Offshore Technology Research Center

 

OTRC Project Summary

Project Title:

Mudslides during Hurricane Ivan and an Assessment of the Potential for Future Mudslides in the Gulf of Mexico- Phase I Final Project Report

Prinicipal Investigators:

Robert Gilbert, Stephen Wright and E. G. Ward

Sponsor:

Minerals Management Service

Completion Date:

December, 2007

Final Report ID#

C175 Phase I Report
C185 Phase II Report (Click to view final report abstract)

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:

Hurricane Ivan resulted in substantial mass soil movements (mudflows and mudslides), a number of which resulted in either large movements or failure of pipelines. The fact that such movements occurred simultaneously with the hurricane suggests that the hurricane was a triggering mechanism. It has long been recognized that in the Gulf of Mexico large hurricanes can result in seafloor movements and submarine slides. In 1969 such movements caused by Hurricane Camille resulted in the complete collapse of an offshore oil production platform and damage to others. Such movements are caused by the increased hydrodynamic pressures on the seafloor produced by large waves at the ocean surface. The potential for such ocean wave induced seafloor movements has long been recognized as an important design consideration in the Gulf of Mexico for facilities in water depths up to approximately 300 - 400 feet.

The phenomena of hurricane ocean wave induced seafloor movements and mudslides has received considerable attention and substantial research has been conducted on the subject. The research has led to procedures for predicting the potential for initiation of seafloor movements and the resulting soil displacements. A majority of this research has been conducted by current and past OTRC researchers at the University of Texas and Texas A&M University. Most notable is the research by Wright (University of Texas), Dunlap (Texas A&M), and Schapery (both Texas A&M and University of Texas). This research led to the development of numerical models for predicting the stability and potential movements of the seafloor caused by large ocean waves. In addition the research showed that large seafloor movements affect the characteristics of the ocean wave and require that the mechanics of the ocean wave and underlying seafloor be coupled. A variety of models were developed and used as part of the research on seafloor slides at the University of Texas and Texas A&M. The models include limit equilibrium, nonlinear finite element, nonlinear layered continua and viscoelastic models. Some models include the coupling between the soil and water motions, while others do not.

The occurrence of what appear to have been ocean wave induced slides in Hurricane Ivan provides an excellent opportunity to better verify the existing numerical models for predicting seafloor movements and to understand better the potential risk for such movements in the future. Although considerable research has been done to develop analytical models, there have been relatively few actual field cases of large-scale movements where the models have been verified ("calibrated"). The failure of the one platform in Hurricane Camille that was described earlier provides one of the few cases where a number of the models have been verified; additional cases are clearly needed. Once verified, the existing models can then used to forecast more reliably where there may be particularly high risks associated with potential seafloor slides.

Development of offshore facilities in water depths up to several hundred (300-400) feet and in locations where large hurricane waves are likely to be generated requires an evaluation of the potential for such waves to initiate seafloor movements in the form of mudslides and mudflows. It is possible to estimate the potential for such movements, including both the potential for initiation and the subsequent displacements, at least up to the point where essentially unrestricted sliding occurs. Knowledge about the potential for mudslides provides valuable information for planning purposes: facilities can sometimes be designed to withstand limited movements provided that the movements are not too large and do not extend too deep; facilities can be moved to reduce the risk; or additional project-specific investigations can be conducted to better understand and/or accept the risks for a particular facility.

Evaluation of the potential for ocean wave induced seafloor movements involves a number of tasks:

• 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 & 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 1:

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 2:

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.

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