Probabilistic approaches are becoming increasingly popular for managing risks and uncertainties associated with the sea floor in the design of offshore facilities. Through a series of case histories, this paper reviews several types of probabilistic approaches and demonstrates how they are applied in practice on offshore projects. It discusses how these approaches have made a positive effect on projects and what can be done in the future to make the technology more effective and practical. The major conclusions from these case histories are that a sound understanding of the physical problem is required in order to apply probabilistic methods appropriately and that there is significant potential for adding value to offshore engineering with probabilistic approaches. The potential benefit is not necessarily associated with making a design less conservative or with cutting costs; it is in rationally and explicitly considering risks and uncertainties in making decisions.
Continued development of deepwater and of remote offshore regions where there is little or no experience will bring new challenges in offshore facility design. The risks and uncertainties associated with the sea floor, generically referred to as geohazards, can be substantial and are a critical part of the development process. Traditional prescriptive methods may not be applicable or will result in designs that are uneconomical. Probabilistic approaches provide an alternative that may make the difference and allow these types of developments to move forward. In addition, there is always room to improve existing design practice in mature areas through a formal evaluation of how well that practice works based on historical experience. Probabilistic approaches also offer the ability to make these improvements happen.
In this paper, five case histories where probabilistic approaches have been implemented in practice are described. These case histories are presented from the perspective of facility engineering to illustrate how the approaches were applied and what the impact was on practice. They span a variety of methods, including reliability-based design, value of information analysis, design-method calibration, risk assessment, and probabilistic hazard analysis. They also span a variety of applications from pile foundations to pipelines to soil borings and include a variety of hazards such as hurricanes, earthquakes and geologic variability.
The objectives in presenting these case histories are to review the types of probabilistic approaches that are available, to show how these approaches have made a positive impact in practice, and to discuss what can be done in the future to make probabilistic approaches more effective and practical in offshore engineering.
The design of a pile foundation for a tension leg platform (TLP) provides a case history of applying reliability-based design methods in practice. This case history demonstrates that conventional probabilistic analyses should be used with caution and that practical insight can be obtained through a probabilistic analysis.
The challenge in designing piles for this facility was that it is located in a frontier area where little was known about the geology and geotechnical properties.