Probabilistic Fault Displacement Hazard Assessment For Flowlines and Export Pipelines, Mad Dog and Atlantis Field Developments, Deepwater Gulf of Mexico

Abstract

Abstract Seafloor faults having strong geomorphic expression and evidence for late Quaternary activity (i.e. < 150,000 years) are common geologic features associated with the Sigsbee Escarpment. Waterbottom maps derived from exploration 3D multichannel seismic data provided an early indication that several zones of seafloor faults are in the vicinity of the Mad Dog and Atlantis prospect areas. As part of the site investigation activities for field development BP initiated a study to characterize the potential hazard due to fault displacement. The fault displacement hazard study consists of five components:a site-wide structural geologic characterization of the style and origin of active faulting and fault-related deformation;development of a late Quaternary stratigraphic model to evaluate the history, recency, and rate of fault activity at the site;detailed characterization of faulting within limited study areas designated to capture fault behavior in areas of potential facilities development;a general description of the relationship between Quaternary active faulting and slope failure processes within the field area; anda probabilistic fault displacement hazard analysis (PFDHA) of the potential for fault rupture within the designated study areas that relates annual frequency of recurrence of faulting events to the size of the event. Changes in the style and origin of faulting and deformation of shallow (suprasalt) sediment across the individual field areas primarily is due to differences in the depth, geometry, and movement history of the underlying Sigsbee Salt Nappe. These relationships and the resulting geologic model for structural evolution of the suprasalt section has been used effectively to assess the site-wide geohazards not only for faulting, but also indirectly for slope failure and mass-gravity flows. Hazard from potential seafloor offset at fault crossings is judged to be moderate to low. Fault offsets of the shallowest horizons (less than 15 thousand years old) are typically less than ten meters to several tens of meters. Fault slip rates are on the order of tenths to several tens of metersper- thousand-years (m/kyr, also millimeters-per-year, or mm/yr), with most values in the range of 2-10 m/kyr Similarly, the probabilistic annual recurrence of 1-meter events is typically less than 10−3. These studies demonstrate that the presence of potentially active faults does not preclude safe development of seafloor facilities. To evaluate risk associated with potential seafloor faulting, integrated hazard studies can and should be conducted in the early stages of project development, with an underlying intent to understand the causative processes and quantitatively and explicitly evaluate the locations, magnitude and recurrence potential of displacement events.