An alternative approach to the structural motion analysis of wedge-beam offshore structures supporting a load

For convenience of dynamic analysis, some offshore structures such as fixed-type platforms are often modeled as the wedge beams supporting tip lumped masses. It is well-known that, due to the effect of the surrounding water, the natural frequencies of a beam in air (or dry beam) are different from those of the same beam immersed in water (or wet beam). However, if the natural frequencies and the associated mode shapes of a dry beam are calculated by taking account of the “added mass” for the immersed beam, then the last natural frequencies and mode shapes will be equal to the corresponding ones of the wet beam. Based on the last concept, the closed form solutions for natural frequencies and the associated mode shapes of the dry beam were determined first, then the partial differential equation of motion for the wet beam was transformed into a matrix equation by using the expansion theorem and the foregoing closed form solutions of free vibration responses for the dry beam. Solving the last matrix equation will give the required natural frequencies and the associated mode shapes of the wet beam. The formulation of this paper is available for the fully or partially immersed double tapered beams with either circular, square or rectangular cross-sections. The taper ratio for width and that for depth may be equal or unequal. The numerical results of this paper were compared with the existing results or the finite-element-method results and good agreement was achieved.     

Distribution of nonlinear wave crests

The distribution of nonlinear wave crests is examined on the basis of a theoretical probability density previously given elsewhere (J. Eng. Mech. 120 (1994) 1009). Certain errors contained in the original theoretical density are corrected, and the corresponding exceedance distribution is derived. The resulting theoretical forms of the probability density and exceedance distribution are then slightly simplified and compared with nonlinear wave data gathered under hurricane conditions. The results indicate that the proposed theoretical forms describe the observed distributions of large wave crests better than the Rayleigh law. However, the quantitative accuracy of the predictions is somewhat poor, as is typical of approximate theories based on Gram–Charlier-type expansions.     

Digital signal processing for precision wide-swath bathymetry

A mathematical model is formulated which accurately represents the envelope function of bottom return signals received from a number of spatial directions comprising a wide swath. The bottom return signals are processed utilizing a digital nonrecursive matched filter whose coefficients are tapered using a Tukey window. High-speed convolution employing the fast Fourier transform is examined for implementation of the digital matched filtering operation. Computer simulation of the signal processing system indicates that, even in the presence of considerable background and fluctuation noises, the processor provides an output signal having a well-defined peak. The error in time of arrival is found to be less than 3 ms, corresponding to an error in depth of less than 0.1 percent, for an average signal-to-noise ratio of 15 dB and a vertical ocean depth of 12 000 ft (3.7 km). These performance figures apply to the most difficult case of mapping at angles ofpm 45degoff vertical.     

A numerically efficient data analysis method with error map generation

The variational inverse model (VIM) for data analysis was already shown to be statistically equivalent to objective analysis (OA) provided the covariance function for OA and the VIM reproducing kernel are identical. The VIM, however does not allow a direct derivation of the error field associated with the analysis. The purpose of the paper is to extend the one-to-one correspondence between the two analysis schemes by proposing a heuristic statistical error expression for the VIM. The numerical efficiency on analysis and error map generation of both methods is compared on quasi-synoptic and climatological data sets. It is shown that the VIM analysis and error map generation offers interesting numerical skills in both case studies.     

Research on the Effects of in-Line Oscillatory Flow on the Vortex-Induced Motions of A Deep Draft Semi-Submersible in Currents

A Deep Draft Semi-submersible (DDS) under certain flow conditions could be subjected to Vortex-Induced Motions (VIM), which significantly influences the loads on and life fatigue of the moorings and the risers. To investigate the VIM of a DDS with four rectangular section columns in waves coupled with a uniform current, a numerical study using the computational fluid dynamics (CFD) method was conducted. The issues of the VIM of multi-column floaters can be conveniently converted to the issues of oscillating cylinders in fluid cross flows. This paper looks into the CFD numerical simulation of infinite cylinders having rectangular sections in a two-dimensional sinusoidal time-dependent flow field coupled with a uniform current. The resulted hydrodynamic forces and motion responses in different oscillatory flows plus currents both aligned in the same direction for the incidence of 135° of the DDS relative to the flow are compared with the ones in current only cases. The results show that the VIM response of this geometric arrangement of a DDS with four rectangular columns in a current combined with oscillatory flows is more evident than that in the current only case. The oscillatory flows and waves have the significant influence on the VIM response, forces and trajectory, in-plane motions of the DDS.     

Natural frequency of offshore wind turbines on rigid and flexible monopiles in cohesionless soils with linear stiffness distribution

A simplified method is introduced to obtain the fundamental frequency of offshore wind turbines supported by monopile foundations. Soil-pile interaction is modeled based on Winkler approach and concept of beam on elastic foundation. The soil is considered to have linearly varying modulus of subgrade reaction along depth which is a typical assumption for cohesionless soils. Rayleigh method which is based on conservation of total energy of the system is utilized. Firstly the natural frequency of the system with rigid pile is derived and then an innovative procedure is introduced to take pile flexural stiffness into consideration. Comparison between results of the present method with those of a numerical FE model for a typical 2 MW wind turbine structure shows excellent agreement for rigid pile and flexible pile with small value of slenderness ratio. The agreement is also good for flexible pile with higher slenderness ratios. A parametric study is carried out to investigate the effect of important parameters of foundation including pile slenderness ratio, pile aspect ratio and pile mass on the system natural frequency.     

一种新型干树式半潜平台设计

开发了一种新型的深吃水干树式半潜生产平台(TCDD-Semi),该平台可用于中国南海深水区域的油气田开发。它的一个显著特点是立柱的截面形状呈现为渐变形式,而非传统的等截面形式。立柱的底部最大,向上逐渐变小,再结合适当的下浮体设计,能够为平台在服役前期及在位状态时提供足够的浮力以及稳性。最重要的是这种新型立柱的设计能够明显地减小平台在恶劣海况时的垂向运动,从而使在平台上布置具有干式采油树的TTR立管系统成为可能。此外,这种渐变式的立柱设计也有利于减弱平台在流作用下的涡激诱导运动(VIM)。分析结果表明这种渐变式立柱半潜平台可以明显降低平台的垂向运动,也可以在码头组装时提供足够的浮力以及在平台下沉过程中提供足够的稳性。平台优越的垂向运动性能可以使顶部张紧式立管的行程范围限制在10.5 m之内(包括平台偏移、垂荡运动、潮汐影响、海床沉降、热膨胀等因素)。分析结果表明平台的垂荡范围、水平偏移、倾侧角度、上部模块重心处的加速度等可以满足设计要求,证明了这种新型干树式半潜平台的可行性。     

Performance of sequential decoding of convolutional codes overfully fading ocean acoustic channels

Sequential decoding of long-constraint convolutional codes is shown to be a feasible technique for digital data telemetry over realistic marine acoustic channels. A computational bound for sequential decoding over a fading dispersive channel is derived for hardlimiting and quantizing decoders. The results indicate that a minimum of 8 dB of bit SNR (signal-to-noise ratio) is required for sequential decoder operation. Simulations indicate that 14-dB bit SNR results in simple and feasible implementations. Diversity methods for coded transmissions over Rayleigh fading channels are examined. The optimal diversity level for minimum error probability of uncoded systems and the diversity level of minimizing the sequential decoder computational load are derived and shown to be different, with the latter requiring a higher order of diversity. Performance differences between fixed-diversity and optimal-diversity systems are presented     

均匀流中深水系泊Truss Spar平台涡激运动试验研究

在工程中广泛应用的Truss Spar平台自诞生之初就为涡激运动问题所困扰,对其运动特性及抑制方法的机理性研究始终都在进行。采用截断系泊系统水池模型试验对设计中的Truss Spar平台在均匀流中的涡激运动响应进行了研究,分析了有无侧板平台的运动轨迹以及不同速度及流向角来流对平台运动的平衡位置的影响。在影响平台涡激运动特性的重要因素中,选取折合速度及流向角分别加以研究,分析了幅值在锁定区间中的变化规律,验证了减涡侧板对涡激运动良好的抑制作用,并对热点问题进行了探讨。为进一步研究Spar平台的涡激运动特性奠定了基础。     

立柱直径比对四立柱平台涡激运动性能影响的数值研究

与单立柱平台(Spar)类似,四立柱平台在洋流作用下也容易发生涡激运动,对锚泊和立管系统的疲劳寿命产生不利影响。一种非对称半潜式平台具有两种不同尺寸的立柱,立柱直径比对非对称平台涡激运动性能的影响尚需深入研究。采用计算流体力学(CFD)方法,对直径比分别为1.0、1.5和2.0的立柱组合在两种不同角度水流作用下的运动响应进行了二维数值模拟,分析发现:立柱直径比不为1时四立柱平台也具有明显的涡激运动,其横荡运动具有显著的锁定现象;在广义折合速度定义下,与相同直径的四立柱平台相比,立柱直径比不为1时其横荡运动锁定区间变宽且滞后,横荡响应幅值的峰值具有明显差异;在所研究的直径比范围内,大直径立柱在上游时涡激运动响应比其在下游时更显著,且这种差异随着直径比变大而变大。     

拖曳线列阵声纳中隔振模块研究

介绍了作为隔振模块研究依据的拖缆抖动特性的试验研究结果及隔振模块理论分析方法 ,给出了隔振模块有关参数变化对隔振量影响的理论值 ,提供了隔振模块隔振量试验方法 ,某隔振模块隔振量的测量结果以及装与不装隔振模块情况下 ,声阵的拖曳噪声声压谱级的测量结果     

New theoretical aspects of statistical properties of narrow-banded random waves

By using a new concept of the discrete amplitude, we examine the statistical properties of narrow-banded random waves. The main results are as follows: (1) the wave height distribution follows the Rayleigh distribution in the case of an infinitely narrow-banded spectrum in the strict sense; (2) the discrete amplitudes, different from the crest heights, are distributed according to the Rayleigh distribution for arbitrary bandwidth spectra; (3) the statistical distribution of the gap length between the discrete amplitude and the crest is examined and derived theoretically. The derived distribution agrees well with the result of numerical simulations; (4) taking into account the gap length distribution, the probability density function of crest heights is derived, which deviates from the Rayleigh distribution.     

Experimental and numerical study on vortex-induced motions of a deep-draft semi-submersible

An experimental study on vortex-induced motions (VIM) of a deep-draft semi-submersible (DDS) was carried out in a towing tank, with the aim to investigate the VIM effects on the overall hydrodynamics of the structure. In order to study the fluid physics associated with VIM of the DDS, a comprehensive numerical simulation was conducted to examine the characteristics of vortex shedding processes and their interactions due to multiple cylindrical columns. The experimental measurements were obtained for horizontal plane motions including transverse, in-line and yaw motions as well as drag and lift forces on the structure. Spectral analysis was further carried out based on the recorded force time history. These data were subsequently used to validate the numerical model. Detailed numerical results on the vortex flow characteristics revealed that during the “lock-in”, the vortex shedding processes of the upstream columns enhance the vortex shedding processes of the downstream columns leading to the rapid increase of the magnitude of VIM. In addition to the experimental measurements, for the two uniform flow incidences (0° and 45°) investigated, comprehensive numerical data of the parametric study on the VIM characteristics at a wide range of current strength will also serve as quality benchmarks for future study and provide guidance for practical design.     

Stream-wise and cross-flow vortex induced vibrations of single tapered circular cylinders: An experimental study

Tapered circular cylinders are employed in a variety of ocean engineering applications. While being geometrically simple, this configuration creates a complex flow pattern in the near wake of the structure. Most previous experimental studies on tapered circular cylinders were dealing with stationary cylinders to explore the wake flow field and vortex shedding patterns past the cylinder. Few studies paid attentions to the vortex induced vibration of the tapered cylinders. This paper reports some results from in-water towing-tank experiments on the vortex-excited vibrations of tapered circular cylinders in a uniform flow. Cylinders with different mean diameters (28 and 78 mm), mass ratios (6.1 and 2.27) and tapers (5–20), along with their equivalent uniform cylinders, have been examined. The single degree of freedom vibrating system has a low mass-damping parameter (m*ξ = 0.0084–0.0279). The Reynolds number, based on mean diameter of the cylinders, ranges from 1400 to 70,200. The reduced velocities vary from 1.5 to 22. Effects of variations in the taper and mass ratios on the lock-in range, the reduced response amplitude, the reduced velocity for the peak vibration response and other stream-wise and cross-flow VIV parameters are reported and discussed.     

基于重叠网格方法浮筒涡激运动特性数值分析

浮筒被广泛应用在海洋工程中,研究浮筒的涡激运动对于减少其对海洋平台构件的疲劳损坏具有指导意义。传统的动网格方法在处理浮筒转动运动时会因网格变形过大导致计算不收敛,采用了重叠网格方法以解决这一问题。数值试验采用了基于开源工具包Open FOAM自主开发的naoe-FOAM-SJTU求解器。分别进行了自由衰减数值试验和涡激运动数值试验。研究表明,随着折合速度的增加,浮筒的顺流向、横流向、垂荡和艏摇运动频率增加,且顺流向与垂荡频率相近,横流向与艏摇频率相近;其次,根据涡量场分布,表明浮筒前一时刻的泻涡会影响到浮筒之后时刻的周向涡量分布;最后,研究发现自由端对于浮筒尾流场泻涡有着显著影响,为将来探究减少浮筒涡激运动的方法提供指导。     

Torsional Dynamic Impedance of a Tapered Pile Considering its Construction Disturbance Effect

The dynamic response of a tapered pile (considering its construction disturbance effect) is investigated when the tapered pile is subjected to a time-harmonic torsional loading. For most engineering conditions, the surrounding soil may be weakened or strengthened owing to the construction disturbance effect of the tapered pile, resulting in the soil becoming radially inhomogeneous. In order to consider this problem, the circumferential shear complex stiffness transfer model is proposed to simulate the radial inhomogeneity of soil. Then, the governing equations of a tapered pile-soil system subjected to torsional dynamic loading are established. By virtue of the circumferential shear complex stiffness transfer method and the impedance function transfer method, the analytical solution of torsional dynamic impedance at the head of the tapered pile is derived. Based on the presented solution, the influence of the construction disturbance effect of the surrounding soil on the torsional dynamic impedance at the pile head is investigated within the low-frequency range concerned in the design of a dynamic foundation. The results show that, even if the hardening range and softening range of the surrounding soil vary within a smaller scale, the hardening effect and softening effect also have a notable influence on the torsional dynamic impedance at the pile head.     

Effect of the tangential component of a force field on convection in a rotating plane layer

Thermal convection in an inclined plane layer having boundaries with different temperatures and rotating around an axis perpendicular to its plane is studied experimentally. It is shown that the convection and heat transfer are determined by two different convective mechanisms—gravitational and thermovibrational ones; they manifest themselves in the threshold excitation of cellular convective structures with different sizes. The thermovibrational convection is caused by the period-averaged mass force arising as a result of tidal oscillations of a nonisothermal fluid with respect to the cavity under the action of the external force-field component tangential to the layer. The map of convection regimes on the plane of governing parameters—the gravitational Rayleigh number and its vibrational analog—has been constructed. It has been found that the thermovibrational convection can develop even in a layer heated from above. The role of the dimensionless velocity of rotation, which has a stabilizing effect on the excitation of both gravitational and thermovibrational convection, has been studied.     

Forced vibrations of a beam elastically restrained against rotation and carrying a spring–mass system

An exact solution for the title problem is obtained in closed form fashion in the case of a Bernoulli–Euler beam. It is assumed that the exciting force is applied to the mass which is elastically mounted on the beam. The mathematical model constitutes a first order approximation to a motor or engine elastically mounted on a structural element. The operation of the machine generates a transverse, sinusoidally varying force. The problem is of basic interest in mechanical, naval and ocean engineering systems from the point of view of the determination of dynamic displacements and stresses; sound radiation calculations, etc. The present problem arose in connection with the mounting of an engine on a structural beam in a small naval vessel and when excessive vibrational level was noted. This study was undertaken in order to understand the physical problem and to correct the mechanical situation     

高光谱碘分子和双边缘多普勒测风激光雷达技术比较

对采用原子滤波器的激光雷达技术和双边缘技术的灵敏度和误差进行了详细的讨论。对于纯 Rayleigh散射 ,双边缘系统的灵敏度高于基于碘分子的激光雷达系统 ,而对于 Rayeigh- Mie混合信号 ,双边缘系统通过采用特殊工作点的办法 ,有效解决 2种信号响应不同的问题 ;但同时该工作点的选取使得系统灵敏度降低 ,误差增加。对于后向散射比不是很大 (<10 )的情况下 ,误差可以控制在1m/s以内 ,完全可以满足测量要求 ,因此对于对流层顶部和平流层测风很适合。基于碘分子的激光雷达系统利用碘分子的高吸收性对 Rayleigh- Mie信号有效的进行分离 ,并且灵敏度随着后向散射比的增加而增加 ,从而对底层大气的探测特别有利。     

Radar estimation of water content in cumulonimbus clouds

The method and results of radar researches of vertically and volumetric integrated water content in powerful cumulonimbus (Cb) clouds obtained for the first time are considered. It is established that in hailstorms of Northern Caucasus vertically integrated liquid (VIL) water content varies in limits from 8 up to 50 kg/m², in shower clouds—from 0.5 up to 12 kg/m², in Nimbostratus (Ns) clouds—it is usual less than 0.5 kg/m² and in clouds with a drizzle—less than 0.05 kg/m². The main water content of hailstorms in a stage of development is concentrated in their supercooled layer, in a maturity stage—in a layer from the ground up to height 8–10 km and in a stage of dissipation—in a ground layer. The ratio of VIL of the supercooled and warm parts of cloud allows estimating hail dangers of clouds and stage of their development. It is shown that the volume of hailstorms varies in limits from 10³ up to 5 × 10⁴ km³ and their volumetric integrated mass (VIM) of water content — from 10⁵ up to 6 × 10⁶ tons. The volume of hail localization seldom exceeds 5–25% from total cloud volume, but its contribution to VIM achieves 30–60%. Speed of precipitation formation in powerful hailstorms achieves 1 × 10⁴−5 × 10⁵ tons/min and the same order of value has speed of recession of VIM in their stage of dissipation.