Drilling hazard assessment for hydrate bearing sediments including drilling through the bottom-simulating reflectors

To achieve the scientific goals of the NGHP Expedition 01 project, it was necessary as a preliminary step to examine the available marine 2-D seismic data covering then-proposed investigation drilling sites for potential drilling geohazards. A total of 19 potential drilling sites, located in offshore areas of the northern Indian Ocean: the Krishna-Godavari Basin, the Kerala-Konkan Basin, the Mahanadi Basin, and the Andaman Islands, were investigated. The NGHP Expedition 01 staff identified several potential drilling hazards to be investigated at each site, including, but not limited to, the common drilling hazards of seafloor slopes and failure conditions, complex seafloor topography and anomalous amplitude response, shallow gas hydrates, shallow free gas, shallow water-flow, and faults/fractures. Specific proposed drilling sites were investigated over a depth range beginning at the seafloor and extending to 350-700 m below the seafloor, depending upon the specific planned depth at each site. These investigations resulted in some sites being interpreted to be clear of potential drilling geohazards, while other sites were determined to have potentials for specific geohazards significant enough for reconsideration. These reconsiderations consisted of moving specific sites to more favorable locations interpreted to be relatively unsusceptible or less susceptible to a specific identified geohazard, and/or reducing the planned depth of drilling, or removing a potential site at least temporarily from consideration. This work contributed to favorable results from the NGHP Expedition 01 in that no drilling geohazards were reportedly encountered.