Summary
Objective
The objective of this research is to examine and review the mudflow/mudslide areas in the Gulf of Mexico caused by Hurricane Ivan. As a result of this effort revised and/or new maps indicating areas of high risk and/or where further detailed evaluations are needed will be developed. This will be accomplished through a review of both historical data as well as new data from Hurricane Ivan, coupled with numerical analyses of mudslides to supplement available and sometimes sparse field data.
Introduction
The objective of this research is to examine and review the mudflow/mudslide areas in the Gulf of Mexico caused by Hurricane Ivan. As a result of this effort revised and/or new maps indicating areas of high risk and/or where further detailed evaluations are needed will be developed. This will be accomplished through a review of both historical data as well as new data from Hurricane Ivan, coupled with numerical analyses of mudslides to supplement available and sometimes sparse field data.
- Identification of potential oceanographic conditions, including the possibility of hurricanes and forecasting the magnitude and extent of large surface waves.
- Description of bathymetry in the area of interest. This includes both an assessment of water depths, which affect the impact of surface waves on hydrodynamic pressures on the seafloor, and the topography (slope inclination) which affects the magnitude of gravitational forces tending to produce lateral (downslope) movements of the seafloor.
- Determination of the subsurface stratigraphy and geotechnical properties, including the unit weights and strength-deformation properties of the seafloor soils in the areas of concern. Very large ocean waves may influence soils to depths of at least 300 feet and, thus, the stratigraphy and soil properties may need to be defined for considerable depths.
- Numerical modeling to evaluate the stability and potential deformations of the seafloor under the anticipated ocean wave loading conditions.
- Probabilistic analyses to quantify the likelihood and magnitude of seafloor movements in the area of interest as a function of seafloor properties and loading conditions.
Once the likelihood and magnitude of seafloor movements are established an evaluation can be made of the likely impact of such movements on offshore facilities and pipelines.
Benefits to MMS and Industry
State-of-the-art methods and tools for predicting storm wave-induced seafloor instabilities and movements (mudslides and mudflows) will be updated and validated with recent data and observations from Hurricane Ivan, one of the most intense storms that has occurred in the Gulf of Mexico.
Maps of potential seafloor instabilities will provide guidance in assessing the potential and likelihood for future seafloor movements throughout areas of interest. These maps will be useful in identifying areas where either seafloor movements are likely or where more detailed investigations may be helpful to better understand the risks. Such maps could be useful for planning purposes in identifying potentially high risk areas for pipeline routes and platforms sites.
Deployment of Results
The results of this work will be conveyed to OTRC, the MMS, and the petroleum industry through interim reports, a final report, conference presentations, and publication in professional and trade journals. All final versions of papers, presentations and reports will be provided to the MMS upon completion of the project.
Project Plan for Phase I
Scope of Work: We propose to examine the mudflows and mudslides that occurred during Hurricane Ivan to determine the validity of the current predictive models and the applicability of design standards for potential mudslide areas.
Task 1: Review existing data from Hurricane Ivan on seafloor movements, including pipeline movements and failures, to identify the locations where movements occurred and the extent of the movements. This task will also involve a review of information from the last 40 years, such as the pipeline or structure failures due to a mudslide in Hurricane Georges (1998) and Hurricane Camille (1969). A 1-day workshop with the industry and the MMS will be held to discuss the available information reviewed and attempt to gather additional information.
Task 2: Review and analyze available soil data including data on soil properties (unit weights, undrained shear strength) for selected areas where large soil movements were observed or expected. MMS data on soil borings have been summarized by OTRC for MMS Project 367, and will also be used for the proposed study.
Task 3: Select representative sites for analyses and further study based on the locations of movements and the available soil data. We will also select and include at least one nearby site where the seafloor appeared to remain stable during Ivan. This approach should help us in determining the validity of criteria and tools used to identify and screen areas of mudslide susceptibility.
Task 4: Obtain Hurricane Ivan oceanographic data and determine wave conditions during Ivan at the selected sites. A major cause of mudflows and mudslides during hurricanes is the hydrodynamic pressures exerted on the seafloor due to ocean waves. Wave data from an available hindcast of Hurricane Ivan will be the primary source of the wave data needed as input to the computational models to predict seafloor movement. An existing hindcast of Hurricane Ivan will be obtained, and the hindcast wave data will be reduced to provide the needed wave information for the sites selected for analysis. The hindcast wave data will be supplemented with any available measurements.
Task 5: Analyze seafloor stability at the representative sites selected in Task 3. Predict the potential for instability and soil movements using data assembled in Tasks 1, 2 and 4. Appropriate numerical models may be used for this task, both to “screen” for potential instability and identify potential depths of soil movement or sliding. Results will be compared with observations of actual behavior during Hurricane Ivan to confirm and validate the numerical models.
Task 6: Prepare Final Report on Phase 1 to document Ivan mudslides and validation of numerical model(s) for predicting seafloor movements.
Project Plan for Phase II
Scope of Work: In the second Phase the validated numerical models to predict seafloor movements will be used to study the potential for mudslides from future hurricanes in areas of interest.
Task 7: Areas of interest will be selected for a study of potential soil movements in future hurricanes. The sites could include the routes of existing and/or expected future pipelines. Areas of future interest will be defined in consultation with the MMS and industry.
Task 8: Analyze the potential for seafloor movements in future hurricanes. The validated models will be used to analyze the potential for seafloor movements due to future hurricanes. Parametric studies of seafloor movement due to hurricane waves will be conducted for sites representing these areas of interest. The parametric study will use seafloor properties estimated from best available sources. Wave conditions will include the range of those that can be expected in future hurricanes.
Task 9: Conduct probabilistic analyses with the calibrated models to provide estimates of the potential and likelihood for future seafloor movements through out the areas of interest. These results can then be used to identify areas where either seafloor movements are likely or where more detailed investigations may be helpful to better understand the risks.
Task 10: Prepare the final project report. The final report will summarize the analyses, results, and data used in this study and include appropriate maps of the Gulf of Mexico identifying areas of potential seafloor movement due to future hurricanes. A final meeting with MMS representatives and industry will be held to discuss study results.
Related Publications
Nodine, M.C., Wright, S.G., Gilbert, R.G., and Ward, E.G. (2006), “Mudflows and Mudslides During Hurricane Ivan,” Proc. Offshore Technology Conference, Houston, Texas, OTC Paper No. 18328.
Gilbert, R.B., Nodine, M.C., Wright, S.G., Cheon, J.Y., Wrzyszczynski, M., Coyne M., and Ward, E.G. (2007), “Impact of Hurricane-Induced Mudslides on Pipelines,” Proc. Offshore Technology Conference, Houston, Texas, OTC Paper No. 18983.
