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Safety and Reliability

Wave Load and Structural Analysis for a Jack-Up Platform in Freak Waves

[+] Author and Article Information
Ould el Moctar

 Germanischer Lloyd, Hamburg 20459, Germanyould.el-moctar@gl-group.com

Thomas E. Schellin1

 Germanischer Lloyd, Hamburg 20459, Germanythomas.schellin@gl-group.com

Thomas Jahnke

 Germanischer Lloyd, Hamburg 20459, Germanythomas.jahnke@gl-group.com

Milovan Peric

 CD-adapco Nürnberg, Nürnberg 90402, Germanymilovan.peric@de.cd-adapco.com

1

Corresponding author.

J. Offshore Mech. Arct. Eng 131(2), 021602 (Mar 12, 2009) (9 pages) doi:10.1115/1.2948952 History: Received July 02, 2007; Revised March 03, 2008; Published March 12, 2009

This paper analyzed the effects of freak waves on a mobile jack-up drilling platform stationed in exposed waters of the North Sea. Under freak wave conditions, highly nonlinear effects, such as wave run-up on platform legs and impact-related wave loads on the hull, had to be considered. Traditional methods based on the Morison formula needed to be critically examined to accurately predict these loads. Our analysis was based on the use of advanced computational fluid dynamics techniques. The code used here solves the Reynolds-averaged Navier–Stokes equations and relies on the interface-capturing technique of the volume-of-fluid type. It computed 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. Lastly, the finite element method was used to apply the wave-induced loads onto a comprehensive finite element structural model of the platform, yielding deformations and stresses.

Copyright © 2009 by American Society of Mechanical Engineers
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References

Figures

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Figure 1

Global structural FEM model of the jack-up rig

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Figure 2

Numerical grid on legs and hull of the rig and on the ocean bottom

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Figure 3

Higher local grid density for wave-structure interaction

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Figure 4

Screen shot of rig in 19.9m wave

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Figure 5

Screen shot of rig in 23.7m wave

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Figure 6

Simulated wave profile of 23.7m wave

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Figure 7

Pressure distribution on rig in 19.9m wave

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Figure 8

Slamming pressures on underside of hull deck

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Figure 9

Time histories of base shear

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Figure 10

Time histories of overturning moment

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Figure 11

Time histories of base shear on a single leg

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Figure 12

Maxima of base shear and overturning moment versus wave height

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Figure 13

Comparative results of base shear for the 11.6m wave

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Figure 14

Comparative results of base shear for the 15.8m wave

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Figure 15

Comparative results of base shear for the 19.9m wave

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Figure 16

Comparative results of base shear for the 23.7m wave

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Figure 17

Comparative results of overturning moment for the 11.6m wave

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Figure 18

Comparative results of overturning moment for the 15.8m wave

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Figure 19

Comparative results of overturning moment for the 19.9m wave

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Figure 20

Comparative results of overturning moment for the 23.7m wave

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Figure 21

Stress distribution at Points 1 and 2 for 15.8m wave

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Figure 22

Equivalent stresses at Point 1

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Figure 23

Bending stresses at Point 2 for 15.8m and 19.9m waves

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Figure 24

Jacking system forces for 19.9m wave

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Figure 25

Vertical leg tip reaction forces for 19.9m wave

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Figure 26

Rigid body hull deformation for 19.9m wave

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