Structural Integrity of Offshore Wind Turbines Oversight of Design, Fabrication, and Installation - Special Report 305 (2011) / Chapter Skim
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A Risk-Informed Approaches to Safety Regulation
A Risk-Informed Approaches to Safety Regulation
Pages 137-147
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From page 137... ...
RISK-INFORMED ANALYSIS AND DESIGN OF CIVIL INFRASTRUCTURE FACILITIES Risk-informed approaches to analysis, design, and condition assessment have reached a state of maturity in many areas of civil infrastructure during the past three decades, particularly in codes, standards, and regulatory guidelines that govern design and construction. These documents are key tools for structural engineers in managing civil infrastructure risk in the public interest, and the traditional structural design criteria they contain address risks in performance as engineers have historically understood them.
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From page 138... ...
FUNDAMENTALS OF RISK ASSESSMENT FOR NATURAL AND MAN-MADE HAZARDS Risk analysis and assessment tools are essential in measuring compliance with performance objectives, in comparing alternatives rationally, and in highlighting the role of uncertainty in the decision process. This section outlines a framework for modern risk-informed decision making, providing the background for the implementation of structural design requirements for civil infrastructure facilities in the current construction and regulatory climate.
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From page 139... ...
The unique nature of each infrastructure type determines how specific risk-informed decision concepts have been implemented. PROBABILITY-BASED LIMIT STATES DESIGN Load and Resistance Factor Design Structural codes and standards applicable to the design of civil infrastructure traditionally have been concerned primarily with public safety (preventing loss of life or personal injury)
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From page 140... ...
Most first-generation probability-based structural design codes focus on that performance objective. Other performance metrics -- direct economic losses from structural damage, indirect losses due to interruption of function, forgone opportunities, and loss of amenity -- have not been addressed in current construction regulations but may be of concern to certain stakeholder groups in certain types of infrastructure facilities.
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From page 141... ...
for a member limit state involving yielding of a tension member or formation of the first plastic hinge in a compact beam was set equal to approximately 3.0 for a service period of 50 years, corresponding to a limit state probability of approximately 0.0013 in 50 years; annualized, this probability is on the order of 10−5. The value of β equal to 3.0 was selected following an extensive assessment of reliabilities associated with members designed by traditional methods and is applicable to load combinations involving gravity loads but not wind or earthquake loads (Galambos et al.
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From page 142... ...
No distinction is made between steel, reinforced concrete, and prestressed concrete girders in terms of their target reliabilities, nor is the target reliability index dependent on the girder span or on whether the girder is simply supported or continuous over internal supports. Offshore Platforms Formal design guidance for offshore structures originated in 1967 with the release of American Petroleum Institute (API)
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From page 143... ...
The commentary to these procedures contains two tables with acceptable reliability levels: the first stipulates annual limit state probabilities and reliability indices for nonseismic events, and the second provides anticipated probabilities of structural failure for earthquakes. These acceptable reliability levels are dependent on the risk category of the structural facility and the nature of the structural failure involved.
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From page 144... ...
CRITICAL APPRAISAL OF EXISTING RISK-INFORMED ANALYSIS AND DESIGN PRACTICES FOR APPLICATION TO OFFSHORE WIND TURBINES Component Versus System Reliability Analysis Most codified reliability-based design for civil infrastructure has focused on individual buildings, bridges, and other industrial facilities for which the hazard can be identified at a point (e.g., Ellingwood 2007)
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From page 145... ...
The associated load factor also plays a key role; for example, the probability of exceedance of some load level, 1.6W, with W determined on the basis of a 50-year MRI wind speed, is about the same as the probability of exceeding 1.0W when W is defined on the basis of a 700-year wind speed. This is also the reason why the IEC-based offshore wind turbine design procedure, which begins with a 50-year wind speed basis and applies load factors of 1.25 or 1.35 when verifying ultimate limit states, might yield the same reliability as the use of an alternative factored load that begins with a 100-year wind speed (as in API RP 2A)
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From page 146... ...
1982. Probability Based Load Criteria: Load Factors and Load Combinations.
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From page 147... ...
2007. Methodology for Development of Design Criteria for Joint Hurricane Wind Speed and Storm Surge Events: Proof of Concept.
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