PSI - Issue 5

Jung Min Sohn et al. / Procedia Structural Integrity 5 (2017) 943–950 Aditya Rio Prabowo et al. / Structural Integrity Procedia 00 (2017) 000 – 000

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element to avoid shear locking and hourglass phenomena that influence the accuracy of calculation result. Inner structure component of the bottom structure consists of several main parts, namely longitudinal stiffener, bottom plate, inner bottom plate, girder and bilge plate. A deformable model and the plastic-kinematic materials are augmented on the bottom. The applied material considers the applied steel of Prabowo et al. (2016b) which has yield strength σ Y = 440 MPa, Poisson’s ratio v steel = 0.3 , density ρ steel = 7,850 kg/m 3 and Young’s modulus E x = 210,000 MPa. The failure criterion is applied to define structural failure during crushing process of the bottom structure by the seabed. The proposed expression of a European classification society, Germanischer Lloyd (2003) is applied in this work. A topology of seabed takes a conical obstacle as the indenter in grounding. Illustration of this indenter is presented in Fig. 2. The seabed is assumed to be a hard rock which is modelled by rigid properties. The material of this seabed itself is taken from a mineral material that can be found in seabed, namely pyroxene. This entity has several main properties, including Poisson’s ratio v rock = 0.281 and density ρ rock = 4,002 kg/m 3 .

Fig. 1. Inner ship structure of the bottom structure. Deformable structure is applied

Fig. 2. The conical indenter for grounding analysis.

3.2. Grounding configuration

Grounding is assumed as a contact between the bottom structure and a conical indenter. Initial distance of two entities is determined to be 0.1 m in longitudinal direction ( x-axis ). Position of the indenter is varied in vertical direction ( z-axis ) which a gap 0.25 m in height and fully parallel on the lower part of the bottom structure and indenter. Both positions are denoted as Position 1 and Position 2 consecutively and set on the grounding scenarios. The conical indenter is given a velocity 10 m/s to move to three target components, namely center girder, side girder and inner shell. In the end of model, the shell is restrained and bottom structure is set to be fix in the centerline during grounding.

4. Results and discussion

This section presents simulation results of several scenarios calculated by the finite element method. Discussions are addressed into two subjects regarding the sequence of structural failure and responses of the bottom structure during penetration of the indenter during grounding.

4.1. Structural failure during grounding

In grounding, the indenter was set to three different targets on the bottom structure. An evaluation to each target strength is performed which indicates that the center girder produced the highest energy. The internal energy (Fig. 3) was taken as consideration to represent energy which was used to crush the target component and surrounding in impact. Similar trend was shared for three targets approximately until 0.05 s. After passing through this time, the center girder produced higher energy than side girder and inner shell which continued until the end of simulation time. Considering impact time, longer similarity was shown by the side girder and inner shell until 0.125 s. These components had similar thickness, but geometry near each component was quite distinct. Surrounding the side girder was strengthened by the longitudinal stiffener on the bottom plate which was similar with the center girder. In other hand, the inner shell connected to the transverse frame which connected the outer and inner shells. In the end of penetration, the results can be concluded that the inner shell has better capability to resist the indenter.