Hydrodynamic Behavior of Fluid Resonance Between Twin Boxes in Moonpool with Various Edge Profiles Under Wave Actions

Fluid resonance in moonpool formed by twin boxes under wave actions is investigated by using a viscous numerical wave flume with ReNormalization Group(RNG)turbulent model.The accuracy of the numerical model is validated by available experimental data.Three types of edge profiles,the sharp edge,concave edge and convex edge are con-sidered.Numerical simulations show that the normalized resonant amplitude in moonpool decreases with the decrease of moonpool opening or increase of incident wave amplitude.The increased reflection coefficients are the major reason for the phenomena,implying less wave energy is able to support the wave resonance in moonpool.With the increase of incident wave height,the energy coefficients increase for convex edges around resonant fre-quency,which are oppositely with those of sharp and concave edges.Various flow patterns of the wave resonance in the vicinity of the moonpool entrance are also identified,which are mainly dependent on the edge profiles.

Quadratic Dissipation Effect on the Moonpool Resonance

This paper adopted a semi-analytical method based on eigenfunction matching to solve the problem of sharp resonance of cylindrical structures with a moonpool that has a restricted entrance. To eliminate the sharp resonance and to measure the viscous effect, a quadratic dissipation is introduced by assuming an additional dissipative disk at the moonpool entrance. The fluid domain is divided into five cylindrical subdomains, and the velocity potential in each subdomain is obtained by meeting the Laplace equation as well as the boundary conditions. The free-surface elevation at the center of the moonpool, along with the pressure and velocity at the restricted entrance for first-order wave are evaluated. By choosing appropriate dissipation coefficients, the free-surface elevation calculated at the center of the moonpool is in coincidence with the measurements in model tests both at the peak period and amplitude at resonance. It is shown that the sharp resonance in the potential flow theory can be eliminated and the viscous effect can be estimated with a simple method in some provided hydrodynamic models.

Numerical Simulation of Fluid Resonance in a Moonpool by Twin Rectangular Hulls with Various Configurations and Heaving Amplitudes

Fluid resonance in a moonpool formed by two identical rectangular hulls during in-phase heaving motion is investigated by employing a two-dimensional numerical wave flume based on OpenFOAM package with Re-Normalization Group(RNG) turbulent model. The focus of the study is to examine the influence of heaving frequency and amplitude with various moonpool configurations on fluid resonant behavior. It is found that the resonant frequency of wave response in moonpool tends to decrease with the increase of moonpool breadth and hulls draft. The decrease of resonant amplitude can be observed for large moonpool breadth. The influence of hulls draft on resonant amplitude is not remarkable, especially for large heaving amplitude. The increase in heaving amplitude results in the decrease of relative resonant amplitude in an approximate power function, implying a complicated dependence of the resonant amplitude on heaving amplitude. Flow patterns in the vicinity of the moonpool are also analyzed, mainly regarding the dependence on the heaving frequency. The negligible influence of vortices on the wave response in moonpool is expected for low-frequency excitation because it is hard to observe the vortex structures. Intensive vortical flow and vortex structure can be identified under resonant condition, which gives rise to significant dissipation and accounts for the smaller relative resonant amplitude in moonpool. As for high-frequency excitation, the vortex motion is rather weak and dissipates rapidly, leading to insignificant effect on wave response amplitude.

Radiation and Exciting Forces of Axisymmetric Structures with a Moonpool in Waves

A highly efficient "hybrid integral-equation method" for computing hydrodynamic added-mass, wave-damping, and wave-exciting force of general body geometries with a vertical axis of symmetry is presented. The hybrid method utilizes a numerical inner domain and a semi-infinite analytical outer domain separated by a vertical cylindrical matching boundary.Eigenfunction representation of velocity potential is used in the outer domain;the three-dimensional potential in the inner domain is solved using a "two-dimensional" boundary element method with ring sources and ring dipoles to exploit the body symmetry for efficiency. With proper solution matching at the common boundary, both radiation and diffraction potentials can be solved efficiently while satisfying the far-field radiation condition exactly. This method is applied to compute the hydrodynamic properties of two different body geometries: a vertical-walled moonpool with a bottom plate that restricts the opening and a spar-like structure with a diverging bottom opening inspired by designs of floating Oscillating Water Columns. The effects of the size of the bottom opening on the hydrodynamic properties of the body are investigated for both geometries. The heave motion of the floater as well as the motion of the internal free surface under incident wave excitation are computed and studied for the spar-like structure.

CFD Simulation of the Vertical Motion Characteristics of the Moonpool Fluid for the Truss Spar

The research purpose of this paper is to estimate the impacts of the parameters of the guide plate on the vertical motion characteristics of the moonpool fluid. With the volume of fluid(VOF) method, three-dimensional models of the moonpool fluid motions of the truss spar platform are established. Simulation results are then presented for the moonpool forced oscillation by employing the dynamic mesh method and user-defined functions in FLUENT. The motions of the moonpool fluid and the loads on the guide plates are obtained for both cases of square-ring and crisscross. The results show that the shape and area of the guide plate at the bottom of the moonpool have a significant impact on the physical parameters of the moonpool, including the load on the moonpool guide plate, motion form of the moonpool fluid and the mass flow rate.

Coupling Response of Heave and Moonpool Water Motion of A Truss Spar Platform in Random Waves

This paper presents the heave responses and the moonpool water motions of a truss Spar platform with semi-closed moonpool in random waves. A 2-DOF（degree of freedom） coupling dynamical equations of the platform heave and vertical motions of the moonpool water are derived. The linear wave theory is used to simulate the random waves. The response statistical values and the power spectrums are calculated to analyze the mutual influences between the platform heave and the moonpool water motions for different opening ratios of the moonpool. The effect of coupling parameters on the platform heave and the moonpool water motions are analyzed. The results show that motions of the moonpool water significantly affected the platform heave when the characteristic wave period is far away from the natural period of the platform heave, and different moonpool opening ratios lead to different heave amplitudes of the platform. In the actual design, an optimized moonpool opening ratio can be designed to reduce heave motions of the platform.

Influence of Moonpool on the Total Resistance of a Drillship by the Effect of Water Motions inside the Moonpool

Moonpools are openings right through the hull from continuous deck to bottom of the ship,allowing equipment or mini-submarines to be put into the water at a location on the vessel with minimum ship motion.Open moonpools in a drillship are causing additional resistance when the ship is in forward speed.It was shown that the water inside the moonpool started to oscillate at forward speed.The water mass in the moonpool is subjected to sloshing and piston modes.The vertical motion is piston mode and the longitudinal one is called as sloshing mode.This water particle motion inside the moonpool is mainly depended on the geometry,moonpool depth,and encountered wave frequency.Out of this,moonpool geometry is one of the key factors for the performance of the moonpool.The varying cross-section geometry is one of the practically possible and economically feasible solutions to reduce the oscillation to a considerable level is attempted in this paper.Also the resistance caused by the moonpool and the free surface generated around the hull is investigated with the use of computer simulation.

简化法月池角隅疲劳强度及载荷敏感性分析

基于某钻井船大小月池结构特点,采用简化分析方法对航行与作业状态下月池角隅疲劳强度进行评估,详细阐述载荷选取与分析计算流程;同时对多种载荷敏感性进行分析,并对结果进行研究得到对月池角隅疲劳强度敏感的设计参数。

环形月池对海洋核能平台拖航阻力的影响

海洋核能平台2~3年需要拖航至港口进行换料,在其设计的初始阶段对平台拖航阻力的研究至关重要。考虑2种海洋核能平台概念,分析环形月池对平台拖航阻力的影响。采用STAR-CCM+软件对分离式海洋核能平台和一体式海洋核能平台在不同拖航速度时的拖航阻力进行计算。结果表明,随着拖航速度的增大,拖航阻力增加明显;月池的存在使平台周围流场趋于紊乱,导致核能平台拖航阻力增加;对月池增阻机理的分析可为核能平台的设计提供参考。

Numerical investigation of edge configurations on piston-modal resonance in a moonpool induced by heaving excitations

The renormalization group (RNG) turbulent model is used to investigate the fluid resonance in a moonpool formed by two identical rectangular hulls under synchronous heaving excitation (e.g., a catamaran or dual pontoon). The numerical model is validated against the available experimental data, and accurate numerical solutions are obtained. The present study focuses on the amplitude of the moving hulls and the edge configuration of the moonpool entrance, as well as their influences on the piston-modal resonant wave in the moonpool. The dependence of the resonant wave amplitude in the moonpool on the heaving amplitude, the characteristic moonpool dimensions and the local velocity magnitude is derived based on a theoretical analysis, and the results are in good agreement with the RNG turbulent solutions. Five different edge profiles are considered, including two convex edges, two concave edges (both with various dimensions), and a sharp edge. Numerical examinations show that the edge configuration has a significant influence on the piston-modal resonant responses, a larger opening size leading to a higher resonant frequency and a larger resonant wave amplitude in the moonpool. Various flow patterns of the piston-modal resonance in the vicinity of the moonpool entrance are also identified, mainly depending on the edge profile. More intensive turbulent and vortical flows give rise to more significant dissipation, accounting for the smaller relative wave amplitude in the moonpool. With the increase of the heaving amplitude, the relative piston-modal resonant amplitude is decreased in an approximate power function. Within the scope of this work, the numerical investigations show that the piston-modal resonant frequency is hardly affected by the heaving amplitude.

A dynamic solution for predicting resonant frequency of piston mode fluid oscillation in moonpools/narrow gaps

The objective of this study is to develop a simple tool for predicting the resonant frequency of the piston mode wave oscillation in a moonpool or a narrow gap between twin floating barges.Based on the dynamic analysis and the domain-decomposition,a linear harmonic oscillator model for the fluid oscillation in a confined space is formulated,which immediately leads to an explicit dynamic solution for the resonant frequency.In the model,it is assumed that the water depth is equal to or greater than a critical value dependent on the ratio of the draft to the breadth of the floating body.The predicted resonant frequencies are compared with the numerical results of the linear potential model and the viscous fluid model,the semi-analytical potential solutions and the experimental measurements.Good agreements are observed for a wide range of geometric parameters of the moonpool and the narrow gap of either sharp or round edge shapes.The present dynamic method is also applied to predict the resonant frequency of the piston mode oscillation in the gap between a floating body and a vertical wall with a reasonable success.

月池开放和封闭状态下钻井船阻力性能试验研究

[目的]为分析多功能钻井船月池封闭或开放状态下月池结构对船体阻力性能的影响,开展带月池结构的钻井船模型水池试验。[方法]以某多功能钻井船为例,研究在规则波和不规则波下的船舶运动响应。试验中,通过挂钩连接拉力传感器测量船模在静水和波浪中的阻力,利用加速度传感器分析船首、舯和船尾的加速度特性。[结果]结果表明:在轻载工况及月池开放时,船舶阻力较大;在设计载重工况及月池封闭时,船舶静水阻力较大。在规则波下,月池封闭使船尾加速度降低了58.2%,船首阻力降低了46.7%,垂荡运动响应最大幅值减小41.8%;在不规则波下,船首阻力峰值约为船尾的10倍,相比于月池封闭时,在相同时间内月池开放时阻力峰值出现的次数更多。[结论]研究表明,设计载重工况的差异使船体水线面面积改变,影响了船舶静水阻力,而月池封闭不仅降低了船舶运动加速度及阻力,而且有效改善了纵摇及垂荡运动响应幅值,这为带月池结构的钻井船结构型式的设计提供了数据支撑。

流速对月池内流体共振影响的数值模拟

本文基于OpenFOAM,对双体船月池中在不同流速下形成的流体共振进行数值模拟。通过与文献中实验和数值结果的比较,验证了该模型的准确性。数值模拟表明,当傅汝德数Fn=0.04和0.08时,流速对共振振幅的影响会减小。当Fn=0.12时,可观察到月池入口附近的速度场与涡流发生显著变化,这导致共振振幅的影响增加。即流速较小时可减小共振振幅,随着流速的增加,月池入口附近区域的流体形态发生显著变化,这种变化会增加对共振振幅的影响。

水平激励下钻井船矩形月池的水动力特性研究

研究了具有矩形月池的钻井船在小幅水平横荡简谐运动激励下的辐射问题.利用简化的底部边界条件,基于线性势流理论建立了钻井船月池二维流体运动方程,利用Galerkin方法求得月池内流体速度势,得到月池对船舶的附加质量表达式.利用半解析解,研究了月池的水动力特性及月池参数对船舶水动力参数的影响.以边界元方法建立具有矩形月池结构的钻井船数值模型,数值模拟结果与半解析解进行对比验证.结果表明,在激励频率穿过月池内流体的固有振动频率时,附加质量会依次出现正、负峰值;自由液面及月池底部无量纲速度分布受激励频率及月池无量纲水深的影响较明显;在不同无量纲水深下,半解析解与数值模拟结果吻合较好.

波浪作用下带月池结构船体运动数值预报

流体在月池内发生共振时,有可能给船舶带来多种危险。建立基于N-S方程和VOF法的三维数值波浪水池,通过实现船体与波浪的耦合运动,对月池内流体发生活塞运动共振情况下的船体运动做出了数值预报,研究了船体运动和月池内流体运动的相互影响。通过与无月池结构船体运动对比,得到了共振情况下月池内流体活塞运动会增大船体垂荡运动的结论;通过与船体静止情况下月池内流体运动的对比,得到了船体运动会增大月池内流体活塞运动的结论。另外还研究了月池内的流场变化,以研究月池振荡的机理。

Spar平台主体垂荡-月池流体耦合运动模型试验研究

针对Spar平台主体垂荡-月池流体耦合运动问题,采用模型试验的方法进行了研究。设计了可拆卸的月池底部挡板及垂荡板,通过静水中自由衰减试验及波浪中强迫运动试验,分析了月池流体对平台主体垂荡运动的影响。结果表明:月池流体显著增加了平台垂荡阻尼;当波浪周期接近平台垂荡固有周期时,月池流体对平台主体垂荡有抑制效应,且抑制程度与月池开孔面积有关;对于较大的底部挡板开孔率,平台主体垂荡RAO有2个峰,当波浪周期接近月池流体垂向振动固有周期时,也会激起平台主体较大的垂荡运动。实际中采用半开口月池时,应考虑月池流体对平台主体垂荡运动的影响。

浪流条件下月池内流体活塞振荡特性实验研究

在浪流实验条件下研究了圆形月池内流体的活塞振荡特性.在浪流的条件下,月池内的流体在不同波浪周期时表现出不同的活塞振荡特性.通过对实验现象及结果的分析,认为在浪流条件下月池内流体的活塞振荡存在着某种分岔现象,当波浪频率相对于月池内流体活塞振荡固有频率为低频时,月池内流体的振荡为强迫振荡,以浪流遭遇频率进行振荡;当浪流遭遇频率不变,波浪频率略有变化,在某一临界值附近时,月池内流体的振荡发生突变,表现出类似于自激振荡的特性,并以活塞振荡固有频率进行振荡.在来流为浪流时,月池内流体的活塞振荡特性由波浪频率和月池内流体活塞振荡固有频率构成的无量纲的Cf所决定,对于该值存在着临界值Cfe,当Cf>Cfe时,月池内流体的活塞振荡类似于自激振荡的特性;当Cf<Cfe时,则表现出强迫振荡的特性.

圆形月池流激振荡实验研究

采用基于经验模态分解(EMD)的本征模态函数(IMF)分析方法和对比研究的实验方法,实验研究了圆形月池在均匀流条件下的流激振荡特性．通过实验研究发现,尽管腔内的流体在不同来流的条件下运动趋势有所不同,但归纳起来对于月池结构,其内部存在两类振荡源,一类是腔内流体的固有振荡,该频率与腔体结构形式、尺寸、腔内流体深度有关,与来流无关;另一类则是流体流经腔口时产生的流体动力振荡,该频率与来流特性、速度有关．当二者频率相近时将会激起腔内流体的共振,即会有“活塞(piston)”现象产生．实验中,还在腔体侧壁上布置了压力传感器,用来分析腔体侧壁受到的流体压力特性及其变化趋势．

Spar平台主体垂荡-月池流体耦合动力学建模及耦合效应分析

考虑月池流体的耦合效应研究Spar平台主体垂荡运动特性。根据可变形控制体质量守恒定律及动量守恒定律建立月池内流体垂向运动方程;考虑顶张紧立管(TTRs)及张紧器刚度,建立平台主体垂荡与月池流体耦合运动模型。采用势流理论求解平台主体受到的波浪激励力,数值计算主体垂荡响应,分析月池底部挡板面积及耦合系数对主体垂荡的影响。结果表明,月池流体对主体垂荡有抑制作用,合理设计挡板面积可减小平台主体的垂荡运动。随着挡板开口面积的增加,主体垂荡RAO逐渐呈现两个峰,波浪周期接近月池流体垂向振动固有周期时也会激起主体较大的垂荡运动;平台主体与月池流体之间的耦合系数对主体垂荡运动影响显著。

二维矩形月池内流体自振特性研究

研究二维矩形月池流体的自振特性。基于线性势流理论,采用Galerkin展开,将问题转化为月池自振频率特征值问题,求解自振频率及速度势函数的系数矩阵,获得二维矩形月池固有频率及振型的半解析解,分析月池几何参数对月池流体自振特性影响。研究表明,由于内部与外流连通,月池流体呈垂向活塞(piston)振动及晃荡(sloshing)运动两种形式。垂向活塞振动模态频率变化主要受液面高度影响,且随液面高度增加而减小;晃荡模态频率主要受月池宽度影响,随月池宽度增加而减小。活塞振动模态振型在液面较浅时出现波峰,随液面高度增加而消失。利用CFD方法计算月池运动,结果与半解析解中振型曲线拟合良好。