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Offshore and Structural Mechanics

Wave-in-Deck Load Analysis for a Jack-Up Platform

[+] Author and Article Information
Thomas E. Schellin

 Germanischer Lloyd, Brooktorkai 18, 20457 Hamburg, Germanythomas.schellin@gl-group.com

Milovan Perić

 CD-adapco, Nordostpark 3-5, 90411 Nürnberg, Germanymilovan.peric@de.cd-adapco.com

Ould el Moctar

 Germanischer Lloyd, Brooktorkai 18, 20457 Hamburg, Germanyould.el-moctar@gl-group.com

J. Offshore Mech. Arct. Eng 133(2), 021303 (Feb 10, 2011) (8 pages) doi:10.1115/1.4002047 History: Received June 19, 2009; Revised May 21, 2010; Published February 10, 2011; Online February 10, 2011

This paper describes the prediction of environmental loads on a typical three-leg jack-up platform under freak wave conditions. Considered were cases where the air gap is small and the hull is subject to impact-related wave-in-deck loads. The technique to predict wave loads was based on the use of a validated CFD code that solves the Reynolds-averaged Navier–Stokes equations. This code relies on the interface-capturing technique of the volume-of-fluid type to account for highly nonlinear wave effects. It computes the two-phase flow of water and air to describe the physics associated with complex free-surface shapes with breaking waves and air trapping, hydrodynamic phenomena that had to be considered to yield reliable predictions. The Stokes fifth-order wave theory initialized volume fractions of water, velocity distributions in the solution domain, and time-dependent boundary conditions at inlet and outlet boundaries. This paper demonstrates that this technique can be a valuable numerical tool for preliminary designs as well as subsequent safety assessments. In particular, it shows that effects of different operating and design parameters on wave-in-deck loads, such as wave direction, wave height, wave period, and wind speed, can be evaluated with an affordable computing effort.

Copyright © 2011 by American Society of Mechanical Engineers
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Figures

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Figure 1

Jack-up platform in a freak wave

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Figure 2

Mesh on platform surface (top) and on vertical plane in the longitudinal direction (bottom)

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Figure 3

The 19.9 m high Stokes fifth-order wave in 33.5 m water depth shortly after initialization (top), after one period (center), and after 1.5 periods (bottom)

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Figure 4

Wave profile (top) and pressures (bottom) on the fine mesh of a longitudinal hull section shortly before the crest of the 19.9 m wave reaches the platform

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Figure 5

Initial horizontal velocity component in the crest region of the 19.9 m wave and 14 s, 15 s, and 16 s thereafter (from top to bottom)

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Figure 6

Wave profile (top) and pressures (bottom) at second wave impact in the 19.9 m wave, computed on the medium grid in a longitudinal section 15 m away from the symmetry plane

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Figure 7

Wave profile (top) and pressures (bottom) at second wave impact in the 19.9 m wave, computed on the fine grid in a longitudinal section 15 m away from the symmetry plane

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Figure 8

Time histories of horizontal forces (top) and vertical forces (bottom) on the simplified platform computed on three grids

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Figure 9

Free surface shape during second wave impact 14 s, 15 s, 16 s, and 17 s after initialization of simulation (from top to bottom)

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Figure 10

Wave profiles on a longitudinal section through the platform in 60 deg incident waves shortly after initialization (top) and at two instants during wave impact (center and bottom) 1.25 s apart

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Figure 11

Free surface shape at two instants 1.0 s apart in 60 deg incident waves (wave height 19.9 m)

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Figure 12

Pressure distributions corresponding to flow fields shown in Fig. 1

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Figure 13

Time histories of horizontal forces on the whole platform (without legs) in 0 deg, 60 deg, 90 deg, and 180 deg incident waves of 15.8 m, 19.9 m, and 23.7 m height (from top to bottom)

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Figure 14

Time histories of vertical force on the whole platform (without legs) in 0 deg, 60 deg, 90 deg, and 180 deg incident waves of 15.8 m, 19.9 m, and 23.7 m height (from top to bottom)

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Figure 15

Time histories of horizontal forces on platform component parts 1–3 (see Fig. 2) in 60 deg incident waves (upper) and 180 deg incidence waves (lower) of 19.9 m wave height

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Figure 16

Time histories of horizontal forces (top) and vertical forces (bottom) acting on the whole platform with and without a 100 kN wind in 19.9 m high waves

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