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1.
Effect of mooring-line stiffness on the performance of a dual coaxial-cylinder Wave-Energy Converter
A point-absorber-type Wave-Energy Converter (WEC) consisting of a floating vertical inner cylinder and an annular outer cylinder that slides along the inner one is considered. The two cylinders heave differently under wave excitation, and wave energy can be harnessed from the relative heave motion between the two cylinders using a Permanent Magnet Linear Generator (PMLG) as the Power Take-Off unit. A mooring cable is attached to the bottom of the inner cylinder. This paper aims to examine the effect of the stiffness of the mooring cable on the performance of the coaxial-cylinder WEC system. The two limiting cases of no mooring cable (freely floating inner and outer cylinders) and an infinitely stiff mooring cable (fixed inner cylinder) were also considered. To perform the analysis, hydrodynamic and interference coefficients of the two heaving cylinders were computed semi-analytically using the method of matched eigenfunction expansions. Experimentally determined viscous corrections on damping were also included in the model in order to have more realistic predictions. The performance of the system in terms of motion responses and capture width were predicted and discussed for both regular and irregular waves. The results of the analysis indicate that both the freely floating design and the design with rigidly moored inner cylinder are viable. The two limiting cases show similar optimal performances, albeit with very different optimal generator damping. However, an ill-chosen mooring-cable stiffness may cause the inner and the outer cylinders to have the same resonance frequency, eliminating the relative heave motion and leading to almost no energy extraction. This situation needs to be avoided when designing the mooring system for a coaxial-cylinder WEC. 相似文献
2.
法向承力锚(Vertically Loaded Plate Anchor,VLA)是一种适用于深水的新型系泊基础,它的拖曳安装过程直接决定了其系泊定位的精度和锚体的最终承载能力。综合考虑VLA锚体、锚泊线和上部船体的运动,建立了一种新的准静力整体分析模型。模型包括不断贯入海床的锚体、锚泊线(土中反悬链段和水中悬链段)和安装船体三部分,针对确定的锚泊线长度,安装船运动张紧锚泊线进行安装的过程,计算了此过程中锚体的运动轨迹、锚泊线形态和作用在船体上的锚泊线张力矢量的变化,重点分析了不同抛链长度和海床土体的参数对安装过程控制的影响,发现链长与水深之比达到5时,接近极限贯入深度。 相似文献
3.
Dynamic Response Analysis for Embedded Large-Cylinder Breakwaters Under Wave Excitation 总被引：4，自引：0，他引：4
1 .IntroductionThestaticmethodsarecommonlyusedinthestabilityanalysisofembeddedlarge cylinderbreak waters .Thestaticanalysismodelscanbedividedintotwocategories :simplifiedmodels (Zhouetal.,1 995;WangandZhu ,2 0 0 2 )andfiniteelementmodels (MengandWang ,1 999) .Thesimplifiedmodelsinheritthetraditionalengineeringexperiencesandareconvenienttouseinengineeringprac tice.However,itisdifficultforthesimplifiedmodelstodepictthegeometricandphysicalpropertiesofastructure soilsystemprecisely .Thefiniteel… 相似文献
4.
具有链—缆—链结构的复合系泊链缆因其相对于全钢链质量和成本上的优势而在深水系泊中得以广泛应用。基于细长杆理论采用有限差分法建立了可以考虑链—缆—链结构的复合系泊缆数值模型，将其应用于不同工况下全钢链和复合链缆运动的数值模拟中，并开展了验证。首先，将单根钢链顶张力数值模拟结果与不同工况下的模型试验结果进行了对比，验证了数值预报程序应用于全钢链的准确性。然后，对于复合系泊链缆开展了静刚度和动刚度迭代数值模拟，并将模拟结果同已发表文章中的算例结果进行比较，验证了该数值模型在复合链缆模拟上的准确性。发现对于单根钢锚链的验证，激励半径越大，激励周期越小，一个周期内顶张力幅值及其极差越大，钢链运动就越剧烈。对于链—缆—链式复合系泊链缆的验证，发现静刚度迭代中数值模拟结果与算例结果差异较小；对于动刚度迭代，除个别大幅慢漂工况外，两者有较高的吻合；且激励周期越小，激励半径越大，复合系泊链缆顶张力越大，弹性模量越小，运动越剧烈。对于聚酯缆刚度的敏感性分析，发现改变动刚度经验公式参数的情况下，杨氏模量的静刚度迭代和动刚度迭代结果误差分别最大达到了60.81%和68.21%，因此合成纤维材料特性对复合系泊链... 相似文献
5.
Drag anchor is one of the most commonly used anchorage foundation types. The prediction of embedded trajectory in the process of drag anchor installation is of great importance to the safety design of mooring system. In this paper, the ultimate anchor holding capacity in the seabed soil is calculated through the established finite element model, and then the embedded motion trajectory is predicted applying the incremental calculation method. Firstly, the drag anchor initial embedded depth and inclination angle are assumed, which are regarded as the start embedded point. Secondly, in each incremental step, the incremental displacement of drag anchor is added along the parallel direction of anchor plate, so the displacement increment of drag anchor in the horizontal and vertical directions can be calculated. Thirdly, the finite element model of anchor is established considering the seabed soil and anchor interaction, and the ultimate drag anchor holding capacity at new position can be obtained. Fourthly, the angle between inverse catenary mooring line and horizontal plane at the attachment point at this increment step can be calculated through the inverse catenary equation. Finally, the incremental step is ended until the angle of drag anchor and seabed soil is zero as the ultimate embedded state condition, thus, the whole embedded trajectory of drag anchor is obtained. Meanwhile, the influences of initial parameter changes on the embedded trajectory are considered. Based on the proposed method, the prediction of drag anchor trajectory and the holding capacity of mooring position system can be provided. 相似文献
6.
The experimental and theoretical investigations on the behaviour of pontoon-type floating breakwaters are presented. A two-dimensional finite element model is adopted to study the behaviour of pontoon-type floating breakwaters in beam waves. The stiffness coefficients of the slack mooring lines are idealized as the linear stiffness coefficients, which can be derived from the basic catenary equations of the cable. The theoretical model is supported by an experimental programme conducted in a wave flume. The motion responses and mooring forces are measured for three different mooring configurations, and the results are reported and discussed in detail in this paper. The wave attenuation characteristics are presented for the configurations studied. 相似文献
7.
采用集中质量法研究了绷紧式系泊系统中系缆由于松弛-张紧过程产生的冲击张力。建立系泊缆绳离散的集中质量模型,对其独立单元进行受力分析并建立了单元的运动方程。给定缆绳上端点简谐激励,通过Ansys中的Aqwa模块,分析了缆绳的运动响应;针对缆绳运动响应过程中的三种状态进行了模拟计算,探讨了冲击张力产生的条件;研究了缆绳初始预张力、上端点激励幅值和频率、拖曳力系数、弹性模量以及单位长度质量对动态张力的影响。研究结果表明:这些影响因素不仅会影响缆绳动态张力的大小,也会对缆绳中的冲击张力产生一定的影响。 相似文献
8.
The drag-induced damping in a mooring cable due to combined first- and second-order wave excited motion of a moored vessel has been determined by statistical linearisation. A dynamic stiffness approach developed elsewhere is used to deal with the dynamics of the mooring cables. The power spectral densities of low- and wave-frequency responses are obtained which clearly show the influence of mooring line damping. The non-Gaussian probability density functions (pdf) and expected crossing rates of vessel responses and dynamic cable tensions are determined using the Kac–Seigert technique, and the influence of drag damping is highlighted. 相似文献
9.
Taut mooring systems have become prospective alternatives for the station keeping of offshore floating facilities in deep water. The associated embedded anchors cause a part of the mooring line to be buried in the seabed − the inverse catenary − which introduces a requirement to predict the load and uplift angle at the padeye, where the chain is connected to the anchor. The padeye load and angle depend on the shape and tension profile of the inverse catenary, which must be assessed in the mooring system design. The dynamic interaction between the embedded chain and the soil in the inverse catenary is not usually considered in this analysis. Instead, the inverse catenary is assessed statically, albeit potentially using cyclically-degraded soil strength parameters. The present study employs the lumped mass method to simulate the dynamic response of mooring lines under different imposed oscillations at the fairlead, where the chain is connected to the floating facility. A new chain-soil interaction model, which includes hysteresis effects associated with irrecoverable relative chain-soil displacement is calibrated by experimental results. Simulations of cyclic vessel motion are then performed, and the resulting chain-seabed interaction is observed. During constant-amplitude vessel motion cycles, the load angle at the padeye significantly decreases due to progressive ratcheting or ‘shakedown’ of the inverse catenary from the initial static profile towards a straighter profile. This effect is due to the hysteretic soil response and creates a less onerous loading condition for the anchor which may be beneficial, but is conventionally overlooked in design. At the end of the present study, an elastic bound method is proposed to estimate the profile of the inverse catenary after shakedown. A parametric study illustrates the performance of this simple method for predicting the steady state condition. 相似文献
10.
This study presents an analysis of a wave energy converter (WEC) system consisting of a buoy, a mooring system, and a power cable connected to a hub. The investigated WEC system is currently under full-scale testing near Runde in Norway. The purpose of the study was to investigate the characteristics of the entire system, primarily with regard to energy performance and the fatigue life of the mooring lines and power cable, considering the effects of marine biofouling and its growth on the system’s components. By means of parametric study, the energy performance and fatigue life of the mooring lines and power cable were investigated considering two mooring configurations, three biofouling conditions, four sea states in a scatter diagram, and three wave and current directions. Hydrodynamic and structural response simulations were conducted in a coupled response analysis using the DNV-GL software SESAM. Energy performance analyses and stress-based rainflow counting fatigue calculations were performed separately using an in-house code. The results show that, for a WEC system which has been deployed for 25 years, biofouling can reduce the total power absorption by up to 10% and decrease the fatigue life of the mooring lines by approximately 20%. 相似文献
11.
This paper describes the life testing of a 35-kV power cable that will connect a single anchor leg mooring (SALM) buoy with a platform in several hundred feet of water. While there are many power cable installations to offshore platforms and between platforms, this system is unique because the cable will be constantly flexing in compression as the mooring buoy moves in the water. The buoy design includes an articulated leg with universal joints to which the cable is clamped. It is the loop in the cable where it crosses a universal joint that is flexed, and is a major cause of concern. To predict the effects of this constant flexure while energized, a flex simulator and electrical circuit were devised to duplicate the service environment as closely as possible. This paper describes the cable system, the test apparatus and procedure, and the results obtained. 相似文献
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The present research aims at clarifying the effects of freak wave on the motion and dynamic responses of a semisubmersible. To reveal the effects of mooring stiffness, two mooring systems were employed in the model tests and time-domain simulations. The 6-DOF motion responses and mooring tensions have been measured and the 3-DOF motions of fairleads were calculated as well. From the time series, trajectories and statistics information, the interactions between the freak wave and the semisubmersible have been demonstrated and the effects of mooring stiffness have been identified. The shortage of numerical simulations based on 3D potential flow theory is presented. Results show that the freak wave is likely to cause large horizontal motions for soft mooring system and to result in extremely large mooring tensions for tight mooring system. Therefore, the freak wave is a real threat for the marine structure, which needs to be carefully considered at design stage. 相似文献
14.
Mark A. Grosenbaugh Walter Paul Dan Frye Norman Farr 《Oceanic Engineering, IEEE Journal of》2006,31(3):574-584
We consider an alternative to traditional high-modulus synthetic electro-optical-mechanical (EOM) mooring cables that are being used in single-point moorings for deep-ocean observatories. The alternative cable design is based on using low-modulus nylon or polyester fibers as the strength member. High-modulus EOM cables such as those that use Vectran fibers as the strength member are usually constructed with the conductors and optical fibers in the core and the strength member on the outside. The key aspect of the new design is that the strength member is placed in the center of the cable and the conductors and fibers are wrapped around the outside at a high helix angle to accommodate stretching of the center-strength-member. A comparison of the static and dynamic responses of moorings constructed with nylon, polyester, and Vectran EOM cables (for mooring scopes of 1.1 and 1.2 and deployment depths of 1800, 3000, and 5000 m) shows that the maximum total tensions of moorings made with nylon EOM cables are lowest under all conditions. Differences between the nylon and the Vectran EOM cable moorings are due principally to the differences in the dynamic tensions. Differences between the nylon and the polyester EOM cable moorings are due mainly to differences in static tensions caused by the higher specific gravity of polyester fibers. Reduction in the scope of all the moorings from 1.2 to 1.1 resulted in significantly higher tensions for the polyester and Vectran EOM cable moorings but only slightly higher tensions for the nylon EOM cable moorings 相似文献
15.
With the application of innovative anchor concepts and advanced technologies in deepwater moorings, anchor behaviors in the seabed are becoming more complicated and pose a great challenge to the analytical methods. In the present work, a large deformation finite element (FE) analysis employing the coupled Eulerian–Lagrangian technique is performed to simulate the installation/mooring line, and then is applied to analyzing comprehensive anchor behaviors in the seabed. By connecting cylindrical units with each other using connector elements, the installation/mooring line is constructed. With the constructed installation/mooring line, FE simulations are carried out to investigate comprehensive anchor behaviors in the seabed, including long-distance penetration of drag anchors, keying of suction embedded plate anchors and non-catastrophic behavior of gravity-installed anchors. Through comparative studies, the accuracy of the proposed method is well examined. A parametric study is also undertaken to quantify the effects of the frictional coefficient, initial embedment depth, and soil weight on the profile of the embedded anchor line and the shackle load. The present work demonstrates that the proposed FE model, which incorporates the installation/mooring line and the anchor, is effective in analyzing the comprehensive anchor behaviors in the seabed. 相似文献
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A two-degree freedom model for an ALT-tanker system is established corresponding to the pitch of the ALT and the surge of the tanker.Tension in the mooring cable between the ALT and the tanker is represented by an unsymmetrical,piecewise-nonlinear function.Wave load on the tower is evaluated by use of the Morison equation.The first order wave load acting on the tanker is calculated by the linear diffraction theory based on the 2-D Helmholtz equation,and the near field approach of Pinkster is used to evaluate the second order drift force.The dynamic equation of motion is established based on the principle of D'lembert.Dynamic response and cable tension of a mooring system composed of an 88.4 m ALT and a 100000 t grade tanker are calculated.The influence of wave frequency,wave excitation amplitude,wind and current force on ALT-tanker motion and cable tension is discussed. 相似文献
18.
Experimental studies of the transverse motions induced in a long flexible cable by axial flows have been conducted in a large blowdown water channel. A cable with a diameter of 1.59 cm and a length of 9 m was employed. Experiments were conducted over a fluid velocity range of 4.6-9.1 m/s. Both free and fixed downstream terminations of the cable were utilized, the former simulating a towed cable and the latter a mooring cable. Measurements of the drag coefficient of the cable with the free end were also made. These results were compared with those obtained previously for a flexible cylinder with a smooth exterior surface, and the following conclusions were drawn. 1) The amplitudes of the flow-induced transverse motions in a flexible cable or cylinder with a free downstream end in general decrease with increasing distance from this end, but they exhibit weak dependence on flow velocity and surface roughness near this end. 2) The amplitudes of the flow-induced motions are in general smaller when the cable is fixed at both ends than when the downstream end is free. 3) The drag coefficient of the flexible cylinder or cable increases with increasing surface roughness. 相似文献
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The key consideration in controlling the dynamic response of an offshore guyed tower subjected to environmental loading is the selection of the stiffness characteristics of the cable mooring array. In the present study, each cable in the array consists of three segments: a leading segment attached to the tower, a short heavy middle segment, and a seabed trailing segment attached at the extremity to an anchor pile. A nonlinear static model is presented in algorithmic form that defines the stiffness characteristics of the cable array. The model involves the inelastic static catenary equations coupled with conditions of geometric compatibility, formulated in terms of seven nondimensional system parameters. The effects of varying each of the system parameters on the stiffness characteristics of example cable arrays are presented and the implications for cable design are discussed. 相似文献