Use of nautical radar as a wave monitoring instrument

Common marine X-Band radars can be used as a sensor to survey ocean wave fields. The wave field images provided by the radars are sampled and analysed by a wave monitoring system (called WaMoS II) developed by the German research institute GKSS. This measuring system can be mounted on a ship, on offshore stations or at coastal locations. The measurement is based on the backscatter of microwaves from the ocean surface, which is visible as ‘sea clutter' on the radar screen. From this observable sea clutter, a numerical analysis is carried out. The unambiguous directional wave spectrum, the surface currents and sea state parameters such as wave periods, wave lengths, and wave directions can be derived. To provide absolute wave heights, the response of the nautical radar must be calibrated. Similar to the wave height estimations for Synthetic Aperture Radars, the so-called ‘Signal to Noise Ratio' leads to the determination of the significant wave height (H_S). In this paper, WaMoS II results are compared with directional buoy data to show the capabilities of nautical microwave radars for sea state measurements.     

Marine sediment acoustic measurement system

This paper describes a system that has been developed to measure compressional wave speed in cored marine sediments onboard ship. The structure enables one to secure an extruded core sample to its base and to move acoustic probes to a desired location, implant them to a specified depth into the sample and perform the measurement. The acoustic measurement system is a pulse-time delay system measuring time difference over a fixed path length and the temperature of the sediment. The time difference and temperature measurement systems are comprised of task oriented components and are housed in a single portable box. The system is adaptable to the various sample sizes obtained with the coring apparatus presently in use. Initial field tests indicate that ship motion has no effect on the system. Data collected from cores has been classified according to sediment type and displays good agreement with data presented by Hamilton (1970). The difference in compressional wave velocity, based on sediment type, for the two studies is 5 m per sec.     

基于Holland台风模型及三重嵌套海浪模式的台风浪数值模拟研究

现有的风场资料存在台风中心附近风速偏低的问题。为改进台风期间风场数据, 使用Holland经验台风模型结合多平台交叉校准数据(cross-calibrated multi-platform, CCMP)及欧洲中期天气预报中心的再分析数据(European Centre for Medium-range Weather Forecasts Reanalysis data, ERA5)风场资料, 研究了不同台风最大风速半径(maximum wind radius of the typhoon, RMW)、Holland B参数对模拟效果的影响, 确定了最优模拟参数, 并以改进后的风场驱动三重嵌套海浪模型对台风“威马逊”发生期间的台风浪进行模拟。模拟结果与实测数据对比表明, (1)改进的风场资料与实测结果更为接近, 作为海浪模式驱动项可更好地模拟台风期间波浪状况; (2)三重嵌套海浪模型的波浪模拟效果优于单独的海浪模型。     

Nonlinear response of a moored buoy

The nonlinear dynamic analysis of a multipoint slack moored buoy is performed under the action of first and second order wave forces. The nonlinearity of the system is caused by the geometric nonlinearity of the mooring lines. The resulting nonlinear equation of motion is solved by an incremental time marching scheme. The nonlinear responses of the system are analysed to investigate different kinds of dynamic instability phenomena that may arise due to the nonlinearity of the system. As an illustrative example, a hollow cylindrical buoy anchored to the sea bed by means of six slack mooring lines is considered. The responses of the system are obtained and analysed for three regular waves namely, 5 m/5 s, 12 m/10 s and 18 m/15 s. The results of the study show that different kinds of instability phenomena like nT subharmonic oscillations, symmetry breaking bifurcation and aperiodic responses may occur in slack mooring systems. Further, a second order wave force may considerably influence the dynamic stability of such systems.     

混响环境中声场重构的实验研究

针对混响环境非自由声场中声源测量的问题,本文以消声水池和混响水槽为实验环境,以换能器辐射的声场为研究对象,以水听器阵列为测量前端,进行了混响环境非自由声场中声源对象的测量、分析和重构的实验研究。通过单层水听器阵列对非自由声场进行声压分布测量,并对测量结果作声波分离处理,将分离前后的声压分布和在消声水池中测量的声压分布进行比较,给出了声源频率为5 000 Hz和7 000 Hz时,声场重构的误差分析结果。结果表明,基于单层水听器阵列声压测量的声波分离方法,能够较精确地对混响环境中的声场进行重构。     

港口内靠码头系泊船运动的计算

本文以英国南海岸处Ｓｈｏｒｅｈａｍ港内系泊船为例，研究了港口内靠码头系泊船运动的数值计算问题。该船的实船实验和模型实验已经完成，为验证本文计算结果提供了依据。运动方程在时域内求解。在计算船体脉冲响应函数时，引入了船体阻尼系数在频率很低时的渐近表达式和一迭代算法。分析了港内共振波浪和其对船体的作用力。讨论了靠码头系泊船运动的非线性特征。计算结果与实验结果符合很好。     

A three-dimensional modeling of multidirectional random wave diffraction by rectangular submarine pits

A three-dimensional modeling of multidirectional random-wave diffraction by a group of rectangular submarine pits is presented in this paper. The fluid domain is divided into N interior regions representing the pit area and an overall exterior region separated by the imaginary pit boundaries. In the interior region, the analytical expressions of the Fourier series expansion for velocity potentials in the pit regions have been derived with the unknown coefficients determined from a series of Green's function based boundary integral equations. The boundary integral approach has also been applied to obtain the velocity potential and free-surface elevation in the exterior region. The Pierson–Moskowitz (P–M) frequency spectrum was selected for the random wave simulation using the superposition of solutions of a finite number of decomposed wave components. Numerical results for the cases of regular waves and random waves are presented to examine the influences of the pit geometry and incident wave condition on the overall wave field. The general diffraction pattern of alternate bands of increase and decrease of relative wave height in front of the pit system can be observed. It is found that, in the shadow region, the relative wave height is reduced. As the number of pit increases, the effectiveness of reducing the relative wave height behind the multiple-pit system increases. However, the relative wave height within the pit area and in front of the leading pit shows increasing trend. It is noticed that under the random-wave condition, the level of increase and decrease of the relative wave height due to the existence of submarine pits is less pronounced than that observed from results in regular-wave condition.     

Deformation of multi-directional random waves passing over an impermeable submerged breakwater installed on a sloping bed

This study investigates the wave deformation of multi-directional random waves passing over an impermeable submerged breakwater installed on the slope. Experiments were conducted in a three-dimensional wave basin equipped with a multi-directional random wave generator. Measurements of the free surface elevations around an impermeable submerged breakwater were carried out using 19 capacitance-type wave gages. In addition, a numerical model is proposed in three-dimensional random wave field. It is shown that the numerical results reproduce the general trend of the experimental results well. Investigations are made to study the effect of the spreading parameter S_max and bottom topography (bottom slope and submerged breakwater) on the wave deformation. It is pointed out that concentration of wave energy with larger values of the spreading parameter S_max is located within narrow limits in onshore side of the submerged breakwater. Furthermore, the supplementary discussion is made by means of numerical results.     

导波式雷达水位计在潮位观测中的应用

文中简要介绍了雷达水位计的测量原理、分类和技术特点,并将其中的导波式雷达水位计和非接触式雷达水位计与传统的浮子式水位计进行了比较,说明了导波式雷达水位计应用于一些特定条件下潮位观测的优势。基于导波式雷达水位计设计了一套水位观测系统。简要介绍了该水位观测系统的系统组成、硬件设计、软件设计。针对该水位观测系统进行了计量检定,并进行了现场安装试验。在现场实验中将导波式雷达水位计数据与浮子式水位计数据进行比较,说明了采用该设计方案进行潮位观测的可行性。     

Computer vision applied to wave flume measurements

A new measurement system for laboratory wave flumes is presented, based on the analysis of digital images by means of computer vision techniques. Unlike conventional wave gauges, the system detects the motions of the free surface along the flume section of interest as opposed to a point. A further advantage lies in that no sensors are necessary within the flume. The system is particularly useful in the case of wave flume tests of floating bodies, such as an oil boom—the motions of the model boom are measured without any sensors in contact with it, which otherwise might place constraints on its displacements. The new method is validated based on free surface measurements carried out with a conventional wave gauge, with excellent results.     

Prediction of ocean waves based on the single-parameter growth equation of wind waves (II) introduction of grid method

The present study is a modification of the wave prediction model presented in the first paper of this title (Kawai et al., 1979) based on the Toba's (1978) single parameter equation of the wind wave growth. The introduction of a grid method reduces the two defects pointed out inKawai et al., i.e., the absence of the prediction of certain instants at fixed points, and the concentration of wave energy at certain points in the wind direction, arising from the lack of treatment of the lateral spreading of wave energy around the wind direction. The new model is applied to the same set of data. The results shows overall improvements, such as the elimination of certain overestimate in the first study and the coincidence of the predicted maximum with the measured one. The swells are separately hindcasted and a very good agreement with measurement is obtained.     

Numerical study of three-dimensional suspended sediment transport in waves and currents

In the present work, a three-dimensional suspended sediment model (SED) is built. A three-dimensional hydrodynamic model (COHERENS) and a third-generation wave model (SWAN) are fully coupled through accounting for mutual influences between wave and current in them. SED is combined with the coupled model built up above. Damping function of suspended sediment on turbulence is introduced into COHERENS. Then a coupled hydrodynamic–sediment model COHERENS-SED incorporating mutual influences between wave and current is obtained. COHERENS-SED is adopted to simulate three-dimensional suspended sediment transport of Yellow River Delta with wave–current co-existing. The simulated tidal current velocities and suspended sediment concentration match well with field measurement data. The simulated significant wave height and wave period for a case with current's effects can give better agreement with measurement data than a case without current's effects. Numerical simulation results of COHERENS-SED are demonstrated to be reasonable though being compared with previous studies and field measurements [Wang, H., Yang, Z.S., Li, R., Zhang, J., Chang, R., 2001. Numerical modeling of the seabed morphology of the subaqueous Yellow River Delta. International Journal of Sediment Research 16(4), 486–498; Wang, H., 2002. 3-dimensional numerical simulation on the suspended sediment transport from the Huanghe to the Sea. Ph.D. Thesis, Ocean University of China, pp. 12–14 (in Chinese)].     

Investigation on Effects of Mooring Line Fractures and Connector Failures for A Hybrid Modular Floating Structure System

The present work reports a Hybrid Modular Floating Structure (HMFS) system with typical malfunction conditions. The effects of both fractured mooring lines and failed connectors on main hydrodynamic responses (mooring line tensions, module motions, connector loads and wave power production) of the HMFS system under typical sea conditions are comparatively investigated. The results indicate that the mooring tension distribution, certain module motions (surge, sway and yaw) and connector loads (M_z) are significantly influenced by mooring line fractures. The adjacent mooring line of the fractured line on the upstream side suffers the largest tension among the remaining mooring lines, and the case with two fractured mooring lines in the same group on the upstream side is the most dangerous among all cases of two-line failures in view of mooring line tensions, module motions and connector loads. Therefore, one emergency strategy with appropriate relaxation of a proper mooring line has been proposed and proved effective to reduce the risk of more progressive mooring line fractures. In addition, connector failures substantially affect certain module motions (heave and pitch), certain connector loads (F_z and M_y) and wave power production. The present work can be helpful and instructive for studies on malfunction conditions of modular floating structure (MFS) systems.

     

Evaluation of methods to remove inertial force from measured model wave impact force signals

The subject of wave impact on offshore structures and their components is important to vessel designers and operators for many reasons. They are often required to quantify these impact loads. Standard methods for wave load prediction will underestimate the forces on these structures due to intermittent loading. This necessitates the use of physical model tests to establish wave impact loads. The model measurement systems are designed to have high stiffness. This ensures that the natural frequency of the structure is above the wave frequency. However, it is widely believed that impacting waves contain high-frequency energy components that cause the structure to vibrate at its modal frequencies. This impact-induced vibration is recorded by the measuring system as a force (inertial force), and corrupts the actual applied force measurement. Before scaling can occur, the inertial force must be removed from the measured signal.A number of techniques for removing inertial force from measured signals are described in the published literature. Three methods are discussed, implemented and compared in this paper. The algorithms and procedures are presented. Each technique contains inherent and unique problems, while some are common to all methods.Neither of the techniques produced results that are fully satisfactory. The main problem is unwanted high-frequency content after the application of the methods. While neither method offers the perfect solution, the use of digital filtering techniques is recommended based on their relative performance and ease of implementation.     

Study on visual control system of acoustic in situ measurement technology for survey of seafloor sediment

Abstract

A new kind of visual acoustic measurement system was developed to obtain the acoustic characteristics of seabed sediment and the working state of in situ measuring equipment in real time. The control unit of the system structure is mainly made up of the underwater control unit and the deck control unit. In comparison with similar traditional systems, the system can transmit the working video image, the measured acoustic data and the waveform synchronous from the underwater control equipment to the deck control unit through the connecting cable. Apart from self-contained in situ measurement, the working modes of it include visual online real-time control. In order to guarantee the stability and measurement precision of the video control system, the process control of in situ measurement was strictly tested in the experimental tank. In 2017, sea trials of the in situ measurement system was carried out in the South China Sea. In the process, data related to measurement, including the acoustic velocity and attenuation coefficient of the seabed sediment, were obtained as well. The results from sea trials show that the control process of the visual system is intuitive, reliable and accurate, and therefore it can be popularized and applied.     

Nearshore directional wave measurements by surface-following buoy and acoustic Doppler current profiler

Directional energy spectra of nearshore surface waves were measured for a 3-year period (2004–2007) at a site with mean depth 14 m and mean tidal range 2.1 m. Triaxys surface-following wave buoys reported hourly directional wave energy spectra and wave parameters near the offshore end of the Savannah River Entrance Channel, Georgia, USA. An acoustic Doppler current profiler (ADCP) was located beside the wave buoy for 3 months. Directional and non-directional surface wave energy spectra and the corresponding bulk wave parameters (height, period, and direction) are compared for the two systems. Most parameters derived from the spectra agree closely; the most significant differences were found at the upper and lower frequency measurement limits, where signal-to-noise ratios were lower. The wave buoy consistently reports a small amount of energy below 0.05 Hz that does not appear in the ADCP-derived spectra and does not appear to be related to the mooring system. This leads to larger mean and peak periods reported by the buoy. All directional spectra were computed using the Maximum Entropy Method for both instruments, but the buoy, with spectra derived from six independent time series, provides lower directional resolving power than the ADCP, which utilizes twelve time series. Both systems gave similar results defining mean and peak wave directions, with the primary difference being that the ADCP indicates energy to be more tightly concentrated around the peak direction.     

近岸带水下沙丘的群体运动

在近岸带,水下沙坝(或沙丘)常以大型沙波的型式作群体运动,从而对多种海岸工程造成威胁。 Choule.J.Sonu等曾以大量实测资料,将进行群体运动的沙波划分为坝式和岬式两类,并分别指出其动力成因;随后又以数理统计方法,确认坝式沙     

基于MEMS加速度计空投浮标的波浪测量方法试验研究

针对基于MEMS加速度传感器的空投波浪浮标存在采样频率与测波精度低的问题，根据频域衰减积分算法，提出一种相应的波浪测量算法，为了验证该算法测波的准确性，开展了多功能水槽试验研究。该算法旨在将MEMS加速度传感器输出的加速度与姿态角转化为浮标运动的波形，首先将加速度与姿态角信号进行竖向处理获得竖直方向的加速度，再利用离散傅里叶变换将竖向加速度转化为频域内的加速度复数序列，然后引入控制函数减弱低频噪声，经过频域积分、离散傅里叶逆变换、时域积分获得竖直方向的位移，最后通过后处理得到最终的波形。多功能水槽试验采取10中不同波高和周期的工况，对比空投波浪测量浮标与波高仪的测量结果，试验结果表明，浮标的测量误差在10%以内，达到测波标准。     

Comparison of offshore wave measurements

Preliminary results of a comparison between several wave measurement systems used during the Atlantic Remote Sensing Land Ocean Experiment (ARSLOE) are presented. Measurements in the offshore region 12-40 km offshore, taken during a storm that occurred October 23-26, 1980, are compared. Disagreement between significant waveheights resulting from the different measurement systems in some cases are larger than can be explained by random variability.