Design and Testing of a Pressure Retaining Sampler
for Sedimentary Gases and Gas-Hydrates

J. Erickson, P. Brewer, G. Friederich E. T. Peltzer
Monterey Bay Aquarium Research Institute, Moss Landing, Ca.

Proceedings of the Marine Technology Society / Institute of Electrical
and Electronics Engineers Oceans Conference (2001) 4: 2396-2399.


ABSTRACT

Marine sediments often contain appreciable quantities of dissolved gases or methane clathrate hydrates. Clathrate hydrates are a physical combination of methane and other low molecular weight gases and water molecules into a substance similar to water ice that occurs at the relatively low temperatures and high pressures found in the deep ocean. Recovery of cores containing hydrates or high concentrations of dissolved gases is often problematic because of the rapid decomposition of the hydrates and the very large gas expansion that occurs due to the great reduction of pressure as the sample transits to the sea surface. In the past, the clathrate samples that have been recovered have dissociated, partially or completely, by the time they reach the sea surface. In order to preserve and experiment on recovered clathrate samples, they must be held at pressures equal to or greater than the conditions where they were collected and within a few degrees of their ambient temperature. This necessitates that the sampling equipment used and the sample storage facilities be specially designed to preserve these conditions. Specifically, any expansion of the pressure vessel during recover must be compensated for as the incompressibility of seawater results in a greatly diminished pressure within the collecting vessel.

A hydraulically actuated pressure-case has been developed to recover "in-tact" samples of methane clathrate hydrate and gassy sediments from the sea floor. This system can be deployed on any submersible or ROV that has hydraulic control lines available for user mounted devices. The pressure-case will hold a small push core (7.6 cm dia., 33.5 cm loa) or similarly sized sample at near in situ pressure and temperature conditions. Pressure is maintained through the use of a gas accumulator compensation system. An exterior containment box, fabricated from clear polycarbonate, surrounds the pressure-case. Once the sample has been collected and the pressure-case securely closed, this exterior box can then be closed capturing 44.5 liters of ambient seawater. The contained seawater serves to buffer the temperature of the pressure-case at in situ conditions during the ROV recovery operation. Quick-connect fittings on the hydraulic lines and quick release pins on the support frame allow for easy removal of the pressure-case from the exterior housing for quick transfer to the shipboard laboratory. There the sample may be drained and / or decompressed under controlled conditions through pressure ports provided in the top and bottom of the pressure case or placed in cold storage for preservation of the sample.

© 2001 by Marine Technology Society.


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