DESIGN CONCEPT OF A WATERWAY DREDGER JIB STRUCTURE

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The 5 th PSU-UNS International Conference on Engineering and 239 Technology (ICET-2011), Phuket, May 2-3, 2011 Prince of Songkla University, Faculty of Engineering Hat Yai, Songkhla, Thailand 90112

1 The 5 th PSU-UNS International Conference on Engineering and 239 Technology (ICET-2011), Phuket, May 2-3, 2011 Prince of Songkla University, Faculty of Engineering Hat Yai, Songkhla, Thailand DESIGN CONCEPT OF A WATERWAY DREDGER JIB STRUCTURE D. Kovacevic*, I. Budak, A. Antic University of Novi Sad, Faculty of Technical Sciences, Novi Sad, Serbia *Authors to correspondence should be addressed via dusan@uns.ac.rs Abstract: This paper is a proposal for an advanced approach for FEM modeling, structural analysis and design of a jib structure which is a typical part of the waterway bucket dredgers facilities. Dredgers with two catamaran-like pontoons, with jib in between, are considered here. The item of the analysis is the jib structure which will be re-constructed for the excavation of grain material from greater depths. Discussion here is oriented to the explanation of advantages and problems in the utilization of various FEM models. Key Words: FEM Modeling, Jib Structure, Optimal Design 1. INTRODUCTION In the classification of the waterway dredgers for the material exploitation under the water surface, a large group is bucket dredgers with the bucket on the continual chain supported by a jib structure, as is shown in Fig. 1. Fig. 1. Bucket dredger with the jib structure The excavation is performed by moving the bucket which plunges into the material at the water bed. Excavation continuity depends on the bucket size and span, as well as on the chain movement length and speed. This paper considers the design and FEM model for the jib structure as a girder for the dredger's working tool. The papers dealing with the FEM analysis of this type of structure (for example, [1]-[4]) are relatively rare. Their main feature is the application of complex models, but only to analyze the critical structural parts. This circumstance has additionally motivated the authors to research the alternatives, i.e. the possibilities for the advanced modeling of the jib structure as a whole. Due to the exploitation conditions, the jib in this type of dredgers should satisfy several opposed demands and its design should be, in the positive engineering sense, a compromise solution. Jib bearing capacity is the main and mandatory performance. Stress condition in all structural elements has to be within the boundaries which exclude the possibility of limit state reaching. Jib structure stability (i.e. buckling stiffness) is another requirement for the structure integrity. Global buckling is questionable, while the appearance of the local stability loss is possible since this is a thin-walled structure. Serviceability is the performance providing the conditions for the real exploitation service of the machine. It is normally connected to the stiffness, i.e. the state of displacement and deformation of the structure that provides the uninterrupted work with the possible failures only as a consequence of so-called "force majeure" circumstances. The above mentioned conditions are opposite to the demand for small mass of a jib structure because the mobility in exploitation and maintenance, as well as for the energy efficiency of the dredger. Furthermore, it is necessary the jib structure to be manufactured with the minimal quantity of steel and to be a simple design, regards to minimization of the manufacturing price. 2. THE JIB STRUCTURE POSSIBLE MODELS In the area of the applied structural design, the objective is to formulate the "optimal" model [5]. This is a model with the largest quality of the approximation achieved in the conditions of the "common designing practice", [6]. The choice of the model finally depends on the structure topology, action configuration and the assumed structure response. Waterway bucket dredger jib is a space structure with the notable length in comparison with other dimensions. It is a thin-walled structure with variable wall thickness, with the lateral stiffeners that increases the bearing capacity and stiffness of the structure. For the preliminary analyses, the satisfactory FEM models are the beam FE models. When the application of

240 these 1D models determines the approximate element dimensions, they are followed by the sophisticated analysis by applying the 2D FEM models with the surface FE.

The state of stress and deformation in the jib thin-walled structure can be approximated rather well by a model in which the stresses in the normal direction on the plate surface that present the jib

2 240 these 1D models determines the approximate element dimensions, they are followed by the sophisticated analysis by applying the 2D FEM models with the surface FE. These 2D models almost completely satisfy the demands of the developmental research, so they can be the final models for the jib structural systems. The state of stress and deformation in the jib thin-walled structure can be approximated rather well by a model in which the stresses in the normal direction on the plate surface that present the jib cover are neglected. Furthermore, it is reasonable to assume that there is no shear in the plate mid-plane. These circumstances indicate the possibility to utilize the model based on the Kirchhoff''s flexural theory of thin plates, [5]. Exceptionally, for the plates with relatively large thickness it is necessary to apply the Reissner-Mindlin models for thick plate bending. If one can avoid the appearance of the so-called "shearlocking" phenomenon, the thick plate models that consider the shear influence (i.e. real shear stiffness) can provide very satisfactory results. Finally, if there is the stress concentration in the local zones ("hot spot area"), when the external action is distributed on a relatively small surface, or if the stresses orthogonal to the plate mid-plane cannot be neglected, the application of a 3D model is an imperative. A simple numerical test will illustrate the advantages of a model with 2D FE in relation to the 1D FE model. The results of this test could be main argument in the final model choice. This and similar, "benchmark test" should become an obligatory part in the model choice methodology. The analysis is performed for the vertical uniformly distributed load equal to the weight of the chain with the buckets. Fig. 2 presents 1D (top) and 2D (botom) models and principal stress values, vertical displacements in characteristic points and the lowest natural frequencies for both models. Fig. 2. 1D and 2D models: displacements principal stresses and the lowest natural frequencies 1D model is formed from the beam FE ( 240x 100x20mm box shape) and in the topological sense it is completely identical to the 2D model with the rectangular shell (isoparametric, nine-node, heterosis) with the FE thickness of t=20mm. Boundary conditions are adapted to the real conditions of the jib support. It is clear that the principal stresses (S1 and S2) in the more accurate 2D model are 1.75 to times greater than in a simpler 1D model. The case with the vertical displacements (Dz) is similar. Here the factors are from 1.54 to 1.80 more beneficial to the more complex 2D model. Furthermore, the lowest natural frequency (f1) in a 1D model is almost 1.8 times lower than the same frequency in the 2D model. It is necessary to note that both models have the natural shape with horizontal displacements. These differences in the response for the same action indicate the necessity of the application of the more sophisticated model with 2D shell FE in modeling the jib behavior. This test shows very clearly that apparently similar models can obtain very diverse data about the structure, and sometimes also a very wrong impression about the bearing capacity, stability and serviceability, thus definitely confirming the demand for applying more complex models. In this sense, all further considerations will apply to the numerical model with 2D shell FE. 3. FINAL MODEL OF THE JIB STRUCTURE Because of new exploitation demands jib will be reconstructed by increasing the length from 35.5m to 49.0m, Figure 3 (can be seen in [6]) shows the jib structure with the marked position of the new additional segment. Fig. 3. Drawing of new length jib structure The increase in the jib length has the following consequences: axial, flexural and torsional stiffness decrease and inertia increase, i.e. natural frequencies decrease. In the jib modeling, the software AxisVM version 10.2h (InterCAD, Hungary) has been used. AxisVM is based on the pre-processor for geometric modeling, the processor for numerical modeling (with the rich library of FE and a large number of various models: linear, nonlinear, dynamic, etc.) and the post- -processor for presentation of the analysis results. 4. MODELING OF THE JIB STRUCTURE TOPOLOGY AND GEOMETRY Data on the jib structure topology and structural elements dimensions are taken from [6]. For the geometric modeling of the jib structure, the non-automatic approach for FE meshing has been selected. The procedure has the following steps: designing stiffeners (only one symmetric side) of diverse type and dimensions, Fig. 4, placing stiffeners at appropriate position, i.e. forming the structure skeleton (symmetric side), Fig. 5, connecting stiffeners with the cover plates (symmetric side), Fig. 6,

241 elaborating the support details - in the zone of the axle around the jib rotates in the vertical plain and in the zone of cable connected for the jib angle changing, Fig.

Furthermore, one obtains, in the largest number, rectangular shape of the FE's, without great shape distortion, which is the most favorable solution from the aspect of the numerical error in

Cover plate segments between frame stiffeners (one side of structure) Fig. 7.

3 241 elaborating the support details - in the zone of the axle around the jib rotates in the vertical plain and in the zone of cable connected for the jib angle changing, Fig. 7, and adding the onother symmetric side, Fig. 8. efficiency of the computation. Furthermore, one obtains, in the largest number, rectangular shape of the FE's, without great shape distortion, which is the most favorable solution from the aspect of the numerical error in computations. Fig. 8. Entire model of the jib structure Fig. 4. Halves of the jib frame stiffeners Fig. 5. Frame stiffeners (one side of structure) Fig. 6. Cover plate segments between frame stiffeners (one side of structure) Fig. 7. Jib structure supports details This approach has been selected with the objective to minimize the FE mesh size, which can lead to the 5. MODELING THE ELEMENTS, BOUNDARY CONDITIONS AND ACTIONS The concept of the model design described in the previous paragraphs has been selected with the objective to obtain a robust and efficient model in the numerical sense: degrees of freedom (DOF) number is for the model with 6525 nodes and 7188 shell FE. Model rationality and calculation efficiency are significant since they enable the following: simple and rapid model changing, various action modeling (loads, temperature changes, support displacements, manufacturing imperfections, accidental actions, etc.) and various types of analyses (linear analysis, geometric/material nonlinear analysis, buckling analysis, free vibrations analysis, time history analysis, etc.). With the structural elements approximation there has not been any great dilemma - rectangular shell FEs are applied with nine nodes of the heterosis type and triangular FE with seven nodes, based on the Reissner- -Mindlin theory of thick plate bending and the theory for membrane stress condition. Each node in this FE element has all six degrees-of-freedom elements - three translations and three rotations. It is important to emphasize that the rotation DOF in the plane of the FE (so-called "drilling" DOF) is introduced implicitly, which is a satisfactory treatment. The size, shape and distribution (and hence the number) of the FE is selected in such a manner as to avoid the structurally and numerically unwanted situations: stress concentration (except when it is a necessity due to the shape of the action) and calculation errors that generate due to the existence of the "distorted" shape of FE. The next step demands a designer's creative efforts regards to the modeling of behavior of the support zones of the structural system, the places with abrupt stiffness change with eccentricity, as well as the areas where the structural elements are connected in a specific manner. It is common in the FEM technology for this modeling to be obtained by defining boundary and interface conditions.

242 For the observed jib structure, the support conditions are simple to model. The axle (Fig.

It is necessary to model almost completely free (or with little friction) only jib rotation around the axle and without any other DOF.

These are 1D FE of special purpose with two nodes and all six DOF in each node. Usually, the connections of the standard FEs are direct - in common nodes.

4 242 For the observed jib structure, the support conditions are simple to model. The axle (Fig. 7, left) around which the jib can rotate freely is the bearing that is by the pedestal supported to the deck structure of the waterway facility. It is necessary to model almost completely free (or with little friction) only jib rotation around the axle and without any other DOF. This can be obtained only by applying the so-called link FE (for details see [7]). Link FEs are used for modeling connections and joints with special characteristics. These are 1D FE of special purpose with two nodes and all six DOF in each node. Usually, the connections of the standard FEs are direct - in common nodes. If the connection between two adjacent FE is without a common node, the link FE is used. By varying the stiffness and position parameters (the so-called interface points), one can model a set of various connections. In the jib structure modeling, the link FEs are applied in two cases: for the axle-jib connection and for large number of eccentrically connected joints between the jibs cover plates. The connection jib/axle is a hinge which allows only the rotation around the own axis. In Fig. 9 there is a model of this hinge with the distribution of link FEs. single link FE hinge part of jib thinner part of axle hinge part of jib FEs thicker part of axle thicker part of axle FE thinner part of axle FE Fig. 9. Modeling the hinged support by link FE As it can be observed, link FEs radially join a node of the FE axle and adjoining nodes of the shell FE of the jib hinge. Similar situation refers to all eccentric joints in the thin-walled cover or diverse thickness stiffeners, as presented in Fig. 10. Here, all six stiffness parameters have a non-zero value that simulates a rigid welded eccentric connection. In some cases there is a necessity for such model configuration of the eccentrically welded sheet metal plates, especially if there are large membrane forces. Fig. 10. Link FEs for connections with eccentricity Jib configurations with various actions are: repair, transportation and three dredging positions. Actions are the following: a) jib self-weight (G= kg), b) empty buckets weight (q E =9.4kN/m), c) filled buckets weight (q C =15.8kN/m), d) bottom wheel weight (G BW =50kN), e) dredging force (F D =273.5kN), f) bucket chain pull force (F BC =560.7kN), g) frontal force of extrude (F FI =150kN), h) lateral force of extrude (F LI =77kN) and i) pending part of chain weight (G PC =220kN). Eight configurations have been observed: jib is on the 12º in relation to the horizontal line - repair and transportation positions, jib is on the 18º - statuses "A", "B", and "C" and jib is on the 45º - statuses "A", "B", and "C". Valid configurations are as follows: repair position: a)+b) - jib is supported by cable to the bow crane (discussed later), transportation position: a)+b)+c) - jib is supported to the waterway bed, status "A": a)+c)+d)+e)+f)+g)+h) - jib is supported by cable to the dredger's bow crane, status "B": a)+c)+d)+e)+f)+g)+h) - jib is supported to the waterway bed and status "C": a)+c)+d)+e)+f)+g)+h)+i) - jib is supported by chain to the dredger's bow crane. 6. ANALYSIS OF THE JIB STRUCTURE RESPONSE Comparing the results of the analysis and the calculations, it is established that, utilizing the criteria of the greatest displacements and support responses, the valid configurations are as follows: "18C" - jib is on the 18º - status "C", "18B" - jib is on the 18º - status "B", "45C" - jib is on the 45º - status "C" and "45B" - jib is on the 45º - status "B". Hence, the characteristic values only for these four configurations will be presented here. In the Table 1 are

243 the maximal displacement values in the jib's own coordinate system. Table 1. Maximum jib displacements (own coordinate system) x-displacement y-displacement z-displacement [mm] [mm] [mm] 18C 32.

It is important to emphasize that the stress overflow in the jib support to the waterway bed zone (configuration "B") is primarily the consequence of simplifying the numerical model in this segment

Principal stresses overflow in "18C" Figure 12. Principal stresses overflow in "18B" Fig. 13. Principal stresses overflow in "18C" Fig. 14. Principal stresses overflow in "18B" 7.

5 243 the maximal displacement values in the jib's own coordinate system. Table 1. Maximum jib displacements (own coordinate system) x-displacement y-displacement z-displacement [mm] [mm] [mm] 18C B L/ C L/ L/937 45B Figures show only the permitted stress (σ perm =16.0kN/cm 2 ) overflow zones. It is important to emphasize that the stress overflow in the jib support to the waterway bed zone (configuration "B") is primarily the consequence of simplifying the numerical model in this segment of the jib structure. Namely, it is justifiable to assume that the equipment not introduced into the model (wheel, axle, engine, etc.) would significantly contribute to the jib stiffness. Figure 11. Principal stresses overflow in "18C" Figure 12. Principal stresses overflow in "18B" Fig. 13. Principal stresses overflow in "18C" Fig. 14. Principal stresses overflow in "18B" 7. CONCLUSIONS From the above stated observations, it is clear that the jib structural analysis, as a very complex task, demand for the attention to be directed towards modeling, not only of the structure elements, but also of the conditions of supports and connections between structural elements. It has been emphasized and presented in the beginning that the simple 1D model has drawbacks that disqualify them in the selection of the final jib model. In connection modeling, a special attention is provided for the link FE that enables the simulation of almost all transitional conditions. A model with this level of complexity and accuracy enables a detailed insight into the jib structure behavior under load. On the other side, the proposed model and modeling algorithm can also be considered optimal since they are adapted to the conditions of everyday design practice. 8. REFERENCES [1] Rusiński, E., Czmochowski, J., Moczko, P. "Numerical and Experimental Analysis of a Mine s Loader Boom Crack", Journal of Automation in Construction, 2008, Vol. 17, pp [2] Shinde, S. D. "Standardization of Jib Crane Design by F.E.M. Rules and Parametric Modelling", International Journal of Recent Trends in Engineering, 2009, Vol. 1, No. 5, pp [3] López, V. D. "Finite Element Crane Analysis According to UNE Standard", 2008, University of Carlos III, Madrid. [4] Vlasblom, W.J. "Dredging Equipment and Technology, Ch. 6 - Bucket (Ladder) Dredger", 2004, Delft University of Technology. [5] Kovačević, D. "FEM Modelling in Structural Analysis" (in Serbian), Građevinska knjiga, 2006, Belgrade. [6] Kovačević, D. "Numerical analysis and computation of the jib structure of waterway bucket dredger - Technical report", Ship Registry of Republic of Serbia, [7] Kovačević, D, Folić, R. "Some Aspects of FEM Structural Modelling by Link FE", Proceedings of the 11th International Conference on Civil, Structural and Environmental Engineering Computing - CC2007, Malta, pp

FEM MODELING OF SPATIAL STRUCTURAL SYSTEMS IN EVALUATION OF THE REAL STRUCTURAL PERFORMANCES UDC : =111

FACTA UNIVERSITATIS Series: Architecture and Civil Engineering Vol. 9, N o 3, 2011, pp. 347-356 DOI: 10.2298/FUACE1103347K FEM MODELING OF SPATIAL STRUCTURAL SYSTEMS IN EVALUATION OF THE REAL STRUCTURAL