Optimum weight design of ship’s structures

Optimum Weight Design of Ship’s Structures

with Application to a Barge’s Deck

By: Mohamed Moanes Abdel Salam

Prof. El-Sayed HegazyProfessor of Ship’s Structures

Department of Marine EngineeringPort-Said University

Dr. Ahmed Naguib

Asst. Prof of Ship’s Structures

Department of Marine Engineering

Arab Academy (AAST)

Supervisors

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Contents

• Chapter One: Introduction• Chapter Two: Principles of Design Optimization• Chapter Three: Reference Deck Stress Analysis• Chapter Four: Optimum Design for Barge’s Deck• Chapter Five: Conclusion

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Chapter One: IntroductionBackground and Motivation

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Minimizing structural weight can have significant impacts on;

• Production cost• Vessel Speed • Lifecycle cost• Pollution

Design for Minimum Weight

Chapter One: Background and Motivation

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Stiffened PanelsStiffened panels are main member in marine structures since half of twentieth century, as they are used in several usages such as; bottom, side, deck and bulkhead constructions etc.


They are more cost-effective by offering a desirable strength/weight ratio.

Why ?

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Stiffened Panels


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Chapter Two: Principles of Design Optimization

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Chapter Two: Design Optimization

Design Optimization Goal

Design Optimization is to design better, more efficient and less expensive systems. By determining the best case without actually testing all possible cases and identify the relationship between the performance of the product (Maximum stress, Deformation, Mass etc.) and the design variables (Dimensions, Material, Loads etc.).

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Strategy of Experimentation

Best guess approach One-factor-at-a-time Factorial Approach



Golf Example

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Best guess approach (trial and error)

• Can continue indefinitely.• Cannot guarantee that best solution has been found.

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One-factor-at-a-time (OFAT)





One-factor-at-a-time (OFAT) approach

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• Inefficient (requires many test runs).• Fails to consider any possible interaction between factors.

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Factorial Experiment



A = = 20 B = = 10

Factorial Sensitivity

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Fractional Factorial Experiment



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Design Of Experiment Techniques

Used to determine the Location of sampling points. The goal is to get as Accurate Response Surface as possible with as few input combinations as possible.

Latin Hypercube Sampling (LHS)

Optimal Space-Filling Design (OSF)

Monte Carlo Sampling Design

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MCS Vs. LHS Vs. OSF

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Multi-Objective Optimization

Screening Optimization Multi-Objective Screening Optimization

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Multi-Objective Genetic Algorithm

Stopping rule

Process use some stopping rule, such as;

o Fixed number of iterations.o Fixed amount of CPU time.o Fixed number of consecutive iterations without any improvement in the best trial solution found so far.


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Chapter Three:Finite Element Analysis for a

Barge’s Deck

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A barge is a flat-Deck boat, built mainly to transport heavy goods such as; containers, gravel and construction equipment.

Chapter Three: FEA for a Barge’s Deck

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Reference Barge’s design

Length Overall 330 ft (100.58m)Breadth Moulded 100 ft (30.48m)Depth Moulded 20 ft (6.10m)

Design Draft 4.5m (Approx.)Deadweight 10,500 Tons (Approx.)

Deck Strength 20 Ton/m2

Principal Dimensions


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Methodology Over view

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Deck Element Model

Name Dimensions (mm)Deck Element Plate 12.8 m x 7.62 m x 18

Longitudinal Stiffeners 150 x 90 x 12Transverse Webs 609 x 203 x 14DK Side Girder 609 x 345 x 12

Spacing of Longitudinal Beams 508Spacing of Transverse Webs 1829

Deck Element Weight is 22.647 T


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Applying Symmetry Boundary Conditions

Reduces the CPU time to the half, while delivering the same level of accuracy in the results.


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Element Size(mm)

No. of Elements

Equ. Stress Max. (MPa)

110 8130 221100 11664 221.3695 12426 222.4790 14954 225.9585 16556 226.3880 17590 226.4975 20622 226.57

More elements in a mesh might give more accurate results but can significantly increase the computational time. 14954 elements (9mm quadratic) used to create a fine mesh.


Meshing

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Maximum Reported Stress is 226 MPa


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Chapter Four:Optimum Weight Design for the

Barge’s Deck

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ABS Barge Rules

The performance of the reference deck and the ABS class minimum requirements for the deck dimensions.

Chapter Four: Optimum Weight Design

Parameter Reference DeckABS Minimum Allowable Limit

Maximum Stress (MPa) 226 ≤ 235Plate thickness (mm) 18 ≥ 17.55

Longitudinal Section Modulus L (mm^3) 54 ≥ 36.04Transverse Section Modulus L (mm^3) 1144 ≥ 778

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Input Parameters

Name DescriptionInitial Value

(mm)P5 Flange width of Long. Stiffener 90P4 Flange thickness of Long. 12P6 Thickness of Long. Stiffener 12P7 Height of Long. Stiffener 150P8 Flange thickness of Trans. Web 14P9 Flange Width of Trans. Web 203

P10 Height of Trans. Web 610P11 Thickness of Trans. Web 14


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Upper & Lower Bounds

The output of the OSF was 82 Design Points.


Initial Value Upper Bound Lower Bound

P4 Flange thickness of Longitudinal Stiffener 12 13 11P5 Flange width of Longitudinal Stiffener 90 99 81P6 Thickness of Longitudinal Stiffener 12 13 9P7 Height of Longitudinal Stiffener 150 165 115P8 Flange thickness of Transverse Web 14 15 11P9 Flange Width of Transverse Web 203 224 156P10 Height of Transverse Web 610 671 469P11 Thickness of Transverse Web 14 15 11

Name Description

Input Parameters

(mm)

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Response Surface


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Sensitivities ChartPositive sensitivity occurs when increasing the input increases the output, negative sensitivity occurs when increasing the input decreases the output.


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Sensitivities Chart

The larger the change of the output parameters, the more significant is the role of the input parameters that were varied.

Output Parameter – Equivalent stress max.


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Number Of Parameters Vs. Number of Design Points


After disabling P4 & P5 the number of design points that created by OSF method reduced from 82 design points for 8 input parameters to 45 design points for 6 parameters.

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Sensitivities Chart

Objectives and Constrains


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3D Tradeoff Chart


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MOGA Stopping rules (Ansys)


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Samples Chart for MOGA Optimization


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The Optimum Design of the Deck


Initial Value Upper Bound Lower Bound Optimum Design

P5 Flange width of Longitudinal Stiffener 90 99 81 90 0P4 Flange thickness of Longitudinal Stiffener 12 13 11 12 0P6 Thickness of Longitudinal Stiffener 12 13 9 10 -20P7 Height of Longitudinal Stiffener 150 165 115 121 -19P8 Flange thickness of Transverse Web 14 15 11 11 -23P9 Flange Width of Transverse Web 203 224 156 187 -8P10 Height of Transverse Web 610 671 469 635 4P11 Thickness of Transverse Web 14 15 11 11 -20

P1 Geometry Mass (kg) 22647 20680 -9P14 Equ. Stress Max (MPa) 226 235 4

Change Percentage

%

Input Parameters

Name Description

(mm)

Output Parameters

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The Optimum Design Vs. ABS Criteria


Parameter Reference Deck Optimized DesignABS Minimum Allowable Limit

Maximum Stress (MPa) 226 235 ≤ 235Plate thickness (mm) 18 17.55 ≥ 17.55

Longitudinal Section Modulus L (mm^3) 54 38.05 ≥ 36.04Transverse Section Modulus L (mm^3) 1144 976.05 ≥ 778

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Conclusion

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Sensitivity tests used to explore the design and understand how each output parameter is driven by input parameters and how the design can be modified. “The Web Height” is playing a major role and has the significant impact in changing the stress with relatively small increase in weight.

On the other hand, two parameters “Flange thickness of Longitudinal Stiffener & Flange width of Longitudinal Stiffener” have a very small impact on the output parameters.MOGA optimization shows its effectiveness, as in the final iteration, the weight of the deck was reduced by 9 percent.

Chapter Five: Conclusion

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Recommendations for Future Work

All FE analyses in this study were made with constant loads. In real life, however, marine structures are subjected to cyclic loads arising from ship motions and encounter with waves.

It is suggested that a more sensitive simulation method be adopted for this task, and assigning different boundary conditions.It is recommended to use different Design of Experiment methods which could lead to better results.

Chapter Five: Conclusion

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Thank You