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1.
The relationship between form drag and the zonal mean velocity of steady states is investigated in a very simple system; a barotropic quasi-geostrophic β channel with sinusoidal topography. When a steady solution is calculated by the modified Marquardt method, keeping the zonal mean velocity constant as a parameter, the characteristic of the solution changes at a phase speed of a wave with a wavenumber higher than that of the bottom topography. For velocities smaller than this critical value, there exists a stable quasi-linear solution similar to the linear solution. For larger velocities, there exist three solutions whose form drag is very large. In addition, the resonant velocity of the mode, whose wavenumber is the same as the bottom topography, has no effect on these solutions. When the quiescent fluid is accelerated by a constant wind stress, acceleration stops around the critical velocity for a wide range of the wind stress. If the wind stress is too large for the acceleration to stop, the zonal current speed continues to increase infinitely. It is implied that the zonal velocity of equilibrium is mainly determined, not by the wind stress, but by the amplitude of the bottom topography and the viscosity coefficient. 相似文献
2.
WANG Nianxiang GU Lei
Senior Engineer Geotechnical Engineering Department Nanjing Hydraulic Research Institute Nanjing P. R. China.
Engineer Plan Statistics 《中国海洋工程》1999,(2)
-According to the Mohr-Coulomb yield criterion,the stress field of the infinite slope is derivedunder a vertical uniform load q on the top of the slope.It is indicated that elastic and elasto-plastic stateswould occur in the slope.When q is smaller than the critical load,q_p,the slope is in the elastic state.Ifq equals q_p,the slope is in the critical state,and the plastic deformation would occur along the critical an-gle.With the increase of q,the plastic zone would extend,and the slope is in the elasto-plastic state.Ifq equals limit load,the slope is in the limit equilibrium state.The slope may be divided into three zones.Some charts of the critical angle,the critical and limit load coefficients are presented in this paper. 相似文献
3.
A simplified box model of the cooling of a salt-stratified ocean has been constructed in the laboratory to test a theory that predicts multiple equilibria if certain conditions exist. An isothermal basin of water had a thin layer of fresh water over salt water. Beside this was a smaller basin connected to the large basin by horizontal tubes at the top, middle and bottom. The small basin was cooled from above. If the top tube has more flow resistance than the bottom tubes, theory indicates that as cooling temperature T* is made colder, there is a sudden transition between two flow states. The velocities in the tubes jump to greater values, while salinity and temperature in the small basin jump to another value. These multiple states are found in the laboratory experiments along with some states that oscillate. Laboratory measurements and layered model calculations for hysteresis and the jump of temperature and salinity agree qualitatively, but there is only rough quantitative agreement. 相似文献
4.
深海悬链线立管涡激疲劳损伤研究 总被引：1，自引：0，他引：1
讨论海洋平台钢质悬链线式立管SCR(Steel Catenary Riser)的涡激疲劳损伤问题。对于悬链线立管外的流体,给出涡脱落频率和升力对立管作用的计算方法。悬链线立管采用索结构模型,进行动力学分析并利用模态叠加法对其进行动力响应分析。根据Palmgren-Miner线性累积损伤准则并结合S-N曲线,分析在不同流速下立管的涡激疲劳损伤。以工程中实际使用的1 500 m Spar海洋平台悬链线立管为例,对立管的涡激疲劳损伤进行了预报。并通过立管的参数研究,分别就立管外不同来流速度、立管壁厚、内部流体密度和柔性接头刚度对其疲劳损伤的影响进行了分析,得到了一些有意义的研究结果。 相似文献
5.
Until now more than 14 subway lines are in operation and some new lines are being built in the coastal city Shanghai. The longitudinal settlement of shield tunnel has significant effect on the safety of the subway operation. In this paper, the deformation of the shield tunnel and the surrounding soil were analyzed by the establishment of a three-dimensional model. The vertical displacements of four paths (Path 1 is on the ground; Path 2 is at the top of the tunnel; Path 3 is in the middle of the tunnel; Path 4 is at the bottom of the tunnel) are affected by the nature of the soil. The horizontal displacement is smaller than the vertical displacement and horizontal displacement of the clay is larger than that of the sand. The distribution of the pore pressure changes with soil properties around the tunnel. The pore pressure of the sand layer is larger than that of the clay layer at the same depth of underlying soil. 相似文献
6.
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
7.
浮体运动是引起钢悬链式立管(steel catenary riser,简称SCR)动态响应和疲劳损伤的关键因素,目前研究SCR问题时,为简化计算往往仅考虑平台一阶运动,忽略二阶运动影响。而实际上不同浮体结构的二阶运动响应特征明显,拟以SCR服役张力腿平台(tension leg platform,简称TLP)为例,探讨浮体二阶运动对SCR触地区动态响应的影响。建立考虑海床刚度退化的管土作用模型以改进现有的CABLE3D RSI程序,通过编写程序接口,将有限元分析得到的平台实际运动响应导入,研究平台不同运动作用下SCR触地区的位移、动力响应及疲劳分布情况。根据波流作用方向将TLP二阶慢漂运动分为近端和远端漂移两种工况,发现二阶运动下立管与海床的作用范围会增大,且触地区不仅发生高频小幅振荡运动,同时伴随低频大幅运动响应;平台远端漂移时,管内张力敏感程度高,而近端漂移时触地区的弯矩显著增大,都会不同程度提高触地区的疲劳损伤率。研究可为服役不同浮体的SCR响应预测与疲劳分析提供参考和借鉴。 相似文献
8.
Yoji Ogawa 《Applied Ocean Research》1984,6(3):140-147
The integrated equations of deep sea mooring lines in static equilibrium under consideration of gravity force and line elongation are obtained. The most useful forms of the equations for designing deep sea mooring system are those in which horizontal displacement and horizontal and vertical components of tension are obtained as functions of line length, water depth, critical tension and unit weight of line.When the dynamic simulation of a buoy system is carried out, line tensions acting on the buoy should be represented as a linear function of displacement. Therefore the linearisation method is introduced by use of linearisation coefficient matrix. Translation formulae for the matrix from two to three dimensions is also presented.The influence of line elongation on the horizontal component of tension in a taut mooring condition is important, therefore the elongation of stretched deep sea mooring line must be considered carefully. Also, the scaling law under consideration of line elongation in static equilibrium, which is necessary in estimation from a small sized experiment, is discussed. 相似文献
9.
基于物理模型试验，探究畸形波和不规则波作用下浮体系泊张力差异问题。讨论相对波高、相对周期和畸形波参数α1对系泊张力的影响。结果显示:畸形波参数α1和浮体系泊张力显著相关。在α1=2.0～2.83范围内，畸形波作用下迎浪侧系泊张力最大值可达不规则波作用的1.9倍。在相对波高Hs/d=0.032～0.097范围内，畸形波作用下迎浪侧系泊张力最大值显著大于不规则波的作用结果，但畸形波和不规则波对应的1/3值及平均值几乎一致。就相对周期影响而言，迎浪侧系泊张力最大差别出现在谱峰周期Tp0p范围内。频域方面采用小波分析方法讨论畸形波和不规则波作用下浮体系泊张力时频谱特征，两种波浪作用下系泊张力时频特征有显著差别。 相似文献
10.
A comparison of two three-dimensional numerical modeling systems for tidal elevations and velocities in the coastal waters is presented. The two modeling systems are: (1) the Princeton Ocean Model (POM) and (2) the MIKE 3 flow model. The model performance results for Singapore's coastal waters show that the predicted tidal elevations from the two hydrodynamic modeling systems are almost identical and are in very good agreement with field measurement data. The simulated tidal current velocities match well with field measurement data at the selected stations, but it seems that the POM provides the slightly better simulation, compared to the MIKE 3 flow model. The depth profiles of the velocities obtained from the two modeling systems may be greatly different at some time, due to the vertical diffusion coefficient calculated from different turbulent sub-models in the two modeling systems. The POM generally predicts larger peak tidal velocities. The maximum speed differences for the model results from the two modeling systems occur in the top and differ from time to time and from location to location, reaching up to 20%. 相似文献
11.
Karl Killian 《Marine Geodesy》2013,36(4):323-335
Let A,B,C denote three mainland‐stations whose positions are unknown. Let D,E,F denote the positions of a moving ship at three different moments of time. At any of the locations D,E,F, the three azimuths to the target‐stations A,B,C are measured. The problem is to determine from these nine measurements the mutual positions of all stations A,B,C,D,E,F up to a common shift and a change of scale. The problem was formulated by J. H. Lambert in 1765. He also specified a mathematical solution. In this paper so‐called “critical configurations”; are investigated. It is shown that there are nontrivial configurations of the six stations A to F, such that solution of the problem is not unique (critical configuration of the first kind). There is a larger set of configurations, such that the linearized problem admits of an infinity of solutions (critical configurations of the second kind). In the latter case, the original nonlinear problem may be solvable, but the solution is highly unstable with respect to perturbations of the measurements. The main result obtained is as follows. If all stations A to F are located on a second degree curve, i.e., on a conic section, then the configuration is critical, at least of the second kind. The configuration continues to be critical if arbitrarily many observations and target‐points are added which are all situated on the second‐degree curve. 相似文献
12.
钢悬链线立管(SCR)在安装过程中,通常会表现出复杂的动态特性,加强其安装技术及分析方法的研究具有重要意义。基于SCR-Spar生产系统,对深水SCR的安装方法进行研究,给出SCR典型安装步骤和各步骤中的关键问题。采用有限元软件OrcaFlex对海底提升和穿越拖拉安装进行了分析。基于整体时域分析,给出SCR安装操作方案和模拟分析方法,分析了控制安装操作的关键参数及其对SCR安装的影响。研究结果表明,SCR提升操作宜在空管状态下进行,初始提升速度和提升结束时吊头最终位置是控制管线应力/应变水平的主要因素。对于穿越拖拉操作,减小吊头与Spar的初始和最终间距,适当减小穿越拖拉缆长度对控制应力/应变水平是有利的,收/放缆速度对安装影响较小。 相似文献
13.
为解决不埋深海管道在温压荷载循环作用下，轴向移动量不断累积的问题，采用数值模拟方法，在论证了钢悬链线立管（steel catenary riser，SCR）张力相比于其他因素对管道轴向移动影响更为显著的基础上，针对一端受到SCR张力作用的短管，分析了不同轴向移动消减方案的作用特点。研究表明，对管道进行锚固能有效约束其轴向移动，且在中部锚固需要的锚固力及管道内产生的有效轴向力均较小；滑动基础可通过释放一定的管道位移量来减少管中的应力累积。因此将中部锚固与端部滑动相结合可优化锚固系统，使锚固力进一步下降54%，管中最大有效轴向压力减小33%，从而最有效地消减轴向移动。 相似文献
14.
A modified Newton-Raphson iterative technique is formulated for obtaining the static configuration of the Lazy "S" flexible marine riser between the floater and mid-arch buoy under its submerged self weight and the applied top tension. The geometrically non-linear problem is solved by finite difference with the above technique. The problem is formulated as a regular boundary value problem with specified moments and deflections at both ends. Usually the bending stiffness of the flexible riser made of Coflexip pipe is very low. By use of the above analysis, several flexible riser configurations are analyzed and their characteristic behaviors are investigated. Also, changes in the riser characteristics due to quasi-static motion of the floater end are estimated for the safety of the riser layout. 相似文献
15.
In order to investigate the effect of internal friction of umbilical cable on its vortex-induced vibration(VIV)responses, the experimental study on VIV of bond umbilical cable(BUC) and un-bond umbilical cable(UBUC) was carried out in an experimental tank. A current generator in the laboratory simulated the uniform current, and the current velocities were observed in real time by using a Doppler Velocimeter. In addition, different sizes of top tension were applied to the umbilical cable model. The VIV responses of the umbilical cable model were measured by using Fiber Bragg grating(FBG) strain sensors. The displacement responses of umbilical cable model were reconstructed based on the experimental strain data processed by modal superposition method. In this paper, the traveling wave characteristics, the spatial-temporal distribution characteristics of frequency and fatigue damage of the BUC and UBUC under VIV are studied. The experimental results show that there are obvious differences between BUC and UBUC in the response characteristics of VIV. The UBUC appears the traveling wave sooner than BUC, but its standing wave characteristics are more obvious than those of BUC at high velocities. Compared with BUC, the spatial-temporal distribution of UBUC frequencies appears wide-band distribution sooner, but has narrower bandwidth in the "lock-in" state. The level of fatigue damage of BUC was approximately the same as that of UBUC. 相似文献
16.
17.
Risers and anchor lines play important roles in offshore oil exploitation activities nowadays. For this reason the proper analysis and design of such slender structures has been of a paramount interest. The principal characteristics to be accounted for in riser and mooring line analysis are the severe nonlinearities involved and the random dynamic effects associated. The Finite Element Method (FEM) is an essential step to cope with this kind of analysis. But the use of the FEM can be computationally very expensive for the solution of the resultant nonlinear differential equations of motion, because the time-domain integration should produce sufficiently long response time-histories using small time-steps in order to obtain reliable time-series statistics of any structural response parameter, e.g., top tension in an anchor line or stresses occurring at a critical section in a steel catenary riser (SCR). This paper presents a very efficient hybrid Artificial Neural Network (ANN)–Finite Element Method (FEM) procedure to perform a nonlinear mapping of the current and past system excitations (inputs) to produce subsequent system response (output) for the random dynamic analysis of mooring lines and risers. Firstly, a quite short FEM-based time-domain response simulation is generated. Then, an ANN is used to predict the remaining structural response time-history simulation. The hybrid ANN–FEM approach can be very efficient for predicting long response time-histories. It has been observed that a 3 h response time-history can accurately be obtained with approximately the computational cost of a 500 s one, i.e., 20 times faster than a complete simulation using finite element-based solution. Roughly, this can represent a reduction of about a dozen of hours of computer time for a single mooring line analysis and about two dozens of hours (or more) for a single SCR analysis, both belonging to a deep-water floating unit. 相似文献
18.
Dynamic Characteristics of Marine Risers Conveying Fluid 总被引：10，自引：0，他引：10
The lateral vibration differential equation for a marine riser conveying fluid is derived by useof the small deflection theory.and the effect of internal flow velocity and top tension on the natural fre-quency of the riser is studied by use of FEM.At the same time,the preliminary relationship between thenatural frequency and riser span under different internal flow velocities is obtained,the effect of riser sup-ports on the vibration frequency is computed.It is found that the natural frequency of the marine riser in-creases with the increase of top tension.however decreases with the increase of internal flow velocity.In ad-dition,the frequency decreases drastically with the increase of riser span. 相似文献
19.
Results of 2DH morphodynamic computations are presented to quantify the temporal evolution of the crescentic patterns emerging in a double nearshore bar system in response to constant wave boundary forcing. Sixteen different conditions varying both offshore wave height and angle of wave incidence were applied. The mean length scales of the emerging irregular crescentic patterns are linearly proportional to the local longshore velocity over the inner and outer bars. For similar longshore velocities, the length scales of the outer bar are larger than of the inner bar. This is explained by accounting for the difference in water depth above the bar crest. The variable morphological response times can be explained by including additional bathymetrical parameters. The active volume of the bar, defined by the breaker index, plays an important role in this response time. With larger active volumes the bar responds more rapidly to identical boundary conditions. Also, bars with a smaller total volume respond more quickly. This faster response is due to the steeper active volume of the bars. Different initial perturbations resulted in different locations of the emerging features, showing that their location is sensitive to the initial bathymetry. However, the range in length scales and response times due to the different perturbations was significantly smaller than those obtained for the different hydrodynamic conditions. Based on the present findings we hypothesize that morphological length scales in the field are rarely in equilibrium with the concurrent offshore wave height and angle of incidence owing to the slow response of the sandbars under constant conditions relative to the stochastic nature of natural wave forcing. 相似文献