This thesis purpose is to study the procedure of optimization of jacket type offshore structure with fatigue reliability constraint. Furthermore, it also studies the applicability and effectiveness of using Genetic Algorithm as the algorithm to optimized the jacket structure under fatigue reliability constraint. The jacket platform must be economical but still reliable in its entire service life.
In this thesis, the multiple variable optimization using Genetic Algorithm (GA) method will be applied in the optimization of jacket type offshore structure with fatigue constraints. The analysis will combine the reliability-based design theory, structural optimization, random vibration, fatigue analysis as well as Finite element analysis method to determine the optimal structure which meet the safety and economic requirement. The research will use Palmgreen and Miner’s rule to predict the fatigue damage in the structural member. The uncertainty will be considered from the parameter in the S-N Curve which represent the material endurance under cyclic loading.
Finally, a case study of jacket structure has been carried out to demonstrate that developed procedure can be applied for the optimization of jacket structure.

This thesis purpose is to study the procedure of optimization of jacket type offshore structure with fatigue reliability constraint. Furthermore, it also studies the applicability and effectiveness of using Genetic Algorithm as the algorithm to optimized the jacket structure under fatigue reliability constraint. The jacket platform must be economical but still reliable in its entire service life.
In this thesis, the multiple variable optimization using Genetic Algorithm (GA) method will be applied in the optimization of jacket type offshore structure with fatigue constraints. The analysis will combine the reliability-based design theory, structural optimization, random vibration, fatigue analysis as well as Finite element analysis method to determine the optimal structure which meet the safety and economic requirement. The research will use Palmgreen and Miner’s rule to predict the fatigue damage in the structural member. The uncertainty will be considered from the parameter in the S-N Curve which represent the material endurance under cyclic loading.
Finally, a case study of jacket structure has been carried out to demonstrate that developed procedure can be applied for the optimization of jacket structure.

Abstract .................................................................................................................... i
Acknowledgment .................................................................................................... ii
Table of Content ..................................................................................................... iv
List of Figures ........................................................................................................ ix
List of Tables ........................................................................................................ xiii
CHAPTER 1 INTRODUCTION
1.1 Background .................................................................................................. 1
1.2 Objective and Scope of research .................................................................. 3
1.3 Thesis organization ...................................................................................... 5
2.1 Preface .......................................................................................................... 7
2.2 Statistical description of waves .................................................................... 7
2.2.1 The definition of sea surface and sea wave .......................................... 7
2.2.2 Wave data ............................................................................................. 8
2.2.3 Significant wave ................................................................................... 9
2.2.4 Wave height spectrum ........................................................................ 11
2.2.5 Scatter diagram ................................................................................... 12
2.2.6 Design wave height and wave period ................................................. 13
2.3 Environment loading analysis in offshore structure................................... 14
2.3.1 Type and description of environmental loading in offshore structure 14
v
2.3.2 Wave Theory....................................................................................... 16
2.3.3 Drag and inertia loading due to sea wave ........................................... 18
2.3.4 Sea state short term and long term characteristic ............................... 21
2.3.5 Uncertainty in wave loading analysis ................................................. 23
2.4 Structural dynamic analysis ....................................................................... 24
2.5 Fatigue analysis .......................................................................................... 25
2.6 Stress concentration factor ......................................................................... 26
2.7 Structural reliability ................................................................................... 29
2.7.1 Design variable, Limit state function and Failure probability in the reliability analysis .............................................................................................. 31
3.1. General ....................................................................................................... 34
3.1.1 Deterministic Analysis of offshore structure design........................... 35
3.1.2 Stochastic analysis of offshore structure design ................................. 36
3.1.3 Basic step procedure of research ........................................................ 36
3.2. Defining the sea state ................................................................................. 38
3.2.1 Main wave direction (θ) ...................................................................... 39
3.2.2 Wave scatter diagram.......................................................................... 40
3.2.3 Wave spectrum model ........................................................................ 40
3.3. Wave loading analysis................................................................................ 41
3.4. Structural analysis ...................................................................................... 43

3.4.1. Stiffness ([K]) and Mass ([M]) matrices............................................. 43
3.4.2. Natural frequency ( ) and mode shape (Ф) of structure ................. 45
3.4.3. Structural response under random loading ......................................... 46
3.4.4. Stress Concentration Factor (SCF) ..................................................... 47
3.5. Reliability fatigue analysis ......................................................................... 50
3.5.1. Fatigue damage model ........................................................................ 50
3.5.2. Fatigue Limit State (FLS) Function .................................................... 52
3.5.3. Uncertainty in material properties (S-N curve) .................................. 53
3.6. Optimization process using Genetic Algorithm ......................................... 54
3.6.1 Problem previews ............................................................................... 54
3.6.2 Design Variable .................................................................................. 55
3.6.3 Defining the objective function .......................................................... 55
3.6.4 Defining the constraint function ......................................................... 56
3.6.5 Implementation of Genetic Algorithm (GA) in optimization process 56
3.6.6 Arranging genetic coding in Gene and Chromosome ......................... 60
3.6.7 Initialization of Parameter .................................................................. 60
3.6.8 Generate the initial population ............................................................ 63
3.6.9 Evaluation of Individu Fitness ............................................................ 65
3.6.10 Natural selection of the individu ......................................................... 65
3.6.11 Cross over ........................................................................................... 68

3.6.12 Mutation .............................................................................................. 70
3.6.13 Generate new population .................................................................... 73
3.6.14 Convergence test ................................................................................. 73
4.1 Description of the problem ........................................................................ 75
4.2 Numerical analysis of wave data................................................................ 76
4.2.1 Wave data ........................................................................................... 76
4.2.2 Analysis of Power Spectral Density (PSD) of Wave Height ( ) ... 78
4.3 Structural modelling ................................................................................... 82
4.4 Drag and inertia loading in the structure due to sea wave ......................... 84
4.5 Numerical evaluation of dynamic structural response ............................... 89
4.6 Reliability analysis for the given structure............................................... 101
4.7 Application of Genetic Algorithm in optimization of structure ............... 105
4.7.1 Genetic parameter ............................................................................. 105
4.7.2 Generate the initial population .......................................................... 106
4.7.3 Application of individu fitness evaluation ........................................ 107
4.7.4 Implementation of natural selection ................................................. 107
4.7.5 Arranging the genetic coding of individual ...................................... 108
4.7.6 Crossover and mutation of the individuals ....................................... 109
4.7.7 New generation from combination of parents and offspring ............ 110
4.8 The result and discussion of optimization ................................................ 119

4.9 Computation time and efficiency ............................................................. 131
5.1 Conclusion ............................................................................................... 133
5.2 Recommendation for the further research ................................................ 134