Wave direction analysis from data buoys

This paper describes a new procedure of directional wave analysis from pitch-roll buoy measurements. The two previous procedures adopted by the National Data Buoy Center (NDBC) [Steele, K. E., Lau, J. C. K. and Hsu, Y.-H. L. (1985) Theory and application of calibration techniques for an NDBC directional wave measurement buoy. IEEE Journal of Oceanic Engineering OE-10(4), 382-396; Steele, K. E., Teng, C.-C. and Wang, D. W. C. (1992) Wave direction measurements using pitch-roll buoys. Ocean Engineering 19(4), 349-375] are relevant to our formulations. In these two studies, an estimate for the total phase shift of the sea surface displacement/slope spectra from the measured buoy heave/pitch and heave/roll spectra was calculated either by a weighted average method or a maximum heave/pitch quad-spectrum method. These two formulations were based on a fundamental assumption of symmetric hull-mooring effect on pitch and roll motions, which will never be true in the oceans. In the present study we essentially incorporate the basic formulations of NDBC, but calculate two estimates for this total phase shift.Examples of directional wave analysis from data measured by a 3 m diameter discus buoy during Typhoon Herb are presented in this paper. This data set was also analyzed by the weighted average method of Steele et al. (1985) which yielded unsatisfactory results of wave directions during severe wave climates.     

Rain measurement on buoys using hydrophones

The impact of rain and spray on the ocean disturbs the sea surface and generates underwater ambient noise. The short scale roughness is influenced by impacting drops due to the momentum transfer. Radar and sonar signals are scattered by the short elements of the sea surface. Spray and rain impact change their characteristics, and consequently affect radar and sonar backscatter. In situ measurements of rain and spray impact are necessary to study their effects on the sea surface. Accurate sea measurements of rain momentum fluxes and drop size distributions are a complex problem, especially on buoys. A new measuring technique has been developed using hydrophones. Exposed to precipitation, these instruments are affected directly by the impact of rain. A drop falling on the hydrophone deforms its surface and is sensed by a piezoelectric transducer. The voltage output of the sensor is a rapidly decaying oscillation. The integral value of this signal is a measure of the drop momentum, and the drop size can be deduced. Laboratory studies of defined drops as well as field measurements of natural rain have shown that hydrophones can be used to determine drop momentums and drop size distributions. Based on simultaneous rain measurements by a Joss-Waldvogel Disdrometer and a hydrophone, an analytical function has been derived which relates drop size and hydrophone voltage output     

Wave measurements by pressure transducers using artificial neural networks

Underwater ultrasonic acoustic transducers are frequently used in ocean wave measurements as they measure surface level using acoustic waves. However, their effectiveness can be severely affected in rough sea conditions, when bubbles generated by breaking waves interfere with their acoustic signals. When the seas are rough, one therefore often has to rely on a pressure transducer, which is generally used as a back-up for the acoustic wave gauge. A pressure transfer function is then used to obtain the surface wave information. Alternatively, the present study employed an artificial neural network to convert the pressure signal into significant wave height, significant wave period, maximum wave height, and spectral peakedness parameter using data obtained from various water depths. The results showed that, for water depths greater than 20 m, the wave parameters obtained from the artificial neural network were significantly closer to those obtained by the acoustic measurements than those obtained by using a linear pressure transfer function. Moreover, for a given water depth, the wave heights estimated by the network model from pressure data were not as good as those estimated by linear wave theory for large wave heights (above a 4 m significant wave height in this study). This can be improved if the training data set has more records with large wave heights.     

Wave direction measured by four different systems

During a March 1977 experiment, four systems were used to provide wave-direction information offshore of Mission Beach, CA: a synthetic aperature radar (SAR) carried aboard a NASA CV990 aircraft, a coastal imaging radar, a pressure-gauge array offshore, and aerial photography aboard two aircraft. The coastal radar, aerial photographs, and SAR provided wave images. From the coastal radar images and the aerial photographs, the direction and length of the principal wavetrains were measured by a manual analysis. The SAR images were also processed using an FFT to give two-dimensional wave spectra. The array at the Naval Ocean Systems Center (NOSC) tower was used to provide directional wave spectra. Scatter diagrams are presented, which intercompare the measurements from these four systems. In addition, radar image spectral information is compared with the array spectra. The intercomparison of the data from these four systems shows good agreement among the imaging systems. Between the imaging systems and the pressure array there is agreement for the most prominent wavetrains and disagreement for several cases where multiple wavetrains from different directions but with similar periods are present.     

Accurate current measurement using drogued buoys employing DeccaNavigator and Argos

Free-floating drogued buoys have been developed for detailed studies of tidal processes, frontal circulation, eddies, and mixing in the North Sea. They are equipped with two independent tracking systems-Decca Navigator and Argos. Decca Navigator, a hyperbolic radio navigation system with extensive coverage in Western Europe, provides accurate and frequent fixes, and the Argos satellite system is used to monitor progress in real time and to assist recovery. The low power consumption microprocessor-based Decca Navigator receiver designed specifically for this application synchronizes to the precisely timed Decca transmission and then goes into sleep mode, waking briefly at 10-min intervals to take position fixes. The position data are stored in a 2-Mb memory card that has a three-month capacity at the 10-min rate. The recovery procedure, using both Argos and a small VHF beacon, is described, together with other recent developments. A typical buoy track from a study in the North Sea is included     

Sea state estimation using multiple ships simultaneously as sailing wave buoys

The article presents initial ideas towards a network-based approach for sea state estimation used for marine operations and other maritime applications. In principle, all available means, ranging from in situ buoys, fleet of ships to remote sensing by satellite and aircraft, could be considered, emphasising that each means and any combinations among may act simultaneously. This study focuses on just one of the means; the use of ships as sailing wave buoys. The article introduces the wave buoy analogy, i.e. ship-as-a-wave-buoy, and it makes a proposal on how to impose (different) weights to the single ship-specific wave spectrum estimates obtained from multiple ships. Moreover, the work includes a discussion about the importance to associate a measure to reflect the (un)certainty of the wave spectrum estimate. The article presents a numerical case study, where multiple ships act simultaneously as wave spectrum-estimators. The case study relies on numerical motion simulations, as appropriate full-scale data is not yet available. In the analysis, it is shown that the use of simultaneous data from multiple ships leads to more accurate wave spectrum estimations.     

Wave prediction using wave rider position measurements and NARX network in wave energy conversion

Several control methods of wave energy converters (WECs) need prediction in the future of wave surface elevation. Prediction of wave surface elevation can be performed using measurements of surface elevation at a location ahead of the controlled WEC in the upcoming wave. Artificial neural network (ANN) is a robust data-learning tool, and is proposed in this study to predict the surface elevation at the WEC location using measurements of wave elevation at ahead located sensor (a wave rider buoy). The nonlinear autoregressive with exogenous input network (NARX NN) is utilized in this study as the prediction method. Simulations show promising results for predicting the wave surface elevation. Challenges of using real measurements data are also discussed in this paper.     

Lagrangian validation of numerical drifter trajectories using drifting buoys: Application to the Agulhas system

The skill of numerical Lagrangian drifter trajectories in three numerical models is assessed by comparing these numerically obtained paths to the trajectories of drifting buoys in the real ocean. The skill assessment is performed using the two-sample Kolmogorov–Smirnov statistical test. To demonstrate the assessment procedure, it is applied to three different models of the Agulhas region. The test can either be performed using crossing positions of one-dimensional sections in order to test model performance in specific locations, or using the total two-dimensional data set of trajectories. The test yields four quantities: a binary decision of model skill, a confidence level which can be used as a measure of goodness-of-fit of the model, a test statistic which can be used to determine the sensitivity of the confidence level, and cumulative distribution functions that aid in the qualitative analysis. The ordering of models by their confidence levels is the same as the ordering based on the qualitative analysis, which suggests that the method is suited for model validation. Only one of the three models, a 1/10° two-way nested regional ocean model, might have skill in the Agulhas region. The other two models, a 1/2° global model and a 1/8° assimilative model, might have skill only on some sections in the region.     

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。     

大型附着生物对近海圆盘浮标污损的特点

为了解大型附着生物对近海圆盘浮标污损的特点,对布设在珠江口东南海域和北部湾东北部海域的4个圆盘浮标的大型附着生物群落进行分析研究。结果表明,浮标侧壁大型附着生物的丰度和生物量分别为400.00～78 296.00 ind./m2和659.42～62 276.00 g/m2,底部的丰度和生物量则为412.00～66 585.00 ind./m2和1 861.60～60 784.00 g/m2,多数情况下浮标底部大型附着生物的丰度和生物量高于侧壁。浮标底部的香农-威纳(Shannon-Wiener)多样性指数(H′)介于2.39～3.06之间,马格列夫(Margalef)丰富度指数(d)为4.02～6.98,皮洛(Pielou)均匀度指数(J′)为0.88～0.91;而浮标侧壁的H′为0.64～2.79,d为1.10～4.89,J′为0.58～0.96,其中H′和d均表现出底部高于侧壁。聚类分析和非度量多维标度分析结果表明,在30%的相似性水平上,可将各站位浮标侧壁和底部的大型附着生物群落分为4个群组,其中浮标底部基本上可归成1个群组,但浮标侧壁之间差异较大。单因子相似性分析和相似性百分比...     

Large angular motions of tethered surface buoys

A three-dimensional coupled analysis of the interaction of a floating buoy and its mooring is studied. External loads include hydrodynamic forces, tether tensions, wind loads and system weight and buoyancy. Nonlinearities include large rotational and translational motions and non-conservative fluid loads. The mooring problem is formulated as a nonlinear two-point-boundary-value-problem. At each instant in time, the mooring problem is solved by direct integration using a successive iterative algorithm to satisfy boundary conditions. Buoy kinetic and kinematic equations are derived assuming large angles represented by Euler parameters. Coupling between the buoy and the mooring is enforced by matching the velocities of the tether and buoy at the attachment point. A predictor-corrector coupling algorithm is used with multiple sizes of time steps used to provide stability for the separate mooring and buoy models. Numerical results are compared to experimental responses of three types of buoys (sphere, spar and disc) subject to both regular and irregular waves.     

Analysis of the surface circulation in the Black Sea performed by using drifting buoys with satellite communications

We present the results of drift experiments with 14LOBAN-TM buoys carried out for the first time in the Black Sea in 1987–1997. We analyze the specific features of application of buoys in summer and winter. We present the main results of investigation of the Main Black Sea Current by drift methods and joint processing of drift contact and remote infrared observations. We demonstrate the possibility of enhancement of the quality of observations due to the application of SVPB drifters of new generation equipped with underwater sails and meters of atmospheric pressure in the near-surface atmospheric layer. Translated by Peter V. Malyshev and Dmitry V. Malyshev     

Surface currents derived from satellite-tracked buoys off Namibia

To study the flow field off Namibia (20–30°S, 10–15°E), 48 satellite-tracked buoys were deployed and tracked in six bimonthly batches between July 1994 to September 1995. In situ supporting wind information was collected from a weather buoy moored off Lüderitz, from coastal stations and from voluntary observing ships. Buoy drift tracks were compared with surface topography data from the TOPEX/POSEIDON satellite and satellite infrared images. Most of the buoys drifted in a northwesterly direction, the buoys deployed in the south generally moving faster and diverging more from the coast than the northern buoys. The overall maximum daily drift velocity was 72 cm s^-1, but typical speeds were 10–30 cm s^-1. In the proximity of the coast some buoys experienced transient southward sets associated with the effect of coastal trapped waves, while tracks north of 23°S showed inertial oscillations.     

Note on wave parameters from moored wave buoys

For decades, the accelerometer wave buoy has been a preferred choice for offshore wave measurements. Although these measurements are accurate and robust, there are some issues of practical character that need to be inspected before using such measurements for detailed time-series investigations. Here three potential sources of inaccuracies are outlined which can appear due to improper mooring, limited high-frequency resolution or overly simple procedures for attaching measurement times (time stamps) to the measurements. The last two of these apply to all types of single-point wave-measuring devices.

An example of a wave-height series is given, in which part of the observed variation seems to be induced by the mooring. It is argued that unexpected semi-tidal modulations in measured wave-height can be an indication of a mooring that is too rigid. By truncating observed wave spectra from a deep-water location, it is demonstrated how the high-frequency cut-off limit of a wave measurement influences the most commonly used wave parameters. It is observed that the accuracy of common wave parameters remains acceptable up to a cut-off limit in the range of 0.30–0.35 Hz if the spectra above the cut-off frequency are replaced by a prognostic f⁻⁵ tail. Finally it is noted that the procedure of connecting time stamps to wave measurements can in some cases introduce an artificial time-lag compared to the real-time sea state.     

A semi-rigid connector for buoys and marine platforms

A system is described which has the capability of maintaining two large structures apart, floating or submerged, but allowing their relative movements due to current, waves and wind.Such a system has been designed and built. The tests carried out show that the developed theory is correct within the specified limits of validity. A practical test in the sea has shown the efficiency of the system and suggested some possible improvements.Its main characteristics are simplicity, reliability and low cost.     

Question of the pitch-roll buoy response to ocean waves as a simple harmonic oscillator?

The assumption that the East and North deck slopes of a pitch–roll buoy respond to East and North sea slopes as simple harmonic oscillators is routinely made by the National Data Buoy Center (NDBC) and others producing directional wave data. Although directional wave data derived with this assumption usually appear to be of good quality, the validity of the assumption has not previously been more directly demonstrated. In this paper, a method is proposed to judge the validity of the assumption for any set of time series records of buoy angular motion. The proposed method is applied to 200 record sets taken by an NDBC buoy located at ocean station 46024, and to five record sets taken by another NDBC buoy at 46051. For the 46024 data, it was demonstrated that the simple harmonic oscillator assumption was near perfectly valid. For the smaller 46051 data set, the simple harmonic oscillator assumption was shown to be slightly less valid.     

垂荡式波浪能装置运动模型数值分析

垂荡式波浪能装置在海洋可再生能源开发中被广泛应用,通过浮子与摇杆在垂荡方向的相对运动吸收波浪能。在以往相关的运动预报数值分析中,通常基于微幅波假设,仅考虑浮子与摇杆在垂荡方向的运动,忽略摇杆其他自由度运动。建立并求解了垂荡式波浪能装置的非线性联合运动方程组,分析垂荡式波浪能装置的波浪载荷、浮子与平台连接处的受力情况。数值计算系统的运动响应,并将计算结果与已有的试验数据进行比较验证,结果表明数值模拟的垂荡式波能装置的运动响应与试验结果相符合。最后,应用本计算方法分析PTO(power take-off)参数对波能装置发电性能的影响。     

海上警戒浮标实时监控系统

为有效保护近海海底观测网观测安全,免受渔船拖网和抛锚等损坏的问题,设计了一套多警戒浮标(直径2.4~3.0 m)的实时监控系统,该警戒浮标可获取浮标本身工作状态,并能实时、动态、连续的将警戒浮标的运行状态发送到陆基岸站,实现对保护目标海域的实时监控,同时该浮标系统可进行扩充实现对海洋环境要素的观测。警戒浮标布设采用正多边形(三角形、正方形和正五边形等)预警保护方式,浮标以保护节点为中心等距布放,组网形成有效的海上保护围栏,保障海底观测系统的安全运行;供电系统采用太阳能电池和蓄电池组合供能方式,可保证在连续阴天的情况下警戒浮标系统运行100天。该警戒浮标技术已在东海海底观测网保护中成功示范运行6个月,该技术可为近海海底观测保护提供有效技术保障。