MUDSLIDES DURING HURRICANE IVAN AND AN ASSESSMENT OF THE POTENTIAL FOR FUTURE MUDSLIDES IN THE GULF OF MEXICO - Phase I Final Project Report
Nodine, Mary C., Stephen G. Wright, and Robert B. Gilbert, The University of Texas at Austin, and E.G. Ward, Offshore Technology Research Center
INTRODUCTION
Objective
The objective of this research was to examine and review the mudflow/mudslide areas in the Gulf of Mexico caused by Hurricane Ivan, and to use this information to develop and evaluate practical methods for quantifying mudslide hazard.
The tasks for Phase I as outlined in the project proposal are as follows.
Task 1: Review existing data on seafloor movements, including pipeline movements and failures, during Hurricane Ivan to identify the locations where movements occurred and the extent of movements.
Task 2: Review pre-Ivan soil data including data on soil properties (unit weights, undrained shear strength) for selected areas where large soil movements were observed or expected.
Task 3: Select representative sites for analyses and further study based on the locations of movements and the available soil data. Also select and include a nearby site where the seafloor appeared to remain stable during Ivan.
Task 4: Determine wave conditions during Ivan at the selected sites. Obtain Hurricane Ivan oceanographic data.
Task 5: Analyze seafloor stability at the representative sites selected in Task 3 to predict the potential for instability and soil movements. Data assembled in Task 1, 2 and 4 will be used in these analyses.
Task 6: Final Report on Phase 1.
Publications
Nodine, M.C., Wright, S.G., Gilbert, R.B., and Ward, E.G. (2006), “Mudflows and Mudslides During Hurricane Ivan,” Proc. Offshore Technology Conference, Houston, Texas, OTC Paper No. 18328.
Background
Large hurricanes in the Gulf of Mexico can generate waves large enough to trigger submarine slope failures, commonly referred to as “mudslides”. This became particularly evident in 1969 when the large (at least 70 feet high) waves generated by hurricane Camille produced a mudslide in the South Pass Block 70 area of the Gulf of Mexico that destroyed one offshore oil platform and severely damaged at least one other (Sterling and Strobeck1). Following this mudslide, considerable attention was devoted by the offshore oil industry to developing numerical and analytical models that could be used to predict when mudslides might occur and for estimating movements as a basis for designing measures to mitigate damage due to such events.
Coleman et al.2 examined sediment instabilities in the offshore Mississippi River Delta region of the Gulf of Mexico and identified a large area of instabilities and mudslide activity. This area of mudslide activity included South Pass Block 70 where the mudslide referred to above occurred. In general the area identified by Coleman et al has been considered a mudslide susceptible area and facilities in this area have generally required special attention in design.
The recent hurricane Ivan in 2004 resulted in large ocean waves that again are believed to have been responsible for triggering mudslides that damaged offshore facilities. Hurricane Ivan is of particular interest because the waves are believed to have been significantly higher than waves that have previously been known to cause mudslides. In addition, the periods of the waves are somewhat longer that those of waves in previous large hurricanes and this may also have aggravated the impact. The wave and mudslide events associated with Hurricane Ivan provide an excellent opportunity to reexamine the areas of the Gulf of Mexico that are expected to be susceptible to mudslides as well as to verify numerical models that have been developed to predict hurricane-induced mudslides.
Scope
This report presents results of the research for MMS Project 552 as outlined in Tasks 1-6, listed above. It includes locations of known mudslide damage caused by hurricane Ivan. It also includes descriptions of data used to analyze the mudslide hazard in the Mississippi Delta region of the Gulf of Mexico, including Hurricane Ivan wave hindcast data, bathymetric data, and soil shear strength data. A limit equilibrium method used to analyze the data is described and presented in two practical forms: a chart that can be used to find the factor of safety against mudslide initiation for a site where the undrained shear strength increases linearly with depth, and an Excel spreadsheet computer program that can be used to calculate the factor of safety for sites with more complicated shear strength profiles. A regional analysis of the data is presented, including ranges in factors of safety for the mudslide locations using all the available shear strength profiles in the Mississippi Delta region of the Gulf of Mexico. Site specific analyses are presented where site-specific shear strength data were available, and a parametric analysis is presented for a site in the Mississippi Canyon lease block 20.