Ocean Engineering

Stern Slamming of a LNG Carrier

[+] Author and Article Information
Jan Oberhagemann

 Germanischer Lloyd AG, Vorsetzen 35, Hamburg 20459, Germanyjan.oberhagemann@gl-group.com

Michael Holtmann

 Germanischer Lloyd AG, Vorsetzen 35, Hamburg 20459, Germanymichael.holtmann@gl-group.com

Ould el Moctar

 Germanischer Lloyd AG, Vorsetzen 35, Hamburg 20459, Germanyould.el-moctar@gl-group.com

Thomas E. Schellin

 Germanischer Lloyd AG, Vorsetzen 35, Hamburg 20459, Germanythomas.schellin@gl-group.com

Daewoong Kim

 Daewoo Shipbuilding & Marine Engineering Co. Ltd., 541 Daewoo Center Building Namdaemunro 5-Ga, Jung-Gu 656-714, South Koreadaewoong@dsme.co.kr

J. Offshore Mech. Arct. Eng 131(3), 031103 (Jun 02, 2009) (10 pages) doi:10.1115/1.3124131 History: Received July 03, 2008; Revised October 05, 2008; Published June 02, 2009

Rational assessment of stern slamming of a large twin screw liquefied natural gas (LNG) carrier comprised prediction of hydrodynamic impact loads and their effects on the dynamic global structural behavior of the hull girder. Linear theory obtained regular equivalent waves that caused maximum relative normal velocities at critical locations underneath the ship’s stern. Reynolds-averaged Navier–Stokes computations based on the volume of fluid method yielded transient (nonlinear) hydrodynamic impact (slamming) loads that were coupled to a nonlinear motion analysis of the ship in waves. At every time step of the transient computation, the finite volume grid was translated and rotated, simulating the actual position of the ship. Hydrodynamic loads acting on the hull were converted to nodal forces for a finite element model of the ship structure. Slamming-induced pressure peaks, typically lasting for about 0.5 s, were characterized by a steep increase and decrease before and after the peak values. Shape and duration agreed favorably with full-scale measurements and model tests carried out on other ships, indicating the plausibility of our numerical predictions. Hull girder whipping was analyzed to investigate dynamic amplification of structural stresses. Short-duration impact-related slamming loads excited the ship structure to vibrations in a wide range of frequencies. Excitation of the lowest fundamental eigenmode contributed most to additional stresses caused by hull girder whipping. Although, for the cases investigated, longitudinal stresses and shear stresses caused by quasisteady wave bending were uncritical, we obtained a significant amplification (up to 25%) due to the dynamic structural response.

Copyright © 2009 by American Society of Mechanical Engineers
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Figure 1

Stern region of the LNG carrier, seen from below

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Figure 2

Locations of investigated slamming loads

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Figure 3

Maximum normal relative velocities obtained from long-term statistics and restricted by wave amplitude

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Figure 4

Volume grid for wave 2

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Figure 5

Time histories of heave and pitch motions (upper graph) and accelerations (lower graph)

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Figure 6

Time history of normal relative velocity

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Figure 7

Time histories of computed slamming pressure in [kPa] for wave 1 (highest values), wave 2 (somewhat lower values), and wave 3 (lowest values)

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Figure 8

Impact peaks of hydrodynamic force

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Figure 9

Time histories of computed and measured pressures in the stern region of a large containership

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Figure 10

Computed pressure distribution for wave 2 occurring at time of peak slamming pressure

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Figure 11

Time histories of vertical acceleration (highly oscillating curve) in [m/s2] and vertical force (other cuve) in [kN] at stern (upper graph) and at bow (lower graph) for wave 1

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Figure 12

Frequency spectra of vertical acceleration at stern (left) and at bow (right) for wave 1

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Figure 13

Hull girder modes at eigenfrequencies of 0.79 Hz (upper graph) and 3.59 Hz (lower graph) for wave 1

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Figure 14

Time histories of elastic (highly oscillating curve), rigid body (thicker curve), and superimposed (total) vertical acceleration (curve oscillating about the thicker curve) in [m/s2] at stern (left) and at bow (right) for wave 1

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Figure 15

Time histories of longitudinal midship quasi-static wave bending stress (unidentified curve), slamming stress (curve identified by 22593 kN/m2), and superimposed (total) stress (curve identified by −105939kN/m2) in [kN/m2] for wave 1

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Figure 16

Longitudinal stress distribution in kn/m2 at time of maximum midship dynamic stress amplification (upper graph) for wave 1

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Figure 17

Elastic (highly oscillating curve), rigid body (thicker curve), and superposed (total) vertical acceleration (curve oscillating about the thicker curve) in [m/s2] at stern (left) and at bow (right) for wave 2




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