Analysis of hydrodynamic characteristics for arbitrary multihull ships advancing in waves

Multihull vessels have emerged as popular alternatives to conventional monohull ships for high-speed crafts. However, the bridging structures connecting the hulls are vulnerable to various wave actions and the wave impact on the bottom of them is the most serious problems associated with multihulled vessels. In this study, prediction of relative wave elevations under the bridging structures is investigated for multihull ships traveling with forward speed in waves. A computer code YNU-SEA using the three-dimensional (3D) Green function method with forward speed has been developed and used to analyze the hydrodynamic radiation and diffraction forces and motion responses for high-speed catamarans in waves. The results of the present calculations are compared with those of previous calculations as well as with experimental results. The numerical results reveal that the present computer code can be used as a powerful tool for the accurate numerical computation of seakeeping problems for multihull ships advancing in waves. Numerical calculations of wave pattern are also carried out including wave interactions between the hulls to analyze the effects of hull form on the free surface flow around catamarans advancing in waves. The analysis of the wave pattern allows the determination of relative wave height including radiation and diffraction waves. Finally, some discussions are included based on these numerical results which may be helpful for the accurate prediction of relative wave height and wave breaking load on the deck associated with multihull ships.     

基于B样条的三维船体水动力数值计算

本文采用基于B样条的一种新的数值方法计算三维船体水动力，用B样条函数表达三维船体表面的几何形状以及流场中未知物理量的分布，为了验证该数值方法的可行性和精确度首先对处于无限流体域中的圆球体绕流问题进行了计算；其次计算了由ITTC所推荐的Wigley船型的兴波阻力以及以一攻角斜航时的操纵水动力；最后在一些假设下对两船作平行航行时的干扰水动力作了相应计算工作。数值计算结果与其它试验或理论结果在定量或定性上吻合良好。     

船体非线性波浪载荷的水弹性分析

本文采用水弹性分析方法研究船舶在规则波及不规则波中迎浪航行时的结构动力响应（运动、剪力和弯矩等）。确定流体载荷时应用了扩展的切片理论，其中计及由于船体的非直舷、剖面吃水的瞬时变化和船体振荡的非简谐特性所导致的非线性，同时还考虑了波浪冲击、出水和上浪的影响。运动方程是在时域内步进求解。数值计算结果与规则波中的模型试验相比较，符合程度令人满意。     

系泊船舶动力学特性的计算机仿真研究

以三阶操纵运动方程为基础，引入定常的风力、潮流作用力和二阶波浪力，建立了系泊系统三自由度的运动微分方程。在此数学模型的基础上．建立了系泊系统的多自由度的计算机仿真模型。在潮流作用、潮流和风作用以及风浪流联合作用等三种情形下，对系泊船舶的动力学响应分别进行了仿真研究。研究表明，系泊系统的动力学行为具有强烈的非线性特征。对于单点系泊船舶而言，在定常的风浪流作用下．顶风顶浪顶流状态并不一定是最为危险的工况。     

Large-amplitude motion responses of a Ro–Ro ship to regular oblique waves in intact and damaged conditions

This article presents a nonlinear time-domain simulation method for the prediction of large-amplitude motions of a Ro–Ro ship in regular oblique waves in an intact and a damaged condition. Numerical computations and model tests have been carried out to investigate the dynamic motion responses of Ro–Ro ship Dextra to various wave amplitudes at three different wave headings. The results of numerical and experimental investigations for stern quartering waves are reviewed. Comparisons between predictions and measurements show good agreement except in the roll-resonant region. Nonlinear effects are significant in horizontal modes of motion, and resonant roll motion, and there is strong coupling between all modes of motion in the roll-resonant region for large-amplitude responses. On the other hand, the time-domain simulation technique suffers from numerical drift in horizontal modes of motion as wave amplitude increases. This is due to nonlinear equations of motion and the lack of a restoring force and moment in horizontal motion. Received: April 30, 2002 / Accepted: August 9, 2002 Acknowledgments. II Programme of the European Community Commission under contract No. BRPR-CT97-0513. Address correspondence to: H.S. Chan (hoi-sang.chan@ncl.ac.uk)     

两圆柱体在波浪载荷作用下的运动响应分析

基于势流理论和波浪辐射/绕射理论,利用水动力分析软件(AQWA)研究两圆柱体在波浪作用下的运动响应特性,给出不同入射角与不同频率规则波作用下,小圆柱体单独运动时的六自由度响应幅值算子(RAOs)及运动响应,并与两个并排圆柱体六自由度响应幅值算子(RAOs)及运动响应作比较。结果表明:大圆柱体的干扰将使小圆柱体的纵摇、横摇、横荡、垂荡运动加剧,对小圆柱体漂移力也有较大影响,但对小圆柱体的纵荡与首摇运动影响不大。计算结果与传统方法计算值相近,证明此软件合理,可为两船在波浪中的干扰模拟计算提供参考。     

波浪中航行两船模池壁干扰效应的数值研究

对规则波中顶浪并行航行的两船模,基于频域移动脉动源格林函数建立了有效表达池壁效应的三维数学模型,该模型采用分布源积分方法求解近距航行两船模的波浪作用力和运动响应,并利用镜像原理将水池格林函数表示为开阔水域的格林函数和由镜像点源组成的无穷级数.分析移动脉动源格林函数的远场传播模态及其特性可知,当τ>0.272(τ为斯特劳哈尔数)时各传播波模态均不在点源的上游出现,故此时点源波系的影响范围可由环形—扇形波的半楔角确定,在此基础上计及船模辐射—绕射波的池壁反射作用对自身和另一船模的干扰,依据船型要素、水池宽度和两船相对位置,采用六个几何临界半楔角表征船体的辐射—绕射波,提出了并行航行两船模的池壁效应判别方法.数值研究表明:不同镜像次数下水池格林函数的数值差异主要体现在距船艏较远处,在兼顾计算精度与效率的原则下可选取有限项级数的水池格林函数作为积分内核,而池壁效应对近距航行两船运动响应的影响主要体现在长波区.     

短波顶浪中KVLCC2船模阻力增加CFD计算研究

波浪中航行船舶阻力增加,特别是短波中的阻力增加,是船舶界关注的焦点之一,也是船舶水动力学界研究的热点之一。论文采用基于RANSE的数值波浪水池技术,针对KVLCC2船型,开展了短波顶浪中船舶阻力增加的数值计算研究。与模型试验结果的比较表明,文中的CFD方法能够相当准确地计算短波顶浪中航行船舶的阻力增加;对船体各部分波浪增阻的分析表明,船体艏段产生的波浪增阻占主导地位,艉段的波浪增阻很小,而平行中体段对波浪增阻几乎无贡献。     

Nonlinear hydrodynamic responses of submerged moving payload in vicinity of a crane barge in waves

The wave interaction with a submerged cylindrical payload subjected to constrained motions in presence of a nearby floating crane barge is investigated in the three-dimensional numerical wave tank using a fully nonlinear potential flow model in the time domain. Numerical simulations are carried out to investigate the hydrodynamic features of this submerged payload under pendulum motion in water waves as well as while it moves towards the sea bed at a constant vertical velocity. It is known that the presence of multiple side by side floating bodies in waves can create significant drift motion. In the present study the similar drift motion is observed for the side by side floating barge and submerged payload and it appears that the submerged payload under constrained motions may face a very large mean drift motion of nearly seven times that of the incident wave amplitude in the beam sea upstream condition. Emphasis is also given towards investigating and understanding the influences of natural frequency of the payload and shielding effect due to the presence of the floating barge. It is found that natural frequency coupled with shielding effect generates remarkable low frequency components in payload responses both in the head sea and the beam sea situations. The effect of different cable lengths, wave maker frequencies and downward moving velocities on payload responses under several geometric setups are studied and compared, and interesting features such as increased low frequency movement of the payload near the natural frequency region and existence of considerable low frequency motions even at a greater depth (while the payload is quite below the free surface) are observed.     

波浪中母船干扰下潜水员梯的运动及水动力特性分析

潜水员梯是通过销轴连接在母船舷侧为潜水员执行潜水作业时上下船的平台,为保证在波浪中使用时的安全性,对其波浪作用力、运动响应及销轴处的动作用载荷设计有较高要求。为研究母船干扰下潜水员梯的动态响应特性,采用基于深水格林函数为积分内核的边界元方法,先计算母船及潜水梯这一整体的运动响应及其平均湿表面的辐射和绕射压力分布,依据刚体运动合成原理求解得到潜水梯重心处的运动。再以潜水员梯为研究对象,采用压力积分方法获得潜水员梯的波浪力作用,并将其运动、波浪力和销轴处的作用力联立构成动力学平衡方程,逆向求解出销轴处的动作用力。基于上述思路,系统研究了波长、航速及潜水员梯与母船相对位置变化时潜水员梯的运动、波浪作用力及销轴处作用力的变化规律,为潜水员梯的布置、销轴的选型及潜水作业时母船的操纵可提供指导意义。     

海洋浮式平台气隙的数值预报

以海洋浮式平台为研究对象,运用三维势流理论结合半经验Morison方程的方法计算了规则波下的水动力系数以及运动响应传递函数,并运用短期预报方法对平台气隙进行了分析。研究结果表明,其水动力性能与波浪频率密切相关,平台的气隙短期预报显示波浪在浮箱首尾部分负气隙严重,即有强烈的波浪砰击现象,平台结构设计时要充分考虑到该区域的波浪砰击载荷。     

两船在波浪载荷作用下的运动响应分析

基于波浪辐射/绕射理论,利用水动力分析软件AQWA研究两船在波浪作用下的运动响应特性,给出驳船受浮式生产储油船(FPSO)影响时的附加质量和辐射阻尼,分析了驳船的漂移力和运动响应。计算结果表明,当两船间距为5 m时驳船的附加质量比间距为10 m时的值大2倍多,驳船的附加质量和阻尼随着间距的减小逐渐增大;在间距30 m时,驳船的速度越小所受的漂移力就越大,并且驳船的横摇和首摇运动就越剧烈。     

带横隔板开孔沉箱所受波浪力的研究

研究了斜向波与无限多开孔沉箱的相互作用。依照结构物的几何形状,整个流域被分成无限多个子域,在每个子域内应用特征函数展开法求解速度势。对于沉箱内的波浪运动,引入相位差概念;同时,在构造反射波模型时,考虑了结构物的几何形状影响。进行了数值试验,分析了开孔沉箱水平波浪力及横隔板受力特点,详细讨论了波浪入射频率、入射角度、相对消浪室宽度及开孔影响系数对开孔沉箱波浪力的影响。     

系泊系统的时域仿真及其非线性动力学特性分析

应用时域仿真的方法研究了系泊系统的非线性动力学特性。以三阶操纵运动方程为基础,引入定常的风力、潮流作用力和二阶波浪力,建立了系泊系统三自由度的运动微分方程。在此数学模型的基础上,建立了系泊系统的多自由度的计算机仿真模型。在风浪流联合作用的情形下,对一艘单点系泊油轮的动力学行为进行了仿真研究。以潮流速度和系缆程度为分岔控制参数,在参数平面上给出了局部分岔集。研究表明,系泊系统的动力学行为具有强烈的非线性特征。在仿真过程中观察到了吸引子的共存和Hopf分岔。局部分岔集将参数平面分为3个系统动力学行为本质不同的区域。极值系泊力水平与系泊系统的动力学行为有着密切的关系。对于单点系泊船舶而言,顶风顶浪顶流的状态并不一定是最为危险的工况。局部分岔集的确定为系泊系统参数的选择提供了决策依据。     

东海涌浪环境下大型起重船吊装组块耦合运动响应数值模拟研究

我国东海海洋环境复杂,经常出现小波高、长周期的涌浪.长周期涌对系泊大型起重船吊装海洋平台上部组块影响比较大,尽管涌的波高较小,但是,吊物组块运动幅度明显比同等波高海况时的要大,严重影响了海洋平台的安装施工效率和进度,甚至威胁作业安全.本文采用水动力势流软件,建立系泊-大型起重船(包括吊臂)-索具-大型组块吊物的耦合运动模型,研究复杂涌浪环境下耦合系统运动响应特性及其机理,并开展了参数敏感性分析,讨论对耦合响应影响因素.最后,对东海涌浪情况下起重安装施工,提出了降低吊装组块运动响应的建议措施.     

Parametrical studies of a new numerical model for controlled ship motions in extreme astern seas

Parametrical studies based on numerical simulations were carried out for very steep regular waves to assess possible improvements in the state-of-the art numerical modelling of the control and capsizing behaviour of ships in following and quartering seas. A nonlinear 6-DOF numerical model has been developed with the inclusion of frequency-dependent terms, the so called memory effects, and a flexible axis system that allows straightforward combination of seakeeping and manoeuvring models while accounting for extreme motions. The previously undertaken validation analyses using extensive model test data provided qualitatively good agreement, whereas the comparison with numerical models without coupling of the vertical motions and frequency-dependent hydrodynamic terms embodied in radiation forces identified improvements in the accuracy. However, to broaden the assessment of the numerical model, further parametrical numerical analyses were carried out using two ships, which had previously been tested in the validation analyses, for various operational and environmental conditions. These parameters were changed in accordance with the recommendations from international organisations and experience from model tests to realise and avoid dangerous conditions that often result in capsizing, such as broaching associated with surf riding and low-cycle resonance. As a result of the parametric analysis, we discuss the sensitivity of the improvements in the numerical model for various critical operational and design parameters and its possible use to provide a link between the ship's behavior and these parameters.     

波浪与系缆方式对系泊船舶响应的影响研究

系泊船舶响应的影响因素很多,且十分复杂。针对影响系泊船舶响应的主要因素（波浪和系缆方式）开展研究,通过船舶系泊物理模型试验,研究波浪条件及系缆方式对系泊船舶响应的影响规律,其中波浪条件包括不同的波向及波浪周期,系缆方式包括不同缆绳配置（有无缆尾索）和不同系缆布置方案。研究结果表明,45°斜向浪作用下系泊船舶的运动量和系缆力均明显小于横浪作用下的结果;试验波浪周期范围（T=3.5~6.0 s）内,系泊船舶的运动量和系缆力均随着波浪周期的增大而变大;从船舶系泊安全角度看,设置缆尾索是有利的;码头前后沿共同系缆布置可明显提高船舶系泊（非装卸作业时）的抗风浪流能力。     

基于设计波的载重3600t干货船结构强度直接计算分析

为准确评估超规范的载重3600t大通舱干货船的弯扭强度及变形水平,采取全船水动力分析及全船有限元直接计算的方法,对各工况下的主要载荷参数进行长期预报,推导出对应等效设计波各参数。根据等效设计波求出各工况全船所受的波浪诱导动载荷,施加到全船有限元模型上,进而对船体弯扭强度及变形水平进行评估。比较了单舱船及货舱中部设一道横舱壁的两舱船,得出单舱船屈曲强度不足的结论,并提出改善屈曲强度的方案。     

基于细长体公式的Truss SPAR在波流中的运动响应预报方法

利用细长体公式和刚体非线性运动方程建立Truss SPAR在波浪与流中运动响应预报方法。通过波浪自由表面和SPAR中心线方程构造辅助函数,迭代计算Truss SPAR瞬时湿长度。根据SPAR主体形状特点和流场中水质点运动规律,分段高效积分Truss SPAR上的水动力载荷。通过Runge-Kutta-Fehlberg方法求解运动方程,得Truss SPAR在波浪与流中的运动响应。对一座Truss SPAR在不同波流工况中的运动响应进行了预报,结果显示波浪和流使Truss SPAR产生了明显漂移运动和振荡运动,漂移运动的大小与流的方向有关,而振荡运动的幅值与波浪的方向有关。     

航行船舶的三维非线性水弹性分析

基于Wu提出的浮体二阶水弹性理论[1],研究大浪中航行船舶的非线性水弹性响应,给出了数值方法和计算结果.在二阶水弹性分析中,主要考虑了大角度刚体转动和瞬时湿表面变化引起的非线性水动力.根据三维线性水弹性理论[2],采用移动脉动源格林函数和Kelvin定常兴波假定,求解了对二阶水动力做主要贡献的一阶速度势及响应.最后以航行于不规则波中的小水线面双体船为例,讨论了航速和定常兴波流场对水弹性响应的影响,并对线性和非线性响应的预报结果进行了比较.