Summary
Objective
The objective of this research project is to assess the vulnerability, operability, and safety of sub-sea production systems in the Gulf of Mexico during earthquakes of varying probabilistic seismic intensities. With an increasing number of sub-sea production systems being used due to the economies they offer in deep water, the evaluation of these systems during earthquakes is warranted, even in zones of small seismic risk. The assessment will involve participation from industrial and regulatory representatives to define current sub-sea systems, technologies, operations, concerns, and potential mitigation actions. The potential impact of the research project will be an understanding of the vulnerability of sub-sea systems during earthquakes, with a focus on those found in the Gulf of Mexico.
Approach
General Approach
The work plan, which includes participation from industrial and regulatory representatives, will consist of the following tasks:
- Review historical seismic activity and identify current seismic risks within the Gulf of Mexico. This will include collecting measured ground motions from previous earthquakes and artificially generated ground motions, as well as elastic response spectra, for varying probabilistic levels of seismic intensity from research efforts in the literature.
- Identify the spectrum of prototype sub-sea structural and non-structural systems.
- Develop representative structural models and analysis procedures that consider fluid-structure interaction for these systems during earthquakes.
- Identify performance limit states for these structural and non-structural systems such as safety, operability, damage control, and collapse prevention.
- Conduct a performance-based probabilistic evaluation of the representative sub-sea systems for earthquakes identified in task 1 and identify potential failure mechanisms in the system that may be critical.
Current State of Knowledge, Existing Data, and Related Work
With regard to the work plan identified above, the current state of knowledge, existing data, and related work will be comprehensively gathered from the literature. Due to the minimal risk potential of earthquakes and because there are few recorded earthquakes in the Gulf of Mexico, it is expected that only limited information on earthquakes in this area will be available in the literature. However, significant efforts have been made regarding the seismic performance of offshore structures in shallow water in other regions of the world, including the North Sea and off the California Coast. The impact of fluid-structure interaction during earthquakes will be reviewed and considered in this research.
For earthquake resistant design of building structures on land, a performance-based design methodology that incorporates various levels of structural performance and probabilistic loading are currently being implemented in codes. In this methodology, performance levels describe limitations on the maximum damage sustained during a ground motion, while performance objectives define the target performance level to be achieved for a particular ground motion intensity. For example, three suggested levels of earthquake intensity are those having a 50%, 10%, and 2% probability of exceedance in 50 years, which correspond to return periods of 73, 475, and 2475 years (also termed standard frequent, design-basis, and maximum considered events). Three performance levels generally considered for building structures are immediate occupancy, life safety, and collapse prevention. For offshore systems, performance levels such as safety, operability and damage control may be more significant. Inherent in the idea of performance levels is the supposition that performance can be measured and either accepted or rejected on the basis of some analytically predicted response values. These response values will be defined in collaboration with industrial and regulatory representatives. In addition, basic performance objectives will be identified through discussions with these groups. For example, the performance objectives for sub-sea structural and non-structural systems might be that they remain operational during design-basis seismic events and that damage control is ensured during maximum considered seismic events.
Expected Form of Results
The results of this study will be in the form of analytical results, which will be interpreted by the research team to develop conclusions and recommendations based on this work.
Deployment of Results
The proposed research will address an important issue for sub-sea systems in the Gulf of Mexico. The research will be conducted through The Offshore Technology Research Center (OTRC) at Texas A&M University. Upon completion of the proposed research, a complete draft report will be prepared and submitted to the Minerals Management Service (MMS) for their review. The report will document the research performed, methods used, results achieved, and recommendations.
Following their review, the report will be published as an OTRC report. The results of research studies performed through the OTRC are disseminated to the OTRC sponsors for use in design of new offshore structures as well as for evaluation of existing structures. Information derived from these activities will also be fully disseminated through reports and technical journal articles. The report will be distributed to the three national earthquake centers established by the National Science Foundation, as well as interested practitioners and researchers. Opportunities to make presentations at conferences and symposia will also be pursued in order to share research findings with the technical community. In addition, this research will be discussed in courses on structural design so that students become knowledgeable in the assessment of offshore structures and sub-sea systems for seismic demands.
