Abstract
Evaluating marine system reliability requires considering the interaction of a limit state with the stochastic ocean excitation. Given a range of operational profiles, a relevant question is which sea states lead to the worst-case system responses, considering the effects of short and long-term variability. If the identified subset of operational profiles indeed leads to the worst-case system responses, it is possible to assess lifetime system performance without unnecessary computational effort via this directed set of conditions. Environmental contour methods identify rare sea states assumed to excite rare responses but generally do not include response dynamics when choosing these sea states. For systems with limit states involving combined loading or with multiple failure modes, rare environmental conditions may not exclusively lead to rare responses. In this case, the response cannot be severed from the identification of relevant sea conditions but should instead drive that identification. This paper illustrates a way to construct response-based reliability contours that identify sea states most relevant for analyzing rare responses of marine systems. These sea states are compared with sea states identified by environmental contours, showing the effect on perceived system risk levels when system dynamics, short-term response variability, and long-term environmental variability are considered.
Highlights
Due to the computational challenges associated with a long-term probabilistic analysis of marine systems, it is of major interest to identify relevant sea state conditions expected to most contribute to the long-term response
Such is the difficulty of identifying relevant sea states associated with rare system responses: effort is either focused solely on the ex citation environment independent of the structure or solely on the structural response given a defined environment (as in, e.g. the dynamic Inverse First Order Reliability Method (IFORM) approach offered by Lutes and Winterstein for load combination problems (Lutes and Winterstein, 2016))
This paper established a method to construct response-based relia bility contours (RBRCs) for complex marine systems excited by combined loading effects using indicators of extreme behavior and extreme value theory
Summary
Due to the computational challenges associated with a long-term probabilistic analysis of marine systems, it is of major interest to identify relevant sea state conditions expected to most contribute to the long-term response Examining such a subset of potential cases, versus all possible cases, allows a more in-depth analysis of the system re sponse without spending unnecessary computational effort. To identify the most relevant sea states to evaluate the reliability or performance of a system with a known or approximate limit state, other methods may give more meaningful results This paper establishes such a method to construct contours of system reliability given a defined design lifetime, range of potential operational profiles, and limit state of interest for design purposes.
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