Risk Assessment and Management for Offshore Geohazards

The state-of-the-art in understanding offshore geohazards has advanced significantly in the past decade. The advances have been mostly driven by the needs of the offshore petroleum industry. The pace of exploration and field development on the continental slopes and in ultra-deep waters worldwide has increased rapidly during the past decade. Activities are concentrated in areas where the sediment thickness is high and accompanied by diapirism and overpressured formations, requiring the offshore petroleum industry to understand and quantify the risk associated with various seafloor geohazards. An increasing number of observations of seabed instabilities, often on slopes with very low inclination, is of concern to the oil industry. The size of the affected areas range from small surficial slumps to enormous retrogressive slide events. The latter may affect field development areas several kilometres upslope as well as downslope of a slide initiation area. Rapid sediment deposition may generate excess pore fluid pressures, resulting in overpressured formations, and reduced sediment strength. This may lead to increased faulting, diapirism and fluid flow processes, with possible impact on local seabed inclination and seabed instability. On the continental slopes, the pore pressure distribution is a key factor in assessment of slide risk. Natural overpressured formations also represent a drilling hazard regarding shallow gas and shallow water flow. The understanding of the geological processes that create hazardous conditions, the geomechanical explanation of observed instabilities, the dating of these events and the evaluation of present and near future conditions are key elements in geohazard investigations. Likewise, human-induced changes have to be understood and evaluated with respect to their risk potential. Geohazard risk assessment is an integral part of the overall risk assessment in offshore oil and gas projects. The assessment of the risk associated with submarine mass movements is thus not just a matter related to commercial interests of oil companies. The societal and environmental consequences of such events could also be enormous for coastal communities. For instance, large submarine slides may generate tsunamis with potential for severe damage along the coastlines. Risk assessment for offshore geohazard requires a systematic search for potentially dangerous scenarios and associated consequences, followed by quantitative assessment of the likelihood of occurrence and vulnerability of the elements at risk.