Fluid-structure interaction models on the hydroelastic analysis of containerships in waves

Commercial vessels have recently been increasing in size to meet the fast-growing demand for transportation and operations. However, this trend may result in more flexible or "softer" hulls. The flexible hull structure and high operational speed requirements bring the ship's natural frequency closer to the wave encounter frequency, increasing the probability of resonance or high-frequency vibrations. Therefore, hydroelastic effects and relevant loads should be considered when designing wave loads and evaluating the strength of large ships. A robust numerical model is in search of ship designers and regulators, intended to predict the impact of hydroelasticity in the initial stages of design as per the design regulations, where there exists a greater opportunity to make modifications and utilise high-fidelity tools to verify the performance of advanced designs. This study aims to fill this gap by performing robust numerical investigations based on open-source software on the seakeeping and hydroelastic analysis of a monohull under wave excitations. Firstly, a detailed literature review is presented to overview the previous theoretical and numerical methods for ship hydroelasticity. This review also includes a general comparison between these hydroelastic techniques and discusses the differences. Following this, two fully coupled CFD-based unsteady FSI numerical frameworks are established: coupled CFD-FEA and CFD-DMB methods, respectively. The physical principle of these FSI models is to treat a ship’s surface hull as an elastic body and interact with its surrounding flow field to form a fully coupled system. Taking advantage of the present numerical models, the hydroelastic behaviours of a containership, such as its vertical bending displacement and corresponding bending moment, can be quantified, and the “springing” and “whipping” behaviour can be measured. It is believed that the present FSI model will exhibit more advantages over the traditional rigid-body method in the ship seakeeping field. Later, the presented CFD-DMB model is further extended for its application to irregular extreme waves and damaged ship conditions. The results achieved from these studies could also help to assess the structural integrity and longitudinal strength of a ship (intact or damaged), which serves as an improved technique for regulations to evaluate conventional ship designs. Finally, the results drawn from each chapter of this thesis are summarised and discussed, and recommendations are made for future research.Commercial vessels have recently been increasing in size to meet the fast-growing demand for transportation and operations. However, this trend may result in more flexible or "softer" hulls. The flexible hull structure and high operational speed requirements bring the ship's natural frequency closer to the wave encounter frequency, increasing the probability of resonance or high-frequency vibrations. Therefore, hydroelastic effects and relevant loads should be considered when designing wave loads and evaluating the strength of large ships. A robust numerical model is in search of ship designers and regulators, intended to predict the impact of hydroelasticity in the initial stages of design as per the design regulations, where there exists a greater opportunity to make modifications and utilise high-fidelity tools to verify the performance of advanced designs. This study aims to fill this gap by performing robust numerical investigations based on open-source software on the seakeeping and hydroelastic analysis of a monohull under wave excitations. Firstly, a detailed literature review is presented to overview the previous theoretical and numerical methods for ship hydroelasticity. This review also includes a general comparison between these hydroelastic techniques and discusses the differences. Following this, two fully coupled CFD-based unsteady FSI numerical frameworks are established: coupled CFD-FEA and CFD-DMB methods, respectively. The physical principle of these FSI models is to treat a ship’s surface hull as an elastic body and interact with its surrounding flow field to form a fully coupled system. Taking advantage of the present numerical models, the hydroelastic behaviours of a containership, such as its vertical bending displacement and corresponding bending moment, can be quantified, and the “springing” and “whipping” behaviour can be measured. It is believed that the present FSI model will exhibit more advantages over the traditional rigid-body method in the ship seakeeping field. Later, the presented CFD-DMB model is further extended for its application to irregular extreme waves and damaged ship conditions. The results achieved from these studies could also help to assess the structural integrity and longitudinal strength of a ship (intact or damaged), which serves as an improved technique for regulations to evaluate conventional ship designs. Finally, the results drawn from each chapter of this thesis are summarised and discussed, and recommendations are made for future research

Development and Application of a Potential Flow Computer Program: Determining First and Second Order Wave Forces at Zero and Forward Speed in Deep and Intermediate Water Depth

A ship traveling in irregular sea with a steady forward speed is a classical hydrodynamics problem which still presents many challenges. An in-house computational code MDLHydroD based on potential theory has been developed to address this problem. A Green function based approach is followed in frequency domain to obtain the linear forces and motion of the vessel. A perturbation approach is then applied to extract the second order forces, and the added resistance on the ship is thus obtained. The numerical method is then extended to consider finite water depth effects. A new finite depth Green function is developed and implemented in the 3D potential code. This allowed analysis of ship motion with forward speed in intermediate water depths. An optimization framework is then developed to solve the inverse problem of ship hull optimization which is classified as a multi variable multi objective problem with nonlinear constraints. The three main problems encountered in the inverse design of ship hull are: automated geometry creation, prediction of forces due to fluid structure interaction and modifying the hull towards a better performing hull form. For this study, a parametric hull form based on typical ship parameters is developed which can be altered to obtain different ship hulls that can be analyzed using the developed hydrodynamic code MDLHydroD. A number of different optimization solvers are studied to understand and select appropriate solver for ship hull optimization. Solvers based on evolutionary algorithms were found to be adequate and used to demonstrate the capabilities of the hull optimization framework. Both single and multi-objective optimization algorithms are implemented. A selected optimized design from the Pareto front is then compared with initial design to show the effectiveness of the optimization method. This study will provide a thorough analysis of hydrodynamic load prediction methodologies and its application in obtaining safer, fuel efficient and more stable hull forms

Cfd optimization of a net design and seakeeping analysis for an offshore fish cage

The present study conducted a comprehensive examination of the design and analysis of an offshore fish cage based on an existing fish cage known as ”shenlan.” The study began with a thorough site selection process, which resulted in the decision to position the fish cage around 20 kilometers from the massachusetts bay shore, at a water depth of 64.6 meters. To optimize the net design, computational fluid dynamics (cfd) simulations were conducted using the tdyn cfd+ht software. Finally, a seakeeping analysis was performed using the tdyn seafem software to assess the fish cage’s motions within acceptable limits

Stability and Seakeeping of Marine Vessels

This book presents the papers accepted into the Special Issue “Stability and Seakeeping of Marine Vessels” and includes nine contributions to this Special Issue published in 2020. The overall aim of the collection is to improve knowledge about the most relevant and recent topics in ship stability and seakeeping. Specifically, the articles cover a wide range of topics and reflect the recent scientific efforts in the 2nd generation intact stability criteria evaluation and modelling of the ship dynamics assessment in intact or damaged conditions. These topics were investigated mainly through direct assessments performed both via numerical methods and tools, and experimental approaches. The book is addressed to individuals from universities, research organizations, industry, government agencies and certifying authorities, as well as designers, operators and owners who contribute to improved knowledge about “stability and seakeeping”

Structural analysis of floating offshore remote terminal for deep sea fishing

Productivity of offshore fishing can increase if there are offshore terminals providing services such as fish unloading and repair of crafts and gears to the fishing fleets. This research proposed the use of FORT (fishing offshore remote terminal) as a very large floating structure (VLFS). Structural analysis is key in the design of VLFS. The research developed an adaptable framework to simulate FORT's hydroelastic interaction and motion using Newtonian's harmonic method. The governing partial differential equation of motion including the effect of deformation and torsional inertia was expressed in a dimensionless form. A finite difference algorithm was employed to transform the differential equations into linear algebraic equations. Linear and nonlinear dynamic responses was obtained using Hamilton principle with modal superposition coupled with finite element methods. Sensitivity tests are performed to quanti$z the effect of changing numerical parameter. Variety of plate models is investigated. Techno-economic model is also developed. The solution for a selected load condition has been presented. The result on hydroelastic response for several wavelength q (0.12, 0.23 and 0.43) to structural length ratios (l:1,2:r and 4:1) revealed longish F9RT experiences higher elastic deformations as comparc a square FORT for higher wavelength. In continuous springing freeboard reaction, the safe margin decreases from 4m to below 2m at higher wavelength ratio. At small wave length the hydroelastic response is the smallest for the lower ratio orientation. It is found that hydroelastic response is minimal as aspect ratio close to 1. Resultant stress on FORT stiffness when aspect ratio approaches 1 amplifies response amplitude by 35%. Sensitivity test indicates, for full load condition, larger structure will experience larger deformation stress (0.928 MN/m2 for 250m, 1.035MN/m2 for 500m, 1.035MN/m2 for 1000m). Permanent plastic deformation starts occurr ing at 20o and worsen at 45o causing higher shear force and moment. Maximum torsional force exceeds 51.25N/m2. For long crest of 0.43 maximum torsional deflection measured are250m(19.32N/m2 for 250m), 500m (27.55N/m2 for 500m), and 1000m 1za.o:\Vm2 for 1000m). Net present value of F9RT is NPV of 146mil, internal rate of return of 22.94% overl5 years. FORT as a new concept is thus techno economically feasible. The analytical model developed is a comprehensive tool for FORT designers

STUDIES ON THE NONLINEAR INTERACTIONS ASSOCIATED WITH MOORED SEMI SUBMERSIBLE OFFSHORE PLATFORMS

The design of moored semi submersible systems constitutes a challenging engineering problem in which, the platform offset, stability, payload and system-optimized cost requirements are to be met simultaneously. This problem is complicated by the incomplete understanding of the nonlinearities associated with the multiple interactions such as wave to wave, wave to platform, platform to mooring, fluid to mooring and mooring to seabed. In this study, an attempt has been made to probe into these nonlinearities through numerical, experimental, and parametric studies. In the numerical study, moored semi submersibles were analyzed in the time domain. The dynamic equilibrium conditions were satisfied through a set of coupled nonlinear differential equations for the six DOF motions. For representing the platform to mooring nonlinear interactions, the 6x6 mooring stiffness matrix was derived based on the mooring stiffness and on the fairlead coordinates relative to the structure CG. For the evaluation of the slow frequency horizontal motions of the platform, the second order wave forces resulting from the second order temporal acceleration and the structural first order motions were formulated. For the assessment of the fluid to mooring and mooring to seabed nonlinear interactions, a deterministic approach for the dynamic analysis of a multi-component mooring line was formulated. The floater motion responses were considered as the mooring line upper boundary conditions. Lumped parameter approach was adopted for the mooring line modeling. Mooring to seabed nonlinear interactions were modeled assuming that the mooring line rested on an elastic dissipative foundation. A numerical dynamic analysis method in the time domain was developed and results for various mooring lines partially lying on different soils were validated by conducting a comparative study against published results. The contribution of the soil characteristics of the seabed to the dynamic behavior of mooring line was investigated for different types of soil. Two phases of experimental studies were conducted to provide benchmark data for validating the numerical methods. In the first phase, the seakeeping performance of a semi submersible with eight circular columns was studied. The model was built to scale of 1:100 using Froud’s law of similitude. The tests were conducted for head, beam and quartering seas. In the second phase, a semi submersible with six circular columns was modeled using the same scale as for the first semi submersible. Linear mass-spring system was arranged to facilitate measurements of the horizontal drift forces. The system natural periods, still water damping, nonlinear viscous damping, drag coefficient and inertia coefficient information were evaluated from the free decay tests. Seakeeping tests were conducted for head and beam model orientations. The measured drift forces were compared to available formulae in the literature to assess the available semi-empirical methods for evaluation these forces. In both experimental phases, twin-hulled conventional semi submersibles were considered. By comparing the results of the numerical and experimental models, the validity of the numerical method was established. Based on the validated numerical algorithm, a number of parametric studies were conducted for investigating the contributions of various design parameters on the dynamics of moored semi submersibles. The effects of pretension, mooring line configuration, clump weight, cable unit weight, elongation, breaking strength and pretension angle on the behavior of multi-component mooring line, were investigated by using an implicit iterative solution of the catenary equations. On the other hand, using linearized frequency domain analysis, the contributions of platform payload, platform dimensions, number of columns, number of mooring lines, the wave environment mathematical model, the wave characteristics and the operating (intact or damage) conditions to the responses of moored semi submersibles were investigated. The experimental and published results verified the efficiency of the developed numerical model for prediction of the wave frequency and low frequency motions and mooring dynamic tension responses of the semi submersible. Moreover, experimental results indicated that in addition to the modeling of the mooring system stiffness, typical or hybrid modeling of the mooring system and attachments are necessary for the critical assessment of the mooring system damaged conditions

Towards 6 Degrees of Freedom Seakeeping Simulations Using a Fully Nonlinear Potential Flow Method

In recent years, the International Maritime Organization introduced a new set of rules in order to try to reduce emissions of ships by improving their efficiency. To assess the energy efficiency of a new ship, the regulations require to estimate the Energy Efficiency Design Index (EEDI), which represent the amount of Carbon Dioxide produced per mile in relation to the amount of cargo carried, and verify that it is smaller than a prescribed value. For a proper evaluation of the EEDI, it is necessary to estimate the added resistance in waves with high accuracy. There are different ways to evaluate added resistance: empirical methods, adding a safety factor to the calm water resistance called sea margin, numerical simulations and model test experiments. Nowadays, the most used way during the design stage to do that is employing numerical simulations. Numerical simulations are not only used for the estimation of added resistance, but also to predict ship motions. If the motions are known at an early design stage, it is possible to modify the design of a ship to minimize them in order to improve the performance of the ship and to increase the safety and comfort of those who are on board.The main objective of the PhD project is to evaluate added resistance and ship motions in oblique waves. In the work presented in this thesis, an existing fully nonlinear unsteady potential flow method is used to perform seakeeping numerical simulations in head and beam sea. Since viscosity is disregarded in potential flow methods but it is still very important for some cases, such as roll motion, viscous damping coefficients were added into the equation of motions. For the last two papers, an unstructured adaptive grid refinement, a nonlinear decomposition of the velocity potential, a formulation for the acceleration potential and a Barnes-Hut algorithm were introduced in the code. The method has been used to simulate roll motion in beam sea, parametric rolling, added resistance and ship motions in head waves as well as ship-ship interaction in calm water. Numerical results were compared with experiments and with other methods. Overall, the method presented here proved to be able to handle the tested scenarios, showing a good agreement between the simulations and the experiments. The work summarized in this thesis contributed to a better understanding of the numerical method used and helped to outline the next steps to be taken in order to achieve numerical seakeeping simulations in 6 degrees of freedom in oblique waves

Numerical and experimental studies of two-body hydrodynamic interaction in waves

Challenges remain in the prediction of hydrodynamic interactions of multiple floating bodies in close proximity, such as side-by-side offloading and ship replenishment. During such operations, large free-surface elevations in the gap and body motions may occur, impacting operation and crew safety. In this thesis, numerical and experimental studies are presented, focusing on the two-body interactions in waves. Linear potential-flow based seakeeping programs have been widely employed to solve hydrodynamic interaction problems due to their high efficiency. However, these methods over-predict body motions, free surface elevations in the gap, and hence low-frequency loadings on the bodies. To suppress the over-predictions, artificial damping is required as input, which is typically obtained from model tests. With objectives of investigating the effects of viscosity and dynamic gap changes in the two-body interaction problem and developing a systematic approach to estimate the artificial damping for use in potential-flow tools, an immersed-boundary method based finite volume method solver has been implemented in the OpenFOAM framework. The pressure implicit with splitting of operators (PISO) algorithm is applied for velocity-pressure coupling. Free surface is captured using the geometrical volume of fluid method. The relaxation zone method is utilized for wave generation and absorption. To provide high-quality experimental data and to validate the numerical method, model tests on two identical box-like FPSO models arranged side-by-side in head waves at zero forward speed have been conducted in the towing tank of Memorial University. Besides, sources of uncertainties in the model test were identified, and comprehensive uncertainty analysis on the test results was conducted. A combined experimental and numerical approach has been developed to estimate uncertainties due to model geometry, model mass properties, and test set-up. Validation studies on the present flow solver were conducted by firstly simulating the present experiment for two-body interactions in head seas without forward speed. Further, the solver was validated by simulating the underway replenishment of a frigate and a supply vessel at a moderate speed. Simulations were also performed using a panel-free method based potential-flow program in the frequency domain. The numerical results from both methods were compared with each other and with the experimental data to identify sources of the discrepancies in potential-flow predictions. A quasi-steady approach, which accounts for the gap changes due to transverse drift forces at zero speed, was adopted to improve the potential-flow simulations