Hello Readers. My name is Dan Kot and I am an ocean bottom seismometer (OBS) technician with Woods Hole Oceanographic. For those of you that are new to the world of oceanography, I thought I would touch upon how we withstand the pressure at the bottom of the ocean.
There are a number of ways to withstand or mitigate the pressure. In this post I’m going to focus on the design that was chosen for the OBS’s used for this project. This OBS design is referred to as a D2, however, unofficially we call them peanuts due to their resemblance of a peanut. A peanut consists of 3 main pressure housings and is rated to withstand the pressures found more than 5,000 meters below the surface of the ocean. For this project they were deployed to depths roughly between 3,000 and 5,000 meters. The D2 has three main pressure housings, two glass sphere pressure housings and a titanium cylinder that houses the seismic sensor. For this post I’m going to focus on the glass sphere pressure housings.
The main body of a peanut houses two glass sphere pressure housings. One glass sphere is 17” in diameter and the other is 12” in diameter. The larger of the two spheres is made by a German based company called Nautilus. The smaller is made by a McLane, a company based in Falmouth, Massachusetts. Glass spheres have been a work horse in oceanography for a long time. When compared to other options with similar pressure ratings, glass spheres have a number of advantages. They provide a significant amount of buoyancy, weigh less in air, and are far less expensive. However there are some catastrophic drawbacks. Despite the glass being rated to withstand such pressures it’s still like all the other glass you are familiar with, fragile. If the glass has a defect or damage it has the potential to implode under pressure. In our compact design an implosion is a catastrophic event. It’ll destroy all the instrumentation we house in the sphere, the force will likely implode the second sphere, and it will shred the yellow hard hat (shell). The majority of the glass turns into what amounts to dust. It’s quite amazing.
Due to this reason, all glass spheres we purchase are pressure tested first. This helps identify defects. It’s not perfect, but further testing is often cost prohibitive and arguable unwarranted due to the successful track record we’ve experienced. Once in our possession the glass is only handled by trained personnel that adhere to our handling guidelines. The glass spheres also go through periodic inspections. When we do fail one, they make interesting bird baths, punch bowls, and terrariums, to name a few ideas.
The glass sphere housings consist of two hemispheres. This allows us to place things in them prior to sealing. In the D2 we have the brains of the operation in one sphere and the primary batteries for the system in another. The glass has drilled holes in specified locations for our penetrators (cables with a bulkhead on one end). These cables connect the two spheres, connect to sensors, and allow us to communicate with the instruments. When the hemispheres are put together (mated), we pull a vacuum to hold the two halves together. There is an additional drilled hole to accommodate a vacuum port which is what we use to pull or release the vacuum. Once we pull a vacuum we seal that mating surface with butyl tape (a sealant). The butyl tape is sticky to the touch so to keep it from sticking to us or other objects we cover it with 2” wide electrical tape.
That in short, is how we use glass to withstand the pressures of the workplace!