Analysis and simulation of wave records through fast Hartley transform

The Hartley transform, a real-valued alternative to the complex Fourier transform, is presented as an efficient tool for the analysis and synthesis of ocean surface wave records. Basic theoretical properties of this real-valued transform are briefly reviewed. Similarities and differences between Fourier and Hartley integral transforms, as well as computational benefits and disadvantages between numerical algorithms used to evaluate their discrete versions, are presented. The fast Hartley transform algorithm is used to simulate stationary Gaussian time series of the sea surface elevation and to estimate the spectral density function, the Hilbert transform and the envelope function of wave records.     

Longitudinal stability and dynamic motions of a small passenger WIG craft

The longitudinal stability characteristics of a Wing-In-Ground (WIG) effect craft are quite different from those of the conventional airplane due to the existence of force and moment derivatives with regard to height. These stability characteristics play an important role in designing a safe and efficient WIG due to its potential danger in sea surface proximity. The static and dynamic stability criteria are derived from the motion equations of WIG in the framework of small disturbance theory and discussed in this paper. The static and dynamic stability analyses of a 20-passenger WIG are conducted based on wind tunnel test data, and dynamic motion behaviors are investigated for changes in design parameters. Finally, the flying quality of the 20-passenger WIG is analyzed at cruising conditions according to the military regulations.     

Propulsive performance of a tanker hull form in damaged conditions

Many disastrous oil spill accidents from damaged vessels become worse especially when the early treatment is not prompt enough. To properly handle this type of accidents and prevent further disasters, International Maritime Organization establishes and imposes various rules and regulations. Better understanding of the propulsive performance of damaged vessels is important for containing the oil spill while the vessels are being towed or self-propelled. In the present study, both experimental and computational methods were used to investigate the flow phenomena around the hull and the hydrodynamic performances of a VLCC in various damaged conditions. From the resistance and self-propulsion test results, it is found that higher power is required to propel the ship especially with the bow trim. Wake measurement data provide physical insight into the factors to be considered for the propeller operation in damaged conditions.     

Marine tether dynamics: retrieval and deployment from a heaving platform

A tether management system of a winch module in a marine environment is proposed. For the purpose of this study the subsea body is considered as a mass attached at the free end of a long tether which is wrapped around a circular drum controlled by an external torque. The winch is considered mounted on a heaving platform. The effect of the heaving platform on the motions of the drum, tether and attached mass are studied with respect to the longitudinal vibrations of the tether in one dimension. The hydrodynamic effects are considered on the deployed portion of the tether and the tethered mass, both of which are assumed submerged in otherwise still water. The resulting nonlinear system of equations of motion is developed and solved numerically for an example of a marine tether system. The effect of the tether extensibility on the operation threshold of the system is identified. Comparison with an inextensible tether case is provided. The numerical results and their analysis for the retrieval/deployment of the system are presented.     

深潜救生艇现状及发展趋势

综述国内外深潜救生艇的发展现状,分析深潜救生艇援救失事潜艇的方法及其优缺点,得出结论:母潜艇驮载深潜救生艇水下援潜救生模式是最佳选择。在对比分析国际上先进深潜救生艇的基础上,预测了深潜救生艇的发展趋势,认为瑞典的S-SRV型深潜救生艇和北约的LR7是成功的典范,代表着未来发展方向。     

Study of non-linear wave motions and wave forces on ship sections against vertical quay in a harbor

The resonance phenomenon of fluid motions in the gap between ship section, seabed and vertical quay wall is studied numerically and experimentally. The natural frequency of the fluid motions in the gap is derived. A two-dimensional time-domain coupled numerical model is developed to calculate the non-linear wave forces acting on a ship section against vertical quay in a harbor. The fluid domain is divided into an inner domain and an outer domain. The outer domain is the area between the left side of ship section and the incident boundary, where flow is expressed by Boussinesq equations. The rest area is the inner domain, which is the domain beneath the ship section plus the domain between the right side of ship section and vertical quay wall. The flow in the inner domain is expressed by Newton's Second Law. Matching conditions on the interface between the inner domain and the outer domain are the continuation of volume flux and the equality of pressures. The numerical results are validated by experimental data.     

波浪产生和运动的二阶计算模型

在快速模拟波浪运动的谱方法基础上,引入造波边界,建立了模拟波浪产生和运动的二阶计算模型。采用摄动展开方法简化了带有造波边界的水波运动问题,将速度势分解,得到了满足造波边界和自由面边界的速度势的一般解,运用快速Fourier变换和时间积分,建立了模拟波浪产生和运动的数学模型。基于该模型,采用不同的数值造波条件,模拟了波浪的产生问题;考虑了波浪的初始运动问题;通过把数值结果与物理实验的比较,验证了波浪计算模型的有效性。     

An Experimental Investigation on the Dynamic Response of a Damaged Ship with a realistic arrangement of the flooded compartment

The dynamics of a damaged ship in waves is a complex phenomenon regarding fluid and structure interactions. Flooded water motions in the damaged compartment could be influenced by decks, obstructions and obstacles in the compartment. This becomes particularly relevant in case of flooding in the engine room that is usually characterized by the presence of large objects such as engines and machineries. In such cases the possibility to better understand the behavior of a damaged ship, influenced by the fluid and structure interactions, could provide novel outcomes and thus enhance the damaged ship safety.In this paper an experimental campaign is conducted on a passenger ferry hull. The effects of obstacles in the engine room compartment, such as decks and engines, on ship roll responses, are studied. Roll decay in still water and steady roll responses in beam regular waves at zero speed are measured for the empty compartment and for the compartment with obstructions, as defined above.The main outcomes from the conducted experiments disclose a mitigation of the resonant behavior of the coupled system, ship with damaged compartment, by having engine shapes occupying the flooded engine room. Moreover it is possible to observe how the resonant frequency of the ship modifies having a more realistic arrangement of damaged compartment and how motion RAOs and roll decay characteristics modify accordingly.     

Linear irregular wave generation in a numerical wave tank

In the design of any floating or fixed marine structure, it is vital to test models in order to understand the fluid/structure interaction involved. A relatively inexpensive method, compared to physical model testing, of achieving this is to numerically model the structure and the wave conditions in a numerical wave tank. In this paper, a methodology for accurately replicating measured ocean waves in a numerical model at full scale is detailed. A Fourier analysis of the measured record allows the wave to be defined as a summation of linear waves and, therefore, Airy's linear wave theory may be used to input the wave elevation and associated water particle velocities. Furthermore, a structure is introduced into the model to display the ability of the model to accurately predict wave–structure interaction. A case study of three individual measured waves, which are recorded at the Atlantic marine energy test site, off the west coast of Ireland, is also presented. The accuracy of the model to replicate the measured waves and perform wave–structure interaction is found to be very high. Additionally, the absolute water particle velocity profile below the wave from the numerical model is compared to a filtered analytical approximation of the measured wave at a number of time-steps and is in very good agreement.     

湛江毗邻海域流场模型在海上搜救中的应用

依据<海岸与河口潮流泥沙模拟技术规范>建立湛江毗邻海域二维流场模型方程组[模拟区是(105°38'E、15°56'N)、(105°38'E、22°14'N)、(113°24'E、22°14'N)、(113°24'E、15°56'N)四点连线范围];采用"等步长"差分网格分割方法和ADI方法数值离散方程组,在确定了边界条件和初始值后,用追赶法求解.模拟出本研究海区的3种流场:一是潮流场;二是考虑了定常风作用后的流场;三是海上遇险目标漂流场.结果为该海域遇险目标搜寻与救助提供了技术支持.     

Comparison of wave load effects on a TLP wind turbine by using computational fluid dynamics and potential flow theory approaches

Tension Leg Platform (TLP) is one of the concepts which shows promising results during initial studies to carry floating wind turbines. One of the concerns regarding tension leg platform wind turbines (TLPWTs) is the high natural frequencies of the structure that may be excited by nonlinear waves loads. Since Computational Fluid Dynamics (CFD) models are capable of capturing nonlinear wave loads, they can lead to better insight about this concern. In the current study, a CFD model based on immersed boundary method, in combination with a two-body structural model of TLPWT is developed to study wave induced responses of TLPWT in deep water. The results are compared with the results of a potential flow theory-finite element software, SIMO-RIFLEX (SR). First, the CFD based model is described and the potential flow theory based model is briefly introduced. Then, a grid sensitivity study is performed and free decay tests are simulated to determine the natural frequencies of different motion modes of the TLPWT. The responses of the TLPWT to regular waves are studied, and the effects of wave height are investigated. For the studied wave heights which vary from small to medium amplitude (wave height over wavelength less than 0.071), the results predicted by the CFD based model are generally in good agreement with the potential flow theory based model. The only considerable difference is the TLPWT mean surge motion which is predicted higher by the CFD model, possibly because of considering the nonlinear effects of the waves loads and applying these loads at the TLPWT instantaneous position in the CFD model. This difference does not considerably affect the important TLPWT design driving parameters such as tendons forces and tower base moment, since it only affects the mean dynamic position of TLPWT. In the current study, the incoming wave frequency is set such that third-harmonic wave frequency coincides with the first tower bending mode frequency. However, for the studied wave conditions a significant excitation of tower natural frequency is not observed. The high stiffness of tendons which results in linear pitch motion of TLPWT hull (less than 0.02 degrees) and tower (less than 0.25 degrees) can explain the limited excitement of the tower first bending mode. The good agreement between CFD and potential flow theory based results for small and medium amplitude waves gives confidence to the proposed CFD based model to be further used for hydrodynamic analysis of floating wind turbines in extreme ocean conditions.     

Handling and launching of heavy concrete caissons in a marine environment

Handling of heavy structures demand unusual crane capacity. Availability of such cranes may be limited in developing countries. An innovative way to construct and launch heavy concrete structures in a developing country, where crane availability is rare, to lift, transport and handle heavy structures, is discussed in this paper. Heavy caissons are used in the marine environment as breakwaters, as berthing facilities for vessels, as gravity structures to absorb energy from waves, etc. The methodology for constructing multi-caissons on-board a barge and then on-board a floating dock is the subject of this paper. The respective feasibilities are discussed.The free floating condition of the caisson and its righting arm stability are described. At first the caisson was considered as its lip wall opened to the sea. The case of lip wall closed for more buoyancy is also considered. The stability of the barge and floating docks is shown for various stages of completion of construction. After investigating many factors, a safe procedure for construction and launching thereafter is described in this paper. The problem of handling a heavy concrete structure from the view point of a naval architect is highlighted.     

国际海上救援效率比较研究 ——以东海特定水域为例

为研究东海海域海上救援效率和国际协作的协调性,以国际救援为视角,基于ArcGIS构建了海事救援效率评价模型。根据巡逻船舶密度数据确定了救援船舶的概率分布,得出了救援船舶与遇险船舶的可能距离,同时考虑了风浪影响下遇险船舶的漂移以及巡逻船舶失速的情况,分别计算了常风向和次常风向下中国、日本和韩国在东海特定水域的海上救援效率。结果表明:以各国现有的巡逻船舶配备对研究区内漂移的遇险船舶展开救援时,常风向下先后抵达的顺序为韩国、日本和中国,驰援时间分别为5.14、8.18和9.67 h;次常风向下抵达的顺序为韩国、日本和中国,驰援时间分别为5.45、8.25和10.08 h;为提高我国的救援效率,需要在东海海域部署9～10艘巡逻船舶开展常态化巡逻。本文研究可为我国有关部门科学制定和完善海上救援方案,调整海上救援力量配备,进一步加强国际间的协调与合作提供参考。     

Analysis of loads,motions and cavity dynamics during freefall wedges vertically entering the water surface

In this paper, theoretical models are developed and numerical methods are used to analyze the loads, motions and cavity dynamics for freefall wedges with different deadrise angles vertically entering the water surface at Froude numbers: 1 ≤ Fn < 9. The time evolutions of the penetration depth, the velocity and the acceleration are analyzed and expressed explicitly. The maximum and average accelerations are predicted. The theoretical results are compared with numerical data obtained through a single-fluid BEM model with globally satisfactory agreement. The evolution of the pressures on the impact side is investigated. Before flow separation, gravity and the acceleration of the wedge have negligible influence on the pressure on the impact side for large Froude numbers or small deadrise angles; with increasing the deadrise angle or decreasing Froude number, the effects of gravity and the acceleration of the wedge tend to become more important. Global loads, with the main emphasis on the drag coefficient, are also studied. It is found that for the light wedge, the transient drag coefficient has slow variation in the first half of the collapse stage and rapid variation in the last half of the collapse stage. For the heavy wedge, the transient drag coefficients vary slowly during the whole collapse stage and can be treated as constant. The characteristics of the transient cavity during its formation are investigated. The non-dimensional pinch-off time, pinch-off depth and submergence depth at pinch-off scale roughly linearly as the Froude number.     

渤海海域搜救困难度研究

为优化海上搜救资源部署、提高救助成功率,结合层次分析法与模糊综合评价法,从海上搜救的自然环境、辖区信息和行动评估3个方面选取11个指标构建海上搜救困难度评价模型,在验证模型可靠性的基础上,首次对渤海海域开展二维搜救困难度评价和研究。使用成功率来表征搜救困难度并进行时空分布特征分析,成功率越低则困难度越高。结果表明：渤海海上搜救成功率全年分布趋势为海岸线周边海域相对较低,渤海中部海域相对较高。整体来看,夏季成功率远高于其他季节,春、秋季次之,冬季最低。冬季辽东湾及秦皇岛沿岸海域成功率为全年和全海域最低。对渤海沿岸港口海域及中部海域各选1个长期观测的站点开展年变化特征分析,发现同一地点的搜救成功率年变化主要受水温影响,证实了冬季是搜救困难度最高的季节。综上所述,冬季渤海沿岸海域,尤其是辽东湾及秦皇岛沿岸海域为高困难度搜救区,需要加大日常巡逻管理,改善搜救系统部署。     

Spatial dynamics of estuarine water surface temperature from airborne remote sensing

This paper examines the applicability of airborne remote sensing to the characterization and quantification of the spatial dynamics of water surface temperature (WST) within estuaries. For this, a series of successive airborne Thematic Mapper thermal images was acquired of Kirkcudbright Bay, an estuary in the United Kingdom, on two dates. Spatial dynamics were determined by two techniques: (1) qualitatively, which involved interpreting features ‘by eye’; and (2) by the velocimetric technique of maximum cross correlation (MCC). Qualitatively, complex spatial dynamics were identified over a wide range of spatial and temporal scales, associated with the creation and dissipation of eddies, and the convergence and divergence of fronts, all of which were superimposed on the synoptic tidal flow. MCC was effective for producing synoptic velocity fields, but the minimum cell size was constrained by image noise, limiting the ability to analyze small-scale spatial dynamics. The effectiveness of MCC was related to the ratio of spatially correlated variance to spatially independent variance, as estimated from the variogram. Suggestions for optimizing the application of airborne remote sensing to estuarine spatial dynamics are presented.     

Application of leeway drift data to predict the drift of panga skiffs: Case study of maritime search and rescue in the tropical pacific

The use of Search and Rescue (SAR) drift forecasting in an operational capacity is demonstrated through two SAR case studies, each predicting the drift of a panga skiff for 120 h (Case 1) and 72 h (Case 2). The leeway characteristics of panga skiffs were previously unknown, until a leeway field study was undertaken in mid-2012 to empirically determine the influence of wind and waves on their drift. As part of the two case studies described herein, four ocean models were used as environmental forcing for a stochastic particle trajectory model, to forecast the drift and resulting search areas for the panga skiffs. Each of the four ocean models were tested individually, and then combined into a consensus forecast to ascertain which ocean model was the most accurate in terms of distance error of modelled positions compared to actual panga skiff locations. Additionally, a hit analysis was undertaken to determine whether the panga skiff was located within the forecast search areas for each ocean model, and for consensus search areas. Finally, an assessment of the search area sizes was carried out to assess the single ocean model forecast search area sizes, and how they compared with the consensus search area size. In both of the case studies, all four ocean model forecast search areas contained the panga skiff at the time intervals tested, indicating a 100% hit rate and general consensus between the ocean models. The consensus search area, where all four ocean models overlapped, was approximately one third the size of the average single model search area. This demonstrates that the consensus search areas provide a more efficient search area compared to individual ocean model search area forecasts.     

Analysis of a simplification strategy in a nonhydrostatic model for surface and internal wave problems

This paper examines the simplification strategy of retaining only the nonhydrostatic effect of local acceleration in a three-dimensional fully nonhydrostatic model regarding the submesoscale wave phenomenon in the ocean.Elaborate scale analysis of the vertical component of the Reynold-averaged Navier–Stokes(RANS) equation was performed, confirming the rationalization of this simplification. Then, the simplification was implemented in a RANS equation-based nonhydrostatic model NHWAVE(nonhydrostat...     

Stochastization of the propagation of a surface wave packet in the field of an internal wave packet

The trapping of a surface wave packet by a packet of internal waves is discussed in this paper. Applying the criterion for non-linear resonance overlap, we have obtained a sufficient condition for stochasticity of the surface wave packet.Translated by Vladimir A. Puchkin.