Sea wave induced sloshing of liquid in floating storage tank

SEA-WAVE-INDUCED SLOSHING OF LIQUID IN FLOATING STORAGE TANK LUONG VAN TUYEN NATIONAL UNIVERSITY OF SINGAPORE 2013 SEA-WAVE-INDUCED SLOSHING OF LIQUID IN FLOATING STORAGE TANK LUONG VAN TUYEN B.Eng. (Hons.), NUCE, Vietnam A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CIVIL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2013 To my family, DECLARATION I hereby declare that the thesis is my original work and it has been written by me in its entirety. I have duly acknowledged all the sources of information which have been used in the thesis. This thesis has also not been submitted for any degree in any university previously. _______________________ Luong Van Tuyen 21st August 2013 ACKNOWLEDGEMENTS First of all, I would like to express my sincere gratitude to my supervisors, Associate Professor Ang Kok Keng and Professor Junuthula Narasimha Reddy for their invaluable advice, wholehearted guidance and encouragement during my research. Whenever I encountered a problem, they have always been there for me. Besides helping me to solve the problems and sharing their inspiring ideas, the most important thing is they make me understand what research is and how to research. I would like to thank Dr. John Halkyard of Technip who has shared his knowledge regarding floating structures which has assisted me a lot in my research. I greatly appreciate Department of Engineering Science, University of Oxford for allowing me to adapt the DIFFRACT program developed by Dr Liang Sun and Prof Rodney Eatock Taylor and the colleagues at University of Oxford 2009. I also want to thank my friends, Dr. Nguyen Hoang Dat, Dr. Pham Duc Chuyen, Mr. Tran Hien and Mr. Chen Mingshen for their discussion and supplied documents related to my research during my study; especially, Dr. Vu Khac Kien and Dr. Luong Van Hai for their proofingread and comments on the draft of this thesis. I also want to acknowledge the research scholarship provided by National University of Singapore and the Centre for Offshore Research & Engineering (CORE) for providing all the necessary recourses to carry out my research. Last but not least, I would like to express my gratitude from the bottom of my heart to my parents. Thank you very much for their continuous and invaluable support in my life. The acknowledgement would not be complete without the mention of my wife who is always my strong support during the whole difficult time of my research. i TABLE OF CONTENTS Acknowledgements i Table of Contents ii Summary vi List of Tables . viii List of Figures ix List of Symbols xiv CHAPTER INTRODUCTION 1.1 Background and Motivation . 1.2 Literature Review . 1.2.1 Dynamic Response of Floating Structures . 1.2.2 Liquid Sloshing in Storage Tanks . 1.2.3 Interaction between Liquid Sloshing and Ship Motions . 11 1.2.4 Mitigation of Liquid Sloshing with Application of Baffles . 15 1.3 Objectives and Scopes . 18 1.4 Outline of Thesis 20 CHAPTER MATHEMATICAL FORMULATION 21 2.1 Introduction 21 2.2 Modeling of Ocean Environment and Irregular Wave Forces . 21 2.3 Kinematics and Coordinate Systems 24 2.4 Modeling of Floating Tank 25 2.5 Modeling of Sloshing Fluid . 28 ii 2.5.1 Governing Equation of Sloshing Fluid 28 2.5.2 Initial and Boundary Conditions 29 2.6 Modeling of Station-Keeping System 31 2.6.1 Forces in a Mooring Line . 32 2.6.2 Restoring Forces from Spread Mooring System 34 2.7 Coupling Tank Motions, Sloshing Fluid and Mooring System . 35 2.8 Concluding Remarks 36 CHAPTER NUMERICAL IMPLEMENTATION 37 3.1 Introduction 37 3.2 Finite Difference Method for Sloshing Fluid . 37 3.2.1 Discretization of Computational Domain 37 3.2.2 Finite Difference Form of the Governing Equations . 38 3.2.3 Pressure Approximation and Solution Algorithm 45 3.2.4 Boundary Conditions . 50 3.2.5 Volume of Fluid Method 53 3.2.6 Numerical Stability 56 3.3 Numerical Solution for Tank Motion . 57 3.3.1 Convolution Replacement 58 3.3.2 Identification Methods for Convolution Replacement . 59 3.3.3 Model Reduction 61 3.4 Algorithm of Fully Coupled Sloshing Fluid - Floating Tank Program 62 3.5 Concluding Remarks 62 iii CHAPTER FULLY COUPLED SLOSHING - FLOATING TANK MOTION PROBLEM 64 4.1 Introduction 64 4.2 Verification of Numerical Model . 64 4.2.1 Liquid Sloshing in Rectangular Tank 65 4.2.2 Dynamic Response of Floating Structures . 73 4.2.3 Effect of Sloshing on Response of Floating Tank . 79 4.3 Dynamic Analysis of Liquid-Filled Floating Rectangular Tank . 84 4.3.1 Parametric Study for Different Wave Frequencies 84 4.3.2 Parametric Study for Different Liquid-Filled Levels . 87 4.3.3 Parametric Study for Different Wave Heights . 93 4.4 Concluding Remarks 99 CHAPTER EFFECT OF BAFFLES IN COUPLED SLOSHINGFLOATING TANK MOTION PROBLEM . 101 5.1 Introduction 101 5.2 Effect of Baffles in Coupled Sloshing - Floating Tank Motion Problem 103 5.2.1 Effect of Baffle Dimension 103 5.2.2 Effect of Baffle under Different Wave Frequencies 107 5.2.3 Effect of Baffle Type 110 5.2.4 Effect of Baffle Location 113 5.3 Concluding Remarks 115 CHAPTER CONCLUSIONS AND FUTURE WORKS . 116 6.1 Summary of Key Points . 116 6.2 Conclusions 117 iv 6.3 Recommendations for Future Work . 120 References . 122 List of Publications . 134 v SUMMARY Sloshing is an important dynamic phenomenon in liquid storage and transportation. Similar to land-based oil storage terminals under earthquake condition, floating oil storage terminals (FOST) in partially filled conditions in waves may also experience violent sloshing in a complex offloading operation where the system has to handle all sea states. The response of a floating storage tank in such operation is of the crucial factors to the safety and operability of the floating oil storage terminal. The main objective of the study is to model and investigate the wave-induced sloshing of liquid in the floating storage tank in a partially filled condition. A suitable numerical model to address the coupled interaction between the floating motions and liquid sloshing is developed and used to study the effects of liquid sloshing on the global responses and stability of the floating tank. In addition, the proposed numerical model is extended to investigate effect of baffles and offloading sequence of multi-compartments floating tanks on this coupled interaction. In conventional floater analyses, the coupled effects of internal sloshing and external hydrodynamics are assumed to be negligible and hence usually ignored because of the complexity of the problems. These studies are only valid when the floater size is much larger than the size of the liquid container and liquid is fully filled. Recent experimental and numerical study has shown that the coupling effect between liquid sloshing and floaters motion is significant at partial filled conditions. Sloshing flow in liquid container is exited by floater motion, but the sloshing flow itself affects the floater motion in return. The liquid sloshing may cause large internal stresses and deformation on the walls of the container as well as affect the global response of the floater, particularly when the external forcing frequencies associate with the floater vi Chapter 6. Conclusions and Future Works • When a vertical baffle is applied, the optimal location of the baffle is at X b / 2a = 0.4 . Shifting the vertical baffle away from the center of the tank may increase the fluid sloshing elevation. 6.3 Recommendations for Future Work Some important contributions of the present study were mentioned above. However, some unexplored problems still remain and need further studies. These problems include: • In the present study, 2-D numerical model with three degree of freedom was developed to simulate coupling effect between sloshing fluid-tank motion and mooring system. In the real situation, floating tanks may experience six degree of freedom under real sea conditions with different heading angle of sea waves. The study may indeed be extended to three dimensions, both within the models of sloshing fluid and floating tank. The 3-D model will find more general applications because of its applicability to tanks of any shape and configuration. The more realistic behaviour of the floating can be simulated with various heading angles of offshore waves. • The focus of this thesis was on the development of the numerical model to simulate coupling interaction between sloshing fluid and floating tank motions. The model can be developed to simulate sloshing effect on other kind of floaters, such as FPSO, FSRU or semi-submersible platforms by adding effect of wind, current and other disturbances in the RHS of Cummins equation. When the floaters operate at deep sea conditions, seakeeping systems by using mooring systems may be unavailable and ineffective. Dynamic positioning method by using thrusters or hybrid positioning method by using combination of mooring and thrusters may be 120 Chapter 6. Conclusions and Future Works applicable. The forces from thrusters need to be considered and added to the model. • In the present model, the filling level of liquid inside the floating tank is assumed to be constant during simulation time. 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State-space modeling of a vertical cylinder in heave. Applied Ocean Research, 17, 265-275. Yu Z. and Falnes J. (1998). State-space modelling of dynamic systems in ocean engineering. Journal of Hydrodynamics; B(1), 1-17. 133 List of Publications LIST OF PUBLICATIONS INTERNATIONAL JOURNAL Tuyen, L.V. and Ang, K.K. (2013). Parametric studies of sloshing-floating tank motion problem. (Under preparation). Tuyen, L.V. and Ang, K.K. (2013). Sloshing mitigation solution for floating oil storage tank. (Under preparation). INTERNATIONAL CONFERENCE Tuyen, L.V., Ang, K.K and Reddy J.N. (2009). Sea-wave induced sloshing of liquid in floating storage tank. Proceedings of the 22th KKCNN Symposium on Civil Engineering, Changmai, Thailand. (Published). Tuyen, L.V., Ang, K.K and Hai L.V. (2012). Hybrid frequency-time domain for coupled sloshing-floating tank motion problem. Proceedings of the International Conference on Advances in Computational Mechanics, Ho Chi Minh City, Vietnam, 656-668. (Published). Tuyen, L.V. and Ang, K.K. (2012). Effect of baffles in coupled sloshing-floating tank motion problem. Proceedings of the 25th KKCNN Symposium on Civil Engineering, Busan, Korea, 143-146 (Published). Tuyen, L.V., Ang, K.K. and Hai, L.V. (2013). Sea-wave-induced sloshing of liquid in floating storage tank with baffles. Proceedings of the 4th International 134 List of Publications Conference of The Euro-Asia Civil Engineering Forum, Singapore, O1-O6 (Published). Tuyen, L.V. and Ang, K.K. (2013). Sloshing effect and mitigation solution of floating oil storage tank. The Thirteenth East Asia-Pacific Conference on Structural Engineering and Construction, Sapporo, Japan. (Accepted). 135 [...]... moves and interacts with the walls of tanks under offshore waves, the dynamic pressures of such an interaction may cause large deformation in the tank walls This phenomenon of liquid in containers is known as liquid sloshing Liquid sloshing phenomenon has been investigated by many researchers from a wide range of disciplines In seismology, the effects of liquid sloshing have been studied on water tanks... Application of baffles to mitigate liquid sloshing effect 102 Figure 5.2 Application of baffles in a floating tank 103 Figure 5.3 Effect of baffle size on dynamic responses of floating tank 104 Figure 5.4 Transient sloshing and wave forces acting on the floating tank 105 Figure 5.5 Effect of baffle height on the sloshing elevation of fluid 105 Figure 5.6 Snapshot of sloshing elevations at time t=... with the presence of a large sloshing effect In the present study, the numerical model for sloshing fluid will be 6 Chapter 1 Introduction developed and added to simulate interaction effects between sloshing in liquid- filled containers and floater motions more correctly 1.2.2 Liquid sloshing in storage tanks Sloshing phenomenon in a storage tank has been widely studied for many years using various theories... Effect of sloshing fluid on the floating tank 83 Figure 4.24 Free surface displacement of sloshing fluid at x = a (right boundary) 83 Figure 4.25 Motion RAO in three DOF of floating tank 85 Figure 4.26 Effect of wave frequencies on maximum sloshing elevation of fluid 86 Figure 4.27 Time history of sloshing free surface at x = a (right boundary) 86 Figure 4.28 Maximum sloshing. .. sloshing phenomena of fluid in partially filled floating tanks have to be considered in the analysis and design of the floating storage tanks 1.2 Literature Review There are four basic areas of literature relevant to this research work The first deals with study of dynamic response of floating structures to wave excitation and interaction between ocean waves and oscillating systems such as ships, floating. .. the natural sloshing frequencies This is of a great concern to the oil tanker (e.g FPSO, FSRU) operation in the production site and offloading operation of floating oil storage terminals In this study, the coupling effects between non-linear fluid sloshing and floating tank motions are investigated by using a hybrid frequency-time domain simulation scheme The hydrodynamic coefficients and wave forces... 4.34 Wave and sloshing -induced force in surge direction with ω = 1.7 rad/s 91 Figure 4.35 Wave and sloshing -induced force in heave direction with ω = 1.7 rad/s 91 Figure 4.36 Wave and sloshing -induced force in pitch direction with ω = 1.7 rad/s 92 xi Figure 4.37 Mooring forces in surge direction with ω = 1.7 rad/s 92 Figure 4.38 Effect of wave height on the tank s... sloshing- floating tank motion interaction and are also investigated vii LIST OF TABLES Table 4.1 Summary of the cylinder’s properties 73 Table 4.2 Floating tank s main parameters 84 Table 4.3 Particulars of the mooring lines (8 cables) 84 Table 4.4 Natural frequencies of sloshing liquid with different filling ratios 87 viii LIST OF FIGURES Figure 1.1 Shirashima Floating Oil Storage Base,... ships, floating storage tanks, wave- energy converters and ocean platforms The second covers the background research on liquid sloshing in storage tanks Various assumptions and different methods to study sloshing phenomenon in a partially filled storage tank will be reviewed The third focuses on the interaction effect between sloshing in liquid 3 Chapter 1 Introduction partially filled containers and ship... In the 2 Chapter 1 Introduction aerospace industry, the influence of liquid propellant sloshing on the stability of jet vehicles has been a major concern to engineers and researchers since the early 1960s (e.g McCarty and Stephens, 1960; Stofan and Pauli, 1962) In the building industry, liquid tanks on roofs are employed as passive dampers to mitigate the movement of the structure due to wind loading . SEA-WAVE-INDUCED SLOSHING OF LIQUID IN FLOATING STORAGE TANK LUONG VAN TUYEN NATIONAL UNIVERSITY OF SINGAPORE 2013 SEA-WAVE-INDUCED SLOSHING OF LIQUID. and operability of the floating oil storage terminal. The main objective of the study is to model and investigate the wave-induced sloshing of liquid in the floating storage tank in a partially. Concluding Remarks 99 CHAPTER 5 EFFECT OF BAFFLES IN COUPLED SLOSHING- FLOATING TANK MOTION PROBLEM 101 5.1 Introduction 101 5.2 Effect of Baffles in Coupled Sloshing - Floating Tank Motion