Overview: Research on hydroelastic responses of VLFS in complex environments

In the past several decades, extensive investigations have been carried out on the hydroelastic responses of Very Large Floating Structures (VLFS) in the deep open sea by researchers around the world. However, when tackling the hydroelastic problems of VLFS used as the protection and exploitation of marine resources near offshore and islands, the effects of complex environments, such as the seabed profile and inhomogeneous wave conditions should be taken into account. Based on the development history of VLFS and the process of classical hydroelasticity of VLFS, the research results of hydroelastic responses of VLFS in complex environments have been systematically summarized in this paper, so as to provide a basis for the further research.     

碰撞事故时超大型浮体模块连接处荷载特性研究

精确评估模块连接处荷载特性对超大型浮体的水动力设计及结构安全性研究具有重要的工程意义。文章以5模块半潜式超大型浮体为研究对象,考虑碰撞荷载和风、浪、流环境荷载的联合作用并采用AQWA软件对超大型浮体模块连接点处的受力特性开展了数值计算。船舶碰撞荷载简化为矩形脉冲,采用定常风、定常流以及JONSWAP波谱,探讨了船舶碰撞角度、撞击位置、环境荷载等因素对模块连接点受力特性的影响。该文的研究成果可为事故荷载作用时超大型浮体连接器结构设计和强度计算提供一定依据。     

Inhomogeneous wave load effects on a long,straight and side-anchored floating pontoon bridge

In this paper, we present a numerical study on the hydroelastic response of a 4.6 km long fjord crossing floating bridge subjected to wave loads. The bridge is straight in design and supported by 35 pontoons along its full length. To limit the response to horizontal loads, four clusters of deep water mooring lines are engaged to increase the transverse stiffness of the bridge. Owing to the very large span across the fjord, inhomogeneity in the wave field exists. This study examines the various effects of inhomogeneous wave loads on the dynamic responses of the floating bridge. These include the spatial variations of the wave direction, significant wave height and peak period as well as the coherence and correlation of waves along the entire length of the floating bridge. For the purpose of comparison, the dynamic bridge responses under homogeneous wave load cases are also studied. In addition, the effects of wave load components and short-crestedness are presented and discussed.     

多模块MOB连接器动力特性研究

以7模块MOB为研究对象,利用RMFC模型,基于势流理论对7个构成模块单元进行3D水动力性能分析,计算过程中考虑了由于遮蔽效应而导致的浮体间相互作用.编制基于频域的多浮体运动和柔性连接器载荷计算程序,计算在规则波和不规则波下的连接器载荷值,并比较不同连接器刚度下各柔性连接器载荷值.计算结果表明,波浪入射角、波频、连接器刚度和海况对连接器载荷响应具有显著影响.本文的计算结果可为MOB柔性连接器设计与模块耐波性设计提供参考.     

Simplified estimation for the dynamic behavior and reliability analysis of very large floating structures under wave-induced loads

This paper deals with the dynamic response and strength of very large floating structures (VLFS) in regular and irregular waves, considering the propagation of the hydroelastic deflection wave of the structure. First, a simplified estimation method is presented for the dynamic response and strength of the structure in regular waves. Then, the validity of the method is demonstrated by comparing its results with analytical results and experimental results for a mat-type floating structure model. Next, a simplified estimation method for dynamic responses under long crested irregular wave conditions is presented by using the above results and by combining them with irregular sea wave spectra. Finally, the applicability of the method is investigated through numerical examples carried out for a 4,800-m class VLFS under trial design. Characteristics of the hydroelastic waves, short-term responses, and reliability levels are numerically identified. Received for publication on April 14, 1999; accepted on Sept. 10, 1999     

近岛礁超大型浮体连接器载荷研究

对于布置在岛礁附近的浮体,复杂海底地形的作用不可忽略.文章基于RMFC模型,同时将地形作为固定物体,研究了复杂海底地形对布置于岛礁附近的超大型浮体的运功响应和连接器载荷的影响.数值计算结果表明,地形的存在对浮体绕射力存在显著的影响,同时增加了低频区域的连接器载荷.     

预报超大型浮式结构物水弹性响应方法的比较

超大型海洋浮式结构物(VLFS)在利用海洋空间和开发海洋资源方面有着很大的实用价值.但如何精确有效地计算和预报其在波浪中的水弹性响应是设计过程中的一个非常重要的技术问题.目前,很多学者提出了不同的简化方法来预报VLFS的水弹性响应.文章全面地介绍了关于计算VLFS水弹性响应的解析方法和数值方法的研究现状,并将相应的数值结果与已有的试验数据相比较,进行了分析讨论.     

超大型浮体模块水弹性响应和结构强度分析

水弹性方法针对超大型浮体的刚度特点,充分考虑了结构变形与流体运动的相互作用,是进行结构安全性分析的有效手段。文章采用水弹性分析方法研究了超大型浮体单模块总体波浪载荷以及结构应力响应。首先基于三维有限元方法分析了模块在真空中的总振动模态,然后结合模态叠加法和边界元法计算了模块在流场里面的谐振和模态响应。在此基础上,研究了各模态下结构的应力响应以及总应力响应,并分析了危险载荷工况,评估了超大型浮体单模块的结构强度,研究结果对超大型浮体单模块结构优化设计和安全性评估具有一定的指导意义。     

Numerical modelling of hydrodynamic responses of Ocean Farm 1 in waves and current and validation against model test measurements

With the continuous growing of the aquaculture industry and increasingly limited fish farming sites at close to shore areas both in Norway and worldwide, there is a need to develop fish farms suitable for aquaculture production in typical offshore environments. For this purpose, SALMAR has developed and deployed the Ocean Farm 1 facility for offshore fish farming.The main purpose of this paper is to develop a reliable numerical model and investigate the motion responses of the Ocean Farm 1 structure in waves and current. The established numerical model consists of the Ocean Farm 1's frame structure (with rigidly-connected circular column components), the net and the mooring system. The hydrodynamic external loads and coefficients of the frame structure are obtained by using potential flow theory. The quadratic drag load on the individual circular columns of the frame structure is formulated by a given drag coefficient. The loads on the net are formulated by using the screen model, where the Reynold number dependent lift and drag forces are formulated as a function of the solidity ratio Sn of the net, relative inflow angle and velocity. The hydrodynamic loads on the mooring lines are formulated using the Morison's equation and the structural responses of the mooring lines are obtained using a nonlinear FE model.With the developed numerical model, time domain simulations are performed. The simulation results are firstly validated against measured data from the decay tests, current tests, and regular wave tests. After the validation, numerical simulations are performed in different irregular wave and current combined weather conditions and the obtained motion response of Ocean Farm 1 are discussed and compared with available measurement data.     

On the hydroelastic responses of a very large floating structure in waves

New numerical methods are presented for hydroelastic analyses of a very large floating structure (VLFS) of several kilometers length and width. Several methods are presented that accelerate computation without an appreciable loss of accuracy. The accuracy and efficiency of the proposed methods are validated through comparisons with other numerical results as well as with existing experimental results. After confirming the effectiveness of the methods presented, various characteristics of the hydroelastic behavior of VLFSs are examined, using the proposed methods as numerical tools. Received for publication on Dec. 3, 1999; accepted on Dec. 15, 1999     

支撑条件对浮动式厚弹性板水弹性行为的影响分析（英文）

The hydroelastic response of very large floating structures(VLFS) under the action of ocean waves is analysed considering the small amplitude wave theory. The very large floating structure is modelled as a floating thick elastic plate based on TimoshenkoMindlin plate theory, and the analysis for the hydroelastic response is performed considering different edge boundary conditions.The numerical study is performed to analyse the wave reflection and transmission characteristics of the floating plate under the influence of different support conditions using eigenfunction expansion method along with the orthogonal mode-coupling relation in the case of finite water depth. Further, the analysis is extended for shallow water depth, and the continuity of energy and mass flux is applied along the edges of the plate to obtain the solution for the problem. The hydroelastic behaviour in terms of reflection and transmission coefficients, plate deflection, strain, bending moment and shear force of the floating thick elastic plate with support conditions is analysed and compared for finite and shallow water depth. The study reveals an interesting aspect in the analysis of thick floating elastic plate with support condition due to the presence of the rotary inertia and transverse shear deformation. The present study will be helpful for the design and analysis of the VLFS in the case of finite and shallow water depth.     

Hydroelastic analysis of oblique irregular waves with a pontoon-type VLFS edged with dual inclined perforated plates

The fluid-structure interaction of oblique irregular waves with a pontoon-type very large floating structure (VLFS) edged with dual horizontal/inclined perforated plates has been investigated in the context of the direct time domain modal expansion theory. For the hydroelastic analysis, the boundary element method (BEM) based on time domain Kelvin sources is implemented to establish water wave model including the viscous effect of the perforated plates through the Darcy’s law, and the finite element method (FEM) is adopted for solving the deflections of the VLFS modeled as an equivalent Mindlin thick plate. In order to enhance the computing efficiency, the interpolation-tabulation scheme is applied to assess rapidly and accurately the free-surface Green function and its partial derivatives in finite water depth, and the boundary integral equation of a half or quarter VLFS model is further established taking advantage of symmetry of flow field and structure. Also, the numerical solutions are validated against a series of experimental tests. In the comparison, the empirical relationship between the actual porosity and porous parameter is successfully applied. Numerical solutions and model tests are executed to determine the hydroelastic response characteristics of VLFS with an attached anti-motion device. This study examines the effects of porosity, submerged depth, inclined angle and gap distance of such dual perforated anti-motion plates on the hydroelastic response to provide information regarding the optimal design. The effects of oblique wave angle on the performance of anti-motion and hydroelastic behavior of VLFS are also emphatically examined.     

Required test durations for converged short-term wave and impact extreme value statistics — Part 1: Ferry dataset

For the design of maritime structures in waves, the extreme values of responses such as motions and wave impact loads are required. Waves and wave-induced responses are stochastic, so such responses should always be related to a probability. This information is not easy to obtain for strongly non-linear responses such as wave impact forces. Usually class rules or direct assessment via experiments or numerical simulations are applied to obtain extreme values for design. This brings up questions related to the convergence of extreme values: how long do we need to test in order to obtain converged statistics for the target duration? Or, vice versa: given testing data, what is the uncertainty of the associated statistics? Often the test or simulation duration is cut up in ‘seeds’ or ‘realisations’, with an exposure duration of one or three hours based on the typical duration of a steady environmental condition at sea, or the time that a ship sails a single course. The required number of seeds for converged results depends on the type of structure and response, the exposure duration, and the desired probability level. The present study provides guidelines for the convergence of most probable maximum (MPM) wave crest heights and MPM green water wave impact forces on a ferry. Long duration experiments were done to gain insight into the required number of seeds, and the effect of fitting. The present paper presents part 1 of this study; part 2 [1] presents similar results for wave-in-deck loads on a stationary deck box.     

Analysis of the slowly varying drift force on a very large floating structure in multidirectional random seas

In designing the mooring system of a very large floating structure (VLFS), it is essential to estimate the slowly varying drift force in random seas. For a small vessel, Hsu's method or Newman's approximation may be used to simulate this slowly varying drift force. However, based on experiments and/or field observations, it was found that the slowly varying drift force acting on a VLFS could be reduced to a great extent from the simulated values based on those methods. Thus, the conventional methods are not applicable for a VLFS. This discovery led to the development of several methods for estimating the slowly varying drift force on a VLFS, e.g., Namba et al. (J Soc Nav Archit Jpn 186:235–242, 1999), and Shimada and Maruyama (J Soc Nav Archit Jpn 190:347–351, 2001). However, Namba's method is only applicable to a pontoon-type VLFS with a shallow draft, and Shimada's method is too simplified to account for the general shape of a VLFS and elastic deformation. These methods have been expanded in this article, and by our proposed method, any shape of VLFS and the effect of elastic deformation of the VLFS can be included. Formulations and several numerical examples are given.     

浮基多体系统展开动力响应的数值模拟（英文）

To simulate the dynamic responses of the multibody system with a floating base when the upper parts spread with a certain sequence and relative speed, the homogeneous matrix method is employed to model and simulate a four-body system with a floating base and the motions are analyzed when the upper parts are spread sequentially or synchronously. The rolling, swaying and heaving temporal variations are obtained when the multibody system is under the conditions of the static water along with the wave loads and the mean wind loads or the single pulse wind loads, respectively. The moment variations of each joint under the single pulse wind load are also gained. The numerical results showed that the swaying of the floating base is almost not influenced by the spreading time or form when the upper parts spread sequentially or synchronously, while the rolling and the heaving mainly depend on the spreading time and forms. The swaying and heaving motions are influenced significantly by the mean wind loads. The single pulse wind load also has influences on the dynamic responses. The torque of joint 3 and joint 4 in the single pulse wind environment may be twice that in the windless environment when the system spreads with 60 s duration.     

Numerical simulation of the spreading dynamic responses of the multibody system with a floating base

To simulate the dynamic responses of the multibody system with a floating base when the upper parts spread with a certain sequence and relative speed, the homogeneous matrix method is employed to model and simulate a four-body system with a floating base and the motions are analyzed when the upper parts are spread sequentially or synchronously. The rolling, swaying and heaving temporal variations are obtained when the multibody system is under the conditions of the static water along with the wave loads and the mean wind loads or the single pulse wind loads, respectively. The moment variations of each joint under the single pulse wind load are also gained. The numerical results showed that the swaying of the floating base is almost not influenced by the spreading time or form when the upper parts spread sequentially or synchronously, while the rolling and the heaving mainly depend on the spreading time and forms. The swaying and heaving motions are influenced significantly by the mean wind loads. The single pulse wind load also has influences on the dynamic responses. The torque of joint 3 and joint 4 in the single pulse wind environment may be twice that in the windless environment when the system spreads with 60 s duration.     

船舶碰撞载荷对移动海上基地连接器载荷的影响

移动海上基地属于超大型浮式结构物,在营运过程中会遇到飞机坠落,船舶碰撞等事故情况。事故载荷不仅会对超大型浮式结构物的局部结构产生破坏作用,而且会对超大型浮式结构物的薄弱环节——连接器产生附加载荷。以船舶碰撞为例,采用简化的RMFC模型,然后通过SESAM软件计算水动力系数,将其代入多刚体运动微分方程后用数值方法求解,研究特殊载荷对连接器载荷的影响。考虑不同连接器刚度以及不同撞击部位工况,估算碰撞引起的连接器载荷的数量级,并与波浪载荷引起的连接器载荷进行了比较。这为今后移动海上基地连接器设计提供了指导和依据。     

Analysis on the applicability of VLFS for China

With the advantages on rapid construction,no limitation of extent,less influence on environment, the VLFS has been turned into the focus of coastal and offshore engineering studying.As the platform,VLFS was built as airport,huge oil house etc,and would play the role of ocean economics,politics and military.The restrictive condition of VLFS is strong wave by monsoon,seismic,serious bomb etc.The floating breakwater should be put forward to shelter VLFS.The analysis of wave condition including typhoon route,archipelago and water depth is performed in this paper.The advantage and disadvantage are compared between VLFS and the marine structure from land.     

Comparing different contour methods with response-based methods for extreme ship response analysis

Environmental contours are often applied in probabilistic structural reliability analysis to identify extreme environmental conditions that may give rise to extreme loads and responses. They facilitate approximate long term analysis of critical structural responses in situations where computationally heavy and time-consuming response calculations makes full long-term analysis infeasible. The environmental contour method identifies extreme environmental conditions that are expected to give rise to extreme structural response of marine structures. The extreme responses can then be estimated by performing response calculations for environmental conditions along the contours.Response-based analysis is an alternative, where extreme value analysis is performed on the actual response rather than on the environmental conditions. For complex structures, this is often not practical due to computationally heavy response calculations. However, by establishing statistical emulators of the response, using machine learning techniques, one may obtain long time-series of the structural response and use this to estimate extreme responses.In this paper, various contour methods will be compared to response-based estimation of extreme vertical bending moment for a tanker. A response emulator based on Gaussian processes regression with adaptive sampling has been established based on response calculations from a hydrodynamic model. Long time-series of sea-state parameters such as significant wave height and wave period are used to construct N-year environmental contours and the extreme N-year response is estimated from numerical calculations for identified sea states. At the same time, the response emulator is applied on the time series to provide long time-series of structural response, in this case vertical bending moment of a tanker. Extreme value analysis is then performed directly on the responses to estimate the N-year extreme response. The results from either method will then be compared, and it is possible to evaluate the accuracy of the environmental contour method in estimating the response. Moreover, different contour methods will be compared.     

Transient responses of a VLFS during landing and take-off of an airplane

The transient elastic deformation of a pontoon-type very large floating structure (VLFS) caused by the landing and take-off of an airplane is computed by the time-domain mode-expansion method. The memory effects in hydrodynamic forces are taken into account, and great care is paid to numerical accuracy in evaluating all the coefficients appearing in the simultaneous differential equations for the elastic motion of a VLFS. The time-histories of the imparted force and the position and velocity of an airplane during landing and take-off are modeled with data from a Boeing 747-400 jumbo jet. Simulation results are shown of 3-D structural waves on a VLFS and the associated unsteady drag force on an airplane, which is of engineering importance, particularly during take-off. The results for landing show that the airplane moves faster than the structural waves generated in the early stage, and the waves overtake the airplane as its speed decreases to zero. The results for take-off are essentially the same as those for landing, except that the structural waves develop slowly in the early stage, and no obstacle exists on the runway after the take-off of airplane. The additional drag force on an airplane due to the elastic responses of the runway considered in this work was found to be small in magnitude.