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Research Papers: Offshore Technology

Wave Run-Up and Air Gap Prediction for a Large-Volume Semi-Submersible Platform

[+] Author and Article Information
Fabio T. Matsumoto

e-mail: fabio_matsumoto@tpn.usp.br

Rafael A. Watai

e-mail: rafael.watai@tpn.usp.br

Alexandre N. Simos

e-mail: alesimos@usp.br
TPN (Numerical Offshore Tank),
Department of Naval Architecture and Ocean Engineering,
Escola Politécnica,
University of São Paulo,
Av. Prof. Mello Moraes, 2231,
Cidade Universitária,
São Paulo, SP, 05508-900, Brazil

Marcos D. A. S. Ferreira

Research and Development Center (CENPES),
Petróleo Brasileiro S.A. (Petrobras),
Rio de Janeiro, RJ, Brazil
e-mail: marcos.donato@petrobras.com.br

Contributed by the Ocean Offshore and Arctic Engineering Division of ASME for publication in the JOURNAL OF OFFSHORE MECHANICS AND ARCTIC ENGINEERING. Manuscript received July 26, 2010; final manuscript received May 11, 2012; published online February 22, 2013. Assoc. Editor: M. H. (Moo-Hyun) Kim. Paper presented at the 29th International Conference on Ocean, Offshore and Arctic Engineering, Shanghai, China, 2010.

J. Offshore Mech. Arct. Eng 135(1), 011302 (Feb 22, 2013) (9 pages) Paper No: OMAE-10-1077; doi: 10.1115/1.4007380 History: Received July 26, 2010; Revised May 11, 2012

This paper addresses the problem of estimating the air gap for a large semisubmersible production platform. Although it has a great impact on the design of the floating unit, many times the minimum deck height is still defined from simplified methods that incorporate relatively large safety margins. The reason for this is the intrinsic complexity of the associated hydrodynamic problem. Nonlinear effects on the incoming and scattered waves are usually relevant and sometimes nonlinear effects on the motions of the floating hull may also play an important role. This discussion is illustrated by means of wave basin tests performed with the model of a large semisubmersible designed to operate in Campos Basin. Significant run-up effects on its squared-section columns were observed for the steepest waves in several design conditions. Also, the unit presented relatively large low-frequency motions in heave, roll and pitch, which also affected the dynamic air gap measurements. In order to evaluate the difficulties involved in modeling such phenomena, simplified tests were also performed with the model fixed and moored in regular waves of varying steepness. Wave elevation in different points was measured in these tests and compared to the predictions obtained from two different numerical methods: a BEM code that incorporates second order diffraction effects (WAMIT) and a VOF CFD code (ComFLOW), the latter employed for fixed model tests only. Results show that a standard linear analysis may lead to significant errors concerning the air gap evaluation. Extending the BEM model to second order clearly improve the results as the wave-steepness increases. Although the VOF analysis is considerably time-consuming, simulations presented very good agreement to the experimental results, even for the steepest waves tested.

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Figures

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Fig. 1

Schematic representation of the experimental setup (dimensions in meters)

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Fig. 4

Location of the wave-probes (values in mm)

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Fig. 5

WAMIT symmetrical model mesh (low order)

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Fig. 6

Heave RAO (experimental and predicted results)

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Fig. 7

Pitch RAO (experimental and predicted results)

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Fig. 8

Convergence analysis for second sum frequencies varying free surface panels

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Fig. 9

Inner free surface mesh (10,600 panels)

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Fig. 10

Inner free surface details near to the column

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Fig. 11

Composition of the total free surface elevation

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Fig. 12

Semisubmersible and symmetrical fluid domain modeled in ComFLOW

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Fig. 13

Nondimensional wave elevation at WP2 (captive test)

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Fig. 14

Nondimensional wave elevation at WP3 (captive test)

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Fig. 15

Nondimensional wave elevation at WP4 (captive test)

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Fig. 16

Wave splash on the stern column in the ComFLOW model

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Fig. 17

Nondimensional wave elevation at WP5 (captive test)

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Fig. 18

Nondimensional wave elevation at WP6 (captive test)

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Fig. 19

Nondimensional wave elevation at WP1 (captive test)

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Fig. 20

Nondimensional wave elevation at WP7 (captive test)

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Fig. 21

Free surface elevation mapping around of the platform

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Fig. 22

Nondimensional wave elevation at WP2 (moored test)

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Fig. 23

Nondimensional wave elevation at WP3 (moored test)

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Fig. 24

Nondimensional wave elevation at WP4 (moored test)

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Fig. 25

Wave run-up (moored-model test)

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Fig. 26

Nondimensional wave elevation at WP5 (moored test)

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Fig. 27

Nondimensional wave elevation at WP6 (moored test)

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Fig. 28

Nondimensional wave elevation at WP1 (moored test)

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Fig. 29

Nondimensional wave elevation at WP7 (moored test)

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