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Ocean Engineering

Structural Response to Sloshing Excitation in Membrane LNG Tank

[+] Author and Article Information
Mateusz Graczyk1

Centre for Ships and Ocean Structures (CeSOS) and the Department of Marine Technology, Norwegian University of Science and Technology (NTNU), Otto Nielsens v 10, 7491 Trondheim, Norwaymateusz.graczyk@marintek.sintef.no

Torgeir Moan

Centre for Ships and Ocean Structures (CeSOS) and the Department of Marine Technology, Norwegian University of Science and Technology (NTNU), Otto Nielsens v 10, 7491 Trondheim, Norway

1

Corresponding author.

J. Offshore Mech. Arct. Eng 133(2), 021103 (Dec 06, 2010) (9 pages) doi:10.1115/1.4001434 History: Received May 18, 2009; Revised September 25, 2009; Published December 06, 2010; Online December 06, 2010

Sloshing in LNG membrane tanks may cause large pressures on the tank structure. To keep the cargo at the required low temperature, the tank structure is covered with an insulation, which has a much less strength than steel. The containment system is a very complex structure, which consists of different materials and requires a careful analysis with due consideration of the load process and dynamic effects in the response. The structural response of the membrane tank wall is investigated in this paper by finite element analyses. First, a modal composition of the structural response is studied. It is shown that many modes contribute to the response, which makes it difficult to establish the simplified DLF approach. The dynamic structural response to a typical sloshing impact is investigated in detail. An important observation is that, although the containment system has traditionally been modeled with a rigid support, the steel plate that supports the insulation may be flexible under the relevant load conditions. It is shown that the flexibility of the steel plate causes significant stress variation in the insulation. Different response patterns of the Mark III containment system are presented, and mechanisms that cause large stress concentrations and different response patterns in the static and dynamic cases are discussed. The scaling issue in view of the response is also investigated. Various scaling formulations may apply in post-processing sloshing experiments. While the Froude law yields conservative scaling for pressure magnitude, its conservatism for scaling the time needs to be investigated in view of the relevant dynamic response. By analyzing the structural response to the differently scaled loads, it is found that the Froude approach is conservative, but the scatter of results may be very large.

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Figures

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Figure 1

Finite element model of the Mark III containment system: (a) a quarter of the 3300×840 mm2 panel with four horizontal locations for assessment of the response; and (b) the exact positions of nodes and integration points for the investigated responses in each of the four locations

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Figure 2

Schematic cross section of the flexible steel plate and the lower plywood supported on the resin ropes (the insulation layers above them are not shown) with a simplified stress distribution due to the two forms of response: steel plate bending and plywood bending; (a) under static loading in the middle of the panel, (d) under static loading above the girder, and (bc) possible combinations under dynamic loading

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Figure 3

The first 30 eigenfrequencies for flexible model with different values of wetted area

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Figure 4

Normalized spectra of the bending stresses in the lowest (○) and the highest (×) fiber of the lower plywood under excitation with 3 ms duration; full added mass. The eigenfrequencies marked as the vertical dotted lines: (a) location I, (b) location II, (c) location III, and (d) location IV; for definition of locations, consult Fig. 1.

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Figure 7

Vertical displacement of the steel plate with adjacent resin ropes (the other layers hidden for a better visibility); the not supported edges in the first plan, magnification factor 20.0: (a) static excitation with a magnitude 1 MPa; (bc) dynamic excitation with a magnitude of 1 MPa and with a 3 ms symmetric triangular temporal pattern; model with full added mass; (b) in t=3.1 ms and (c) in t=13.4 ms

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Figure 6

Time history of S11 in three fibers of lower plywood under dynamic excitation with a magnitude 1 MPa and with a 3 ms symmetric triangular temporal pattern; model with full wetted area: (a) location II and (b) location IV

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Figure 5

Time histories of containment system response in location II under dynamic excitation with a magnitude of 1 MPa and with a 3 ms symmetric triangular temporal pattern; model with full wetted area: (a) S22 in the foam and (b) S12 in the lower plywood

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