Observations and Numerical Modeling of Debris Flows in the Na Kika Basin, Gulf of Mexico
Authors: Pirmez Carlos , Marr Jeffrey , Shipp Craig , Kopp Frans 
Published: October 2008

The Na Kika Basin in the eastern Gulf of Mexico experienced debris-flow events in geologically recent times (Late Pleistocene to Recent). To assess the potential risk of such debris-flows to the pipelines and structures associated with the Na Kika field development we performed a semi-quantitative risk assessment of the potential impact of debris-flows to the proposed pipeline network. The numerical model BING was used in conjunction with available sediment core information, seismic, and bathymetric data to model a number of sediment failure scenarios. Calibration of the model was done using data from two existing debris flow deposits. After calibration the model was used to simulate specific scenarios, using bathymetric profiles extending from potential source failure areas to proposed pipeline locations. We found that flows with sufficient volumes to impact the proposed engineering structures are generally inconsistent with observed geologic conditions in the area.

Introduction

This study was motivated by the proposed development of the oil and gas reservoirs located in the greater Na Kika Basin. Figure 1 shows the study area location in the eastern Gulf of Mexico. Seafloor attributes illustrate the complex topography associated with salt domes surrounding the basin (Figure 2). A previous studies of the seafloor in the greater Na Kika Basin identified two subsurface debris flow deposits (GEMS report, 1998). Deep-tow data acquired recently as part of pipeline route evaluation showed a debris flow deposit located on the seafloor in block MC 470 (Figure 3). These observations suggest the Na Kika Basin may be susceptible to future debris flow events that may pose risk to engineered structures in the area, such as pipelines, flexible tubing and anchors.

Results of simulations using the numerical model BING (Imran et al., 2001) are used to better understand the risks associated with potential debris flow events in the Na Kika Basin. The inputs parameters for the model are estimated from available field data. These input conditions include information about sediment rheology, flow-path bathymetry (Figure 4), and size of failure.

The simulations were performed in two phases. In Phase I we perform a calibration of BING using available seismic and core data from two debris flow deposits located in the Na Kika Basin. In Phase II, we use the calibrated model and fieldderived input parameters to simulate several "attack" scenarios, i.e., forward simulations to determine the conditions at which a pipeline or seafloor structure would be impacted in case a debris-flow occurred. For a given flow-path we explore a range of failure positions and failure volumes and gauge risk to the proposed engineering structures in the area.

The present study does not include an assessment of slope stability, or probability of a seafloor failure occurring at a particular location. Instead, our approach is to evaluate what happens to the failed sediment after failure. The results are evaluated vis-Ã -vis the local geological conditions. The volume of failed sediment, the rheological properties of the failed mass, and to a lesser extent the geometry of the failure, influence the runout distance of the flow. In the simulations, we iteratively vary these parameters until a best match is found between the model and field data for both runout distance and thickness, given the rheological conditions and initiation point assigned as input.