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1.
A solution is presented for the wave induced drift forces acting on a submerged sphere in a finite water depth based on linearised velocity potential theory. In order to obtain the velocity potential, use has been made of multipole expansions in terms of an infinite series of Legendre functions with unknown coefficients. The series expression for the second order mean forces (drift forces) is provided by integrating the fluid pressure over the body surface. The horizontal drift force is also expressed by a series solution obtained using the far-field method. 相似文献
2.
回顾了近几年来深海平台二阶波浪力的研究成果。对几种典型深海平台结构二阶波浪力研究作了详细的阐述,包括理论分析、数值计算和实验验证等。最后指出目前该领域主要研究方向,作为进一步研究的参考。 相似文献
3.
An iterative frequency domain method of analysis is presented for determining the response behaviour of Guyed Offshore Towers to low-frequency, second-order wave drift forces generated in a random sea environment. For the response analysis, the tower is idealized as a shear beam with a rotational spring at the bottom support. The guylines are replaced by a non-linear spring. The second-order drift force is considered to be proportional to the square of the wave elevation and is simulated using a drift force coefficient and the time history of a slowly varying wave envelope in random sea. The responses due to drift forces are obtained in frequency domain by incorporating the non-linearities produced due to non-linear guy lines. An example problem is solved under different random sea states to compare the response behaviour of the tower obtained by the second-order wave force, the first-order wave force and a combination of the two. 相似文献
4.
Analytical expressions of the bifurcation boundaries for symmetric spread mooring systems 总被引：4，自引：0，他引：4
For a general symmetric Spread Mooring System .(SMS), the five necessary and sufficient conditions for stability are derived analytically. It is shown that only two conditions are dominant in symmetric ship moorings. The equations derived in this paper provide analytical means for determining static and dynamic loss of stability, as well as elementary singularities and roots to chaos, of symmetric SMS configurations, such as those recommended by the American Petroleum Institute. Thus, first it becomes easy to identify the morphogenesis occurring when a bifurcation boundary is crossed; and second, it is possible to determine the dependence of a symmetric SMS on any design parameter—such as mooring line length, orientation, pretension, etc. Theoretical results are verified by comparison to numerical results generated with nonlinear dynamics and codimension one and two bifurcation theory. The mathematical model of the SMS consists of the nonlinear third order manoeuvring equations without memory of the horizontal plane slow motion dynamics—surge, sway, and yaw—of a vessel moored to several terminals. Mooring lines are modeled as synthetic nylon ropes, chains, or steel cables. External excitation consists of time independent current, wind, and mean wave drift forces. 相似文献
5.
The dynamic analysis of a deepwater floating structure is complicated by the fact that there can be significant coupling between the dynamics of the floating vessel and the attached risers and mooring lines. Furthermore, there are significant nonlinear effects, such as geometric nonlinearities, drag forces, and second order (slow drift) forces on the vessel, and for this reason the governing equations of motion are normally solved in the time domain. This approach is computationally intensive, and the aim of the present work is to develop and validate a more efficient linearized frequency domain approach. To this end, both time and frequency domain models of a coupled vessel/riser/mooring system are developed, which each incorporate both first and second order motions. It is shown that the frequency domain approach yields very good predictions of the system response when benchmarked against the time domain analysis, and the reasons for this are discussed. It is found that the linearization scheme employed for the drag forces on the risers and mooring lines yields a very good estimate of the resulting contribution to slow drift damping. 相似文献
6.
为了有效地考虑浮体慢漂运动对钢悬链线立管疲劳损伤的影响,提出了波频和慢漂运动组合作用下钢悬链线立管疲劳损伤简化计算的位置组合叠加法。其核心是:基于浮体慢漂运动概率分布选取若干典型慢漂位置,进行波频运动作用下钢悬链线立管动力响应分析;根据钢悬链线立管运动位置变化特征,截取若干慢漂位置对应的波频应力时程叠加到慢漂应力时程上,得到波频和慢漂运动的组合应力时程;编写基于雨流计数法的MATLAB程序处理立管各节点应力,采用海水环境下Do E.E型S-N曲线和Palmgren-Miner累积损伤准则计算立管各节点疲劳损伤。应用位置组合叠加法对某海域500 m水深的立管进行了疲劳分析,并与全耦合法、权重组合叠加法以及波频和慢漂疲劳损伤简单相加法的结果进行了对比,结果表明该方法具有较高的精度和效率。此外,进行了区域设定系数、波浪高度、波浪周期和土壤表面剪切强度等参数对组合作用下立管疲劳损伤的敏感性分析。 相似文献
7.
Tom Marthinsen 《Applied Ocean Research》1983,5(3):141-144
A new method for approximating time dependent drift forces in irregular, long or short crested sea is presented. The approximation is based on the assumption that the wave energy spectrum is narrow-banded. The drift forces may be computed from the spectrum or directly from a time record of waves by using Hilbert transforms. Numerical calculations are made for drift forces and corresponding motions of a floating box, and the method is compared to a commonly used method due to Newman. The two methods are found to agree very well. 相似文献
8.
Based on the second-order random wave theory, the joint statistical distribution of the horizontal velocity and acceleration is derived using the characteristic function expansion method. From the joint distribution and the Morison equation, the theoretical distributions of drag forces, inertia forces and total random wave forces are determined. The distribution of inertia forces is Gaussian as that derived using the linear wave model, whereas the distributions of drag forces and total random forces deviate slightly from those derived utilizing the linear wave model. It is found that the distribution of wave forces depends solely on the frequency spectrum of sea waves associated with the first order approximation and the second order wave–wave interaction. 相似文献
9.
An approximate method is presented for calculating drift forces on arrays of vertically axisymmetric bodies in a wave tank. It is assumed that the wave scattering properties of an isolated element and of the array in the open sea are known. The procedure described allows the open-sea results to be post-processed to give good estimates of drift forces when a structural array is placed in a wave tank. For the particular case of an array of vertical circular cylinders extending throughout the depth, the method is compared with accurate results from the full linear problem for scattering by the array both when the array is in the tank and when it is in the open sea. The results show how the mean forces on the array when in the tank may differ considerably from those experienced in the open sea. 相似文献
10.
K.R. Drake 《Ocean Engineering》2011,38(5-6):810-814
An analytical solution is presented for the horizontal drift force acting on a uniform circular cylinder that is undergoing surge and pitch motion in regular waves. The solution is exact to second order, for a perturbation expansion in terms of wave steepness, but is limited in a strict sense to the hypothetical case when the draught is equal to the water depth. The formulation is used to provide an analytical approximation for investigating the influence of key parameters on the horizontal drift force acting on a deep draught spar. 相似文献
11.
W. Koterayama 《Ocean Engineering》1980,7(3):399-412
This paper presents a method of estimating wave forces acting on a submerged horizontal circular cylinder fixed in oblique waves.The experiments show that drag and inertia coefficients in beam sea are available for calculating the wave forces in oblique waves.Wave forces exerted on a vertical circular cylinder in deep waves are also investigated.The experimental results show that wave forces acting on the vertical cylinder coincide approximately with hydrodynamic forces acting on a submerged circular cylinder in an oscillating fluid. 相似文献
12.
The hydroelastic responses of a very-long floating structure (VLFS) in waves connected to a floating oscillating-water-column (OWC) breakwater system by a pin are analyzed by making use of the modal expansion method in two dimensions. The Bernoulli–Euler beam equation for the VLFS is coupled with the equations of motions of the breakwater taking account of the geometric and dynamic boundary conditions at the pin. The Legendre polynomials are employed as admissible functions representing the assumed modes of the VLFS with pinned-free-boundary conditions. It has been shown numerically that the deflections, bending moments and shear forces of the VLFS in waves can be reduced significantly by a pin-connected OWC breakwater. The time-mean horizontal drift forces of the VLFS equipped with the breakwater calculated by the near-field method are also presented. 相似文献
13.
The motions and time-mean horizontal drift forces of floating backward-bent duct buoy wave energy absorbers in regular waves are calculated taking account of the oscillating surface-pressure due to the pressure drop in the air chamber above the oscillating water column within the scope of the linear wave theory. The present numerical results show that the time-mean drift forces of backward-bent duct buoys are in the reverse direction of propagation of the incident waves over specific frequency ranges as found by McCormick through his experimental work. The drift force has been calculated by the near-field method. A brief discussion on Maruo’s formula which shows that the time-mean drift force must be in the direction of propagation of the incident waves, has also been presented. 相似文献
14.
合成孔径雷达(SAR)海面场景原始数据仿真是研究海洋动力参数(表面波浪、风矢量和洋流)的有效工具。目前海面场景原始数据仿真方法已经基于逆Omega-K算法实现了海洋运动参数的空间变化。但是目前仅仅讨论了正侧视情况下的海面场景仿真,应用范围有限,同时没有考虑Stokes漂流以及Bragg相速度的影响,而这两者都是存在于真实海面的。通常情况下为了反演得到海面流场的二维速度矢量,雷达需要从两个不同的方位方向观察海面的同一个区域,因此这就需要考虑大斜视的雷达波束,同时Stokes漂流和Bragg相速度是SAR海表面流场观测不容忽视的两种运动。本文在不改变原有正侧视逆Omega-K算法的情况下,通过增加重新计算零方位时刻的斜视波束中心位置坐标,并据此确定SAR原始数据在多普勒域的位置来将其扩展到大斜侧视逆Omega-K算法,并通过时域Stokes漂流公式到频域内离散化Stokes漂流公式的推导来加入Stokes漂流,以及根据Bragg散射机制加入了Bragg相速度。仿真结果表明,经过聚焦成像后的SAR图像很好的体现了真实海面波浪场的形状,同时能够很好地反演出设定的雷达径向流场速度,且流速精度误差控制在6%以内。最后也证明了Bragg相速度以及Stokes漂流对于海面流场的影响不可忽视。 相似文献
15.
R.S. Langley 《Applied Ocean Research》1984,6(4):182-186
A starting point in the statistical analysis of the slow drift motions of moored offshore structures is the determination of the statistical distribution of the second order exciting forces. In the past2,6 an exponential probability density function has been used, this being the limiting case as the incident seastate becomes infinitely narrow banded. In the present note the second order force distribution is found for a general seastate and a significant deviation from an exponential distribution is found for those seastates which are likely to occur in practice. The statistical theory is based on a heuristic approximation to the second order force, which is shown to be consistent with a frequently used frequency domain approximation which has been suggested by Newman.8 Although Roberts2 has shown that for very lightly damped systems the response statistics are independent of those of the exciting force, the present results may have a significant effect on the response statistics for higher levels of damping. 相似文献
16.
An analytical solution is presented for calculating the regular wave induced respones of an articulated column in a wave tank. Extension of the procedure leads to calculation of second order mean drift forces and moments. Hydrodynamic interaction between a cylindrical column and the parallel walls of the tank is shown theoretically to be highly significant and experimental data are presented to corroborate this finding. 相似文献
17.
Adrian W. K. Law 《Ocean Engineering》1999,26(11):301
In this study, the surface drift of an inextensible film due to a series of progressive gravity waves is investigated both analytically and experimentally. A second approximation of the conformal mapping that transforms the progressive sinusoidal surface to the horizontal axis is applied, thus allowing the analysis to formally accommodate a boundary layer thickness that is much less than the wave height. By computing the stream function to the third order that is an order higher than the past analysis, a pressure component in phase with the wave slope is revealed. The pressure force generated is comparable to the bottom shear on the thin film induced by the moving fluid and thus cannot be ignored. Based on the combined forces on the surface firm, a mean drift is estimated by assuming that the opposing force is due to the viscous drag induced by the drift motion. The experimental results show that the computed drift velocity based on the present study mostly underestimates the experimental observations, but it is in closer agreement than the classical Phillips' 7/4 estimate. 相似文献
18.
In this paper,the effects of a quay or a solid jetty on hydrodynamic coefficients and vertical wave excitation forces on a ship woth or without forward speed are discussed.A modified simple Green function technique is used to calculate the 2D coefficients while the strip theory is used to calculate the 3D coefficients. Wave excitation forces are also calculated with the strip theory. Numerical results are provided for hydrodynamic coefficients and vertical wave excitation forces on a 200000 DWT tanker ship. It is found that the quay has a considerable effect on the hydrodynamic coefficients and wave excitation forces for a ship. 相似文献
19.
Safety is the topmost priority considered by the designers of floating systems or any other structural systems. Reliability, economy and environmental pollutions and the liability of the structure in the case of accidents are also considered by the designers. The paper concentrates on the non-linear behavior of a moored floating platform in stochastic seaway generated using the Pierson–Moskowitz spectrum. Second-order wave forces (slow drift force) acting on the structure is considered as they contribute to a major percentage of the excursion of a large platform. Wave drift damping and skin friction damping have also been considered.It has been shown that the principal frequency of the second-order motion of the platform due to drift forces closely matches the natural frequency of the system in surge motion. This has been subsequently used in tuning a PID (proportionate integral and differential)-based control system for the surge mode, where reduction in the order of 90% has been observed. 相似文献
20.
Vertical distribution of underwater scattering radiances of the first, the second and the third order is estimated by the Jerlov's method. In the estimation the volume attenuation coefficient and the volume scattering function are assumed to be constant in near surface layer, and the sky radiation separated from solar radiation to be a parallel beam and to fall into the sea in the vertical direction only.In the zenith direction the second scattering radiance is more intense than the first scattering radiance in near surface layer. As for the total radiance which is a sum of radiances up to the third order, the zenith radiation is largest, the nadir radiation is moderate and the horizontal radiation is least. 相似文献