Nonlinear hydroelastic analysis of a moored floating body

By integration of the second-order fluid pressure over the instantaneous wetted surface, the generalized first- and second-order fluid forces used in nonlinear hydroelastic analysis are obtained. The expressions for coefficients of the generalized first- and second-order hydrodynamic forces in irregular waves are also given. The coefficients of the restoring forces of a mooring system acting on a flexible floating body are presented. The linear and nonlinear three-dimensional hydroelastic equations of motion of a moored floating body in frequency domain are established. These equations include the second-order forces, induced by the rigid body rotations of large amplitudes in high waves, the variation of the instantaneous wetted surface and the coupling of the first order wave potentials. The first-order and second-order principal coordinates of the hydrelastic vibration of a moored floating body are calculated. The frequency characteristics of the principal coordinates are discussed. The numerical results indicate that the rigid resonance and the coupling resonance of a moored floating body can occur in low frequency domain while the flexible resonance can occur in high frequency domain. The hydroelastic responses of a moored box-type barge are also given in this paper. The effects of the second-order forces on the modes are investigated in detail.     

动力定位船舶二阶低频慢漂力模型试验研究

对一艘动力定位船舶二阶低频慢漂力进行了模型试验,并将试验得到的纵向慢漂力谱、横向慢漂力谱与势流理论方法得到的理论值进行比较,结果表明,该模型试验方法与理论计算较为吻合。可为动力定位系统的设计和应用提供参考。     

Experimental studies on the slowly varying drift motion of a berthed container ship model

The effects of free-surface waves on the floating structures are of great importance in the offshore industry. Among the six degrees of motions of a surface ship the absence of restoring forces in surge, sway and yaw led to critical situations for moored ships in the recent times. The order of forces in horizontal plane and their exciting frequencies are matters of interest. The resonance with the presence of moored chains led to many accidents in the recent past. The lines in dry conditions may not give good damping and in wet condition they may trigger the system to chaotic motions and jumps. Two different loading conditions of a container ship model are tested with waves in laboratory conditions in two different drafts. The mooring lines are chosen as per scale law and the energy under the response spectrum is determined from the plots. The results give new insights into the movement of a berthed ships subjected to waves. Response of the moored ship to different loading conditions in different water depths are discussed in this paper. The paper gives the order of energy due to first-order and slowly varying movement of a berthed container model in a towing tank.     

海洋导管架平台随机响应混合分析方法

采用将结构的解析分析和数值求解方法相结合的固定式海洋平台随机响应混合求解方法，考虑波浪－结构相互作用和基于最小二乘法原理的非线性曳力的影响。采取两个步骤，第一步进行与结构有关的特征值分析；第二基于谱方程进行响应估计，进行海洋平台的线性和非线性数值分析。     

一种新型Semi-Spar式海上风机平台系泊系统优化分析

参考英国的Kincardine风机采用的新式的Semi-Spar概念,结合spar式基础和半潜式基础的特点,提出了一种新式海上浮式风机平台模型,并基于三维势流理论,利用AQWA软件进行水动力计算,验证新式平台可靠性。分析了在风、浪、流荷载联合作用下,锚链竖向夹角、系缆数量对风机浮式平台运动性能和系泊张力的影响,对系泊系统进行优化,并验证极端工况下的可靠性。结果证明风机平台水平运动和纵摇运动幅值较小,但垂荡幅值略大,而通过减小锚链竖向夹角可以控制平台运动响应幅值,增加系缆数量可以同时减小系泊张力大小。计算结果证明了新型Semi-Spar式海上风机平台可行性,为浮式风机平台及系泊系统的设计提供参考。     

极限海况下Spar平台系泊系统耦合动力分析

在复杂的风浪流海洋环境中,Spar及其系泊系统的耦合作用受多方面的影响。对极限海况下的某Classic-Spar平台及其系泊缆索的动力响应做了时域非线性耦合研究。重点分析了极限海况下一根或者多根系泊缆索断裂时平台运动及其缆索张力动力响应的统计特性。分析结果表明,在风浪流同向且共线的极限海况下,不同位置处的系泊缆索发生断裂,对整个平台系泊系统安全的影响程度不尽相同,尤其以迎浪方向系泊缆索的断裂影响显著。     

Volterra series-based system analysis of random wave interaction with a horizontal cylinder

The response of a long flexible cylinder excited by random waves in a large model basin was investigated. The linear and non-linear physical mechanisms associated with the wave–cylinder interaction were analysed using system identification and modelling techniques. A third-order frequency domain Volterra model and its orthogonalized counterpart were used to analyse the relationships between wave elevations at various locations in the vicinity of the cylinder and cylinder acceleration data at various cylinder longitudinal locations. It was found that linear mechanisms dominate, particularly at the frequency band where the majority of the wave energy is located. At higher frequencies, the cubic component of the Volterra model is the main contributor to the total model coherence, i.e. the fraction of the measured output power that can be approximated by the model output, whereas the quadratic component's contribution to the total model coherence was in general quite small. This process of identification and quantification of the non-linear mechanisms of the unknown physical system can lead to the design of improved parametric models for the cylinder response, which should by design simulate non-linearities such as the ones identified by the Volterra model. The estimated linear and non-linear Volterra transfer functions were also used to predict the cylinder acceleration under excitation inputs not used in the estimation of the model transfer functions. The good match between predicted and measured output auto-power spectra suggests that the estimated transfer functions are indeed true models of the underlying physical mechanisms of the interaction. However, the latter can only be achieved if a minimum number of data segments, as determined by an error analysis involving modelling and prediction errors, is used in the estimation of the Volterra transfer functions.     

浮基多体系统自激运动响应的时域分析方法

浮基多体系统上部机构作业时不仅存在多体系统内部各物体间的耦合作用 ,还存在浮基与流场间的耦合作用。本文将浮基在流场中引起的辐射势表示为浮基加速度与时域规范化速度势乘积的卷积形式 ,导出流体动压力及静水系泊恢复力与浮基运动的关系。使用多刚体力学的凯恩方法得到系统的动力学方程。最后给出了在时域中数值求解系统运动响应的具体步骤     

Dynamic modeling and vibration suppression for an offshore wind turbine with a tuned mass damper in floating platform

The worldwide demand for renewable energy is increasing rapidly. Wind energy appears as a good solution to copy with the energy shortage situation. In recent years, offshore wind energy has become an attractive option due to the increasing development of the multitudinous offshore wind turbines. Because of the unstable vibration for the barge-type offshore wind turbine in various maritime conditions, an ameliorative method incorporating a tuned mass damper (TMD) in offshore wind turbine platform is proposed to demonstrate the improvement of the structural dynamic performance in this investigation. The Lagrange's equations are applied to establish a limited degree-of-freedom (DOF) mathematical model for the barge-type offshore wind turbine. The objective function is defined as the suppression rate of the standard deviation for the tower top deflection due to the fact that the tower top deflection is essential to the tower bottom fatigue loads, then frequency tuning method and genetic algorithm (GA) are employed respectively to obtain the globally optimum TMD design parameters using this objective function. Numerical simulations based on FAST have been carried out in typical load cases in order to evaluate the effect of the passive control system. The need to prevent the platform mass increasing obviously has become apparent due to the installation of a heavy TMD in the barge-type platform. In this case, partial ballast is substituted for the equal mass of the tuned mass damper, and then the vibration mitigation is simulated in five typical load cases. The results show that the passive control can improve the dynamic responses of the barge-type wind turbine by placing a TMD in the floating platform. Through replacing partial ballast with a uniform mass of the tuned mass damper, a significant reduction of the dynamic response is also observed in simulation results for the barge-type floating structure.     

An extensive analysis of numerical ship self-propulsion prediction via a coupled BEM/RANS approach

The paper deals with the self-propulsion problem, i.e. the solution of the flow around the hull that advances at uniform speed due to the action of its own propeller. A coupled BEM/RANS approach, previously proposed for a simpler case with only rudder and propeller, has been extensively analysed to highlight the strength and the weakness of the method. The proposed analyses consider the influence of different turbulence modelling, the role of the interpolating algorithm for the inclusion of body forces into the RANS domain, a mesh and simulation time step sensitivity study and the influence of the extrapolation procedure for the definition of the effective wake to the propeller in the light of the lightest and the most affordable computational setup for daily accurate calculations. At first, the well-known Kriso Container Ship (KCS) test case is considered. This ship has been widely investigated in the context of different research projects and a large amount of data (both measurements and numerical calculations) is available to validate the solution approach and to highlight the benefits, as well as the weaknesses, of the proposed coupled BEM/RANS approach versus established but computationally demanding calculations based only on RANS simulations. Once the approach has been developed and validated via the KCS test case, calculations have been repeated in the case of completely different ships, in order to evaluate its general applicability and to test the robustness and the reliability of the proposed procedure.     

内孤立波中半潜平台动力响应特性

基于内孤立波mKdV理论,采用时域有限位移运动方程,结合改进的Morison公式,研究了两层流体中内孤立波与带分段式悬链系泊约束半潜平台的相互作用问题。针对东沙群岛南部海域附近某实测内孤立波特征参数,计算分析了在该内孤立波作用下SEDCO-700型半潜平台的内孤立波载荷、运动响应及其系泊张力的变化特性。研究表明,内孤立波不仅会对半潜平台产生突发性冲击载荷作用,使其产生大幅度水平漂移运动,并导致其系泊张力显著增大,因此在半潜平台等深海平台的设计与应用中,内孤立波的影响是不可忽视的。     

海上环形观光平台内域水体共振响应研究

海洋空间资源开发中,海上观光旅游平台具有较广的应用前景。浮式观光平台可设计为圆环形结构,环形内部区域可以为游艇停泊和亲水场所提供掩护。此类结构与船舶月池结构类似,有内域半封闭水体,也存在水体共振问题。为分析此类结构水体共振响应,评估内域水体掩护条件,对以圆环形为基础增加弦与所截圆弧围成区域平台模型(观光平台)的内域水体共振模态进行模拟。观光平台共有6个特征模态,分布在0.2～0.6 rad/s之间。模态峰值主要分布在中轴线附近位置。平台圆弧和直边界组合导致共振模态分布包括圆形水体环向模态特征和矩形水体模态特征;不同弦心距对应不同内域水体尺寸。随着内域水体尺寸减小,模态响应频率向高频移动;通过增加环形宽度和设置主迎浪面在突出平台位置,可降低波浪透射,从而减弱内域水体共振响应幅值。工程设计中可调整上述参数降低内域水体共振幅值,达到较好的泊稳条件。     

随机波浪中张力腿平台耦合运动及系泊系统特性研究

张力腿平台(TLP)是一种垂直系泊的半顺应半刚度式平台,预报平台的运动响应及锚泊系统的张力是张力腿平台结构设计的重要基础。应用挪威船级社SESAM软件在频域和时域内研究了张力腿平台在随机波浪中的非线性运动响应及系泊系统特性,并在试验室中通过缩尺比为1∶40的模型进行了试验验证。在试验验证的基础上,将仅考虑浪与考虑浪、流联合作用下的张力腿的张力平均值、幅值及标准差作了对比。结果表明,在较低海况时,考虑浪和流时的张力腿与不考虑流的情况变化不大;在较大海况时,流的影响不可忽略,考虑流的张力腿变化幅度要略大于不考虑流的情况。     

八角形FPSO串靠外输系统耦合动力响应分析

针对100 m作业水深的八角形FPSO,提出采用穿梭油轮串靠的外输方案,研究串靠外输在南海的适用性。建立由八角形FPSO及其系泊系统、穿梭油轮及FPSO与穿梭油轮之间的系泊大缆等组成的浮式多体动力学模型,根据多浮体动力学理论进行耦合时域模拟。在外输海况条件下,分析了串靠连接的环境适应性及研究大缆载荷的响应特性,对连接大缆的长度、刚度等关键参数进行了敏感性分析。研究表明,串靠外输的形式对于八角FPSO具有足够的安全性和可靠性,系泊大缆受到明显的冲击张力,张力的幅值受大缆的长度和刚度影响较大。     

Truss Spar平台桁架管节点疲劳分析

Truss Spar平台在风、浪、流的作用下,结构受力十分复杂,准确分析其疲劳强度相当困难。通过建立Truss Spar平台三维有限元模型,利用HydroD软件进行载荷预报、波浪搜索,得到产生最大应力幅响应的最不利浪向和相位角。运用Sestra软件进行整体有限元分析,得到的结果作为桁架管节点细化模型的边界条件,再用Nastran软件进行局部分析求得节点的热点应力,修正壁厚影响。最后根据DNV规范的S-N曲线计算了管节点的疲劳寿命。     

深海锚泊浮标的二阶动力分析

计算了规则波上深海锚泊浮标的运动响应和锚泊线的动力响应。在对浮标的二阶漂移力计算时考虑了锚系的影响,并将浮标平均漂移的计算结果与不考虑锚系影响的结果进行了比较。本文计算所用浮标为单点系泊浮标,锚链由不同重量的分段组成。     

两层流体中内波作用下Spar平台运动响应

研究两层流体中Spar平台在内波作用下的运动响应问题。在线性势流理论框架,提出在内波作用下Spar平台运动响应及分段式悬链线系泊张力特性的计算方法。数值分析两层流体内界面位置、入射内波的波长以及系泊索初始预张力对Spar平台运动响应及其系泊索张力特性的影响规律,结果表明内波对Spar平台纵摇运动响应的影响是小的,但对Spar平台纵荡与垂荡运动响应及其系泊索张力的影响是不可忽视的。因此,在Spar平台的设计中,考虑内波的影响是重要的。     

Stability and capsizing analysis of nonlinear ship rolling in wind and stochastic beam seas

Considering the actual seaway condition, stability and capsizing of nonlinear ship rolling system in stochastic beam seas is of significant importance for voyage safety. Safe zone are defined in the phase space plan of the unperturbed Hamilton system to qualitatively distinguish ship motions as capsize and noncapsize. Capsize events are defined by solutions passing out of the safe zone. The probability of such an occurrence is studied by virtue of the random Melnikov function and the concept of phase space flux. In this paper, besides conventional wave excitation, the effect of wind load is also taken into account. The introduction of wind load will lead to asymmetry, in other words, it transforms the symmetric heteroclinic orbits into asymmetric homoclinic orbits. For asymmetric dynamical system, the orbital analytic solutions and its power spectrum are not readily available, and the technique of discrete time Fourier transformation (DTFT) is used. In the end, as verification of theoretical critical significant wave height, capsizing probability contour diagram is generated by means of numerical simulation. The contour diagram shows that these analytical methods provide reliable and predictive results about the likelihood of a vessel capsizing in a given seaway condition.     

均匀流作用下三立柱轻型半潜式生产平台涡激运动特性数值研究

三立柱轻型半潜式生产平台是一种新型多功能高效平台,可广泛用于中等水深及深水边际油气田开发。但在一定的来流条件下,立柱后方会产生漩涡脱落,导致平台发生涡激运动现象,从而影响平台的海上作业。基于分离涡模拟(detached eddy simulation,简称DES)方法,对具有三立柱和深吃水特点的非传统型半潜式生产平台的涡激运动响应特性开展数值模拟研究,从运动响应幅值和频率、运动轨迹及水动力系数等多个角度分析其在不同流向角和不同折合速度下涡激运动响应的关键特征和锁定范围。研究表明,三立柱轻型半潜式生产平台的横向运动响应由其受到的脉动升力主导,在折合速度7.0≤Ur≤10.0区间内发生锁定现象,且在180°流向角时幅值达到最大,约为立柱直径的59%;首摇运动在0°流向角时达到最大响应幅值4.9°,且锁定区内的响应频率接近其首摇运动固有频率;其运动轨迹主要表现为沿着横向的直线往复运动。     

Mooring tension and motion characteristics of a submerged fish reef with net in waves and currents using numerical analysis

A numerical model was used to analyze the motion response and mooring tension of a submerged fish reef system. The system included a net attached to a rigid structure suspended up from the bottom with a single, high tension mooring by fixed flotation. The analysis was performed by using a Morison equation type finite element model configured with truss elements. Input forcing parameters into the model consisted of both regular and irregular waves, with and without a steady current. Heave, surge and pitch dynamic calculations of the reef structure were made. Tension response results of the attached mooring line were also computed. Results were analyzed in both the time and frequency domain in which appropriate, linear transfer functions were calculated. The influence of the current was more evident in the tension and heave motion response data. This is most likely the result of the large buoyancy characteristics of the reef structure and the length of the mooring cable. Maximum mooring component tension was found to be 13.9 kN and occurred when the reef was subjected to irregular waves with a co-linear current of 1.0 m/s velocity. The results also showed that the system had little damping (in heave) with damped natural periods of 2.8 s. This combination of system characteristics promotes a possible resonating situation in typical open sea conditions with similar wave periods.