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Research Papers: Polar and Arctic Engineering

Simulating Ice-Sloping Structure Interactions With the Cohesive Element Method

[+] Author and Article Information
Wenjun Lu

Sustainable Arctic Marine
and Coastal Technology (SAMCoT),
Centre for Research-Based Innovation (CRI),
Norwegian University of Science and Technology,
Høgskoleringen 7A, 7491,
Trondheim, Norway
e-mail: wenjun.lu@ntnu.no

Raed Lubbad

Sustainable Arctic Marine
and Coastal Technology (SAMCoT),
Centre for Research-Based Innovation (CRI),
Norwegian University of Science and Technology,
Høgskoleringen 7A, 7491,
Trondheim, Norway
e-mail: raed.lubbad@ntnu.no

Sveinung Løset

Sustainable Arctic Marine
and Coastal Technology (SAMCoT),
Centre for Research-Based Innovation (CRI),
Norwegian University of Science and Technology,
Høgskoleringen 7A, 7491,
Trondheim, Norway
e-mail: sveinung.loset@ntnu.no

Usually, to rigorously test a particular numerical method, a much simpler test case and smaller scale is employed (e.g., uniaxial tension test, beam bending test, etc.).

1Corresponding author.

Contributed by the Ocean, Offshore, and Arctic Engineering Division of ASME for publication in the JOURNAL OF OFFSHORE MECHANICS AND ARCTIC ENGINEERING Manuscript received August 20, 2012; final manuscript received February 18, 2014; published online April 1, 2014. Assoc. Editor: Arne Gürtner.

J. Offshore Mech. Arct. Eng 136(3), 031501 (Apr 01, 2014) (16 pages) Paper No: OMAE-12-1084; doi: 10.1115/1.4026959 History: Received August 20, 2012; Revised February 18, 2014

The major processes that occur when level ice interacts with sloping structures (especially wide structures) are the fracturing of ice and upcoming ice fragments accumulating around the structure. The cohesive zone method, which can simulate both fracture initiation and propagation, is a potential numerical method to simulate this process. In this paper, as one of the numerical methods based on the cohesive zone theory, the cohesive-element–based approach was used to simulate both the fracturing and upcoming fragmentation of level ice. However, simulating ice and sloping structure interactions with the cohesive element method poses several challenges. One often-highlighted challenge is its convergence issue. Numerous attempts by different researchers have been invested in this issue either to prove or improve its convergence. However, these researchers work in different fields (e.g., fracture of concrete, ceramic, or glass fiber) with different scales (e.g., from a ceramic ring to a concrete block). As an attempt to study the cohesive element method's application in the current ice-structure interaction context (i.e., an engineering scale up to hundreds of meters), the mesh dependency of the cohesive element method was alleviated by both creating a mesh with a crossed triangle pattern and utilizing a penalty method to obtain the initial stiffness for the intrinsic cohesive elements. Furthermore, two potential methods (i.e., introduction of a random ice field and bulk energy dissipation considerations) to alleviate the mesh dependency problem were evaluated and discussed. Based on a series of simulations with the different aforementioned methods and mesh sizes, the global ice load history is obtained. The horizontal load information is validated against the test results and previous simulation results. According to the comparison, the mesh objectivity alleviation with different approaches was discussed. As a preliminary demonstration, the results of one simulation are summarized, and the load contributions from different ice-structure interaction phases are illustrated and discussed.

Copyright © 2014 by ASME
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References

Figures

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Fig. 1

Difference between the shared nodes approach and the currently employed tied surface approach

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Fig. 2

Detailed illustration of the cross triangle structured mesh pattern

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Fig. 3

Physical model test setup (Note that all the numbers are in model scale with a scaling number of 10. Left: schematic drawing of the cone and load measuring system; right: the actual test setup during the experimentation).

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Fig. 4

The numerical model setup (note that the numbers given are in full scale)

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Fig. 5

Fracture energy's spatial distribution with a 0 m correlation

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Fig. 6

Fracture energy's spatial distribution with a 1 m correlation

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Fig. 7

Fracture energy's spatial distribution with a 5 m correlation

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Fig. 8

Bulk energy allocations

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Fig. 9

Fluid effects in the vertical direction

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Fig. 10

Illustration of the smeared pressure on the structure surface ((a) initial contact and (b) nonsimultaneous contact during the interaction)

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Fig. 11

Mises stress in the ice sheet during the initial contact, crack formation, and fragment interactions

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Fig. 12

Visual illustration of the cohesive element method based simulation (a) the ice rubble accumulation during the interaction and (b) the channel behind the cone structure

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Fig. 13

A typical channel shapes behind an ice breaker (reproduced based on Fig. 38 in Ref. [5] but originally from Ref. [60])

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Fig. 14

Bending failure observation made in (a) numerical simulation and (b) physical model test

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Fig. 15

Global horizontal load history considering bulk energy dissipation

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Fig. 16

Mesh dependency of the mean horizontal load

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Fig. 17

Illustration of peak load sampling (circle: sampled peak values; dark curve: simulated load history; gray line: mean value of the load history)

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Fig. 18

The mean value of the peak horizontal loads

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Fig. 19

(a) Ice breaking patterns and (b) ice breaking length in the radial direction (note here that the radial direction of the ice breaking length is labeled in red color, and the ice thickness in model scale is 33 mm). See online version for color figure.

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Fig. 20

Comparison of the ice load history

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Fig. 21

Mean horizontal load in different integration layers (mesh size: 0.625 m)

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Fig. 22

Illustration of the cantilever beam bending case study: (a) Original problem and (b) FEM meshing with cohesive elements in between the bulk elements

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Fig. 23

The illustration of structural softening with the presence of intrinsic cohesive elements (note here N = ⌈l/Lm⌉ is an integer number describing the number of cohesive elements and inversely the mesh size)

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