Design of Deep Foundations in Marine Gas Hydrate-Bearing Sediments

摘要

Design of deep foundations within oceanic gas hydrate-bearingsediments (HBS) can be problematic and calls for special attention toincorporate the effect of gas hydrate melting. As naturally occurringgas hydrates in the seabed sediments begin dissociation process due totemperature or pressure changes, the pore pressure increases associatedwith volume expansion of the gas released during gas hydratedissociation will likely result in fractures filled with pressurized fluidproducts open accompanied by softening of soil in gas and waterintrusion zone together with seafloor and subsurface sediment motions.In this paper, a numerical approach is described to investigate theeffects of gas hydrate dissociation induced by constant heating from ahot wellbore on sediment engineering properties and the radial extent ofhydrate dissociation influence zone. The effects of the followingprocess during gas hydrate melting on soil parameters are discussed: (1)thermal stresses; (2) gas exsolution during heating; (3)seepage- induced swelling effects and (4) fracturing. Estimation ofhydrate dissociation influence zone employs numerical assessment offracture formation taking into account of pore seepage of fluid and gasfrom fractures, which is premised on either (1) development of apattern of laterally expanding horizontal fractures evenly spacedvertically throughout the hydrate-bearing zone and centered on thewellbore, or (2) development of vertical fractures which will intersectthe seafloor to allow released gasses to vent into water column. If thefractures form vertically from the origin of hydrate dissociation, theeffects of hydrate melting on sediment parameters for determining deepfoundation capacities will be essentially confined within thedissociation front; on the other hand, in the event of horizontal soilfracture formation, the radial extent of dissociation influence zone onsoil properties will extend beyond the dissociation front especially forrelatively high gas hydrate concentration.Discussion of deep foundation design (tension-leg platform (TLP)piles) in hydrate environment considering effects of hydratedissociation due to elevated temperature is also presented herein. Thepotential sediment strength loss and soil loads exerted on the pileresulting from sediment motions induced by fracture growth should beincorporated in the design process.