ABSTRACT Artificial reef (AR) can be regarded as a perforated breakwater when it performs as an auxiliary structure in beach nourishment. In this paper, a kind of artificial reefs made of cuboid reinforced concrete blocks, which has already applied practically in beach nourishment projects in Beidaihe, has been experimentally investigated on the wave attenuation process in terms of small and destructive waves. Regular waves with five sets of wave height and period were used. The reflection coefficient, transmission and dissipation coefficients were obtained from measurements. Wave and velocity processes near the reef were compared and analyzed. The reflection coefficient ( K r ) is 0.10~0.29, with transmission coefficient ( K t ) of 0.31~0.65 and the dissipation coefficient ( K i ) of 0.05~0.59. It is a definite and stable regime that the AR attenuates the incident wave. The AR performs as a submerged breakwater for small wave. But it impacts on moderate waves by more dissipation than reflection, when the porosity character highlights. As for big incident wave, the dissipation and reflection effects of the AR can reach the limit due to the extra energy loss, which maybe in large proportion, caused by the natural breaking of big incident wave. INTRODUCTION Coastal erosion is a worldwide problem and now largely intensified due to human activities. The available options of shoreline management to deal with erosion problems are: to accept retreat in areas where beaches and dunes are wide and high; to maintain the coastline at a fixed position by of hard structures and/or by soft nourishment (Van Rijn, 2011). If a long term erosion trend is captured at the problem area by profile analysis versus time, it may consider to nourish the area, either as beach or shoreface nourishment or both. Although beach nourishment often is regarded as the most effective method for shoreline stabilization, it is not economically or environmentally suitable for some sites or even the ‘hot spots’ in a successful nourishment project (Harris, 2003).

Abstract Two-dimensional gap resonance between oppositely leaning floating twin barges in proximity subjected to normal incident waves is investigated by both model test and numerical simulation. The experimental data and numerical results of the gap resonance between twin oppositely leaning barges are in good agreement generally, and both of which show the resonant frequency of the fluid oscillation in the narrow gap decreases with the increase of the leaning angle relative to the vertical and the resonant wave height in the narrow gap decreases with the increase of the leaning angle. Introduction Side-by-side arrangement of floating structures in proximity is often applied in marine field operations, for example, the offloading from an FPSO (Floating Production Storage and Offloading) to an oil tank or LNG (Liquid Natural Gas) ship. As the incident wave frequency is close to the natural frequency of the confined fluid bulk between floating structures, large amplitudes of fluid oscillation in the narrow gap can be observed. With a two-dimension assumption, the wave-induced gap resonance between fixed floating barges has been extensively studied by theoretical analysis, laboratory test and numerical simulation. Miao et al. (2000 and 2001) studied the wave interaction with two floating caissons with a small gap between them and examined the resonant phenomenon of the oscillation inside the gap theoretically. Saitoh et al. (2006) and Iwata et al. (2007) conducted two-dimensional experiment to investigate the gap resonance between two and three fixed boxes, respectively. Their experimental results indicated that the maximal amplitude of resonant wave motion excited in the narrow gap can approach up to about five times of the incident wave amplitude. As for the numerical simulation employed for the gap resonance, there are two main numerical models. One is the potential flow model and the other is the viscous flow model. Based on the potential flow theory, various numerical codes (Zhu et al., 2005; Li et al., 2005; Teng et al., 2006; He et al., 2006; Zhu et al., 2008; Sun et al., 2010) have been developed. The potential models can predict the resonance frequency at the gap between boxes accurately, and run very fast. It is widely used to find the resonant frequencies at the gaps of various floating bodies in practices. However, for predicting the wave amplitude in the narrow gap accurately, a viscous model must be used, as the physical energy dissipation due to fluid viscosity, vortex shedding and even turbulence plays a dominate role for the amplitude of resonant oscillation. Employing the viscous flow theory, Computational Fluid Dynamics (CFD) methods have also been utilized to investigate the fluid resonance in the narrow gaps between fixed bodies due to incident waves (Lu et al., 2008; Lu et al., 2010a; Lu et al, 2010b; Lu et al., 2011). The CFD models have a good performance in predicting both resonant frequency and wave height in the narrow gap. Though the fluid viscosity and vortex shedding are considered in those models, the application for the interaction between the incident waves and the moving floating barges are rarely seen due to the complex in meshing for moving and inclined bodies.

Abstract In this paper, three typical floating offshore wind turbines (FOWTs) were studied, with the generic 5MW wind turbine. The dynamic responses of three FOWTs are calculated in both frequency and time domain in operational and survival case under the same environmental conditions. During the calculation, the coupled load effects of aerodynamic and hydrodynamic loads are considered as well as the structure and mooring lines. By comparing the motions and mooring line tensions of three FOWTs to conclude that Semi-submersible shows better dynamic response and safer factor of strength of the mooring systems. And the sensitivities of three FOWTs to the environmental conditions as wave height and wind speed are also studied.

Abstract Damage level of coastal structures has been scaled up according to the increase of wave height and duration of a storm due to the global climate change. Therefore the design criteria for a new breakwater are being intensified, and structural strengthening works are being also conducted for securing the existing breakwaters. Recently, an interlocking caisson-type breakwater has gained much attention to enhance the stability of the conventional breakwaters. This study investigated the dispersion characteristic of wave forces using an interlocking system which connects the upper part of caisson with cables in the normal direction of a breakwater. From the numerical analyses, the higher wave forces are transmitted through the interlocking cables as the angle of an incident wave is larger, and the maximum allowable wave force was capable to be increased by sharing the wave forces with the adjacent caissons. Further, it was also found that the larger the stiffness of the interlocking cable becomes, the larger wave dispersion effect.

ABSTRACT: A second order frequency-domain model implemented by a boundary element method was developed for wave elevation around structures. The diffraction of monochromatic waves by a square array of truncated cylinders and a simplified TLP was studied with this model, and the wave run up and the free surface elevation around the structures were presented. The computation results show that the near-trapping phenomena occurs inside the structures, which leads to increased wave height. Special concerns were paid on the examination of the crucial frequencies and maximum wave height of the near-trapping phenomenon. INTRODUCTION An arrangement of four truncated cylinders centered at the corners of a square is a simple model of a typical TLP platform. The prediction of the run up on the cylinders is of great interest for the offshore industry, e.g. to determine the height of the platform deck above sea level. Evans and Porter (1997) indicated that wave interaction with four-cylinder structures can result in a considerable enhancement of the local free surface for particular incident frequencies over a very narrow range and the phenomenon is regarded as the near-trapping phenomenon. If these large free surface elevations were to occur in practice, then this would have serious implications for the design of large arrays of offshore structures. It is therefore important to understand when these effects occur and how they might be affected by factors, such as structure form and nonlinearity. Although some interesting interaction affects that arise from nonlinearity have been observed, nonlinear effects are difficult to analyze with complex geometries. In present study, the wave interaction with a square array of truncated cylinders and a TLP platform is investigated in frequency domain based on a Stokes expansion approach. The velocity potential is obtained by a boundary-integral equation method.

ABSTRACT: Many analytical methods have been proposed to calculate hydroelastic responses of a very large pontoon-type floating structure in waves. In this paper, variational principles considering wave radiation condition at infinity related to motions of a plate in waves, which are very important in calculations of the elastic response of the pontoontype floating structure, are discussed. First, Sommerfeld radiation condition at infinity is extended to treat a case with an incident wave. Second, four kinds of variational principles related to motions of the elastic floating plate on a water surface considering the incident and radiated waves are proposed and clarified the mutual relationship of these variational principles. Third, numerical results for elastic response of a floating plate of rectangular and L-shaped plan geometry in waves, which are obtained by using these proposed variational principles, are shown. INTRODUCTION A Pontoon-type VLFS is one of the typical structural types of very large floating structures (VLFS). Various numerical methods have been proposed to predict the hydroelastic response of this structure in waves (Watanabe (2004); Chen (2006)). These methods are classified into the modal expansion method and the direct method. These analyses are carried out in the frequency domain or in the time domain. Finite element method is used for the structure in order to analyze actual complicated floating structure (Seto et al. (1998); Utsunomiya et al. (2002)). In relation to the elastic response of Pontoon-type VLFS in waves, four kinds of variational principles related to elastic motions of such a floating plate were derived and made clear the mutual relationship of them (Isshiki (2000), Isshiki and Nagata(2001)). However, in these variational principles, wave radiation condition at infinity was not included and the normal velocity on a vertical cylinder surface of finite size surrounding the plate was designated.

ABSTRACT: This study aims the investigation of depth effects in the motion response of floating structures. To this end, a Rankine panel method adopting higher-order B-spline basis function is applied in time domain. The topology of sea bottom is assumed to be either constant or varied. Taking the advantage of the Rankine panel method, any bottom topology near floating structures can be considered by distributing the solution panels on the bottom surface. The numerical analysis includes the radiation, diffraction problems and floating motion responses for typical hull forms, e. g. LNG carrier and barge. The result is compared with other numerical solution for validation purpose. The motion RAOs are observed for different water depth and varying bottom topology. INTRODUCTION Recently moving inland facilities into coastal area is seriously considered. In coastal area, water waves and floating-body motion have different property by restricted water depth. Therefore considering bottom effect is important to predict the motion responses of such coastal platforms. The motion responses of floating bodies in constant depth have been considered as one of classical problems in marine hydrodynamics, and several methods have been introduced. For instant, strip method has been utilized by Tuck (1970), Tasis et al. (1978), Andersen (1979), Perunovic and Jensen (2003). This method is still used nowadays for practical purpose, however it has limitation as a two-dimensional theory. To complement such limitation, Kim (1999) introduced a new unified theory for the finite-depth effect, showing much improved accuracy. Nowadays, three dimensional panel method programs such as WAMIT (Lee, 1995), which contains solution procedure for constant depth, are available. For an offshore structure, Teigen (2005) has showed motion responses of floating barge over constant and sloped bottom. Ship motion over sloping bottom has been considered by Buchner (2006), Ferreira and Newman (2008) and Hauteclocque et al. (2009).

ABSTRACT: Overtopping Wave Energy Convertor (OWEC) is an offshore wave energy convertor for collecting the overtopping waves and converting the water pressure head into electric power through the hydro turbines installed in the vertical duct which is fixed in the sea bed. The numerical wave tank based on the commercial computational fluid dynamics code Fluent is established for the corresponding analysis. The Reynolds Averaged Navier-Stokes and two-phase VOF model are utilized to generate the 2D numerical linear propagating waves, which has been validated by the overtopping experiment results. Several incident wave conditions and shape parameters of the overtopping device are calculated. The straight line type and parabolic type of the sloping arm are compared in the optimal designing investigation of the overtopping characteristics and discharge for OWEC device. The numerical results demonstrate that the parabolic sloping arm is available for wave running up and the overtopping discharge increasing. INTRODUCTION Among various ocean renewable resources, wave energy is most abundantly available and applicable in most coastal and offshore areas. Due to the advantages of simple converting techniques and producing cost over other types of ocean energy, the wave energy conversion system is feasible to be established for the commercial power production. Plenty of wave energy absorption devices and plants have been invented, and several of them have been utilized in the electricity generation. Recently, the oscillating water column type has been widely employed in the application for the wave energy conversion. The disadvantage of this type is the low wave energy converting efficiency. Overtopping Wave Energy Convertor (OWEC) has the sloping walls and reservoirs to lift water waves to the levels above the average surrounding ocean. The released reservoir water is used to drive hydro turbines or other converting devices.

ABSTRACT: This paper outlines the numerical and experimental investigation of wave interactions with fully submerged and floating dual buoy/vertical membrane breakwaters. To assess the efficiency of this dual system with non-porous or porous membranes, two-dimensional multi-domain hydro-elastic formulation was carried out in the context of linear wavebody interaction theory and Darcy's law. It is found that transmission and reflection characteristics are sensitive to design parameters of breakwater system; size of buoy, membrane tension and spacing, submergence depth, gaps between seafloor and end of membranes, and mooring type and stiffness. These parameters offer the opportunity to tune the breakwater system, for archiving mutual cancellation to the coming waves, and to reflect incident waves over a wide range of wave frequency. The energy dissipations through fine pores on membranes attenuate the transmitted and reflected waves simultaneously, and reduce dramatically the displacement of buoy and membranes at resonance frequencies. INTRODUCTION Flexible and floating membrane breakwater system has desirable characteristics of a quickly deployable, easily re-locatable, sacrificial with less environmental impacts, reusable, and comparably economical system. Thus, it may be an ideal candidate as a portable temporary breakwater for various coastal construction works, demanding sheltered areas or seasonal protection. In this regard, a number of vertical floating buoy/membrane breakwaters have been investigated by Thompson et al. (1992), Kee & Kim (1997), Cho et al. (1997, 1998). Kee et al (2003) extended the work by Kee & Kim (1997) and developed a boundary integral method solution for fully submerged dual buoy/porous-membrane system with gaps between seafloor and membranes structures. System performance was found to be highly dependent on the buoy radius, which is directly related to the membrane stiffness, submergence depth from the free surface to the top of buoy, gaps between seafloor and membranes, asymmetric system with different mooring types.

ABSTRACT: The northeast coast of Taiwan is well-known for its amazing nature beauty. However, the tourism infrastructure is relatively underdeveloped, specially on the seashore side. Furthermore, the region suffers from beach erosion as well as sedimentation in fishing harbour. A database of the wave climate is vital for understanding and predicting of the morphodynamic processes in this region. In this way, suitable measures to maintain a sustainable coastal environment can then be proposed. This paper was written with the hope that a small contribution can be made toward this goal. Two sets of wave data and one set of wind record were used for the study. The wave energy as well as the interrelationships among different parameters of both the wind and wave fields are studied here. For a better understanding of the possible effects of refraction and shoaling in this region, simulation of the wave field is also carried out using the PMS (Parabolic Mild Slope) model from DHI. BACKGROUNDS OF THE MEASUREMENTS The shoreline discussed in this paper consists of rock beach interchanged with sandy beaches. The water depth in the bay can reach 60 m. Generally speaking, the coast in the north region of Ou Dee consists mostly of coral reefs, whereas sandy beaches are found in the south region. Wave data from two measuring stations were used for the study. The first one, Long Dong, is located in the northern part; whereas the second one, Long Men, is situated in the southern part of the region. Since parts of the two wave records were measured concurrently, a comparison can be made. Wave data from measuring station Long Dong was measured by a buoy, where the water depth is 32 m. Water depth at Long Men is 14 m, and a pressure type wave gauge was used.

Abstract: The transient surge motion of a moored ship among random waves is studied with emphasizing the role of the time-memory effect and the initial condition, Since the wave damping of monochromatic waves with low frequency is extremely small, the contribution from the time-memory function to damping is vital to the motion response within the framework of potential theory. It is preferred to derive the time-memory function from the Fourier inversion of the wave damping for the sake of numerical accuracy. Formally the equation of surge motion represents a damped linear dynamic system and the response tends to a stable point attractor irrespective of the initial condition. But in the case of slow drift, there exists an inset due to the small amount of damping, which bifurcates two different attractors entirely depending on the initial condition of motion. 1. Introduction A moored ship in a sea is subjected to second-order wave forces as well as to linear oscillatory ones. The second-order force contains slowly-varying components, of which the characteristic frequency can be as low as the natural frequency of horizontal motions of the moored ship. As a consequence, the slowly-varying force can excite an unexpectedly large horizontal excursion of the ship, which may result in a serious damage on the mooring system. A lot of investigators have tackled with this problem under respective interests. To mention a few among them, Havelock considered second-order forces on a ship due to total reflection on one hand(1940) and relative linear motion on the other hand (1942). Maruo (1960) rigorously derived it in a general form using momentum theorem. In a similar fashion, Remery & Hermans (1972) developed a method in frequency domain. In these approaches, the low-frequency drift force has been approximated by the steady component at zero difference frequency.

ABSTRACT: Once a mathematical model is developed for any type of problem, it is hard to cope with a certain modification of the original boundaries in the model. This also happens frequently in harbor design and other ocean related works which have very complex geometries. This paper discusses an approach to expansion of the boundaries in a numerical model without changing the created finite element mesh which fits for the given area. The introduced model is an hybrid element model and the accuracy and applicability of the model are demonstrated for harbor design. The model is dealing with monochromatic waves under linear wave theory. In this study the boundary value problem of water waves scattering under the effects of bottom friction and boundary absorption is introduced to the model. INTRODUCTION Prediction of the responses of the bay or harbor to possible incident waves both with and without the intended design is essential for the evaluation of an existing harbor or a new harbor and its future development. Therefore, in order to more efficiently answer such questions, it is valuable to develop the best possible methods of engineering analysis. Accurate and efficient wave transformation models, either analytlcal, physical or numerical, are the tools needed for coastal engineering analyses. The objective of this study is to show a numerical model for prediction of waves propagating into a bay and/or harbor with water of varying depth. Moreover, the effects of the open channel are discussed without changing the introduced grid pattern. The foundation of any wave transformation model is the wave theory upon which it is based. One of the formulations for the prediction of wave evolution is the mild-slope formulation, applicable to general linear wave theory, first developed by Berkhoff (1972).

Abstract: Due to the constantly varying sea-states with which any wave energy conversion device must contend in order to efficiently extract energy the ability to control the device" s position relative to the incident waves is critical in achieving the creation of a truly functional and economical wave energy device. In this paper, the authors will propose methodology based on a variable structure system theory to utilize a three dimensional source distribution as a model to estimate anticipated surge, sway and yaw of a wave energy conversion device relative to varying angles and characteristics of incident waves and there from derive a feedback to a sliding mode controller which would reposition the device so as to maximize its ability to extract energy from waves in constantly varying ocean conditions. Introduction Previous attempt to efficiently extract energy from ocean waves did not achieve the desired results due to the high degree of structural complexity which must be incorporated into any wave energy conversion device attempting to extract energy from the non-linear hydrodynamic phenomena that characterize ocean waves. Ocean waves may come from any direction of incidence and have different characteristics of wave height length etc. The combination of these two sources of complexity make it extremely difficult to estimate or promote extracted wave power. as previously demonstrated both theoretically and experimentally(Kim and Park.1989) (Kudo. 1984). Therefore. a three dimensional source distribution method is used to estimate the hydrodynamic forces on a wave energy conversion device and to calculate the hydrodynamic forces according to the variations in length and direction of incident waves (Xim and Park.1989).(Kim and Park and Xim,1988). And the authors believe is possible to control the position of a wave energy conversion device by using a "water column" model" s three dimensional source distribution.