Searching for Solid Gas at 2000 Fathoms

The Monterrey Bay Aquarium Research Institute used COSMOSWorks to protect a precision instrument from deep ocean pressures.

By Louise Elliot

Ocean science research offers many challenges. One of the greatest is identifying objects at great depth without collecting physical samples. This is a primary focus for the Monterrey Bay Aquarium Research Institute (MBARI) of Moss Landing, California (about 20 miles north of the famed aquarium), where engineers develop instruments for unmanned vehicles used to perform such identification.

ROVing for Frozen Gases

MABRI has two ROVs (remotely operated vehicles) that it uses for a variety of experiments. One vehicle, the Ventana, operates to a depth of 1,850 meters (6,068 feet). The other, Tiburon, operates down to 4,000 meters (13,123 feet).

One of MABRI’s experiments involves carrying a laser Raman spectrometer (LRS) to a depth of 4,000 meters. The LRS is a laboratory instrument manufactured by Kaiser Optical Systems, a developer of sensors, scientific instrumentation, and applied holographic technologies. Researchers use the LRS, which was modified for deep ocean application by MBARI, to determine the makeup of certain gases that freeze at great depth. This data, in turn, is used in ongoing research of ocean temperatures and currents.

The LRS works by shining monochromatic laser light at an object and using its spectrometer to examine the light scattered by the sample. The majority of the photons making up the light scatter at the same frequency as the laser; however, a small band shifts in the spectrum to a frequency that is specific to a certain molecular structure allows oceanographers to identify different deep gases and other matter without requiring physical samples.

Making Equipment Function Under Pressure

But at 4,000 meters, the LRS requires a specially designed pressure case to withstand 6,000psi—pressure that would damage a sensitive scientific instrument. To counteract these forces, Mark Brown, who was a mechanical engineer for design at MBARI prior to a recent promotion to manufacturing group leader, used COSMOSWorks to iterate designs for the pressure housing.

This trio of sceenshots shows an electronics housing component undergoing weight reduction iterations in COSMOSWorks. Click on images to enlarge. 

“COSMOSWorks let me iterate designs to reduce the weight of the housing and stiffen the case so that it did the job properly,” Brown says. Other factors affecting the LRS case design included thermal issues, vibration, and movement under load.

The equipment actually fits into three compact pressure housings. An electronics housing contains a single-board computer, power components, and the 100mW 532 nm excitation laser. Made of glass filament-reinforced epoxy, this housing is a little lighter and less costly than a conventional aluminum or titanium design rated for the same depth.

The electronics housing.

An off-the-shelf titanium housing holds the holographically filtered probe head. This connects to the laser and spectrometer by MBARI-built penetrating fiber optic cables.

Finally, the spectrometer’s optical bench CCD camera made by Andor Technology, a manufacturer of scientific imaging and spectroscopy equipment, and its associated electronics go into a separate housing made of 7075-T6 grade aluminum. The final selection of material was based primarily on thermal capacity for cooling, budget, and weight restrictions imposed by the ROV’s carrying capacity.

The laser housing.

Robustness, Corrosion-Resistance Key

Brown concentrated his efforts on the design of the aluminum spectrometer housing. The housing has a 90-degree angle going into a tube with domed ends, and a circular perforation that enables placement of the camera connected to the optical bench. “The electronic equipment needs to pushed into the tube, and it’s a tight fit,” he says.

The laser housing, uncapped, showing the perforation for the CCD camera.

“The largest portion of the housing consists of three tubes bolted together,” Brown says. “The middle one has the hole that enables containment of the CCD. If we just wanted to hold all the parts in one place, we could have used a big can, but at 6,000 psi pressure, in a remotely operated vehicle, the overall weight is as big an issue as it would be for a spacecraft.”

MBARI chose aluminum for the housing because of its robustness and resistance to corrosion; big issues in the ocean’s harsh operating environment. “The instrument will be handled roughly during launch and recovery from the research vessel,” says Brown. “I always design for robustness, because of the ocean’s tough environment. In high sea states it’s easy to break instruments…. COSMOS analysis gave us a way to reduce the weight and increase the strength.” He aimed for a factor of safety in excess of two, and says that he also made sure to account for buckling issues because the design has a perforation near the middle of the assembly.

The design features a modified ring-stiffened cylinder, for which, says Brown, “we needed to beef up the flange to accommodate for the penetration used by the CCD.” He iterated on the radius of the flange to meet his pressure and stiffness requirements, and made a variety of geometric changes, going back and forth between SolidWorks and COSMOSWorks. The overall design went through four geometric iterations, as well as a number of studies of the appropriate thickness of the aluminum.

“The housing design started out as a weldment, but in the end we used a single large aluminum billet to avoid potential inclusions or laminations that can happen with any welding process,” he says. “Because the potential for stress … to occur at geometric imperfections is high, we try to design with such possible problems in mind. We had to machine the middle node on a four-axis CNC machine to get some of the contours.”

Unpredictable Conditions

All these considerations had to be met for the device to operate reliably in an environment that precluded maintenance. Brown reports that the scientists wanted to identify a number of gases, such as carbon dioxide, methane, and higher hydrocarbons, which appear in solid (ice) form at these depths because of both temperature and pressure. Raman spectroscopy enables study of these substances as gases, in dissolved form, or incorporated in clathrates.

“They need to find out the variables that are involved in these substances which, as they rise, change state from solid to gel to gas.” The tools also analyze minerals and other solid targets, including sulfides, anhydrite, calcium carbonates, silicates, feldspars, magnetite, and hematite.

“Conditions at the bottom aren’t predictable,” says Brown. “Ices occur. Why are they there? Why do they change state? The whole field of oceanography is changing as a variety of organizations start to put ocean observatories in place, just as we’ve had astronomical observatories for so long.”

Contributing Editor Louise Elliott is a freelance writer based in California. Offer Louise your feedback on this article through [email protected].