Oil and gas developments often require placing equipment, e.g., subsea wells, pipelines and flowlines, foundation systems for floating structures, in areas with sloping seafloors. Submarine slope failures can occur in such areas and create soil slides. Thus the stability of submarine slopes must be considered in selecting the site for installing and designing seafloor equipment.
Assessing submarine slope stability requires estimating the likelihood, extent, and impact of a slide during the lifetime of the facility. This assessment is difficult due to the large difference in time scales between the project life (10’s of years) and the geologic processes and triggering mechanisms that cause the slides (10,000’s of years) Such an assessment is best approached through a probabilistic risk analysis that considers the risks to the equipment; the causes, likelihood, and behavior of submarine slides; and the uncertainties.
A forum of experts from industry, government, and academia was held in 2002 (1, 2) to discuss the current state-of-the-art and state-of-the-practice and to identify areas where future research was needed to advance capabilities to assess submarine slope stability and the impact of submarine slides. That forum concluded that a comprehensive data base should be developed for historical slides containing information on the seafloor characteristics (soil properties, slope topography, geology), triggering mechanisms, and the characteristics and extent of the slide. This database could then be used to develop, improve, and test models for predicting slope stability, slide occurrence and behavior, and to assess the impact of uncertainties in seafloor characteristics and triggering mechanisms in predicting the likelihood and behavior of slides. By looking for similarities between a new site under investigation for a subsea installation and sites of historical slides, data from the historical slides might also be useful in assessing the slope stability and risks of a slide for the new site. This was the basis for the project reported here.
Development of a Database and Assessment of Seafloor Slope Stability based on Published Literature This work resulted from a research project conducted by J.J. Hance for his Master of Science in Engineering at the University of Texas under the supervision of Dr. Stephen G. Wright. Hance’s thesis (3) is attached
Based on published literature, a database was compiled the includes 534 submarine slide events. The database contains information on the geographic location, water depth, date and type of failure, potential triggering mechanisms, dimensions, slope angle, and soil types and properties. The data were examined to identify important characteristics of seafloor slope failures. While the database is substantial, significant geotechnical information was not available for many slope failures.
Fourteen different triggering mechanisms were identified and included in the database. Earthquakes are the most commonly reported trigger.
Slope stability analyses were performed to assess the likelihood of slides being triggered by gravity, rapid sedimentation (underconsolidation) and earthquakes. The analyses revealed that it is unlikely that most of the seafloor slope failures were triggered by gravity loads alone. Earthquake loading was confirmed as a common trigger, and rapid sedimentation (underconsolidation) was also a likely trigger of many slope failures.
It is important to note that the study revealed that a relatively large number of submarine slides occurred on much flatter (less than 10 degree) slopes and traveled much greater distances than slope failures on land. This strongly suggests that different mechanisms are prevalent for submarine slides, compared to those on land.
Hydroplaning is one mechanism that may explain such large runout distances. The mechanism of hydroplaning is summarized, and a simple sliding block model is presented to illustrate how conditions for hydroplaning can be developed. Rheological models have also been developed to explain slide runout, and several models are described. However, the rheological models do not seem to explain some of the very large runout distances observed in both experiments and actual seafloor slides. For many slides, hydroplaning appears to be the mechanism that can best account for large runout distances.