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TECHNICAL PAPERS

FPSO Conference—Estimating Wind-Sea and Swell for FPSO Operability

[+] Author and Article Information
K. C. Ewans

 Shell International Exploration and Production, Rijswijk, The Netherlandskevin.ewans@shell.com

L. Vanderschuren

 Ocean Wave Engineering Ltd., Liss, Englandluc.vanderschuren@skynet.be

P. S. Tromans

 Ocean Wave Engineering Ltd., Liss, England

J. Offshore Mech. Arct. Eng 128(4), 314-321 (Jun 06, 2005) (8 pages) doi:10.1115/1.2166653 History: Received January 11, 2005; Revised June 06, 2005

The motion response of an FPSO is sensitive to the relative intensities and directions of the wind-sea and swell components in a sea state, and the operability of the FPSO is a function of the long-term variation in these components. Estimations of the operability are therefore dependent both on how the sea state is described in terms of its constituent wind-sea and swell components, and on how the long-term variability of the sea state is captured. However, there is currently no consensus on how either the sea state or its long-term variability should be described. We investigate these issues by means of a study of the responses of a typical FPSO to the wave fields at a location offshore Namibia and a location off the west coast of New Zealand. We make use of a state-of-the-art program for splitting a directional wave spectrum into wind-sea and swell components, and we examine the effect on the motion responses of allowing the spectra to be split into many swell partitions or constraining the spectral split to a maximum of two partitions, as is often assumed in response calculations. The resulting decompositions are used to examine the effects of swell on hull motions and, hence, to identify methods for generating sea state criteria for operability. In addition, one-year metocean conditions are estimated; these are relevant for analysis of the limits on operations.

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Copyright © 2006 by American Society of Mechanical Engineers
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Figures

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Figure 8

Scatter plots of response parameters for the Namibia data: (a) roll against heave, (b) heave at centerline of turret against heave, (c) heave at centerline of turret against pitch, and (d) roll against pitch

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Figure 9

Bimodal conditions giving the one-year response for heave at mid-ships “h,” pitch “p,” roll “r,” and heave at centerline of turret “t”

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Figure 10

Time series plot of New Zealand data

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Figure 11

Time series plot of Namibia data

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Figure 4

Scatter plots of bimodal against multimodal response parameters, for the Namibia data (a) ship heading angle, (b) heave at mid-ships, (c) heave at centerline of the turret, (d) roll, (e) pitch, and (f) significant wave height

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Figure 5

Q-Q plots of bimodal against multimodal response parameters, for the New Zealand data (a) ship heading angle, (b) heave at mid-ships, (c) heave at centerline of the turret, (d) roll, (e) pitch, and (f) significant wave height

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Figure 6

Q-Q plots of bimodal against multimodal response parameters, for the Namibia data (a) ship heading angle, (b) heave at mid-ships, (c) heave at centerline of the turret, (d) roll, (e) pitch, and (f) significant wave height

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Figure 7

Scatter plots of response parameters for the New Zealand data: (a) roll against heave, (b) heave at centerline of turret against heave, (c) heave at centerline of turret against pitch, and (d) roll against pitch

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Figure 1

Scatter plots of modeled multimodal and bimodal Hs and T2 against measured Hs and T2 for the New Zealand location. The quantities cm and cb are the correlation coefficients for the multimodal and bimodal values with the measured values.

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Figure 2

Scatter plots of modeled multimodal and bimodal Hs and T2 against measured Hs and T2 for the Namibia location. The quantities cm and cb are the correlation coefficients for the multimodal and bimodal values with the measured values.

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Figure 3

Scatter plots of bimodal against multimodal response parameters, for the New Zealand data (a) ship heading angle, (b) heave at mid-ships, (c) heave at centerline of the turret, (d) roll, (e) pitch, and (f) significant wave height

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