Summary
Gas hydrates are solid, icelike materials formed by gas molecules included in a crystalline water lattice. They belong to a class of compounds termed “clathrates”. The term clathrate refers to the fact that the water molecules surround the gas molecules in a cage-like structure.
The phase equilibrium diagram of many hydrates has been determined through laboratory experiments. For a given pressure, they enable the calculation of the temperature range for which the hydrate remains stable. At the bottom of the oceans, under 500 meters of water, the temperature is around 3°C and the pressure is around 50 bars. For those thermodynamic conditions, methane gas hydrates remain stable. Therefore, methane hydrates can be encountered in deep waters at relatively small depths below the mudline of the oceans. Within the seafloor, the increase in temperature due to the geothermal gradient prevents hydrates from remaining stable beyond a certain depth below the mudline.
It has been known for a long time that gas hydrates can form during drilling for oil or in pipelines. These are man made gas hydrates. Gas hydrates were first discovered in 1965 in the permafrost region of the Soviet Union by Makogon.
Today they are recognized to be widespread in the outer continental margins of the oceans and in a portion of the permafrost region. In the case of offshore platforms, oil is pumped from a reservoir located typically around 5000 meters below the mudline.
Due to the geothermal gradient, the temperature in this reservoir is around 100°C. Thus, the conductor pipes through which the oil rises are heated; this may cause the temperature in the soil around the conductors to exceed the upper stable temperature of hydrates and the hydrates can melt.
The effect of melting hydrates on foundation soils is complex. Melting may occur slowly as the heating front advances and significant quantities of gas drain to the seafloor. On the other hand, if gas is trapped with the soil strata, then considerable soil heave may result, possibly affecting the pile foundation. The possible decrease in soil strength and stiffness would lead to a decrease in the ultimate capacity of the foundation and to excessive settlements.
In trying to solve this problem, the first step is to develop a technique to detect the hydrate layers. This can be done by using the difference between the hydrates engineering properties and those of the surrounding soil. In Geotechnical Engineering, the Cone Penetrometer Test has been used successfully for forty years to investigate the stratigraphy of soil deposits. Layers of either pure gas hydrates or frozen sand saturated with hydrates may lead to cone penetrometer profiles which are different from those of the surrounding deposits. Furthermore, the cone penetrometer is readily available for offshore investigations. Therefore, the first logical step in gas hydrates detection was to use the Cone Penetrometer Test.
The objective of this research program is to determine whether or not the Cone Penetration Test is suitable for the detection of natural hydrate layers.
