Design of deep foundations within oceanic gas hydrate-bearing sediments (HBS) can be problematic and calls for special attention to incorporate the effect of gas hydrate melting. As naturally occurring gas hydrates in the seabed sediments begin dissociation process due to temperature or pressure changes, the pore pressure increases associated with volume expansion of the gas released during gas hydrate dissociation will likely result in fractures filled with pressurized fluid products open accompanied by softening of soil in gas and water intrusion zone together with seafloor and subsurface sediment motions.
In this paper, a numerical approach is described to investigate the effects of gas hydrate dissociation induced by constant heating from a hot wellbore on sediment engineering properties and the radial extent of hydrate dissociation influence zone. The effects of the following process during gas hydrate melting on soil parameters are discussed:
thermal stresses;
gas exsolution during heating;
seepage- induced swelling effects and
fracturing.
Estimation of hydrate dissociation influence zone employs numerical assessment of fracture formation taking into account of pore seepage of fluid and gas from fractures, which is premised on either (1) development of a pattern of laterally expanding horizontal fractures evenly spaced vertically throughout the hydrate-bearing zone and centered on the wellbore, or (2) development of vertical fractures which will intersect the seafloor to allow released gasses to vent into water column. If the fractures form vertically from the origin of hydrate dissociation, the effects of hydrate melting on sediment parameters for determining deep foundation capacities will be essentially confined within the dissociation front; on the other hand, in the event of horizontal soil fracture formation, the radial extent of dissociation influence zone on soil properties will extend beyond the dissociation front especially for relatively high gas hydrate concentration.
Discussion of deep foundation design (tension-leg platform (TLP) piles) in hydrate environment considering effects of hydrate dissociation due to elevated temperature is also presented herein. The potential sediment strength loss and soil loads exerted on the pile resulting from sediment motions induced by fracture growth should be incorporated in the design process.
