The 5th PSU-UNS International Conference on Engineering and

Technology (ICET-2011), Phuket, May 2-3, 2011 Prince of Songkla University, Faculty of Engineering

Hat Yai, Songkhla, Thailand 90112

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

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 email: dusan@uns.ac.rs

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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 "shear-

locking" 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 11.75 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,

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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.

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

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

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 140838 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.

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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.

thinner part

of axle

thicker part

of axle

hinge part

of jib

thinner part

of axle FE

thicker part

of axle FE

hinge part

of jib FEssingle

link 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=76545.8kg),

b) empty buckets weight (qE=9.4kN/m),

c) filled buckets weight (qC=15.8kN/m),

d) bottom wheel weight (GBW=50kN),

e) dredging force (FD=273.5kN),

f) bucket chain pull force (FBC=560.7kN),

g) frontal force of extrude (FFI=150kN),

h) lateral force of extrude (FLI=77kN) and

i) pending part of chain weight (GPC=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

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the maximal displacement values in the jib's own

coordinate system.

Table 1. Maximum jib displacements (own

coordinate system)

x-displacement

[mm]

y-displacement

[mm]

z-displacement

[mm]

18C 32.14 112.11 36.08

18B 44.62≈L/1098 -71.66 48.74

45C 12.49 12.53≈L/435 52.32≈L/937

45B 6.94 -71.61 -34.50

Figures 11-14 show only the permitted stress

(σperm=16.0kN/cm2) 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. 271-277.

[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. 145-149.

[3] López, V. D. "Finite Element Crane Analysis

According to UNE 58132-2 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, 2009.

[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, 2007. pp. 211-226.

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