Lander Lab #4: Underwater Releases
“We can get anything to the bottom of the ocean,” quipped Don Walsh, Pilot #1 of Bathyscaphe Trieste. “The trick is getting it back.” A release is how we get our lander back. The anchor weight needs to be discarded, allowing the self-buoyant lander to float back to the surface. That’s the primary task of a release.Releases serve other secondary purposes. They are used to initiate action such as deploying a drop arm, closing a water sampler or animal trap, or releasing a surface buoy or hydrophone string.A clever at-sea adaptation of timer releases was reported by Scripps researchers Mantyla and Reid in 1977. Making measurements of salinity, oxygen and silicates in the Mariana Trench, they found the tapered wire rope on the hydrographic winch was of insufficient length to reach below 10km. Engineer Dave Muus proposed modifying a John Issacs “Monster Camera” free vehicle, a chain of components more like a short mooring than an ocean lander, to carry a string of Niskin bottles. A timer release of the messenger was set to occur 30 minutes after ETA on the seafloor. A second timer release was set to drop the anchor 10 minutes later. Dave rigged and deployed the free vehicle. It worked. Researchers found secondary benefits: the vehicle descended and rose twice as fast as the winch payout and inhaul, while the water samplers could be placed much closer to the bottom. They rigged and deployed a second time. It worked again.Releases are available both as commercial products, and in-house customized solutions. They can be sorted into functional categories: Acoustic, Timer, Magnetic, Electrolytic, Galvanic, Fusible Link, Pressure, Melt and Dissolvable.Expendable electrical release elements can be a burnwire (aka corrosive link, galvanic link) and fusible link or “flash wire”.A release can be broken down into a handful of elements.There’s the trigger that starts the release function, such as a countdown timer, acoustic command, bottom contact, or event sensor.There is the load bearing release element. This could be a burnwire, a flash wire, a rotating thread, a rotating rod with a semicircular segment on the end, or a retracting plunger. This in turn may release a force multiplier, such as a lever or pelican hook.There’s the power source, usually batteries, which could be supplemented by compressed air or squib gas generating cartridge.There’s the housing, a sphere or cylinder.Acoustic ReleasesToday many commercial acoustic releases are available.Most commercially available releases come as a fully integrated package, such as the EdgeTech 8242XS. The design allows the in-line load to be taken directly though the release housing. In-line tension is routed around the hydrophone by a heavy metal cage on the top. The cage also serves to protect the ceramic transducer from impact. The transducer is placed on top to provide a clear view to the surface for acoustic communication.Figure 2. The EdgeTech 8242XS acoustic release. (Photo by Edgetech)The release can be dualed with a second release, providing a back-up for critical functions, or paired with a strong back to substantially increase the load rating from 12,000-lbs to multiples higher. Typical lander anchor weights, however, are under 100-lbs, so the size, cost and weight are a little overkill. Some consider rotating shaft seals to be a weak point, though I’ve never seen those fail in the scores of sea trips I’ve been on.The release also functions as a transponder, providing a ranging function. Anyone who’s been to sea will agree that’s another benefit: one ping means a lot when it’s coming from the bottom of the ocean. Acoustic releases have an immense set of unique transmit/receive frequencies, command codes, and multiple frequencies to negate ambient noise fields, making them safe to use within acoustic range of each other. It’s wise to make a chart of manufacturers, release codes and frequencies then comparing those, especially if multiple groups are involved in a joint operation.Acoustic releases are preferred for long term deployments, where the lander remains safely on the bottom until the ship returns. This allows for changes to ship schedules, bad weather, unexpected ship problems, crew health, plus Murphy and his prankster friends.The top side part of an acoustic release is a matching deck communications box with overboard hydrophone.Figure 3. Teledyne Benthos UTS-9500 Universal Topside, an acoustic release deck command unit and overboard hydrophone. (Photo Teledyne Benthos)Manufacturers of the integrated acoustic release design include:Applied AcousticsDesert Star Systemsdevelogic GmbHEdgeTechiXblueTeledyne Benthos Sonardyne Subsea SonicsDownsides include this type of release can be heavy, with a water weight of up to -62 lbs, limited to 6km (Benthos has a 12km version), and only a single release function. The air weight is 79-lbs, plus the weight of a 17” glass sphere to lift it (+57-lbs buoyancy, 51-lbs air weight), then add additional buoyancy, counterweights, scientific payload, and the frame to tie all together. The assembled weight of the lander on deck becomes something more than can be hand deployed, suggesting the larger release designs are better suited to moorings than small landers. For shallow water, long term deployments, biofouling of the release mechanism can compromise a clean release. EdgeTech developed alternative release techniques including a “push-off” mechanism, where a threaded shaft unscrews a threaded link (PORT-LF) that holds the anchor weight, and a dual burnwire release (BRT-6000).SubSeaSonics