Summary
Objective
The challenge of seafloor characterization for deepwater facilities is that the mooring foundations, subsea well trees and flowlines are spread over large areas (tens of thousands of feet across) while the cost of obtaining high-quality geotechnical data for the seafloor is high. Therefore, information from a handful of soil borings is typically extrapolated over thousands of feet to design foundations. This extrapolation leads to uncertainty that could potentially lead to excessively conservative designs or to unreliable designs.
The goal of this research is to develop a reliability-based methodology to design offshore foundations with limited seafloor characterization data and to apply this methodology to optimize geotechnical investigation programs. Specific research objectives are:
- Develop models describing spatial variability in foundation design parameters for different geologic profiles encountered in deepwater fields.
- Relate uncertainty in foundation design parameters to partial safety or resistance factors required to achieve target reliability levels for different foundation types and loading conditions.
- Quantify the added value of geologic and geophysical information in reducing uncertainty in foundation design parameters.
- Quantify the added value of foundation installation information in reducing uncertainty in foundation design parameters.
Approach
These research objectives are being addressed through completing the following tasks:
Task 1: Develop geologic-dependent models for spatial trends and variability in design data.
Task 2: Develop a methodology to determine the reliability of a foundation in deepwater considering uncertainty in the predictive model and soil properties as well as in the loading conditions.
Task 3: Calibrate models of foundation capacity based on installation data.
Task 4: Develop design charts that relate partial safety or resistance factors to uncertainty in design data for a specified level of reliability and for different foundation configurations and loading conditions.
Deployment of Results
Results will be documented in a final report and be published in conference proceedings and journal articles. Results will be useful in developing and updating industry guidelines and standards.
Project Plan
Scope of Work:
Task 1 – Developing geologic-dependent models for spatial (both horizontal and vertical) trends and variability in design data, such as foundation capacity for floating structures and pipelines/flowlines. In addition, statistical correction factors and weights are being developed to relate data obtained from other sources (specifically, geophysical information and foundation installation information) to that obtained from a conventional soil boring. The development of these models is being guided by data from various sources and locations that have been compiled through separate industry-sponsored projects (specifically, Unocal, BP and ExxonMobil have sponsored projects at UT to generate, compile and analyze offshore geotechnical in the past several years). We have been able to compile conventional geotechnical data for a number of different offshore fields. Continuing efforts will focus on developing a reliability framework to accommodate non-normal probability distributions for soil properties and foundation capacity in a practical way.
Task 2 – A methodology is being developed to determine the reliability of a foundation in deepwater considering uncertainties in the predictive model, soil properties, and the loading conditions. Foundation capacity models are being calibrated with laboratory and field tests. We have developed a practical methodology to accommodate highly non-normal probability distributions to accommodate the finite lower bound of foundation capacity and generally non-normal spatial variability in soil properties. Information on the uncertainty in loading conditions is being obtained from an ongoing OTRC-sponsored project on mooring system reliability. Continuing efforts will focus on calibrating models to predict suction caisson capacity with laboratory and field data. This effort will incorporate information from a related OTRC project where model tests are being performed on suction
Task 3 – Models of foundation capacity are also being calibrated with field data obtained during installation, using information that was compiled in a separate project sponsored by API. Installation information is being related to the ultimate capacity using available models and information on time-dependent increases in capacity after installation. We are also relating installation information to a lower bound or minimum possible capacity. This lower bound can play a significant role in the reliability of the foundation since the load would theoretically need to reach this value before failure is even possible. This work is also significant because the ability to install the foundation is an important design consideration. Driven pile data has been analyzed, and data form suction caisson installations is now being studies. Continuing efforts will focus on developing a methodology to update foundation capacity (both the lower bound and the expected value) with installation information.
Task 4 – Design charts are being developed that relate partial safety or resistance factors to uncertainty in design data for a specified level of reliability and for different foundation configurations and loading conditions. Foundation types being considered include deep foundations, such as driven piles and suction caissons, as well as shallow foundations, such as for flowlines and well systems. This task has been completed for simple cases (normal or lognormal probability distributions, no lower bound, and no incorporation of installation data). Research will continue to extend and complete this work for more general and realistic cases.
Anticipated Project Results: The project deliverables include the following:
- A set of geologic-dependent models to account for spatial variations in foundation capacities for both deep and shallow foundations. These models, when combined with user-supplied information for a specific field or project, will provide valuable information to manage situations where foundations are spread out and not located adjacent to soil borings.
- Information about the uncertainty in conventional models to predict suction caisson capacity and a methodology to updated the predicted capacity on a project-specific basis using installation data.
- A reliability-based design framework for suction caissons that can ultimately be used to guide the development of recommended practice guidelines.
Related Publications
Gilbert, R. B., Choi, Y. J., Dangyach, S. and Najjar, S. S. (2005), “Reliability-Based Design Considerations for Deepwater Mooring System Foundations,” Proceedings, ISFOG 2005, Frontiers in Offshore Geotechnics, Perth, Western Australia, 317-324.
Gilbert, R. B., Najjar, S. S. and Choi, Y. J. (2005), “Incorporating Lower-Bound Capacities into LRFD Codes for Pile Foundations,” Proceedings, Geo-Frontiers 2005, Geotechnical Special Publication No. 132 (CD-ROM), ASCE, Reston, Virginia.
Gilbert, R. B., Najjar, S. S. and Shields, M. K. (2005), “Importance of Residual Strengths in Factors of Safety and Reliability,” Proceedings, Geo-Frontiers 2005, 18th GRI Conference on Geosynthetics Research and Development in Progress (CD-ROM), ASCE, Reston, Virginia.
Gilbert, R. B. and Puskar, F. (2005), “Geohazards: An Overview of Probabilistic Approaches and Application to Engineering Design and Risk Assessment for Offshore Facilities,” Proceedings, Offshore Technology Conference, OTC 17676.
Najjar, S. S. and Gilbert, R. B. (2005), “Lower-Bound Estimates of Capacity in Geotechnical Reliability Assessments,” Proceedings,ICOSSAR 2005, Intl. Conf. on Structural Safety and Reliability, Rome, Italy, 955-962.
Gilbert, R. B. (2003), “Reliability-Based Design as a Decision-Making Tool,” Proceedings, International Workshop on Limit State Design in Geotechnical Engineering Practice, Phoon, Honjo and Gilbert (eds.), Cambridge, Massachusetts.
Gilbert, R. B. and Murff, J. D. (2001), “Design Methodologies and Criteria for Suction Caissons for Deepwater Mooring Applications,” Workshop Report, Offshore Technology Research Center, 136 pp.
Gilbert, R. B. and Murff, J. D. (2001), “Identifying Uncertainties in the Design of Suction Caisson Foundations,” Proceedings, International Conference on Geotechnical, Geological and Geophysical Properties of Deepwater Sediments Honoring Wayne A. Dunlap, OTRC, Houston, Texas, 231-242.
Olson, R. E., Rauch, A. F., Gilbert, R. B., Tassoulas, J. L., Aubeny, C. P. and Murff, J. D. (2001), “Toward the design of New Technologies for Deep-Water Anchorages,” Proceedings, Annual International Offshore and Polar Engineering Conference, Stavanger, Norway, in press.
Smith, G. L. and Gilbert, R. B. (2001), “A Simplified Model of Spatial Variability to Evaluate Effects of Spatial Averaging on Foundation Capacity,” Proceedings, International Conference on Structural Safety and Reliability, Huntington Beach, California, in press.
Gilbert, R. B., McBrayer, M. C. and Tang, W. H. (2000), “Statistical Considerations in Calibrating Performance Models with Field Data,” Proceedings of Performance Confirmation of Constructed Geotechnical Facilities, ASCE, GSP No. 94, Amherst, 307-321.
Gilbert, R. B. (2000), “Optimizing Investigation Programs for Offshore Platform Foundations – Effect of Geology on Axial Pile Capacity,” Proceedings,International Young Geotechnical Engineers Conference 2000, University of Southampton.
Gambino, S. J. and Gilbert, R.B. (1999), “Modeling Spatial Variability in Pile Capacity for Reliability-Based Design,” Analysis, Design, Construction and Testing of Deep Foundations, Roesset Ed., ASCE Geotechnical Special Publication No. 88, 135-149.
Gilbert, R. B., Stong, T. J., Lang, J. T., Albrecht, R. S. and Dupin, R. M. (1999), “Optimizing Investigation Programs for Offshore Foundations – Effect of Geology on Axial Pile Capacity,” Proceedings, 2nd International Conf. on Seabed Geotechnics, IBC Ltd., London.
Gilbert, R. B., Gambino, S. J. and Dupin, R. M. (1999), “Reliability-Based Approach for Foundation Design without Site-Specific Soil Borings,” Proceedings, Offshore Technology Conference, Houston, Texas, 631-640.
