Progress Reports: June 2003 December 2002 June 2002 December 20012001-2003 OTRC PROJECT: Seafloor Slope Stability under Static and Seismic Loading Conditions - Numerical Analyses and Model Calibration
OBJECTIVE:
At the outset of this project the intent was to calibrate one or more of the existing models for sea-floor slope stability using either results from centrifuge model tests performed by C-CORE or data from actual sea-floor slope failures documented in the literature. Particular attention was to be devoted to applicability of the work performed by C-CORE and the Phase I Project Offshore Deep Slopes (PODS) project to U. S. waters, including the Gulf of Mexico. After initiation, the project scope was expanded by the addition of a "Forum" on "Risk Assessment of Slope Stability", which has now been successfully completed. Since completion of the Forum the work plan has been modified for several reasons. Principal among the reasons for modification was the lack of data from centrifuge model tests and the limited availability of case history data for actual slope failures that could be used to verify and calibrate very sophisticated numerical models. Work currently is focused on identifying a more complete set of data and establishing a database of these documented submarine slope failures along with any pertinent information related to the failures. One of the objectives of this effort is to provide preliminary guidance for the development of Recommended Practices for investigation of sea-floor slopes and assessment of risk of submarine slope instability.
PROGRESS:
The Forum on Risk Assessment of Submarine Slope Stability has been organized and successfully conducted. A draft report was prepared and submitted to all attendees for comment. The report is now being finalized for final printing and distribution. A technical paper on the workshop finding has been offered and accepted for the Deep Offshore Technology conference in New Orleans, in November 2002. This paper is now being prepared for that conference.
Because of the lack of information on well-documented case histories suitable for analyses and calibration of existing numerical models of sea-floor stability, a substantial portion of the recent project effort has been directed to compiling a comprehensive collection of data from the published literature on sea-floor stability. Although C-CORE had identified several case histories as part of the Phase I PODS project, these case histories lacked important geotechnical data that would be needed to provide verification of most of the more sophisticated numerical models. Thus, it was felt that a more thorough investigation was needed to determine if case histories could be found with more complete geotechnical data. To date in excess of over 100 case histories have been identified and data from them have been summarized in a database.
The over 100 case histories that have been identified to date all seem to lack much of the geotechnical information needed to provide a reliable assessment of the validity of numerical models. This finding is consistent with what we concluded from the several case histories that C-CORE had investigated and also with what was generally indicated by participants in the recent Forum that was held as part of this project. Unfortunately most case histories that are well documented are the proprietary property of oil companies and not in the public domain. However, the collection of case histories that has been developed as part of the current project is still useful. One of the conclusions that was reached from the Forum was that there was a need for either "standards" or "guidelines" for assessment of slope stability similar to the API RP2A recommended practices for pile foundations. The case history data that has been compiled provides a useful framework for developing recommended practices for assessment for slope stability. Clearly any guidelines must enable potential causes of slope failure to be identified and provide steps for assessment of the associated risks.
PLANNED WORK THROUGH PROJECT COMPLETION:
Work will continue to complete and submit the paper to the D.O.T. conference and present the paper at their meeting in New Orleans in November. We will also work to complete the compilation of case history data, including the case history database. Finally, we plan to use the information from the case histories to develop preliminary requirements for a "Recommended Practice" on submarine slope stability assessment. The emphasis of these recommendations will be on deep water. We are hopeful that this will provide a basis for developing a complete Recommended Practice or guideline for assessment of slope stability in deep water.
ESTIMATED COMPLETION DATE: June/September 2003
PRINCIPAL INVESTIGATOR: Dr. Stephen G. Wright
2001-2002 OTRC PROJECT: SEAFLOOR SLOPE STABILITY UNDER STATIC AND SEISMIC LOADING CONDITIONS – NUMERICAL ANALYSES AND MODEL CALIBRATION
OBJECTIVE:
To develop methodologies applicable to slope stability in deep water areas like the Gulf of Mexico (USA) and offshore Nova Scotia (Canada). Relatively steep slopes are known to exist in these areas and there is geological evidence that slope failures have occurred in the past. A need exists for reliable numerical models for predicting when slope instabilities may occur under both static (gravity) and dynamic (earthquake) loading. The objective of this project is to develop and/or identify such models and to calibrate them with experimental results from centrifuge modeling experiments.
APPROACH:
This project is being undertaken as part of C-CORE’s Projects on Deepwater Slopes (PODS), which is a JIP. The project will investigate numerical models for predicting sea-floor slope instabilities under both static gravity loads and earthquakes. C-CORE's centrifuge and planned tests on underwater slopes provides an excellent opportunity for validating the numerical models. Several models exist for both static slope stability and dynamic (earthquake) stability. For static slope stability analyses there are "infinite slope" models and models based on procedures of slices that employ both circular and noncircular slip surfaces. For earthquake stability there are pseudo-static limit equilibrium analyses and a number of simple one-dimensional dynamic models that can be used. One objective is to identify which models are most suitable and where possible to calibrate these models for offshore slopes. An important aspect of such calibration is to determine what role if any the overlying water column has on stability during earthquake shaking.
Once candidate numerical models and suitable software are identified numerical analyses will be performed of slopes that have been tested in the C-CORE centrifuge under both static and dynamic loading. Particular attention will be given to the suitability of simple models and the effect of the overlying water, especially with regard to its influence on seismic slope stability. Presently water loads are usually treated as static (constant) loads when assessing seafloor stability, even under earthquake events. In addition, for pseudo-static analyses submerged (buoyant) soil unit weights are usually used to compute the gravity loads (gravity shear stresses), while total unit weights are used to compute the seismic forces. Analyses of actual tests on submerged slopes should provide a much better insight into the validity of this practice and lead to improved methods for treatment of water in analyses of slope stability.
DEPLOYMENT OF RESULTS:
Results will be presented in technical reports and papers. Also as a part of this effort we will develop an Internet web site to disseminate the research results and recommendations, including possibly making appropriate software available.
ANTICIPATED PROJECT DURATION: 1 Year
PROJECT PLAN FOR YEAR 1 (2001-2002):
Scope of Work: Work will involve review of results from C-Core centrifuge model tests, and numerical analyses of the results to validate and/or calibrate existing static and seismic models for seafloor slope stability. Suitable modifications and/or adaptation of existing models for earthquake loading may be required for applications to underwater slopes.
Anticipated Results: Results will be in the form of recommendations of analytical models and procedures for computing static and seismic slope stability of seafloor slopes. These will include recommendations for any special procedures required for seafloor slopes and/or deep water.
Sponsorship: MMS
PRINCIPAL INVESTIGATOR (S) & OTHERS INVOLVED IN PROJECT:
PI(s): Dr. Stephen G. Wright
Date: June, 2003
Project Name: Seafloor Slope Stability Under Static and Seismic Loading Conditions with Forum on Risk Assessment of Submarine Slope Stability
Project Number: 32558-5888Z1 Task Order: 18217
Principal Investigators: Dr. Stephen G. Wright
Estimated Completion Date: June/September 2003
Project Description:
A 1-1/2 day Forum on "Risk Assessment for Submarine Slope Stability" with special emphasis on deepwater was organized and conducted with participation by both domestic and international participants from government, academia and industry.
In addition, an extensive search and compilation of data from the literature is being conducted to develop a database on submarine slope failures. Because the information required to perform a complete risk assessment is frequently limited, the objective of developing a database is to provide information that can be used to supplement site-specific data for making risk assessments of sea floor stability.
Progress:
A workshop on Risk Assessment of Submarine Slope Stability in Deepwater has been completed and a final report has been issued. A paper summarizing the important findings of the Workshop was prepared and presented at the 2002 D.O.T. conference in New Orleans in November. Currently a database containing pertinent information on approximately 400 submarine slope failures is being completed and documented. Preliminary analyses of the data are also currently underway.
Reports & Publications:
The following reports and publications have been prepared:
Wright, Stephen G., "Forum On Risk Assessment For Submarine Slope Stability - Report Of A Forum Held In Houston, Texas - May 10 And 11, 2002", June, 2002 (Report In Final Preparation By Offshore Technology Research Center).
Wright, Stephen G., Gilbert, Robert B., and Smith, Charles E., "Risk Assessment of Submarine Slope Stability in Deepwater - Issues and Priorities", Proceedings, 14th Deep Offshore Technology Conference, New Orleans, Louisiana, November 13-15, 2003.
Date: 22 November 2002
Project Name: Seafloor Slope Stability Under Static and Seismic Loading Conditions with Forum on Risk Assessment of Submarine Slope Stability
Project Number: 32558-5888Z1 Task Order: 18217
Principal Investigators: Dr. Stephen G. Wright
Estimated Completion Date: June/September 2003
Project Description:
A 1-1/2 day Forum on "Risk Assessment for Submarine Slope Stability" with special emphasis on deepwater was organized and conducted with participation by both domestic and international participants from government, academia and industry. In addition, an extensive search and compilation of data from the literature is being conducted to develop a database on submarine slope failures. Because the information required to perform a complete risk assessment is frequently limited, the objective of developing a database is to provide information that can be used to supplement site-specific data for making risk assessments of sea floor stability.
Progress:
The Forum on Risk Assessment for Submarine Slope Stability was held on May 10 and 11, 2002 in Houston, Texas. Fifty-five invited participants, of whom 36 were from industry, attended the Forum; Fifteen participants were from outside the United States. Results of the Forum have been summarized and presented in a final project report.
Most of the literature search and data entry for the database on submarine landslides has been completed. Currently approximately 400 submarine slope failures have been documented and included in the database. Of these approximately 400 failures, only about 15 percent have any significant geotechnical data. Preliminary analysis of the data in the database is anticipated to begin shortly. More extensive analyses of the data are anticipated for the future and extending beyond the current project year.
Reports & Publications:
The following reports and publications have been prepared:
WRIGHT, STEPHEN G., "FORUM ON RISK ASSESSMENT FOR SUBMARINE SLOPE STABILITY - REPORT OF A FORUM HELD IN HOUSTON, TEXAS - MAY 10 AND 11, 2002", JUNE, 2002 (REPORT IN FINAL PREPARATION BY OFFSHORE TECHNOLOGY RESEARCH CENTER).
Wright, Stephen G., Gilbert, Robert B., and Smith, Charles E., "Risk Assessment of Submarine Slope Stability in Deepwater - Issues and Priorities", Proceedings, 14th Deep Offshore Technology Conference, New Orleans, Louisiana, November 13-15, 2003.
Date: July 2002
Project Name: Seafloor Slope Stability under Static and Seismic Loading Conditions - Numerical Analyses and Model Calibration
Task Order: 18217 Project Number: 5888Z
Principal Investigators: Dr. Stephen G. Wright
Estimated Completion Date: September 2002
Project Description:
This project is being conducted in conjunction with a larger project on seafloor slope stability in deep water being carried out by C-CORE. One of the objectives of the OTRC portion of the study is to evaluate the applicability of C-CORE's work to Gulf of Mexico and other U. S. territorial waters. The OTRC portion of the study will also identify appropriate models for static and seismic stability and calibrate them with results from centrifuge model tests being performed by C-CORE. We will also be assisting C-CORE in identifying triggering mechanisms for slope instability and planning model tests needed to calibrate models and verify various triggering mechanisms of seafloor instability.
Progress:
In response to an October meeting with C-CORE and the MMS, the need for a workshop or Forum was identified to define the state-of-the-art, state-of-practice, and research needs for developing probabilistic -based methods to evaluate the risk and reliability associated with submarine slopes failures.
A one and one-half day Forum was organized and held in Houston, Texas on May 10 and 11, 2002. The Forum was planned and organized by the OTRC in cooperation with C-CORE). About 55 invited US and international experts from industry, government organizations, and academia participated the Forum. The Forum featured Keynote addresses on (1) an industry prospective on submarine slope stability and (2) risk assessment Mini-panels, which allowed experts to address particular aspects of risk assessment and participants to generate ideas and issues for discussion in subsequent breakout sessions. Topics for the four mini-panels were: (1) Facilities & Design Issues (impact of a slide); (2) Geologic Environment (processes, triggering mechanisms, stratigraphy); (3) Geotechnical Properties; and (4) Modeling (response given trigger). Breakout Sessions allowed all participants to address issues identified by the mini-panels as well as other issues the felt were significant. Each session identified and prioritized a list Contributing Elements (to risk assessment of slope stability in deep water) and the associated Importance, Current State of Knowledge, and Knowledge Needs for each Element. A final Plenary Session that discussed the finding of the Breakout Sessions, and then voted on the top six most important causes/reasons for uncertainty in risk assessment of slope stability.
The Forum was very successful in increasing the awareness of participants to the broad range of the state-of-the-art and problems in slope instability, and identifying and prioritizing research needs. The final report has been drafted and sent to participants for review.
Date: December 7, 2001
Project Name: Seafloor Slope Stability under Static and Seismic Loading Conditions - Numerical Analyses and Model Calibration
Task Order: 18217 Project Number: 5888Z
Principal Investigators: Dr. Stephen G. Wright
Estimated Completion Date: September 2002
Project Description:
This project is being conducted in conjunction with a larger project on seafloor slope stability in deep water being carried out by C-CORE. One of the objectives of the OTRC portion of the study is to evaluate the applicability of C-CORE's work to Gulf of Mexico and other U. S. territorial waters. The OTRC portion of the study will also identify appropriate models for static and seismic stability and calibrate them with results from centrifuge model tests being performed by C-CORE. We will also be assisting C-CORE in identifying triggering mechanisms for slope instability and planning model tests needed to calibrate models and verify various triggering mechanisms of seafloor instability.
Progress:
This project was initiated in October of this year and has been underway approximately two months. During this time the principal investigator (Wright) attended a two-day meeting at C-CORE in St. Johns, Newfoundland along with two representatives of MMS to review the results of the first phase of C-CORE's research on deep ocean slope stability and to formulate plans for future work. One of the outcomes of this meeting was identification of the need for a workshop ("forum") with industry, government and academic representatives to define the current state-of-the-art and state-of-the-practice and the associated research needs for developing a probabilistic-based methodology for evaluating risks due to submarine slope instabilities. Approval has been given by OTRC/MMS for additional funding for the workshop and a separate proposal for the additional work has been written and submitted for processing.
Another outcome of the November meeting at C-CORE was the suggestion by Dr. Wright of several additional model tests that C-CORE can perform in their centrifuge to obtain useful insight into mechanisms of seafloor stability. Results from these model tests will be used in the OTRC portion of this study to verify triggering mechanisms and to calibrate numerical models.
Prior to the November meeting at C-CORE a preliminary report by C-CORE on their Phase 1 research study was reviewed. Based on this report an oral presentation was prepared and presented by Dr. Wright to C-CORE, identifying several important issues and directions for further research. We are continuing to review the report of C-CORE's completed phase one research. Emphasis in the review is being placed on applicability of the research findings to Gulf of Mexico and U. S. territorial waters.
Reports & Publications: oral report at C-CORE meeting (see above)