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Research Papers: Offshore Technology

Bivariate Extreme Value Statistics of Offshore Jacket Support Stresses in Bohai Bay

[+] Author and Article Information
Zhang Jian

Naval Architechture and Offshore
Engineering Department,
Jiangsu University of Science and Technology,
Zhenjiang 212003, China
e-mail: justzj@126.com

Oleg Gaidai

Naval Architechture and Offshore
Engineering Department,
Jiangsu University of Science and Technology,
Zhenjiang 212003, China
e-mail:oleg.gaidai@yahoo.com

Junliang Gao

Naval Architechture and Offshore
Engineering Department,
Jiangsu University of Science and Technology,
Zhenjiang 212003, China
e-mail: gaojunliang880917@163.com

Contributed by the Ocean, Offshore, and Arctic Engineering Division of ASME for publication in the JOURNAL OF OFFSHORE MECHANICS AND ARCTIC ENGINEERING. Manuscript received May 23, 2017; final manuscript received February 21, 2018; published online April 24, 2018. Assoc. Editor: Marcelo R. Martins.

J. Offshore Mech. Arct. Eng 140(4), 041305 (Apr 24, 2018) (7 pages) Paper No: OMAE-17-1080; doi: 10.1115/1.4039564 History: Received May 23, 2017; Revised February 21, 2018

This paper presents a generic Monte Carlo-based approach for bivariate extreme response prediction for fixed offshore structures, particularly jacket type. The bivariate analysis of extremes is often poorly understood and generally not adequately considered in most practical measurements/situations; that is why it is important to utilize the recently developed bivariate average conditional exceedance rate (ACER) method. According to the current literature study, there is not yet a direct application of the bivariate ACER method to coupled offshore jacket stresses. This study aims at being first to apply bivariate ACER method to jacket critical stresses, aiming at contributing to safety and reliability studies for a wide class of fixed offshore structures. An operating jacket located in the Bohai bay was taken as an example to demonstrate the proposed methodology. Satellite measured global wave statistics was used to obtain realistic wave scatter diagram in the jacket location area. Second-order wave load effects were taken into account, while simulating jacket structural response. An accurate finite element ANSYS model was used to model jacket response dynamics, subject to nonlinear hydrodynamic wave and sea current loads. Offshore structure design values are often based on univariate statistical analysis, while actually multivariate statistics is more appropriate for modeling the whole structure. This paper studies extreme stresses that are simultaneously measured/simulated at two different jacket locations. Due to less than full correlation between stresses in different critical jacket locations, application of the multivariate (or at least bivariate) extreme value theory is of practical engineering interest.

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Figures

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Fig. 1

Annually averaged spatial distribution of wave height and period in Bohai bay [1]. Star indicates jacket location.

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Fig. 2

An example of offshore jacket platform operating in the Bohai continental shelf

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Fig. 3

Sea state scatter diagram for the Bohai bay area from Ref. [3]. Numbers are fractions of 1000.

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Fig. 4

Finite element model of the jacket

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Fig. 5

Schematic jacket illustration with two stress monitoring location spots

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Fig. 6

Stresses in MPa for two different sea states. Mean stresses are subtracted for each stress.

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Fig. 7

Contour plot of the empirically estimated Ê2(ξ,η) surface (•), optimized Gumbel logistic G2(ξ,η) (∘) and optimized asymmetric logistic A2(ξ,η) (--) surfaces. Negative numbers indicate probability levels on a logarithmic scale.

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Fig. 8

Contour plot of the return periods for optimized Gumbel logistic G2(ξ,η) (∘) and optimized asymmetric logistic A2(ξ,η) (--) surfaces. Boxes indicate return periods in years.

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Fig. 9

Comparison between contour lines for 10 yr return period

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