Direct time domain analysis of floating structures with linear and nonlinear mooring stiffness in a 3D numerical wave tank

In this paper, motion response of a moored floating structure interacting with a large amplitude and steep incident wave field is studied using a coupled time domain solution scheme. Solution of the hydrodynamic boundary value problem is achieved using a three-dimensional numerical wave tank (3D NWT) approach based upon a form of Mixed-Eulerian–Lagrangian (MEL) scheme. In the developed method, nonlinearity arising due to incident wave as well as nonlinear hydrostatics is completely captured while the hydrodynamic interactions of radiation and diffraction are determined at every time step based on certain simplifying approximations. Mooring lines are modelled as linear as well as nonlinear springs. The horizontal tension for each individual mooring line is obtained from the nonlinear load-excursion plot of the lines computed using catenary theory, from which the linear and nonlinear line stiffness are determined. Motions of three realistic floating structures with different mooring systems are analyzed considering various combinations of linear and approximate nonlinear hydrodynamic load computations and linear/nonlinear mooring line stiffness. Results are discussed to bring out the influence and need for consideration of nonlinearities in the hydrodynamics and hydrostatics as well as the nonlinear modelling of the line stiffness.     

海洋系泊缆索非线性有限元静力分析

采用几何非线性有限元方法研究海洋系泊缆索在重力、浮力和水流流体阻力作用下的静态平衡位形和张力分布.基于完全的Lagrangian格式,采用二节点等参索单元建立系泊缆索的静态平衡方程;采用增最迭代法求解得到的静平衡方程.数值算例证实此方法的有效性和准确性.计算了不同水流流体阻力作用下缆索的静平衡位形和张力分布,得到一些有意义的结论.     

不同布缆方式锚碇沉管管段运动的数值模拟

通过建立波浪作用下锚碇沉管管段运动的时域数值计算模型,对锚碇沉管的运动响应及锚碇缆受力特性进行了研究。应用集中质量法求解锚碇缆力,应用四阶Runge-Kutta法求解管段时域运动方程,计算了在不同沉深、不同周期、不同波高和不同波向条件下沉管管段的运动幅值和锚碇缆力。数值计算的结果表明:锚碇沉管的锚碇缆对沉管管段的运动起到一定的约束作用,在沉放深度较浅、波浪周期较大时,锚碇缆对沉管管段运动幅度的制约更为明显。通过五种布缆方式的计算与分析,探讨了布缆方式对沉管管段运动响应及锚碇缆受力的影响,并给出了较为合理的布缆方案。     

Hydrodynamic interactions and relative motions of two floating platforms with mooring lines in side-by-side offloading operation

The hydrodynamic interaction and mechanical coupling effects of two floating platforms connected by elastic lines are investigated by using a time-domain multi-hull/mooring/riser coupled dynamics analysis program. Particular attention is paid to the contribution of off-diagonal hydrodynamic interaction terms on the relative motions during side-by-side offloading operation. In this regard, the exact method (CMM: combined matrix method) including all the vessel and line dynamics, and the 12×12 hydrodynamic coefficients in a combined matrix is developed. The performance of two typical approximation methods (NHI/No Hydrodynamic Interaction: iteration method between two vessels without considering hydrodynamic interaction effects; SMM/Separated Matrix Method: iteration method between two vessels with partially considering hydrodynamic interaction effects, i.e. ignoring off-diagonal cross-coupling terms in the 12×12 hydrodynamic coefficient matrix) is also tested for the same side-by-side offloading operation in two different environmental conditions. The numerical examples show that there exists significant discrepancy at sway and roll modes between the exact and the approximation methods, which means that the cross-coupling (off-diagonal block) terms of the full hydrodynamic coefficient matrix play an important role in the case of side-by-side offloading operation. Therefore, such approximation methods should be used with care. The fender reaction forces, which exhibit large force with contact but no force without contact, are also numerically modeled in the present time-domain simulation study.     

Strength modelling in stiffened FRP structures with viscoelastic inserts for ocean structures

The purpose of this paper is to investigate the static structural response of a new type of composite stiffener containing a viscoelastic insert. The introduction of this material has proven benefits in terms of noise and vibration attenuation across the joint. House, 1997 describes the use of this material in sonar dome/hull connections — equipment sensitive to noise and vibration. Structural stiffeners incorporating this material would have positive implications for not only marine and ocean structures but for structural applications in general. The effects of introducing this new material on the structural response of the joint are numerically examined by using a progressive damage model. Application of this method allows the initiation and progression of failure and ultimate failure load to be predicted. Experimental results show good qualitative and quantitative agreement with the predictive damage model.     

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.     

Bifurcation analysis of wind-driven flows with MOM4

In this paper, the methodology of bifurcation analysis is applied to the explicit time-stepping ocean model MOM4 using a Jacobian–Free Newton–Krylov (JFNK) approach. We in detail present the implementation of the JFNK method in MOM4 but restrict the preconditioning technique to the case for which the density distribution is prescribed. For a prescribed density field case, we present bifurcation diagrams, for the first time in MOM4, for the wind-driven ocean circulation. In addition, we show that the JFNK method can reduce the spin-up time to a steady equilibrium in MOM4 considerably if an accurate solution is required.     

Finite element analysis of ship structures using a new stiffened plate element

A new stiffened plate element is developed for the three-dimensional finite element analysis of ship structures. The plate element can accommodate any number of arbitrarily oriented stiffeners and obviates the use of mesh lines along the stiffeners. The new element provides a very economic global analysis of the complete ship structure with fewer elements and without any loss of accuracy. The global analysis of a rectangular box shaped vessel is carried out with the present element and compared with the general-purpose finite element software NISA. An Offshore Tug/Supply Vessel is analysed for crest at perpendiculars.     

A finite element method for dynamic analysis of long slender marine structures under combined parametric and forcing excitations

This paper presents a numerical analysis of lateral responses of a long slender marine structure under combined parametric and forcing excitations. In the development of the 3-D numerical program, a finite element method is implemented in the time domain using the Newmark constant acceleration method. Some example studies are performed for various water depths, environmental conditions and vessel motions. The relative amplitudes of combined excitations to a conventional forcing excitation are examined. The response amplitude of a combined excitation is much greater than that of a forcing excitation in the even number of instability regions of the Mathieu stability chart. The results demonstrate that a combined excitation needs to be considered for the accurate dynamic analysis of long slender marine structures subjected to a surface vessel motion.     

Buckling of circular, solid and annular plates with an intermediate circular support

The title problem is solved in the case where uniform applied loading is present at the plate outer boundary. Two independent solutions are obtained: the optimized Rayleigh–Ritz method and the finite element algorithmic procedure. Good engineering agreement is shown to exist. The proposed analytical procedure is convenient from a designer's viewpoint. Admittedly exact analytical solutions can be found in terms of Bessel functions, the procedure being rather complex from an engineering viewpoint.     

Dynamic analysis of vehicle–deck interactions

This paper presents the review and studies at various levels of problems concerning the authors’ previous research on the dynamics of vehicle–deck interactions. The various levels of study include the dynamic structural behavior of vehicle–deck systems, vehicle vibrations, damping effects of vehicles on structural systems, dynamic interactions between tire and deck surface, and vehicle securing on decks during ship motions, etc. The study includes analytical, numerical and experimental analysis. Practical problems encountered by Ro–Ro ship designers are addressed by discussing those analysis. It is shown that influences from the dynamics of vehicle–deck interactions are relevant to a number of aspects of issues, such as the excitation frequency range, how detailed information of the structural system response is required, the structure characteristics, and positions and orientations of vehicles on decks, etc. The study contributes to the knowledge for the naval architect and vehicle engineer on how significant the dynamics of vehicle–deck interactions are when dealing with relevant problems.