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Research Papers: Structures and Safety Reliability

Extreme Response Statistics of Fixed Offshore Structures Subjected to Ringing Loads

[+] Author and Article Information
Oleg Gaidai

MARINTEK AS,
NO-7491 Trondheim, Norway

Jørgen Krokstad

STATKRAFT AS,
NO-7037 Trondheim, Norway

Contributed by the Ocean, Offshore, and Arctic Engineering Division of ASME for publication in the JOURNAL OF OFFSHORE MECHANICS AND ARCTIC ENGINEERING. Manuscript received November 21, 2010; final manuscript received April 23, 2014; published online May 19, 2014. Assoc. Editor: Bernt J. Leira.

J. Offshore Mech. Arct. Eng 136(3), 031604 (May 19, 2014) (9 pages) Paper No: OMAE-10-1112; doi: 10.1115/1.4027542 History: Received November 21, 2010; Revised April 23, 2014

This paper describes an efficient Monte Carlo based method for prediction of extreme response statistics of fixed offshore structures subjected to random seas. The nonlinear structural response known as “ringing” is studied, which is caused by the wave impact force on structural support units. Common challenge for design of such structures is a sound estimate of the hydrodynamic load including diffraction effects. The aim of the work was to develop specific methods which make it possible to extract the necessary information about the extreme response from relatively short time histories. The method proposed in this paper opens up the possibility to predict simply and efficiently both short-term and long-term extreme response statistics. The results presented are based on extensive simulation results for the large fixed platform operating on the Norwegian continental shelf. Structural response time histories, measured in MARINTEK (MT) wave basin lab, were used to validate numerical results.

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References

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Krokstad, J., Nestegård, A., Mathinsen, J., and Stansberg, C. T., 1998, “A New Nonslender Ringing Load Approach Verified Against Experiments,” ASME J. Offshore Mech. Arct. Eng., 120(1), pp. 20–29. [CrossRef]
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Figures

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Fig. 1

General structural layout, North and West view

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Fig. 2

Torsethaugen wave spectrum

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Fig. 3

Load calculation procedure

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Fig. 4

Finite element mesh for WADAM simulation

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Fig. 7

Time histories of base shear force for 90 deg wave heading. Experimental (Basin) versus numerical

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Fig. 8

qq-plot, normalized experimental base shear versus numerical

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Fig. 10

ACER declustering steps, n=1,..,4. Response ξ is base shear force.

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Fig. 11

ACER3 log plot for the base shear force. Dotted line is Monte Carlo data. Arrow and dashed line indicate 90% fractile response level.

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Fig. 12

Transformed plot, corresponding to Fig. 11

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Fig. 13

Gumbel plot, corresponding to the same dataset as in Fig. 11. Arrow and dashed line indicate 90% fractile response level.

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