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Research Papers: Structures and Safety Reliability

Probabilistic Analysis of Extreme Riser Responses for a Weather-Vaning FPSO in Tropical Cyclones

[+] Author and Article Information
Curtis Armstrong

The Australian Maritime College,
University of Tasmania,
Maritime Way,
Newnham, Tasmania 7248, Australia
e-mail: curtis.armstrong@utas.edu.au

Yuriy Drobyshevksi

WorleyParsons, Ltd. (INTECSEA, Pty. Ltd.),
600 Murray Street,
West Perth 6005, Australia
e-mail: yuriy.drobyshevski@intecsea.com

Christopher Chin

The Australian Maritime College,
University of Tasmania,
Maritime Way,
Newnham, Tasmania 7248, Australia
e-mail: c.chin@utas.edu.au

Irene Penesis

The Australian Maritime College,
University of Tasmania,
Maritime Way,
Newnham, Tasmania 7248, Australia
e-mail: i.penesis@utas.edu.au

Contributed by the Ocean, Offshore, and Arctic Engineering Division of ASME for publication in the JOURNAL OF OFFSHORE MECHANICS AND ARCTIC ENGINEERING. Manuscript received March 8, 2018; final manuscript received July 26, 2018; published online October 12, 2018. Assoc. Editor: Zhen Gao.

J. Offshore Mech. Arct. Eng 141(2), 021602 (Oct 12, 2018) (16 pages) Paper No: OMAE-18-1021; doi: 10.1115/1.4041069 History: Received March 08, 2018; Revised July 26, 2018

The probability distributions of extreme responses of a flexible riser connected to a weather-vaning floating production storage and offloading (FPSO) are developed and investigated numerically for two tropical cyclones. Statistical properties of riser responses provide the foundation for response based analysis (RBA), a comprehensive approach for the prediction of extreme responses and design metocean conditions of offshore systems. The storm-based probabilistic analysis is applied to responses of flexible risers with the objective to develop their distributions in a storm and to determine their most probable maximum (MPM) values. An asymptotic form of the response distribution in a storm is formulated, which can be used in the random event, method of Tromans and Vandersohuren (1995, “Response Based Design Conditions in the North Sea: Application of a New Method,” Offshore Technology Conference, Houston, TX, May 1–4). The methodology is illustrated by two case studies for an FPSO in cyclonic storms at a location offshore Australia. Time domain simulations are employed to predict the FPSO motions, critical riser responses, and their probability distributions. It is shown that the maximum storm responses can be reproduced by governing “equivalent” metocean intervals with increased percentiles or inflated durations. Effects of different environmental excitation upon the risers and their impact on the statistical properties of responses are discussed, providing important insights for extension toward a multistorm RBA approach. The study also discusses issues with practices such as the analysis for a 3 h design event and presents observations on the variability of several types of responses, which reveal their environmental sensitivities.

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References

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Figures

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Fig. 1

OrcaFlex riser model (floating circles indicate locations of responses)

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Fig. 2

Storm 1 metocean parameters

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Fig. 3

Storm 2 metocean parameters

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Fig. 4

Metocean heading angle

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Fig. 5

Turret movement tracks from Ariane time histories

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Fig. 6

Sample Gumbel distribution fit CDFs

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Fig. 7

CDF of tension response

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Fig. 8

Truncated CDFs of tension response

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Fig. 9

LF + WF storm 1 sag bend curvature interval CDFs and storm CDF

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Fig. 10

LF + WF storm 1 turret heave interval CDFs and storm CDF

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Fig. 11

LF + WF storm 2 hangoff curvature interval CDFs and storm CDF

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Fig. 12

LF + WF storm 2 MWA curvature interval CDFs and storm CDF

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Fig. 13

LF + WF storm 2 sag-bend curvature interval CDFs and storm CDF

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Fig. 14

WF-only storm 2 effective hangoff tension interval CDFs and storm CDF

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Fig. 15

LF + WF storm 2 effective hangoff tension interval CDFs and storm CDF

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Fig. 16

Equivalent metocean parameters for hangoff tension, MWA curvature and turret heave: Storm 1 (interval 29)

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Fig. 17

Metocean parameters of benign storm 1 interval 30. Equivalent metocean parameters for hangoff curvature, sag-bend curvature, and hangoff declination angle.

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Fig. 18

Equivalent metocean parameters for turret heave: Storm 2 (interval 73)

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Fig. 19

Metocean parameters of severe storm 2 interval 74. Equivalent metocean parameters for hangoff tension.

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Fig. 20

Equivalent metocean parameters for hangoff curvature and hangoff declination angle: Storm 2 (interval 75)

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Fig. 21

Equivalent metocean parameters for sag bend curvature: Storm 2 (interval 83)

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Fig. 22

Equivalent metocean parameters for MWA curvature: Storm 2 (interval 92)

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Fig. 23

Storm 2 effective hangoff tension MPMs

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Fig. 24

Storm 2 sag-bend curvature MPMs

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Fig. 25

Storm 2 MWA curvature MPMs

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Fig. 26

Storm 2 hangoff curvature MPMs

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Fig. 27

Storm 2 turret heave MPMs

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Fig. 28

Storm 2 declination angle MPMs

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Fig. 29

Asymptotic and exact CDFs of storm 2 s effective hangoff tension storm distribution

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Fig. 30

Asymptotic and exact CDFs of storm 2 s hangoff curvature storm distribution

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