Time Domain Simulation of Transient Responses of Very Large Floating Structures Under Unsteady External Loads

A time domain finite element method （FEM） for the analysis of transient elastic response of a very large floating structure （VLFS） subjected to arbitrary time-dependent external loads is presented. This method is developed directly in time domain and the hydrodynamic problem is formulated based on linear, inviscid and slightly compressible fluid theory and the structural response is analyzed on the thin plate assumption. The time domain finite element procedure herein is validated by comparing numerical results with available experimental data. Finally, the transient elastic response of a pontoon-type VLFS under the landing of an airplane is computed by the proposed time domain FEM. The time histories of the applied force and the position and velocity of an airplane during landing are modeled with data from a Boeing 747-400 jumbo jet.

Hydroelastic Analysis of Very Large Floating Structures Using Plate Green Functions

Great attention has been paid to the development of very large floating structures. Owing to their extreme large size and great flexibility, the coupling between the structural deformation and fluid motion is significant. This is a typical problem of hydroelasticity. Efficient and accurate estimation of the hydroelastic response of very large floating structures in waves is very important for design. In this paper, the plate Green function and fluid Green function are combined to analyze the hydroelastic response of very large floating structures. The plate Green function here is a new one proposed by the authors and it satisfies all boundary conditions for free-free rectangular plates on elastic foundations. The results are compared with some experimental data. It is shown that the method proposed in this paper is efficient and accurate. Finally, various factors affecting the hydroelastic response of very large floating structures are also studied.

Time Domain Calculation of Connector Loads of a Very Large Floating Structure

在飞机坠毁，轮船碰撞，和另外的事故以后产生的负担可以破坏很大的漂浮的结构(VLFS ) 并且创造另外的接头负担。在这研究，负担被考虑建立的轮船碰撞和波浪的联合效果打手势为多身体 VLFS 的微分方程。一个时间领域计算方法被建议在波浪计算 VLFS 的接头负担。LonguetHiggins 模特儿被雇用模仿随机的波浪负担。液体力量和水动力学系数用 DNV Sesam 软件被获得。运动当频率域水动力学系数被变换成运动的存储器函数时，微分方程被使用时间域方法计算时间域的微分方程。由于波浪和影响负担的联合行动，高周波的摆动在接头负担的时间历史曲线被观察。在 0 的波浪方向？

Reliability-Based Optimal Design for Very Large Floating Structure

Costs and losses induced by possible future extreme environmental conditions and difficulties in repairing post yielding damage strongly suggest the need for proper consideration in design rather than just life loss prevention. This can be addressed through the development of design methodology that balances the initial cost of the very large floating structure (VLFS) against the expected potential losses resulting from future extreme wave induced structural damage. Here, the development of a methodology for determining optimal, cost effective design will be presented and applied to a VLFS located in the Tokyo bay. Optimal design criteria are determined based on the total expected life cycle cost and acceptable damage probability and curvature of the structure, and a set of sizes of the structure are obtained. The methodology and applications require expressions of the initial cost and the expected life cycle damage cost as functions of the optimal design variables. This study includes the methodology, total life cycle cost function, structural damage modeling, and reliability analysis.

A method to Estimate Dynamic Responses of VLFS Based on Multi-Floating-Module Model Connected by Elastic Hinges

Based on the elastic foundation beam theory and the multi-floating-module hydrodynamic theory,a novel method is proposed to estimate the dynamic responses of VLFS(Very Large Floating Structure).In still water,a VLFS can be simplified as an elastic foundation beam model or a multi-floating-module model connected by elastic hinges.According to equivalent displacement of the two models in static analysis,the problem of rotation stiffness of elastic hinges can be solved.Then,based on the potential flow theory,the dynamic responding analysis of multi-floatingmodule model under wave loads can be computed in ANSYS-AQWA software.By assembling the time domain analysis results of each module,the dynamic responses of the VLFS can be obtained.Validation of the method is conducted through a series of comparison calculations,which mainly includes a continuous structure and a three-part structure connected by hinges in regular waves.The results of this paper method show a satisfactory agreement with the experiment and calculation data given in relative references.

台风浪下海上机场VLFS动态响应与能量转换机理

超大浮式结构(very large floating structure,简称VLFS)是集空港和海港为一体的大型海上多功能浮式结构,现有研究大多关注规则波下刚性模块柔性连接模型的动力响应,忽略了台风浪极端环境下海上机场自身柔性引起的非线性振动特性。针对此问题,提出了一种新型多柔-刚性混合模块建模方法,采用Jonswap谱特征参数对台风“鲇鱼”过境实测海浪谱开展了精细化仿真模拟,分析了台风浪下海上机场VLFS整体和局部非线性动态响应特性,揭示了海上机场与环境荷载之间的能量转换机理。结果表明:海上机场多柔-刚性混合模块模型可以较好地反映此类VLFS结构动力响应特性;海上机场超长柔性及台风浪场不均匀性使其结构呈现显著非线性,位移、转角和水弹性变形分别以沿波向、绕展向和沿垂向为主,极值应力主要分布于撑杆附近;环境荷载能量和结构重力势能在初始阶段主要转换为系泊势能,稳定阶段则主要转换为结构动能和弹性势能。

A Novel Conceptual Telescopic Positioning Pile for VLFS Deployed in Shallow Water:Structure Design

A conceptual design of using novel telescopic piles to position a multi-modular very large floating structure(VLFS),which is supposed to be severed as a movable floating airport,is proposed.The telescopic piles can automatically plug in the soil to resist the environmental loads and pull out from the soil to evacuate or move on to the next operational sea.The feasibility demonstration of the conceptual design includes two parts:function verification and structure design.In the latter part of the conceptual design,a time-domain structural analysis is firstly conducted by using Abaqus software.The simulation results suggest that the preliminary structure scheme is not optimum due to the insufficient structure utilization,although both structure safety of the piles and positioning accuracy are guaranteed.To realize a cost reduction of construction and installation,a Genetic Algorithm-Finite Element Analysis(GA-FEA)method is employed to perform structural optimization.After optimization,31 percent of the weight of each pile is reduced and higher structure utilization is maintained.The difference of the self-weight and allowable buoyancy of a single module(SMOD)of a semisubmersible-type VLFS is much larger than the weight of the piles.Combined with the function verification in our previous work,the conceptual design of using the novel telescopic pile to position VLFS is demonstrated to be feasible.

兼具波能转换与减振作用的OWSC-VLFS集成系统的水动力性能研究

摆式波能装置与超大型浮体集成系统在间隙流体共振作用下其浮式弹性板的水弹性响应和波能装置的水动力性能属于非线性范畴,尤其当非线性多体相互作用时,这种非线性行为更加复杂,因此采用完全非线性分析技术是唯一可获得浮板精确水弹性响应以及波能装置俘获效率的方法。本文利用计算流体软件耦合有限元方法建立模拟摆式波能装置与超大型浮体相互作用的完全非线性三维时域数值水槽模型;将弹性板模型加入到流固耦合交界面,对流体结构相互作用进行建模并对超大型浮体进行离散,再利用数据交互实现耦合计算;采用有限体积法对Navier-Stokes控制方程进行空间离散,使用流体体积法追踪流体与空气两相之间的自由界面变化。结果表明:浮板中线各点处的垂向位移随摆板水下深度的增加而减小,而摆板俘获效率随其水下深度的增加而增大;波幅的增加会加剧波浪的非线性作用,当波幅增加时,集成系统中浮板的水弹性响应明显减小,摆板的俘获效率逐渐降低。

台风-浪-流耦合作用超大浮体水弹性响应分析方法

海上机场超大浮体结构在台风等极端海洋环境下的动态响应准确预测是保障其安全性能的关键前提,其难点在于准确构建中/小尺度复杂流场与浮体结构的流固耦合计算模型。本文提出了一种基于中/小尺度嵌套的海上机场超大浮体水弹性响应分析方法,二次开发实现了台风-浪-流耦合模拟,分析了超强台风“莫兰蒂”途经海上机场超大浮体全过程台风-浪-流时空演变规律;采用中/小尺度流场嵌套技术计算水动力和气动力,代入结构动力学方程进行双向迭代解耦求解;通过算例分析了台风-浪-流耦合作用下浮体结构水弹性响应及影响机制。研究表明:本文提出的方法能够有效地预测此类极端环境下超大浮体结构水弹性响应。

融合有限元理论的DMB方法及其在复杂超大型浮体上的应用

[目的]将有限元理论融入离散模块梁单元(DMB)方法,优化集中质量刚度矩阵的求解方式,以使DMB方法更易于处理复杂超大型浮体(VLFS)问题。[方法]使用三维势流理论进行水动力分析并给出水弹性方程;引入有限元理论,将每个子模块离散为若干微小梁单元以求解刚度阵,并基于子结构法和矩阵理论推导集中质量刚度矩阵;处理复杂边界条件时,根据边界约束模式,采用划零置一法或是直接在刚度阵中增加约束项进行处理;处理复杂连接条件时,首先更改节点编号,然后再根据连接件的特点建立连接件约束矩阵并将其融入总刚度阵中。[结果]使用DMB方法计算复杂超大型浮体的位移响应时,在边界条件选取固定端和弹簧−阻尼端、连接件选取铰接、刚性连接和弹簧阻尼连接等情况下,所得结果与直接法吻合较好。[结论]研究表明,融合有限元理论的DMB方法能够方便、快速、准确地处理复杂超大型浮体在多种复杂工况下的水弹性响应。

基于离散模块梁单元理论的超大型浮体在非稳态载荷下的水弹性响应

使用离散模块梁单元(Discrete-Module-Beam,DMB)理论计算超大型浮体在非稳态载荷下的水弹性,包括位移响应和结构力响应。使用三维势流理论进行水动力分析,并基于Cummins方程将DMB方法扩展至时域;使用子梁法进行位移响应还原。在求解结构力响应时,结构被视作无支持、两端自由的无质量梁,将其所受外载荷按沿结构纵向分布是否已知划分为2类。对于已知分布的外载荷,直接对其进行积分即可获得结构力分布;对于未知分布的外载荷,使用改进后的三次曲线插值法求解结构力分布。计算定点货物移动和飞机降落工况下的水弹性响应。采用DMB方法可准确计算结构位移响应,也可给出非稳态加载过程中初始阶段的弯矩分布,并体现由集中力作用引起的弯矩突变。

大尺度异形镂空浮体疲劳强度分析方法研究

海洋环境复杂多变,海上结构物需长期承受随机波浪引起的交变载荷,极易产生疲劳破坏。本文以某海上大尺度异型镂空浮体为研究对象,通过计算分析找出了异形浮体结构容易产生疲劳破坏的位置。为使结构疲劳谱分析计算结果更加准确,本文提出一种数值规则波模拟法,用来计算浮体结构在工作时应力的传递函数。在此基础上,分别采用基于热点应力与名义应力的谱分析方法计算了浮体的疲劳寿命。结果显示,基于热点应力法得到的结构疲劳寿命略低于基于名义应力法得到的疲劳分析结果,更偏保守。以上研究结果可为大型海上浮式结构的设计提供参考。

大尺度异形镂空浮体疲劳强度分析方法研究

海洋环境复杂多变,海上结构物需要长期承受随机波浪引起的交变载荷,极易产生疲劳破坏。本文以某海上大尺度异型镂空浮体为研究对象,通过计算分析找出异形浮体结构容易产生疲劳破坏的位置。为使结构疲劳谱分析计算结果更加准确,本文提出一种数值规则波模拟法,用来计算浮体结构在工作时应力的传递函数。在此基础上,分别采用基于热点应力与名义应力的谱分析方法计算浮体的疲劳寿命。结果显示,基于热点应力法得到的结构疲劳寿命略低于基于名义应力法得到的疲劳分析结果,更偏保守。研究结果可以为大型海上浮式结构的设计提供参考。

海上大型结构物的装卸与安装技术研究

海上大型结构物的装卸与安装技术是海洋工程中的重要环节,然而,海上工程环境复杂、作业难度大以及安全性要求高,现有的装卸安装技术但仍存在诸多问题和挑战。针对这一缺陷,研究引入了一种新的智能测控系统进行参数控制,以提升装卸过程的安全性和效率。实验结果表明:传统测控系统在驳船浮态控制中横纵倾角产生的平均压载舱水位分别为1.34和0.92,驳船高度差平均值为1.19。而智能测控系统下驳船横纵倾角产生的平均压载舱水位降至1.08和0.81,驳船高度差平均值降至0.98。研究设计的智能测控系统明显改善了驳船的浮态稳定性,提高了驳船在装卸过程中的稳定性,对提高海上大型结构物装卸过程的安全性和效率具有实际意义。

Influence of Gaps Between Multiple Floating Bodies on Wave Forces

The present study aims to give general hints about hydrodynamic interactions for water wave diffraction on a super large floating structure composed of a large number of box-shaped modules with many small gaps in between. And meanwhile, it also aims to seek for an effective way to take the gap influence into consideration without numerical difficulties existing in conventional methods. An asymptotic matching technique is exploited by virtue of the smallness of gaps. Formal potential solutions are established for the near field around the gap ends and the far field away from gap ends, respectively, and the unknowns in those solutions are uniquely determined by asymptotic matching. The eigen-function expansion method is used for the outer far field and a series of pulsating sources at each gap end is introduced to simulate the gap influence. Strong hydrodynamic interaction is observed and a new resonant phenomenon, the mechanism of which differs absolutely from any known ones, is revealed in the present study. Sharp peak responses for both vertical and horizontal wave-exciting forces on each block are found around some special resonance frequencies, which depend on the draft of the structure and the gap width. The present results are of practical significance to the design of links (connectors) of modules for super large floating structures. And the importance is also closely related to the hydro-elasticity analysis for super large floating structures, in which local loads may be as important as the integrated loads.

An Improved Simplified Method for Predicting the Hydroelastic Response of Mat-Like VLFS

Very Large Floating Structures (VLFS) have received considerable attention recently. Efficient and accurate estimation of their hydroelastic responses in waves is very important for the design. The most efficient approach would obviously be the analytical one, Within the category of analytical approaches, the simplified method proposed by Ohkusu and his colleague are of special characteristics. However, when one studies their methods, several questions arise. The purpose of this paper is to critically study the simplified methods proposed by Ohkusu and his colleague in order to answer these questions. Some problems in their original methods have been found and possible improvements are suggested. It is concluded that the improved simplified method using the same idea of Ohkusu and his colleague could provide a reasonable estimate of the hydroelastic response of mat-like VLFS in a certain range of incident angles of waves.

Comparison of Linear Level I Green-Naghdi Theory with Linear Wave Theory for Prediction of Hydroelastic Responses of VLFS

Very Large Floating Structures (VLFS) have drawn considerable attention recently due to their potential significance in the exploitation of ocean resources and in the utilization of ocean space. Efficient and accurate estimation of their hydroelastic responses to waves is very important for the design. Recently, an efficient numerical algorithm was developed by Ertekin and Kim (1999). However, in their analysis, the linear Level I Green-Naghdi (GN) theory is employed to describe fluid dynamics instead of the conventional linear wave (LW) theory of finite water depth. They claimed that this linear level I GN theory provided better predictions of the hydroelastic responses of VLFS than the linear wave theory. In this paper, a detailed derivation is given in the conventional linear wave theory framework with the same quantity as used in the linear level I GN theory framework. This allows a critical comparison between the linear wave theory and the linear level I GN theory. It is found that the linear level I GN theory can be regarded as an approximation to the linear wave theory of finite water depth. The consequences of the differences between these two theories in the predicted hydroelastic responses are studied quantitatively. And it is found that the linear level I GN theory is not superior to the linear wave theory. Finally, various factors affecting the hydroelastic response of VLFS are studied with the implemented algorithm.

Transient Hydroelastic Response of VLFS by FEM with Impedance Boundary Conditions in Time Domain

A time-dependent finite element method (FEM) is developed to analyze the transient hydroelastic responses of very large floating structures (VLFS) subjected to dynamic loads. The hydrodynamic problem is formulated based on the linear theory of fluid and the structural response is analyzed based on the thin plate theory. The FEM truncates the unbounded fluid domain by introducing an artificial boundary surface, thus defining a finite computational domain. At this boundary surface an impedance boundary conditions are applied so that no wave reflections occur. In the proposed scheme, all of the procedures are processed directly in time domain, which is efficient for nonlinear analyses of structure floating on unbounded fluid. Numerical results indicate acceptable accuracy of the proposed method.

Hydroelastic interaction between water waves and thin elastic plate floating on three-layer fluid

The wave-induced hydroelastic responses of a thin elastic plate floating on a three-layer fluid, under the assumption of linear potential flow, are investigated for two-dimensional cases. The effect of the lateral stretching or compressive stress is taken into account for plates of either semi-infinite or finite length. An explicit expression for the dispersion relation of the flexural-gravity wave in a three-layer fluid is analytically deduced. The equations for the velocity potential and the wave elevations are solved with the method of matched eigenfunction expansions. To simplify the calculation on the unknown expansion coefficients, a new inner product with orthogonality is proposed for the three-layer fluid, in which the vertical eigenfunctions in the open-water region are involved. The accuracy of the numerical results is checked with an energy conservation equation, representing the energy flux relation among three incident wave modes and the elastic plate. The effects of the lateral stresses on the hydroelastic responses are discussed in detail.

Hydroelastic Response Analysis of Mat-Like VLFS over A Plane Slope in Head Seas

Very large floating structures (VLFS) have an extremely large size of several kilometers in length, thus, the environment at one end of the platform may be different from that at the other end. The importance of such an inhomogeneous environment to the hydroelastic response of a VLFS is of obvious concern for practical application. Some studies have been carried out to investigate the effects of shoreline proximity, breakwaters and harbor walls. In this paper, the impact of the variable depth on the hydroelastic responses of a VLFS is investigated. For simplicity, an ascending plane slope is taken to simulate the varying bottom although the method is capable of treating a bottom of arbitrary variation. The long wave theory and the thin plate theory are employed to model the wave field and the mat-like VLFS respectively. The finite difference method is used to numerically solve the boundary value problem. The results for the zero inclination slope are compared with experimental data and an analytical method to validate the present numerical method. Finally the effect of the inclination of the slope on reflection and transmission coefficients and plate deflections are investigated thoroughly.