Design and operation of a pressure retaining sampler for sedimentary gases and gas hydrates

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 pressure and temperature conditions below their phase change boundary. 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 "intact" 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.