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Reducing conservatism in highest density environmental contours.

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Bibliographic Details
Title: Reducing conservatism in highest density environmental contours.
Authors: Haselsteiner, Andreas F.1 (AUTHOR) a.haselsteiner@uni-bremen.de, Mackay, Ed2 (AUTHOR) e.mackay@exeter.ac.uk, Thoben, Klaus-Dieter1 (AUTHOR)
Source: Applied Ocean Research. Dec2021, Vol. 117, pN.PAG-N.PAG. 1p.
Subjects: Offshore structures, Rogue waves, Conservatism, Density, Wind speed, Ocean waves
Abstract: Environmental contours are often used to define design loads acting on a marine structure. An environmental contour describes joint extremes of variables such as wave height, wave period and wind speed that are exceeded with a target return period. These extreme environmental conditions should lead to a structural response with a similar return period. Environmental contours can be defined based on different probabilistic definitions, one of them being that a contour surrounds a so-called highest density region. Highest density contours are a conservative concept that implies that any structure that is designed based on it will have an extreme response that has a return period that is higher than the contour's return period (when short-term variability is accounted for). For some structures, however, the design loads are overly conservative because in the contour construction phase also environmental conditions, which will clearly not lead to an extreme response, are counted as exceedance. Here, we show how this over-conservatism can be avoided by predefining a region in the variable space that will not lead to extreme loads. We give an example, where we define such "mild regions" for sea state contours. By assuming a structural response, we explore the effect of mild regions on the estimated extreme response. In the presented example, a normal highest density contour leads to a design response that is 13% too conservative, while a contour that is adjusted using a reasonable mild region can reduce conservatism to 7%. • Highest density contours are a conservative way to define joint extremes for loads • Over-conservatism can be avoided by defining conditions that will not cause failure • Environmental conditions in this "mild region" are not counted as exceedance. • An example with a sea state environmental contour is presented. [ABSTRACT FROM AUTHOR]
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Database: Engineering Source
Description
Abstract:Environmental contours are often used to define design loads acting on a marine structure. An environmental contour describes joint extremes of variables such as wave height, wave period and wind speed that are exceeded with a target return period. These extreme environmental conditions should lead to a structural response with a similar return period. Environmental contours can be defined based on different probabilistic definitions, one of them being that a contour surrounds a so-called highest density region. Highest density contours are a conservative concept that implies that any structure that is designed based on it will have an extreme response that has a return period that is higher than the contour's return period (when short-term variability is accounted for). For some structures, however, the design loads are overly conservative because in the contour construction phase also environmental conditions, which will clearly not lead to an extreme response, are counted as exceedance. Here, we show how this over-conservatism can be avoided by predefining a region in the variable space that will not lead to extreme loads. We give an example, where we define such "mild regions" for sea state contours. By assuming a structural response, we explore the effect of mild regions on the estimated extreme response. In the presented example, a normal highest density contour leads to a design response that is 13% too conservative, while a contour that is adjusted using a reasonable mild region can reduce conservatism to 7%. • Highest density contours are a conservative way to define joint extremes for loads • Over-conservatism can be avoided by defining conditions that will not cause failure • Environmental conditions in this "mild region" are not counted as exceedance. • An example with a sea state environmental contour is presented. [ABSTRACT FROM AUTHOR]
ISSN:01411187
DOI:10.1016/j.apor.2021.102936