Research Papers: Structures and Safety Reliability

Airgap Statistics for a Tension Leg Platform

[+] Author and Article Information
Oleg Gaidai, Carl Trygve Stansberg

Trondheim NO-7491, Norway

Arvid Naess

CeSOS and Department of
Mathematical Sciences,
Trondheim NO-7491, 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 September 30, 2013; final manuscript received July 3, 2014; published online October 21, 2014. Assoc. Editor: Lance Manuel.

J. Offshore Mech. Arct. Eng 137(1), 011602 (Oct 21, 2014) (5 pages) Paper No: OMAE-13-1092; doi: 10.1115/1.4028734 History: Received September 30, 2013; Revised July 03, 2014

The paper discusses a method for estimating extreme value statistics of the airgap for floating offshore platforms subjected to random seas. It is an adaptation of a recently developed method, which is based on the mean upcrossing rate (MUR) function for univariate time series combined with an optimization procedure that allows prediction at extreme response levels by extrapolation. Extensive model tests were performed in a large wave basin for a tension leg platform (TLP) operating in the Norwegian Sea. Among several critical parameters, the airgap was measured at a number of locations under the platform deck. The wave in deck impact is a critical safety issue with respect to the deck damage and occurrence of extreme tether tensions. The authors have utilized experimental data to look at critical airgaps under the deck in random waves. Conclusions are drawn about extreme airgap statistics, and consequently about the wave impact probability in severe seas.

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Fig. 1

Heidrun TLP as seen from the side

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Fig. 2

Snapshot of model testing of the Heidrun TLP in a high sea state in the MARINTEK ocean basin

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Fig. 3

TLP deck and measurement locations, selected only under the deck

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Fig. 4

Distribution of maximal wave height per measurement location, see Fig. 3. Only true under-deck locations are kept. Dataset from 20 storms.

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Fig. 5

Prediction, 2 storms dataset. 95% confidence bands are dashed lines.

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Fig. 6

Prediction, 20 storms dataset. 95% confidence bands are dashed lines.

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Fig. 7

Free field wave elevation upcrossing rate, 20 storms dataset. 95% confidence bands are dashed lines.

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Fig. 8

Prediction for station #11, 20 storms dataset. 95% confidence bands are dashed lines. Prediction P=10−4 and CI = (0.5, 1.6) × 10−4.




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