading towards the seafloor off Anacapa island in an area known simply as the “footprint,” a deep-water fish, coral and sponge hot spot in the California Channel Islands. I was lying prone on my stomach looking out a porthole as the pilot, Doug Privitt, sat on a chair straddling my thighs. It’s a bit cramped, and definitely not for the claustrophobic, but it’s a fairly comfortable sub for an hour or two. By twisting around and craning my head I could just barely glimpse the depth gauge: 1200 ft. — the maximum operating depth for the sub. As Doug wasn’t answering the repeated surface queries, and I couldn’t see the bottom, I was starting to get a bit concerned. As we passed 1250 ft., and all I could see was darkness below us, the sub let out a low groan and I suddenly had a premonition of what it might be like to die underwater. Just what was the crush depth of the Delta, I wondered, and why did I not know this suddenly key piece of information? After all this was hardly my first dive in Delta.

footprint-map

Map showing the footprint off the California Channel Islands. It’s a small rocky reef surrounded by deep water.

Anyone who has ever been in a sub knows very well the concept of crush depth: the depth at which the submersible’s hull collapses due to building water pressure. On previous Delta cruises off Oregon in the late 1980s, I, along with the scientific crew, delighted in watching such incidences portrayed in films like U-571, Das Boot, and everyone’s favorite, the Abyss. But experiencing it first-hand was another matter. We laughed at the ridiculous launches in the Abyss, where the sub was dropped from 20 ft. up into a raging ocean; not to mention the colliding underwater sub battles. Submersibles are precisely engineered machines, designed to withstand strong isostatic pressures, those equally applied in all directions, but collisions or sudden directional stresses would likely result in severe leaks, at best. But hey, they were in a hurry and there were aliens underwater!

Watching the sub begin to implode beyond crush depth in the movie Abyss. Photo: 20th Century Fox Studios.

Just for reference, in U-571 in order to evade depth charges, the sub red lines at 160m (525 ft.) but they push it to 200m (656 ft.) with great trepidation — and survive, but just barely. But the truth is, subs like the Delta are largely based on technology not all that different from WW II subs. You see scientific diving, unlike commercial and military endeavors, are largely done on a shoe-string budget, so their aren’t a lot of fancy systems nor backups. One of my graduate students went out on a cable-laying ROV (remotely operated vehicle) cruise once and in addition to having two large, state-of-the-art ROVs (one for backup) the ship had a full bar, large 24-hr staff, and an on-board hot tub. If something breaks on a scientific sub operations cease for hours or days if needed and you just stay out in the ocean rolling with the swells. With no bar or hot tub to pass the time. Life is rough for us scientists!

screenshot-2016-09-09-21-41-04

Depth gauge approaching 200 m in the movie U-571. Photo: Universal Pictures.

Anyway, these were my thoughts as we passed 1280 ft. and once again I hear the surface, now almost pleading for a response “What’s your depth?” However, at this point Doug hands me the microphone and says “Here, talk to these guys. I can’t hear a word they are saying!” Then it suddenly occurred to me that Doug had been losing his hearing for years and must by now be quite deaf (why didn’t I notice this topside?). The irony of course was that I have had hearing difficulties my entire life and was just barely making out the conversation on the radio with my two hearing aids. Although I took comfort in knowing that Doug had designed and built Delta, and certainly knew its limits, my confidence level immediately dropped another notch. So just where is the bottom?

Delta

Delta, cruising underwater

So I twisted around as best I could and looked at another gauge, which read an altitude (distance off the bottom) of 125. But 125 what? meters? feet? Scientist mostly use metric so I thought it must be meters. Then by quick calculation (math does come in handy!): 1280 ft + 125m x 3.2 ft/m is about 1,680 ft. at the bottom. Now that may not seem very deep for most people, especially if you know the fact that the oceans are 10,000-15,000 feet deep on average, but in terms of pressure they are enormous. Here’s where you really want to remember your basic physics, particularly Boyle’s Law: at the surface the pressure is 14.7 lbs/in2, or one atmosphere. But as you descend in the ocean, pressure increases by 1 atmosphere every 33 ft. So, at 1200 feet it about 36 atmospheres or about 534 lbs/in2, increasing to 51 atmospheres or 748 lbs/in2 at 1680 ft. — a 40% increase and certainly if anything went wrong, lethal pressure. These calculations, in much rougher form I might add, flashed through my head in an instant as I stared at the gauge. We all knew the risks.

screenshot-2016-09-09-21-25-46

NOAA Scientist Tom Laidig inside Delta and looking out the starboard port-hole.

As I was struggling to communicate with the surface, suddenly, the seafloor came into view and a minute later we touched down at 1425 ft. on a flat, featureless muddy bottom. So it was feet after all! Later I learned that Delta had been pressure tested in a tank to 1700 ft. and was built to survive twice her operating depth, to 2400 ft. But that key piece of information was small comfort later on the surface when it had no immediate consequences.

delta-035

The front of a model of Delta. Looking through the ballast chamber at the observer’s port holes. It was through these ports that I could see the bottom.

But now the dive was officially starting so Doug blew the ballast tanks to achieve neutral buoyancy. For the Delta that meant using a Scuba tank filled with compressed air (there were two on board) to displace water out of the front outside ballast chamber and fill it was air. This is how the sub goes up and down; the electric motor primarily pushes it forward. But now, here on the bottom, Boyle’s Law came back to bite us: at 1425 ft., beyond the normal diving depth, Doug drained an entire tank of air into the ballast and nothing happened. The sub just sat there. Then I quickly realized why: that the deeper you go, the more the gas is compressed due to pressure (see above). The question now was: do we have enough compressed air to get off the seafloor? So Doug popped on another tank — our last I might add — and began to fill the ballast some more.

pisces

Story of the Pisces III diving accident in 973.

At his point I thought back to two well-known submersible accidents involving crew stuck on the ocean bottom: the Sea Link and Pisces III. In 1973 a tow line wrenched open a compartment on the Pisces III which flooded and sent the sub down to the seafloor at 175 ft. When they were rescued 75 hours later they only had 12 minutes of oxygen left. But fortunately nobody died. In contrast, the 1973 Johnson Sea Link accident did not have a happy ending. The sub became tangled on a cable and the crew was rescued after spending 24 hr at 360 ft. but two of the four occupants died from carbon dioxide poisoning. These horrific events flashed through my mind as the ballast tank continued to slowly fill. Luckily, after about half of the remaining tank had been used, Delta slowly lifted off the bottom and began to ascend. Time to move to shallower water.

The rest of the dive was fantastic scientifically but technically uneventful. Both great outcomes. The “footprint” is an amazing place and is now fully protected as a State and Federal Marine Reserve in the Channel Islands Marine Protected Areas.

foorprint corals

The seafloor at the “Footprint” off Anacapa Island. Photo by Oceana.

But this was just one dive among the 20-30 I have had the pleasure and privilege of making on the Delta, all of which were completely safe. In fact Delta has performed over 6,700 dives since 1982 and has a perfect safety record. The important truth about submersibles such as these is that they are key to the future of our oceans: we’ve only explored about five percent of our ocean seafloor — so the deep-sea is largely unknown.

And this is because we have only spent a tiny fraction of the money on ocean exploration that we’ve spent on space exploration. Don’t get me wrong, NASA is very important and essential, but we need a NASA-like organization for ocean exploration, because we need to be exploring and protecting our oceans, which sustains life for us all. Unfortunately things are headed in the wrong direction as the nations’ top undersea funding agency, the National Undersea Research Program (NURP), was merged into another program in 2007 and eventually eliminated in the 2014 federal budget. Its successor, Ocean Exploration and Research is a good program but doesn’t support the kind of long-term underwater research that was supported by NURP. Luckily, private companies. such as the Schmidt Ocean Institute are entering the ocean exploration arena. And while this is good news I can only wonder where all this is going. Hopefully deep ocean research is headed — as we finally surfaced at the end of the dive — off the bottom and above crush depth.

Diving in Delta off Southern California.

References:

  • Tissot, B.N., M.M. Yoklavich, M.S. Love, K. York, and M. Amend. 2006. Benthic invertebrates that form habitat structures on deep banks off southern California, with special reference to deep sea coral. Fish. Bull. U.S. 104:167-181. PDF
  • Tissot, B. N., W. W. Wakefield, M. A. Hixon and J. E. R. Clemons. 2008. Twenty years of fish-habitat studies on Heceta Bank, Oregon. pp. 203-217 In J.R. Reynolds and H.G. Greene (eds). Marine Habitat Mapping Technology for Alaska, Alaska Sea Grant College Program, Fairbanks, AK. CD ROM.doi:10.4027/mhmta.2008.15 PDF
  • Yoklavich, M. and V. C. O’Connell. 2008. Twenty Years of Research on Demersal Communities Using the Delta Submersible in the Northeast Pacific. Marine Habitat Mapping Technology for Alaska, J.R. Reynolds and H.G. Greene (eds.) 143 Alaska Sea Grant College Program, University of Alaska Fairbanks. doi:10.4027/mhmta.2008.10 PDF

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