系泊系统非线性恢复力研究及其应用(英文)

Mooring system plays an important role in station keeping of floating offshore structures.Coupled analysis on mooring-buoy interactions has been increasingly studied in recent years.At present,chains and wire ropes are widely used in offshore engineering practice.On the basis of mooring line statics,an explicit formulation of single mooring chain/wire rope stiffness coefficients and mooring stiffness matrix of the mooring system were derived in this article,taking into account the horizontal restoring force,vertical restoring force and their coupling terms.The nonlinearity of mooring stiffness was analyzed,and the influences of various parameters,such as material,displacement,pre-tension and water depth,were investigated.Finally some application cases of the mooring stiffness in hydrodynamic calculation were presented.Data shows that this kind of stiffness can reckon in linear and nonlinear forces of mooring system.Also,the stiffness can be used in hydrodynamic analysis to get the eigenfrequency of slow drift motions.     

A stability analysis of tandem offloading systems at sea

In this article, we analyze the linear stability of tandem offloading systems in wind, current, and waves. The wind and current forces are evaluated with the help of published experimental data, while the hydrodynamic coefficients and wave drift forces are rigorously estimated by using a three-dimensional singularity distribution method based on potential theory. The bow hawser and mooring lines are described quasistatically by elastic catenary equations. In order to examine the linear static and dynamic stability of the system, the equations for surge, sway, and yaw are linearized. The effect of design parameters such as turret position, mooring stiffness, and hawser length and stiffness on stability is investigated based on linearized model. The stability analysis clarifies the mechanism of the limit cycle for tandem offloading systems, which is known as fishtailing motion. The theoretical results of the shape and amplitude of the limit cycle are found to be in good agreement with those of simulations and experiments.     

生产驳船多点系泊定位系统动态响应

导出了多点系泊系统的船舶运动方程,在船舶运动方程中计入了非线性恢复力。缆索分段的三维动态方程中采用了凝集参数模型与平均切向量非线性流体动力载荷处理技术,对某生产驳船系泊问题进行了计算,对受有风、浪、流环境力的系泊船运动方程进行了数值求解,得到了随时间变化的船舶运动和系泊索动态张力,考查了缆索动力学对船舶运动和链张力的影响,并与以前未计入缆索动力影响的准静态结果进行了比较。     

Validation of a dynamic mooring model coupled with a RANS solver

Standard design procedures and simulation tools for marine structures are aimed primarily for use by the offshore oil and gas. Mooring system restoring forces acting on floating offshore structures are obtained from a quasi-static mooring model alone or from a coupled analysis based on potential flow solvers that do not always consider nonlinear mooring-induced restoring forces, fluid structure interactions, and associated hydrodynamic damping effects. This paper presents the validation of a dynamic mooring system analysis technique that couples the dynamic mooring model with a Reynolds-averaged Navier-Stokes (RANS) equations solver. We coupled a dynamic mooring model with a RANS equations solver, and analyzed a moored floating buoy in calm water, regular and irregular waves and validated our motion and mooring force predictions against experimental measurements. The mooring system consisted of three catenary chains. The analyzed response comprised decaying oscillating buoy motions, linear and quadratic damping characteristics, and tensile forces in mooring lines. The generally favorable comparison of predicted buoy motions and mooring forces to experimental data confirmed the reliability of our implemented coupling technique to predict system response. Additional comparative results from a potential flow solver demonstrated the benefits of the coupled dynamic mooring model with RANS equations. The successful validated tool of coupling the dynamic mooring model with the RANS solver is available as open source, and it shows the potential of the coupled methodology to be used for analyzing the moored offshore structures.     

船体湿表面脉动压力预报的三维非线性方法

本文给出了一种预报船体湿表面脉动压力响应的三维非线性时历模拟方法.计及瞬时物面变化引起的各种非线性因素,在时域内建立船体非线性运动方程.在每一时刻,认为船体以其瞬时平均湿表面为平衡位置做简谐振荡运动,利用三维线性频域解来简化运算.在此基础上,进一步将绕射和辐射问题在静平衡位置求解,仅考虑占主导地位的入射波和静恢复力变化的非线性,作为一种兼顾工程需要的计算精度和计算效率的简化方法.以此方法为基础编制了计算机程序,对实船进行了计算,并与模型试验进行了对比.分析比较表明,该方法能够给出令人满意的船体湿表面脉动压力响应预报结果.     

Wave load computation in direct strength analysis of semi-submersible platform structures

A wave load computation approach in direct strength analysis of semi-submersible platform structures was presented in this paper. Considering the differences in shape of pontoon, column and beam, the combination of accumulative chord length cubic parameter spline theory and analytic method was adopted for generating the wet surface mesh of platform. The hydrodynamic coefficients of platform were calculated by the three-dimensional potential flow theory of the linear hydrodynamic problem for platform with low forward speed. The equation of platform motions was established and solved in frequency domain, and the responses of wave-induced loads on the platform can be obtained. With the interpolation method being utilized, the pressure loads on shell elements for finite element analysis(FEA) were converted from those on the hydrodynamic computation mesh, which pave the basis for FEA with commercial software. A computer program based on this method has been developed ,and a calculation example of semi-submersible platform was illustrated. Analysis results show that this method is a satisfying approach of wave loads computation for this kind of platform.     

一种船舶运动和载荷响应的非线性预报方法

首先采用半解析的方法得到时域水动力系数,并给出一些数值计算时实用性的建议,然后计及瞬时湿表面变化引起的非线性恢复力和入射波力并在时间域内计算运动方程,最后对实船进行了运动和载荷预报,数值结果与模型试验数据对比表明,载荷非线性要比运动非线性明显;采用的计及非线性影响的预报方法是切实可行的。     

轻型码头结构疲劳可靠性分析*

采用随机波浪谱和Miner线性疲劳累积损伤原理对轻型码头结构在波浪荷载和船舶撞击荷载共同作用下的疲劳可靠性进行研究。根据轻型码头结构变形大的特点,计算船舶撞击力时考虑了结构刚度对撞击力的影响,波浪荷载使用Morison方程计算,并以Airy波浪理论计算水质点速度。结果表明:波浪荷载和船舶撞击力对轻型码头结构的疲劳寿命均产生一定的影响,在进行疲劳可靠性分析时应考虑两种荷载的共同作用。     

一种基于Taylor展开的系泊刚度矩阵计算方法

系泊系统对于船舶及海上浮式结构物的定位十分重要。钢质系泊缆索是业界广泛采用的一种形式。由于其提供的恢复力与浮体位移之间呈现非线性关系,故针对此种系泊方式基于Taylor展开方法得到了非线性的刚度系数及相应的系泊刚度矩阵,研究了其在系泊定位及水动力计算方面的应用。分析表明该方法可以较为方便地计及刚度非线性的影响。     

系泊多浮体系统流固耦合和浮体间耦合动力分析

利用多刚体力学运动学方法进行系统铁数学描述。以系统摇荡时各浮体质心位移和浮体角位移表达广义速率。进而表达出有关偏速度、偏角速度、加速度和角加速度。单个系泊浮体在规则波作用下，所受的流体动压力用势流理论计算。多浮体系统摇荡时除入射规则波外，每一浮体的摇荡辐射波对其他浮体也构成了入射波并作用以流体动压力。表达出系统的广义主动力和广义惯性力后代入Kane方程，补充以浮体间的运动约束方程，消去时间因子后得到求解各浮体质心位移和浮体角位移的复数线性代数方程组。     

浅水沉管船的锚泊定位系统设计

浅水锚泊系统的设计增加了诸多挑战,恶劣的环境使船舶具有较大的低频漂移运动,同时锚泊系统需要有足够的恢复力特性。文章针对深中通道沉管船,基于三维势流理论,在时域下对安装船和沉管系统进行分析,设计出合适的锚泊系统,并验证了锚泊系统设计的安全性。同时为以后类似系统的设计提供借鉴。     

Truss Spar平台在不同系泊线组合下的动态响应分析（英文）

The dynamic responses of any floating platform are dependent on the mass, stiffness and damping characteristics of the body as well as mooring system. Therefore, it is very essential to study the effect of individual contributions to the system that can finally help to economise their cost. This paper focuses on the effect of mooring stiffness on the responses of a truss spar platform, obtained by different grouping of lines. The study is part of our present researches on mooring systems which include the effect of line pretension, diameter and azimuth angles. The platform is modelled as a rigid body with three degrees-of-freedom and its motions are analyzed in time-domain using the implicit Newmark Beta technique. The mooring lines restoring force-excursion relationship is evaluated using a quasi-static approach. It is observed that the mooring system with lines arranged in less number of groups exhibits better performance in terms of the restoring forces as well as mean position of platform. However, the dynamic motions of platform remain unaffected for different line groups.     

浮式风机气动-水动耦合特性数值模拟（英文）

The exploration for renewable and clean energies has become crucial due to environmental issues such as global warming and the energy crisis. In recent years,floating offshore wind turbines(FOWTs) have attracted a considerable amount of attention as a means to exploit steady and strong wind sources available in deep-sea areas. In this study, the coupled aero-hydrodynamic characteristics of a spar-type 5-MW wind turbine are analyzed. An unsteady actuator line model(UALM) coupled with a twophase computational fluid dynamics solver naoe-FOAM-SJTU is applied to solve three-dimensional Reynolds-averaged NavierStokes equations. Simulations with different complexities are performed. First, the wind turbine is parked. Second, the impact of the wind turbine is simplified into equivalent forces and moments. Third, fully coupled dynamic analysis with wind and wave excitation is conducted by utilizing the UALM. From the simulation, aerodynamic forces, including the unsteady aerodynamic power and thrust, can be obtained, and hydrodynamic responses such as the six-degrees-of-freedom motions of the floating platform and the mooring tensions are also available. The coupled responses of the FOWT for cases of different complexities are analyzed based on the simulation results. Findings indicate that the coupling effects between the aerodynamics of the wind turbine and the hydrodynamics of the floating platform are obvious. The aerodynamic loads have a significant effect on the dynamic responses of the floating platform, and the aerodynamic performance of the wind turbine has highly unsteady characteristics due to the motions of the floating platform. A spar-type FOWT consisting of NREL-5-MW baseline wind turbine and OC3-Hywind platform system is investigated. The aerodynamic forces can be obtained by the UALM. The 6 DoF motions and mooring tensions are predicted by the naoe-FOAM-SJTU. To research the coupling effects between the aerodynamics of the wind turbine and the hydrodynamics of the floating platform, simulations with different complexities are performed. Fully coupled aero-hydrodynamic characteristics of FOWTs, including aerodynamic loads, wake vortex, motion responses, and mooring tensions, are compared and analyzed.     

船体在波浪中的非线性水动压力

本文提出了一个预报船体在波浪中大幅运动时非线性水动压力场的二维时域理论。船体扰动势用时域自由面格林函数和在入射波下的瞬时湿表面上的分布源求解；与非线性水动压力场相匹配的船体运动用差分法求得。为提高计算效率和避免数值过程发散，采用了改进的数值模型和方案。通过线性理论计算与模型试验结果的比较，指出了线性切片理论在预报水动压力场时的不足，水动压力与波高的非线性关系及正负水动压力沿船体表面的分布在Wigley船的计算比较中得到了说明。初步计算表明，该理论的实用化发展前景是令有鼓舞的。相应的计算机程序可在PC机上运行。     

LNG旁靠FSRU码头系泊的水动力性能试验研究

本文对码头系泊的FSRU和LNG船的水动力性能进行了研究,分别对FSRU单船码头和FSRU-LNGC并排码头系泊进行了模型试验.码头与船和船与船之间均采用系泊缆和靠垫进行连接.试验中考虑了风、流、浪等环境因素对码头系泊系统的影响.在风浪流模型试验中,分别测量了不同海况下系泊系统的动态响应,其中包括FSRU和LNG船的六自由度运动响应、系泊缆张力和靠垫接触力.相比较其它浪向,横浪(90°)引起的船舶运动更大,进而导致系泊缆和靠垫的受力增大.结果还表明,FSRU单船码头系泊在所有海况下安全性较好,但对于LNG船旁靠FSRU码头系泊系统,其对环境的响应非常敏感,尤其是横浪环境.     

Comparison of numerical and experimental results of nonlinear wave-induced vertical ship motions and loads

 A nonlinear time-domain procedure is presented which is used to calculate the vertical responses of a container ship advancing in head waves. The method assumes linear radiation forces represented by time convolution of memory functions, infinite frequency added masses, and radiation restoring coefficients. The nonlinear hydrostatic restoring and Froude–Krilov forces are computed exactly over the instantaneous wetted surface of the ship's hull. Forces due to green water on deck are calculated using the momentum method. Nonlinear effects are identified on different vertical ship responses, namely on the heave and pitch motions, the vertical accelerations, and the vertical bending moment. These non-linear effects are expressed by the variation of the transfer function with the wave amplitude, the higher-order harmonics of the time signals, the offset of the time series, and the asymmetry of the peaks. The numerical results and the quantified nonlinear effects are compared with experimental results showing an ability to reproduce the main nonlinear effects. Received: December 17, 2001 / Accepted: January 31, 2002     

双浮体波浪能装置的振荡和能量转换特性研究（英文）

In this study, we investigated the hydrodynamic and energy conversion performance of a double-float wave energy converter(WEC) based on the linear theory of water waves. The generator power take-off(PTO) system is modeled as a combination of a linear viscous damping and a linear spring. Using the frequency domain method, the optimal damping coefficient of the generator PTO system is derived to achieve the optimal conversion efficiency(capture width ratio).Based on the potential flow theory and the higher-order boundary element method(HOBEM), we constructed a threedimensional model of double-float WEC to study its hydrodynamic performance and response in the time domain. Only the heave motion of the two-body system is considered and a virtual function is introduced to decouple the motions of the floats. The energy conversion character of the double-float WEC is also evaluated. The investigation is carried out over a wide range of incident wave frequency. By analyzing the effects of the incident wave frequency, we derive the PTO's damping coefficient for the double-float WEC's capture width ratio and the relationships between the capture width ratio and the natural frequencies of the lower and upper floats. In addition, it is capable to modify the natural frequencies of the two floats by changing the stiffness coefficients of the PTO and mooring systems. We found that the natural frequencies of the device can directly influence the peak frequency of the capture width, which may provide an important reference for the design of WECs.     

Formulation of consistent nonlinear restoring stiffness for dynamic analysis of tension leg platform and its influence on response

Shortcomings of the traditionally used nonlinear restoring stiffness of TLPs, i.e. unrealistically high stiffness of horizontal motions, their uncoupling and secant formulation are pointed out. Therefore, new consistent restoring stiffness is derived. The platform is considered as a rigid body moored by flexible pretensioned tendons. Global horizontal low frequency motions (surge, sway and yaw) with large amplitudes as a result of dominant second order wave excitation and small stiffness, and vertical local motions (heave, roll and pitch) of higher frequency and small amplitudes excited by the first order wave forces, are distinguished. Hence, horizontal displacements represent position parameters in analysis of vertical motions. First, the linear restoring stiffness, which consists of the tendon conventional axial stiffness, the tendon geometric stiffness and the platform hydrostatic stiffness, is established. Then it is extended to large displacements resulting in new secant restoring stiffness. It depends on surge, sway and yaw displacements and is the same in any horizontal direction. Also, the tangent stiffness, which gives more accurate results, is derived. Heave is defined as vertical projection of axial tendon vibrations and platform tangential oscillations, which are analyzed in their natural moving coordinate system. Inertia force due to setdown, as a slave d.o.f. of the master horizontal motions, is taken into account in the dynamic equilibrium equations. As a result the complete tangential stiffness matrix of horizontal and vertical motions includes 7 d.o.f. The known secant restoring stiffness matrices are compared with the new one and noticed differences are discussed. All theoretical contributions are illustrated by relatively simple numerical example.     

波浪作用下单体网格式浮架受力的数值模拟

模型试验表明重力式网箱在波浪作用下，单纯浮架的受力贡献比例较大。本文运用波浪理论，通过建立浮架、锚绳及浮子的运动方程，采用五阶龙格-库塔进行数值求解，得到了单体网格式锚碇单纯浮架的数值计算模式。将计算模型简化为四点锚碇，并将计算结果与实验结果进行了比较，二者符合程度较好。在此基础上，比较了单体网格式锚碇与四点锚碇单纯浮架的受力，结果表明网格式锚碇方式浮架受力较小。     

Simplified analysis for estimation of the behavior of a submerged floating tunnel in waves and experimental verification

To achieve rational design in waves for a submerged floating tunnel which has emerged as a new offshore transportation infrastructure, it's necessary to understand its hydrodynamic behavior. For simple but accurate estimation of hydrodynamic forces, a theoretical method is proposed and the tests with physical models in a wave flume were carried out for verification. Morison's equation was used to estimate wave loads composed of inertia force and drag force. Forces calculated by applying the linear wave theory to Morison's equation coincided well with those measured by the tests. The test results showed that mooring systems played a significant role in the movement of the submerged floating tunnel in waves. A pendulum model could be used to describe the motion of the submerged floating tunnel with a single vertical mooring. Based on the verified relations, a simple slack condition which causes the submerged floating tunnel to be unstable was also proposed. The simplified approach proposed by this study proved to be useful in designing the submerged floating tunnel in the initial stage.