Hydroelastic analysis of flexible floating interconnected structures

Three-dimensional hydroelasticity theory is used to predict the hydroelastic response of flexible floating interconnected structures. The theory is extended to take into account hinge rigid modes, which are calculated from a numerical analysis of the structure based on the finite element method. The modules and connectors are all considered to be flexible, with variable translational and rotational connector stiffness. As a special case, the response of a two-module interconnected structure with very high connector stiffness is found to compare well to experimental results for an otherwise equivalent continuous structure. This model is used to study the general characteristics of hydroelastic response in flexible floating interconnected structures, including their displacement and bending moments under various conditions. The effects of connector and module stiffness on the hydroelastic response are also studied, to provide information regarding the optimal design of such structures.     

基于CFD-FEM的浮式结构水弹性响应数值模拟

开发并验证了一种基于CFD-FEM耦合的弹性浮体水弹性响应计算模拟方法。采用CFD方法建立黏性数值水池模拟非线性波浪，弹性浮板进行有限元离散，并在交界面进行数据交互实现耦合计算；通过与水池试验数据和三维板理论在各种波浪环境下的浮体垂向位移结果对比，证实CFD-FEM耦合方法的有效性。并进一步研究了浮板的厚度、入射波波幅和浮板的三维效应对浮板水弹性响应的影响。结论表明，波幅的增加会加剧弹性浮板的水弹性响应，浮板各点处的垂向位移随波幅的增加而增大；当浮板厚度改变时，不同厚度浮板自由端处的垂向位移差异较小，而在中部等位置处，厚度对浮板的水弹性响应有较大的影响。     

波浪作用下铰接式层合浮体的水弹性响应研究

在工程设计中,通常采用模块化方式制造超大型浮式结构物,将巨大的单体结构分割成多个较小模块,后期通过合适的连接器拼装形成。为了明确多模块超大浮体在波浪作用下的水弹性响应,以两个相邻层合浮体（高刚度面板和低密度芯材）为研究对象,建立波浪作用下铰接层合浮体水弹性响应的高阶势流模型。采用匹配特征函数展开法求解流体运动的速度势,探讨了铰接处弹簧刚度对浮体的反射系数、透射系数、挠度、弯矩和剪力的影响规律。研究结果表明：迎浪侧浮体的存在可以有效降低背浪侧浮体的挠度、弯矩和剪力幅值;与垂直弹簧相比,扭转弹簧刚度的增加可以更加有效抑制铰接层合浮体的水弹性响应;当扭转弹簧刚度大于一定值时,继续增大弹簧刚度对浮体的动力响应不产生影响。     

A Study on Hydroelastic Response of Box-Type Very Large Floating Structures

The application of very large floating structure (VLFS) to the utilization of ocean space and exploitation of ocean resources has become one of the issues of great interest in international ocean engineering field. Owing to the advantage of simplicity in structure and low cost of construction and maintenance, box-type VLFS can be used in the calm water area near the coast as the structure configuration of floating airport. In this paper, a 3D linear hydroelastic theory is used to study the dynamic response of box-type VLFS in sinusoidal regular waves. A beam model and a 3D FEM model are respectively employed to describe the dynamic characteristics of the box-type structure in vacuum. A hydrodynamic model (3D potential theory of flexible body) is applied to investigate the effect of different dry models on the hydroelastic response of box-type structure. Based on the calculation of hydroelastic response in regular waves, the rigid body motion displacement, flexible deflection, and the short term and long     

Hydroelastic analysis of floating structures with liquid tanks and comparison with experimental tests

In this study, we develop a numerical method for a 3D linear hydroelastic analysis of floating structures with liquid tanks subjected to surface regular water waves and compare the numerical results with experimental tests. Considering direct couplings among structural motion, sloshing, and water waves, a mathematical formulation and a numerical method extended from a recent work [1] are developed. The finite element method is employed for the floating structure and internal fluid in tanks, and the boundary element method is used for the external fluid. The resulting formulation completely incorporates all the interaction terms including hydrostatic stiffness and the irregular frequency effect is removed by introducing the extended boundary integral equations. Through various numerical tests, we verify the proposed numerical method. We also performed 3D hydroelastic experimental tests of a floating production unit (FPU) model in an ocean basin. The measured dynamic motions are compared with the numerical results obtained using the proposed method.     

Hydroelastic optimisation of a composite marine propeller in a non-uniform wake

The purpose of this paper is to optimise the hydroelastic performance of a composite marine propeller to reduce vibration and dynamic stress. A hydroelasticity method based on the finite element method (FEM) coupled with computational fluid dynamics (CFD) is used to simulate the composite marine propeller in a non-uniform wake. Composite blades can be considered as a cantilever-like laminated structure experiencing an unsteady hydrodynamic load and centrifugal force. The objective of the improved design is to minimise the vibratory hub loads. The ply angle and stacking sequence are considered as the design variables. The nonlinear periodic transient responses and vibration hub loads of the composite blade are obtained by solving coupled equations using the Newton–Raphson numerical procedure. Compared to the starting design of the propeller, the optimum solution results in a 49.6–70.6% reduction of the 7/rev hub loads.     

Investigation of ship hydroelasticity

The importance of hydroelastic analysis of large and flexible container ships of today is pointed out. A methodology for investigation of this challenging phenomenon is drawn up and a mathematical model is worked out. It includes definition of ship geometry, mass distribution, structure stiffness, and combines ship hydrostatics, hydrodynamics, wave load, ship motion and vibrations. Based on the presented theory, a computer program is developed and applied for hydroelastic analysis of a flexible segmented barge for which model test results of motion and distortion in waves have been available. A correlation analysis of numerical simulation and measured response shows quite good agreement of the transfer functions for heave, pitch, roll, vertical and horizontal bending and torsion. The tool checked in such a way can be further used for reliable hydroelastic analysis of ship-like structures.     

Fully Coupled Fluid-Structure Interaction Model Based on Distributed Lagrange Multiplier/Fictitious Domain Method

This paper,with a finite element method,studies the interaction of a coupled incompressible fluid-rigid structure system with a free surface subjected to external wave excitations.With this fully coupled model,the rigid structure is taken as "fictitious" fluid with zero strain rate.Both fluid and structure are described by velocity and pressure.The whole domain,including fluid region and structure region,is modeled by the incompressible Navier-Stokes equations which are discretized with fixed Eulerian mesh.However,to keep the structure's rigid body shape and behavior,a rigid body constraint is enforced on the "fictitious" fluid domain by use of the Distributed Lagrange Multiplier/Fictitious Domain(DLM/FD) method which is originally introduced to solve particulate flow problems by Glowinski et al.For the verification of the model presented herein,a 2D numerical wave tank is established to simulate small amplitude wave propagations,and then numerical results are compared with analytical solutions.Finally,a 2D example of fluid-structure interaction under wave dynamic forces provides convincing evidences for the method excellent solution quality and fidelity.     

A time-domain method to evaluate the use of moving weights to reduce the roll motion of a ship

A method to evaluate the use of actively controlled moving weights on board ships to reduce roll motion is developed. The weights can simulate in principle anti-roll-tank systems, or they can be considered a possible anti-roll device in their own right. The ship, the moving weight, and the control device are considered components of a single dynamic system. The full eight-degree-of-freedom set of coupled governing equations for the complete dynamic system is derived. And a three-degree-of-freedom non-linear approximation for the roll motion only (MOTSIM) is derived from these eight equations. The reduced set of equations is used to determine the influence of various parameters and to evaluate control strategies. A PID controller is developed to command the position of the weight and a servomechanism model is used to predict its actual position. Then, the moving-weight system is incorporated into LAMP (Large–Amplitude–Motion Program), a computer code that integrates the governing equations of the sea and the motion of the ship interactively and simultaneously and predicts the motion of the ship in the time domain. A comparison of the results from the two simulations shows that there is fairly good correlation between the simple and complex models, but the simple model is a little optimistic in predicting the effectiveness of the moving-weight system. The results predict that the moving-weight system can be an excellent roll-suppressing device when the moving weight is as small as 1% of the displacement of the ship and the maximum distance the weight moves is as small as 15% of the half-beam.     

Transient Hydroelastic Response of VLFS by FEM with Impedance Boundary Conditions in Time Domain

A time-dependent finite element method (FEM) is developed to analyze the transient hydroelastie responses of very large floating structures (VLFS) subjected to dynamic loads. The hydrodynamic problem is formulated based on the linear theory of fluid and the structural response is analyzed based on the thin plate theory. The FEM truncates the unbounded fluid domain by introducing an artificial boundary surface, thus defining a finite computational domain. At this boundary surface an impedance boundary conditions are applied so that no wave reflections occur. In the proposed scheme, all of the procedures are processed directly in time domain, which is efficient for nonlinear analyses of structure floating on unbounded fluid. Numerical results indicate acceptable accuracy of the proposed method.     

完全非线性波浪与二维固定结构物作用的IGN-BEM耦合分析模型

为高效准确地对完全非线性波浪与二维固定结构物的相互作用进行模拟分析,建立了二维完全非线性时域耦合模型。耦合模型将计算域划分为靠近结构物的内域和远离结构物的外域,每个区域均采用满足完全非线性自由水面边界条件的波浪模型进行求解。在内域使用Laplace方程描述流体运动并采用高阶边界元法（BEM）对其进行求解;而在没有结构物的外域,波浪运动的控制方程为Irrotational Green-Naghdi（IGN）方程并采用有限元法（FEM）对其进行求解。内域和外域通过一段重叠区域进行耦合,从而实现模型间变量的传递。首先利用耦合模型分别对规则波的传播、直墙前立波的生成以及相关物理模型试验进行模拟,数值结果与精确解和试验结果的良好吻合验证了耦合模型耦合方式的合理性以及处理非线性问题的准确性;然后使用耦合模型模拟分析了波浪与固定结构物间的相互作用,并将结果与线性解析解以及完全非线性BEM模型的结果进行了对比分析,进一步证明了耦合模型的正确性与高效性。     

内孤立波与深海立管相互作用数值模拟

提出了一种对内孤立波与深海立管相互作用耦合数值模拟方法。流场采用内孤立波数值水槽方法进行模拟,结构响应采用基于薄壳理论的有限元方法进行计算,采用一种将流场和结构响应数据进行实时传输的方法,实现了流体与固体之间的耦合数值模拟。对内孤立波作用下某长径比为1 200的深海立管载荷及其动力响应特性进行了数值模拟与分析。结果表明内孤立波不仅会对深海立管产生突发性剪切载荷作用,而且还会使立管产生大幅度变形响应现象,因此在深海立管设计与应用中,内孤立波的影响是不可忽视的。研究表明,该方法为研究内孤立波作用下深海立管动力特性及其工程预报相关问题提供了一种有效的手段。     

Hydroelastic response of a very large floating structure over a variable bottom topography

An influence of sea-bottom topography on the hydroelastic response of a Very Large Floating Structure (VLFS) is considered. When the floating structure is constructed near the shore, the sea-bottom topographical effect should be considered. In this study, the effect of sea-bottom topography is investigated for four different bottom cases. To calculate the sea-bottom effects rigorously, the finite-element method based on the variational formulation is used in the fluid domain. The pontoon-type floating structure is modeled as the Kirchhoff plate. The mode superposition method is adopted for the hydroelastic behavior of the floating structure.     

Hydroelastic interaction in panel-water impacts of high-speed craft

The paper considers the hydroelastic interaction involved in panel-water impacts for high-speed craft. Hydroelastic panel-water impacts are simulated by using the finite element code LS-DYNA and a simplified in-house developed method. The mechanisms involved are studied through a systematic series of simulations comprising different impact velocities, deadrise angles, boundary conditions, membrane effects, and panel properties. The terms inertial hydroelastic effects and kinematic hydroelastic effects are introduced to categorize the different mechanisms involved in the hydroelastic problem. It is shown that hydroelastic effects can both increase and reduce the panel responses depending on the impact envelope considered, and it is made clear that the impact envelope has to be carefully considered when establishing the significance of hydroelasticity in a hull-water impact problem. Membrane effects are shown to have large influences on the hydroelastic effects even for moderate panel deflections. The results generated are further used to evaluate and discuss hydroelastic criterion and methods for hydroelastic characterization.     

Wave-induced seabed response in shallow water

To simulate the wave-induced response of coupled pore fluids and a solid skeleton in shallow water, a set of solutions with different formulations (fully dynamic, partly dynamic, and quasi-static) corresponding to each soil behavior assumption is presented. To deal with Jacobian elliptic functions involved in the cnoidal theory, a Fourier series approximation is adopted for expanding the boundary conditions on the seabed surface. The parametric study indicates the significant effect of nonlinearity for shallow water wave, which also enhances the effect of soil characteristics. The investigation of the applicability of reduced formulations reveals the necessity of a partly or even fully dynamic formulation for the wave-induced seabed response problem in shallow water, especially for thickened seabed. The analysis of liquefaction in the seabed indicates that the maximum depth of liquefaction is shallower, and the width of liquefaction is broader under cnoidal wave loading. The present analytical model can provide more reasonable result for the wave-induced seabed response in the range of shallow water wave.     

A pseudo-coupled analytic fluid-structure interaction method for underwater implosion of cylindrical shells

Underwater implosion, the rapid collapse of a structure caused by hydrostatic pressure, is a fully coupled, highly dynamic and nonlinear fluid-structure interaction (FSI) problem. The primary motivation behind studying implosion is the short-duration, high-pressure pulse generated in the surrounding water. This paper presents a simplified analytic method to estimate the energy in the pressure pulse, based on potential flow theory. The method accounts for the varying fluid pressure and accompanying FSI. The focus is on long, thin, unstiffened metallic cylindrical shells that collapse in mode 2. The implosion pulse energy is shown to be equal to the maximum system kinetic energy developed during collapse. The kinetic energy is calculated using an energy balance approach and analytic solutions for plastic energy dissipation and energy required to compress the internal air. The time-varying fluid pressure, and subsequently the work done by the fluid on the cylinder, is found using a novel explicit time-stepping methodology. The result is a pseudo-coupled analytic solution for the fluid pressure time history and implosion pulse energy. Solutions for pulse energy agree with RANS numerical simulations within 5%.     

Comparison of Linear Level I Green-Naghdi Theory with Linear Wave Theory for Prediction of Hydroelastic Responses of VLFS

Very Large Floating Structures (VLFS) have drawn considerable attention recently due to their potential significance in the exploitation of ocean resources and in the utilization of ocean space. Efficient and accurate estimation of their hydroelastic responses to waves is very important for the design. Recently, an efficient numerical algorithm was developed by Ertekin and Kim (1999). However, in their analysis, the linear Level I Green-Naghdi (GN) theory is employed to describe fluid dynamics instead of the conventional linear wave (LW) theory of finite water depth. They claimed that this linear level I GN theory provided betler predictions of the hydroelastic responses of VLFS than the linear wave theory. In this paper, a detailed derivation is given in the conventional linear wave theory framework with the same quantity as used in the linear level I GN theory framework. This allows a critical comparison between the linear wave theory and the linear level I GN theory. it is found that the linear level     

Ray theory for predicting hydroelastic behavior of a very large floating structure in waves

The conceptual design of a very large floating structure (VLFS) requires a convenient computer code for predicting hydroelastic behavior of it. The code should not be time consuming, but it should be flexible for all environmental conditions. In order to meet the needs, we apply the ray theory for predicting hydroelastic behavior of a mat-like VLFS. The hydroelastic behavior of the VLFS is treated as wave propagation in the platform. The theory itself is based on the classical ray theory, which yields a quick computational scheme. The parabolic approximation is applied to smoothing the discontinuous deformation obtained by the classical ray theory. An experimental technique in a small wave tank with a mini scale model has been developed. Through comparisons with the mini scale experiment and other data found in literatures, it is confirmed that the ray theory has enough accuracy for the conceptual design, unless the assumptions of the ray theory are completely violated.     

Numerical analysis of large-scale surface-piercing propellers

Surface-Piercing Propellers (SPPs) are the preferred propulsion system for light to moderately loaded high-speed applications due to the high fuel efficiency. For highly loaded applications, the efficiency of SPPs tends to decrease because of the limited submerged blade area and the presence of large suction side cavities. Moreover, it is a challenge to design large-scale SPPs that can maintain reliable fatigue strength and avoid vibration issues while maximizing the propeller thrust for a given power input. In this work, three SPP designs are presented for different size Surface Effect Ships (SESs) that can attain a maximum advance speed of 25.72 m/s (50 knots). A previously developed and validated three-dimensional (3-D) coupled boundary element method-finite element method (BEM-FEM) is used for the transient hydroelastic analysis of SPPs. The method is validated by comparing the predicted hydrodynamic performance with those obtained using a vortex-lattice method (VLM) and a Reynolds Averaged Navier-Stokes (RANS) solver. The hydrodynamic and structural dynamic performance of the SPPs are presented. Finally, challenges associated with the design related analyzes of large-scale SPPs are discussed.     

Field and Theoretical Study of the Response of Super-Long Bored Pile Subjected to Compressive Load

In this article, two full-scale pile loading tests were conducted to observe the field performance of the super-long bored piles, and a simplified approach for nonlinear analysis of the load-displacement behavior of a single pile was presented. The field tests on piles indicates that, under the maximum test load, more than 70% of the pile top settlement is caused by the compression of pile shaft. For practical purposes, the pile top settlement can be reduced through improving the pile shaft strength. When the load reaches the maximum test load, the proportion of the load carried by the pile tip is approximately 30%. The super-long pile is functioning as an end-bearing friction pile. The skin friction at shallow depth is fully mobilized and decreases from a peak value with increasing load. However, the skin friction of deeper soil is not fully developed due to less relative displacement. Furthermore, a BoxLucas1 model is used to capture the relationship between unit skin friction and pile-soil relative displacement, whereas a hyperbolic model is used to describe the relationship between toe stress and pile base displacement. Based on the BoxLucas1 model and the hyperbolic model, a load transfer method is used to clarify the response of a single pile, and a computational flow chart is developed. The efficiency and accuracy of the present method is verified using the field tests on piles. The proposed simple analytical approach is economical and efficient, resulting in savings in time and cost.