Probabilistic hazard analyses, which have been successfully used to evaluate various geohazards in both onshore and offshore environments, provide results that feed directly into project risk analyses. Evolving regulatory environments likely will incorporate more probabilistic and risk-based analyses in the future and this paper describes some of the key issues that need to be addressed in such studies and illustrates how these methodologies have been implemented successfully to address ground motion, fault displacement, and volcanic hazards in regulatory environments.
Probabilistic approaches and methodologies provide a framework to incorporate explicitly geologic, seismologic, and geophysical data into assessments of a variety of geohazards that may impact the development and design of offshore facilities. The results of such analyses, which commonly are given as annual probabilities or frequencies of exceedance, feed directly into risk assessments. Further, the expression of various types of hazards in a comparable format (e.g., hazard curves) allows for design criteria to be developed at consistent annual frequencies of exceedance. Probabilistic approaches to evaluate seismic hazard related to strong ground motion, which were developed over three decades ago [1], [2], are well established and widely used as the basis for determining seismic design for critical facilities onshore as well as for offshore engineered facilities. Probablistic approaches also have been used to evaluate the hazard posed by both intrusive and extrusive volcanic events [3], [4], [5]. More recently, probabilisitic methodologies, which were developed in conjuction with seismic hazard assessments for the proposed nuclear waste repository at Yucca Mountain, Nevada [6], [7], have have been employed to evaluate fault displacement hazard to aid in the design of proposed offshore facilities in deep-water [8], [9].
In complex geologic settings, geohazard assessments must address a variety of uncertainties and limitations of available data. Despite the advances that have been made in the application of probabilistic analyses for assessment of various types of geohazards, there are still controversies and misunderstandings surrounding their use. In this paper, we describe the methodologies for hazard analyses and some of the issues that should be considered in outlining the scope of a probablisitic study to address these limitations and uncertainties. Through case studies we highlight some of different approaches and levels of effort that may be considered to meet specific project design and regulatory guidelines. In a companion paper by Angell and Hanson [10], we describe the types of characterization studies that are needed to provide basic inputs to the analysis.
The case studies are based on probablisitic methodologies that have been shown to be particularly effective in modeling and evaluating hazards from strong ground motion (probablisitic seismic hazard analysis, PSHA), fault displacement (probabilisitic fault displacement hazard analysis, PFDHA), and volcanic events (probabilisitic volcanic hazard analysis,PVHA). Similar types of studies to evaluate the hazard posed by slope failures are still being developed.
