ABSTRACT A semi-submersible type floating wind turbine platform with 5MW power rate called "Y-Wind" is newly developed to improve the dynamic performances by employing damping plates. The platform is moored at a water depth of 200m with three catenary chain lines. Time domain coupled simulations are conducted to assess the dynamic responses for the various design load cases including power production, extreme and survival conditions. The present Y-Wind design is also validated with the ABS design requirements. Significant reductions of motions are observed due to the damping plates, which also results in reducing the mooring line tensions, compared to the values with no damping plate hull. INTRODUCTION The majority of the current offshore wind turbine farms are located in the shallow water with fixed foundations although there are many opportunities to take advantages of deepwater wind farms with floating foundations. The offshore floating foundations to harvest the wind energy could be designed by applying the mature technologies of the oil and gas platforms in the hull, mooring, installation, and many associated areas. Various types of floating foundations have been developed and can be categorized into semi-submersible, spar, TLP, and barge. The designs have been evolved mainly to minimize the wave-wind coupled interactions by typically implementing large displacement hull, heave plate, deep draft hull or high tensioned vertical mooring system. Example designs are OC4 Semi (Robertson et al., 2014), Windfloat (Rodierr et al., 2010), Trifloater (Antonutti et al., 2014), CSC-Semi (Xu, 2015), VolturnUS (Viselli, 2015b), Hywind (Statoil website; Jonkman, 2010), and Pelastar (Hurley and Nordstrom, 2015). Like the oil and gas platforms, the cost drivers of the floating wind foundations are the fabrication, mooring and installation costs so that the platform design should consider the acceptable hull size to be fabricated in the existing yards, integrated at quayside, and towed and installed with no expensive vessel in order to lower the cost. In addition, disconnection from the mooring and tow to yard can be considered for a critical damage repair to the turbine: Large sized or deep draft hull may suffer from the fabrication yard limitations or installation vessel requirement whereas foundations with insufficient self-buoyancy like TLP may need dedicated vessel from tow to installation. As for those factors, the floating foundation designs suggest a comprehensive design approach with provisions in a series of phases from the concept design to installation and maintenance.

ABSTRACT An innovative floating wind foundation, a TLP with inclined legs, was tested in MARIN Offshore Basin in 2015 at scale 1/40. The MARIN Stock Wind Turbine is used to mimic the NREL 5MW performances with Froude similitude. The tests have been simulated with the aero-servo-hydro-elastic solver DeepLinesWind, starting with simple tests with a fixed turbine up to tests with the floater submitted to wind and waves. The work on the fixed turbine appears to be critical to accurately simulate the fully coupled tests. Finally, the numerical and experimental data show a good agreement, in terms of both mean values and spectral content. INTRODUCTION SBM Offshore and IFPEN are developing an inclined-leg TLP to support floating wind turbines (Melis, 2016a, 2016b). The inclined-legs locate the rotation center slightly above the nacelle, as depicted on Fig. 1, and thus reduce its motion and acceleration. A pilot farm based on this technology is being developed in Mediterranean Sea for 2020. Before going to full scale, the system has to be tested at model scale in order to validate the technology and increase its maturity. A 3 weeks campaign was carried out in summer 2015 at MARIN Offshore Basin. The campaign also enables to validate and to improve the numerical model used to design the system. A step-by-step methodology is adopted to carry out the tests and the simulation work, aiming at increasing confidence on each part of the system (hydrodynamic, aerodynamic, structure, environment) before studying the complex fully coupled system. The whole campaign specifications, the methodology and the results of the hydrodynamic calibration and of the comparison simulated/experimental data are presented in Bozonnet (2016) and Caille (2017). This paper will focus on the aerodynamic aspects and their effect on the system. Two main problematics related to aerodynamics are addressed. Performing model tests for floating wind turbines is a challenge since aerodynamic and hydrodynamic phenomena are at stake and should be downscaled with different law of similitude: respectively, Reynolds and Froude ones (Martin, 2014). This challenge is partly overcome thanks to the use of Froude similitude combined with the MARIN Stock Wind Turbine (MSWT) (DeRidder, 2014). The representativeness of such tests, especially for a TLP type solution, will be discussed by analyzing the experimental results and the numerical simulations.

Abstract In this work we consider the problem of detection of stationary intervals for random waves using clustering techniques for time series. The detection of changes in time series from an spectral point of view has been studied in several areas, but the methods proposed usually focus on detecting instantaneous changes, and the results are not satisfactory when changes are slow, which is frequently the case for waves. In the method used in this work there is a change of viewpoint: instead of looking for change-points, the method looks for intervals having similar oscillatory behavior as candidates for stationary periods. The procedure is based on the comparison of normalized estimated spectral densities using the total variation distance. This method is applied to a long data series of buoy measurements and the results are statistically analyzed. This analysis gives statistical characteristics for the duration of stationary intervals, which may vary for different periods of the year. Introduction Stationary time series have frequently been used as models for sea surface height measurements at a fixed point on the ocean surface. However, it is well known that the hypothesis of stationarity is only valid for short periods of time. The problem of segmentation of time series into stationary intervals has usually been looked at from the point of view of change-point detection. However, this approach is not adequate when changes are not instantaneous but rather take some time to develop, as is frequently the case with waves. The method employed in this work divides the series into fixed-length intervals and then uses a clustering algorithm to identify intervals having similar oscillatory behavior. Looking at clusters of time series switches the viewpoint from the search of break points in the evolution of the series to finding intervals where the series has similar behavior. If those intervals are contiguous in time, they are assumed to belong to a (longer) stationary period of the series.

Abstract This paper describes the spectral analysis of waves based on a set of experiments undertaken at the Catalonia University of Technology in the large wave flume of the Maritime Engineering Laboratory. The scope of this study is to compare the effect of various wave regimes on very low frequencies generations. The influence of seiching related to the interpretation of the low-frequency for both cases studied erosive and accretive wave conditions was quantified.

ABSTRACT Random sea waves are often modeled as stationary processes for short or moderately long periods of time and therefore the problem of detecting changes in the sea state is very important. We look at this problem from the spectral point of view, proposing a method based on the total variation distance. The method considers processes normalized to have unit variance and looks at changes in the energy distribution through the energy spectra, by looking at their total variation distance. This distance measures the difference between two probability densities by determining how much they have in common, or equivalently, how much one of them has to be modified to coincide with the other, and the spectrum of a normalized process can be seen as the probability density of the energy distribution. The problem of detecting changes in the spectral distribution of energy for processes which are piecewise stationary has been considered in several areas, but the focus has been mainly on instantaneous or nearly instantaneous changes, and the methods developed usually give unreliable results when changes occur slowly, over a period of time. Our method takes into account this phenomenon, by considering the total variation distance not only for contiguous intervals but also for intervals separated by a time interval and using a global optimization method. We present examples of segmentation of wave records and compare our results with alternative methods for detecting spectral changes.

ABSTRACT The two-scale approximation (TSA) method is designed to overcome the time consuming, expensive problem of computing the full Boltzmann integral (FBI). The accuracy of TSA depends mainly on the accuracy of the spectral representation of the broad-scale component, which is the dominant component of quadruplet nonlinear wave-wave interactions. In this study, an improved JONSWAP spectrum is applied as the broad scale parametric spectral form; it is evaluated by fitting it to realistic hurricane generated wave spectra, and used to determine spectral parameters (peak-enhancement factor, energy scale parameter, peak frequency). Finally, the effect of the spectral parameters on quadruplet nonlinear wave-wave interactions is analyzed.

ABSTRACT The fatigue damage of a long flexible riser undergoing vortex-induced vibration (VIV) in deepwater is numerically studied using pseudo-excitation method (PEM) in present paper. For evaluating the fluid-structure interaction problem of vortex-induced vibration of bluff bodies at high Reynolds number, the strip theory approach is taken into consideration, and the discrete vortex method (DVM) is used to calculate the vortex-induced vibration of each strip to obtain the load spectrum for the calculation of pseudo-excitation, while the finite volume method (FVM) is employed for evaluating the structure dynamics of a long flexible riser. The DVM model is validated by its comparison with the experimental results. The vortex-induced vibration is considered as a stationary random process. The response of riser to vortex induced excitation is calculated using pseudo-excitation method (PEM). The fatigue damage rate of the riser per year is evaluated and the service life of the riser can be predicted based on the Palmgren-Miner Rule.

ABSTRACT Waves at 10 m water depth in the south coastal waters of Jiangsu province were measured, including a complete winter monsoon period, and the characteristics were described. The study showed that the significant wave height H1/3 varied from 0.05 to 2.22m with average of 0.62m. During the monsoon period, the main wave direction varied from WNW to ESE. Relatively high correlations exist between some wave parameters and relationships between them were identified. Most of wave spectra were single peaked. The tentative spectral model agreed well with the measured spectra for high waves.

We propose a novel Sparse Spectrum (SS) model of the sea surface, where each surface realization contains a finite, possibly random, number of sinusoidal components with random frequency, phase and amplitude. Unlike the FFT-based model, the number of spectral components forming the surface is determined by the sea state, but not the desired spatial resolution and domain size. A single constraint on the probability distribution of the wave vectors, and the amplitude variances of the components allows the SS surface to conform to any prescribed spectral density. The SS model is capable of providing a well-defined statistics of the individual waves that are important for the marine engineering and remote sensing applications. INTRODUCTION Traditionally statistical modeling of the sea surface elevations is based on the Fast Fourier Transform (FFT) technique. In this approach the surface is represented as a sum of sinusoids with equally spaced wave vectors and random amplitudes and phases. Statistics of the amplitudes and phases is designed to match the required observation-based spectral density. Higher spatial resolution and/or larger coverage domain are achieved by increasing the number of the spectral components in the model. For some remote sensing applications the required number of components can be as high as 10 11 . The undesirable outcome of the FFT models is that the amplitudes of the individual waves decrease with the number of spectral components used. We proposed a novel Sparse Spectrum (SS) model of the sea surface, where each surface realization contains a finite, possibly random, number of sinusoidal components with random frequency, phase and amplitude. Unlike the FFT-based model, the number of spectral components forming the surface is determined by the sea state, but not the desired spatial resolution and domain size. A single constraint on the probability distribution of the wave vectors, and the amplitude variances of the components allows the SS surface to conform to any prescribed spectral density.

In this work we look at the spectral evolution of waves from the point of view of the total variation (TV) distance. There are several methods for determining changes in the variance of a random process, which correspond to changes in the total energy of the waves. We look instead at changes in the distribution of the energy as given by the energy spectra, after they have been normalized to correspond to a process with unit variance, using the total variation distance. This corresponds to looking at changes in the distribution of energy instead of changes of the total energy present. The TV distance has been successfully used for the comparison of random samples and probability distributions and measures the difference between two probability distributions determining how much one of them has to be modified to coincide with the other. We consider several sets of waves measured at fixed locations over periods of several hours or days and calculate the wave spectrum for periods of 30 minutes. After all the spectra have been normalized the TV distance between all spectra is calculated and used to determine change points in the sequence of normalized energy spectra. We show examples in which although there is considerable change in the significant wave height for the different time intervals, the energy distribution remains essentially unchanged. The method could be used to determine periods of stationarity for sea waves. INTRODUCTION The problem of the evolution in time of wave spectra, and in non-stationary stochastic processes in general, is an important and challenging one. In this work we introduce the total variation distance as a new tool that can help the detection and analysis of changes in the energy distribution of the process. This distance has been used for the comparison of probability densities and quantifies the differences between two densities in a scale ranging from 0 to 1.

ABSTRACT: The paper discusses the issues related to modeling of hydrodynamic flow induced by simultaneous propagation of waves and current in test tank under shallow water conditions. For this purpose the paper considers the transformation of wave spectral density in presence of opposite and following currents under shallow-water conditions using well-known algorithms. The paper presents and analyses the results of experimental studies on hydrodynamic loads induced by waves and current on fixed structures of various architecture. INTRODUCTION The modeling of hydrodynamic flow induced by simultaneous propagation of waves and current in test tanks has always been considered as a challenging task. This problem is addressed by different methods. Some test tanks are capable to physically model a waveinduced flow which is superimposed with a constant-velocity fluid flow. The equipment and studies carried out in such test tanks are rather costly. Moreover, the current speed in this case is quite limited, and the current is generated only in one direction. The most common method for simultaneous modelling of wave and current effects on test object is a reversed motion when the object is moved against waves. However, this method often fails to realistically reproduce the true physical pattern of the interactions under consideration. The modelling technique discussed in this paper is based on the transformation of variations in wave ordinates generated in test tank. The studies performed by a number of authors have made it possible to suggest an algorithm for transformation of surface wave density due to constant current in deep water. This paper is using this algorithm as a basis for treating the transformation of wave density in shallow water By way of example the authors present and analyse the results of experimental hydrodynamic load studies for a fixed platform subject to simultaneous effect of irregular waves and current.

ABSTRACT The analytical 3D model of Renzi and Dias (2012) developed to investigate the resonant behaviour of an Oscillating Wave Surge Converter (OWSC) in a channel is used here to study the hydrodynamic loading on an array of devices in random seas. Within the framework of a linear theory, separation of variables and the application of Green's theorem yields a hypersingular integral equation for the velocity potential in the fluid domain. The latter is solved with a series expansion in terms of the Chebyshev polynomials of the second kind. The physical behaviour of the system is investigated demonstrating the effects of five different random sea states on the hydrodynamic loading on the converters. The most effective of the five sea states examined is shown to be a realistic case for a nearshore OWSC configuration off the west coast of Ireland. INTRODUCTION The wave scattering by an infinite array of thin plates in the open ocean, used for the purpose of wave energy extraction is investigated here. Among the tasks necessary to describe the behaviour of such a system and to optimise its efficiency, the analysis of the scattering of the incident waves by the plates is of particular importance. Within the framework of a linear wave theory, the wave power P extracted by a single element of the array when pitching in incident waves depends on the square of the excitation torque FD acting on the element when it is fixed in incoming waves (Mei, Stiassnie and Yue; 2005). The model adopted here is based on the theoretical work of Renzi and Dias (2012), where a linear inviscid potential flow theory is devised for a single plate, either moving or fixed, in a channel.

ABSTRACT: This work presents a numerical method able to take into account wind wave coupling and energy dissipation related to wave breaking in a deterministic sea state model. We carry out such simulations with an HOS model developed at LMF-ECN since 2002 and based on the work of West et al. (1987) and Dommermuth & Yue(1987). This model performs direct numerical simulations of the fully nonlinear primitive equations by mean of a spectral method in term of decomposition on a basis of eigenfunctions. A parametric coupling for wind forcing is achieved under Miles(1957)'s formulation, and an associated parameterization following Hasselmann(1974)'s theory enables to account for dissipation. The formulation and set parameters referred as BAJ [Bidlot et al.(05)], widely used in phase averaged spectral model solving the balance equation of energy spectra, are implemented and analyzed. INTRODUCTION The time evolution of deep ocean surface waves in realistic conditions is mainly related to the non-linear interactions, wind forcing and dissipation through white capping. Due to the nearly infinite degree of freedom for such a system, usual modeling tools prefer to focus on the statistical (i.e. stochastic) properties of the wave field more than on the deterministic description. In an attempt to improve the knowledge in the second approach, we present here a first step toward realistic modeling by introducing both processes of wind forcing and dissipation in a phase resolved context. The formulation of a High Order Spectral method presented independently in West et al.(1987) and Dommermuth & Yue (1987) enables to capture properly the non-linear evolution of the free surface elevation and velocity potential. This approach has been developed at LMF-ECN since 2002 (e.g. Bonnefoy et al.(2010)) and the improvement of computational performances enables to perform largescale oceanic simulations in a parallel environment at reasonable computational costs.

Extreme motion analysis is vital in shipping industry for finding the inertial loads on the structure. Extreme motions are generally found for zero forward speed, where spectral analysis imposes no problems. But significant motions of the vessel may also be required to be found for speeded vessels. For speeded vessel wave induced motion Response Amplitude Operator needs to be transformed to encounter frequency. As well as sea spectrum needs to be transformed to encounter frequency. In the present work an algorithm for finding the significant (and extreme) motions of forward speed vessels is discussed. INTRODUCTION Initially, spectral analysis technique is discussed in short. Wave induced vessel motions are the input to the spectral analysis. Computation of these motions (Response Amplitude Operator - RAO) is a very standard and well established technique, hence is not discussed in this paper. Only the interpolation of RAO in angular direction is discussed in brief. The main focus is the sea spectrum and the motion spectrum computations. Standard sea spectrum, viz. Joint North Sea Wave Project (JONSWAP) and Pierson - Moskowitz (PM) definitions are given for the reference. Spreading function and the encounter sea spectrum are discussed in sequence. The significant values of motions are finally found out. The results are compared with the Rankine panel based sea keeping code SWAN. METHODOLOGY - SPECTRAL ANALYSIS Ship motion in an irregular seaway is determined as (Bhattacharya, 1978) 1. Response Amplitude Operators (RAO) for wave induced motions is found out analytically or experimentally. RAO is defined as ratio of motion amplitude to unit wave amplitude. A plot of RAO square versus the encountering frequency is first obtained. 2. A suitable wave spectrum is chosen for the particular seaway in which the vessel is to operate. 3. Wave spectrum is transformed to encounter wave spectrum. 4. Multiplication of the ordinates of the (RAO) 2 with the encounter spectrum for corresponding frequencies results in a motion spectrum. Area under the motion spectrum is found to obtain various motion characteristics.

A hindcast wind wave simulation by WAVEWATCH III (WW3) in Eastern North Pacific is performed with the calculation period from 1 st to 30 th January 2006. In-situ data from National Oceanic and Atmospheric Administration's National Data Buoy Center (NOAA/NDBC) and altimeter data from Jason-1 satellite are used for the model evaluation. Hindcast results agree well with the observations. The correlation coefficient (cc) of significant wave height (SWH) between them is 0.89. One-dimensional spectra under intense and rapidly changing winds are well simulated. The results show that it takes about 70 hours for the WW3 model to get rid of the influence of its initial conditions. The big deviations between model and observation appear when the significant wave heights are changing rapidly. This supplement of the WW3 validation is helpful for a better understanding about the WW3's simulating performance and thus may provide a useful reference for a variety of scientific investigation, such as operational wave forecast, wave model development, data assimilation and wave study using wave model. INTRODUCTION The study of wind waves plays an important role in the investigation of the process in air-sea interface. Numerical simulation of ocean waves has been developed quickly and widely in recent years. From the firstgeneration wave models developed in 1960s to the third-generation wave model WAM (WAMDI Group, 1988), numerical modeling had made great development and contribution in wave simulation and forecast. The third-generation wave models overcome the shortcomings in first and second wave models which had been identified and discussed in detail in the SWAMP (1985) wave-model intercomparison study. Improved, new parameterized source terms were added and there were no prior assumptions on spectral shape in the third-generation wave models. WAVEWATCH III (WW3) (Tolman, 1999) was developed subsequently after WAM. The WW3 model used in this study was developed at NOAA/NCEP in the spirit of the WAM model.

The spectral analysis of nonlinear random waves in shallow waters is carried out in this paper by theoretical and numerical methods. This is done by expressing nonlinear wave profile as nonlinear combination of the first order wave component. Under the assumption of the first order wave component being zero-mean Gaussian processes, the relationship between autocorrelation functions of wave profile and the first order wave component is established using the nonlinear spectral analysis. The spectral densities of nonlinear random waves are then obtained. An experience formula is proposed for estimating wave spectra in shallow waters by numerical method. The wave force spectrum is calculated by using the results of the analysis to show the applications of the method. INTRODUCTION With the development of offshore oil exploration and exploitation, a large number of offshore structures have been constructed. Some of them were built in the shallow waters. Several offshore structures built in Chinese Bohai Bay in the past few years crushed in the wave forces, though the calculated wave forces on the structures are relatively small. This fact results in active researches for the wave force calculation in shallow waters. The offshore structures serviced in offshore environment will endure to enormous environment loadings. Since the sea waves are of random property, the stress in the structures induced by the waves is alternated. The fatigue strength and fatigue life of the structures will be suffering from severe threat. It is well known that the random sea wave spectrum is the basis to the dynamic responses, stress in structures and fatigue life estimation of offshore structures in random sea waves. Therefore, the properties of random waves have been being studied by a quite number of offshore engineering researchers (Chuntao, 1996, Beji, 1999). Previous studies indicated that for small amplitude waves, the spectral properties can be studied by linear method.

ABSTRACT The response behavior of a coupled mini-TLP / barge system in head and beam West Africa sea conditions is studied. Both of the vessels are independently moored and coupling is introduced through the connection system, consisting of a fender and breast line configuration. The dynamic response of the mini-TLP / barge system in uncoupled and coupled configurations is of particular interest. The experimental data obtained during the model test program is characterized using statistical parameters, distribution functions, correlation functions, spectrum and coherence functions. It is confirmed that the connection system reduces the horizontal vessel motions; and that the forces exerted on the fender system are quite sensitive to sea heading conditions. INTRODUCTION The use of two or more closely positioned platforms for offshore operations is quite common. Examples of two-body hydrodynamic systems include a tender barge floating adjacent to an offshore structure, and more recently the side-by-side loading or offloading of LNG vessels. It is important to have a good understanding of the basic two-body hydrodynamic behavior before considering even more complex systems. For instance, in the case of an LNG tanker moored to a deep water terminal, the ability to more accurately predict tanker's heave and roll motions could better facilitate the necessary product transfer between the platforms. Similarly, improving the prediction of the sway and roll motions could help designer to avoid collisions between the platform and the vessel. The hydrodynamic behavior of a two-body system is different from that of a single-body system since the motions of each body will be influenced by the presence of its neighboring structure. Besides tie-offs and soft or hard connections for product transfer, the hydrodynamic interaction effects such as diffraction / radiation and amplitude of the wave field between the platforms present significant design challenges.

ABSTRACT We consider the evolution of spectra of random waves over periods of three days. Two segmentation methods are used: Detection of Changes by Penalized Contrasts (DCPC) proposed and developed by Lavielle (1998, 1999) and Smooth Localized complex EXponentials (SLEX) proposed in Ombao et al. (2001). We compare the results obtained with both methods. In each case the intervals obtained are considered stationary and the corresponding spectra are obtained. For both sets of intervals the classical Fourier spectrum is obtained using the WAFO software. We compare some of the spectra obtained. We also apply both methods to the Hurricane Camille wave height data. INTRODUCTION A simple approach to building long-term models of random waves is to assume that they are piecewise stationary random processes, i.e. that there are instants where the ‘state’ of the waves changes but in-between these change-points the waves are a stationary process. One advantage of this approach is that the classical spectral analysis can be used in each stationarity interval with the usual interpretation of the spectrum as the distribution of energy in a range of frequencies. To implement this approach it is necessary to have ways of detecting changes in the state of the process, and since the spectra characterizes the covariance structure of a stationary process it is reasonable to look for methods based on changes of the spectra. In this work we compare two such methods: Detection of Changes by Penalized Contrasts (DCPC) Lavielle (1998, 1999), Lavielle and Ludeña (2000), and Smooth Localized complex EXponentials (SLEX) Ombao et al. (2002). Both methods have been implemented by their authors in Matlab and have been successfully used in other areas, particularly for the analysis of EEGs. To compare their performance we considered three sets of data.

ABSTRACT Variability of surface waves and the atmospheric near-surface layer was investigated over various ocean areas (above submarine rises, at depths "dumping", etc.). It is found out that the bottom topography features transforming a field of currents vary surface waves in a wide frequency range. Surface wave transformation results in variability of the mesoscale component of meteorological fields in the atmospheric near-surface layer. Theoretical calculations are in good agreement with observations. INTRODUCTION Remote radar and optical facilities combine high spatial resolution and a wide scanning band. Thus their application is most promising for determining bottom topography in coastal regions. However, radiophysical remote sensing methods enable one to find out only characteristics of water surface or a thin surface layer. That is why great attention has been paid recently to investigations of nature of the surface wave transformation above an inhomogeneous bottom relief (Shuchman, Lyzenga and Meadows, 1985; Vogelzang, 1997; Hennings, 1998; Hennings, Metzner and Calkoen, 1998; Vogelzang, 2001; Calkoen, Hesselmans, Wensink and Vogelzang, 2001; Hennings, Herbers, Prinz and Ziemer, 2004). The main flow, for example, tide or ebb, is modulated by bottom irregularities. This may cause variability of surface waves. In this case irregularities of the bottom relief may be mapped in surface roughness at a depth of several hundreds meters. Besides, turbulent motion of an air stream essentially depends on its interaction with the underlying surface. Turbulent currents above immobile underlying surfaces are rather well studied now. The interaction of the atmosphere with sea surface is more difficult for study, since a sea surface state depends on air mass motion above it. An important aspect in research of wind characteristics above basins is to study the properties of an underlying water surface, which affect the aerodynamic structure of an air stream, including roughness parameters.

ABSTRACT Numerical simulations of irregular waves propagate over a set of submerged obstacles with various conditions are investigated in this study by means of boundary element method. The algorithm was based on the Lagrangian description with finite difference adopted for the approximation of time derivative. The accuracy of the model was first verified by studying the case of irregular waves propagating the water tank without any obstacle. Moreover, sets of submerged breakwaters are fixed under the water tank for investigation. An artificial absorbing beach represents here as sponge zone is employed at the other end of the flume to minimize the reflection effects. Power spectrum of Brestschneider-Mitsuyasu type defined by significant wave height, H1/3, and significant wave period, T1/3, employed for the condition of incident waves was chosen for the generation of irregular waves. Time histories of water elevations are measured with numerous pseudo wave gages on the free water surface, by comparing the spectra of these gages with the target, also refer to the methods for estimation of irregular incident and reflected waves in random sea presented by Goda & Suzuki, the dissipative efficiency of the breakwaters are therefore investigated. Gauges with different position are tested for their suitability of the estimations of reflection coefficients for irregular wave. In this paper, the results demonstrated the effectiveness of the estimation of reflection coefficient for random waves, which clarify the numerical model a feasible scheme. INTRODUCTION In virtue of the high-speed development of science and technology of electronic calculator nowadays, numerous numerical models are being enthusiastically established for the estimations of oceanic physical characteristics, which provided several reliable statistics for the design and construction of coastal structures. The sea wall, jetty and detached breakwaters are traditionally adopted as absorbing facilities for the elimination of water wave energies.