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TECHNICAL PAPERS

An Approach to Calculate the Probability of Wave Impact on an FPSO Bow

[+] Author and Article Information
C. Guedes Soares

Unit of Marine Technology and Engineering, Technical University of Lisbon, Instituto Superior Técnico, Avenida Rovisco Pais, 1049-001 Lisbon, Portugalguedess@mar.ist.utl.pt

R. Pascoal, E. M. Antão

Unit of Marine Technology and Engineering, Technical University of Lisbon, Instituto Superior Técnico, Avenida Rovisco Pais, 1049-001 Lisbon, Portugal

A. J. Voogt, B. Buchner

 MARIN, Haagsteeg 2, NL-6708 PM Wageningen, The Netherlands

J. Offshore Mech. Arct. Eng 129(2), 73-80 (Mar 28, 2006) (8 pages) doi:10.1115/1.2426983 History: Received July 16, 2004; Revised March 28, 2006

This work aims at characterizing the probability of wave impact and determining the position of impact on an FPSO (floating production storage and offloading platform) bow geometry. In order to determine the instants when impact occurs, an experimental program was performed on a specific bow shape. The bow was instrumented with pressure transducers and the test program, also making use of video recordings, was designed such that it was possible to determine the correlation between undisturbed wave shape and the impact pressure time traces. It has been found that the wave impact at the bow is highly correlated with the local wave steepness, which for very high waves has at least second-order effects. A comparison between the probability distributions of local wave steepness of the experimental undisturbed wave time trace and numerical simulations of second-order wave theory is provided and it confirmed that the latter is very adequate for calculations. The experimental results were further used to determine how the probability of impact varies with free surface vertical velocity. It was found that the significant wave height of the sea state itself does not have significant influence on the result and a regression model was derived for the bow type in the experiments. The proposed model for determining the probability of having an impact is based on combining distributions, adjusted a priori to the numerically generated second-order free surface vertical velocity, and the experimental probability of impact of a known certain seastate and free surface velocity. The analytical description makes it fast and easy to expand to other cases of interest and some example calculations are shown to demonstrate the relative ease of the procedure proposed. The position of the impact is determined by the nonlinear wave crests and the ship motions. The ship motions can be determined based on a linear response to the nonlinear waves considered.

Copyright © 2007 by American Society of Mechanical Engineers
Topics: Waves , FPSO , Probability , Seas
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Figures

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Figure 1

Front view of schematic bow setup and its instrumentation

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Figure 2

Schematic of the probe frame

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Figure 3

Wave impact on the instrumented fixed bow (top right corner shows mirrored underwater view)

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Figure 4

Traced impacts versus time, superimposed on the free surface vertical velocity plot

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Figure 5

Crest height as function of wave vertical velocity

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Figure 6

Probability of impact versus free surface vertical velocity, average for group 1

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Figure 7

Probability of impact versus free surface vertical velocity, average for group 2

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Figure 8

Probability of impact versus free surface vertical velocity, average for group 3

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Figure 9

Probability of impact given a free surface vertical velocity

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Figure 10

Comparison of the cumulative probability of exceedance of field data and numerical simulation

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Figure 11

Comparison of the cumulative probability of exceedance of field data and numerical simulation

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Figure 12

Relative error between basin data and numerical simulation, Hs=8m, Tp=8s

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Figure 13

Relative error between basin data and numerical simulation, Hs=12m, Tp=12s

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Figure 14

Relative error between basin data and numerical simulation, Hs=14m, Tp=14s

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Figure 15

Scatter diagram plot for a European area with contour probability levels

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Figure 16

Definition of slam center (H) with respect to crest height (C) of incoming undisturbed wave

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Figure 17

Slam center (H) on horizontal axis as function of crest height (C)

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Figure 18

Complete wave impact prediction methodology

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Figure 19

Time traces of measured vessel response compared to calculated response assuming linear motion response

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