Characterizing Site Geologic Conditions for Quantitative Hazard Analyses

Abstract This paper describes a methodology for characterizing geologic features to support quantitative project risk assessment and decision modeling. The methodology contains the basic elements of a geohazard characterization to develop input for a probabilistic analysis of hazard (see companion paper by Hanson et al., OTC paper number 17675). We are suggesting this approach is also appropriate for supporting deterministic hazard analysis. It has the advantage of being able to withstand the close scrutiny of technical experts, and can provide the hazard results in a format that expresses the qualities of uncertainty and likelihood, both of which are fundamental components of decision modeling and risk assessment. Introduction Marine engineering projects, particularly in the deepwater environment can face severe technical challenges to design, construct and operate safe and reliable systems. The challenges are being met by expanding the application of existing technology and by innovation and proving of new designs. The result is often a complex system installed at great cost that must operate reliably for decades in a potentially hostile natural environment. Geohazards are geological processes that have the potential to adversely affect the planned facilities. The deepwater marine environment is a particularly geohazardous place, and he contribution of geohazard to overall project risk has increased as a result. The investigation and characterization of site geological conditions in the marine environment has undergone rapid growth in response to these increasing demands. The application of 3D seismic data, AUV technology, and deepwater geotechnical borings are examples of the new generation of site investigation tools that are now being used routinely, and in many cases required, for deepwater site characterization. Theze tools are capable of obtaining high resolution and very high resolution information that can be used to quantify the geohazard potential of a site or region. At the same time, there have been rapid changes in the decision-making process for both technical and economic issues associated with these projects prompted by the increasingly complex nature of the technical and managerial systems and the greater risk/reward factor. This paper describes a site investigation methodology for geohazard analysis that is well-suited to take advantage of the new site investigation techniques, and also can provide the content and format most useful to project risk assessment and decision modeling. The aims of a hazard analysis are to develop information regarding the "where", "how big" and "when" for any event that could potentially impact project safety. In a geohazard assessment the geologic characteristics that determine the level of hazard are:the location of a hazard feature;the potential magnitude of a hazard event; andthe rate of occurrence (recurrence) of hazard events. The location of hazard features is represented on maps, magnitude is typically expressed either on maps or as values in table format and rate of occurrences are as values based on past events in table format, and/or an areal extent in map view.