The present paper reports a numerical study concerning the modeling and geometric optimization of a two body heave oscillating point absorber wave energy converter (WEC) using the force reacting principle. The device work principle is based on a surface body (or floater) force-reacting against a submerged body, as the excitation force of both, the floater and the submerged body, are in phase opposition. The relative motion between the two bodies generates electrical power through a linear power take off system (PTO). From the hydrodynamic point of view, the main purpose of the concept is to emphasize the radiation capabilities of the body placed under the free surface, as it has both excitation force components (diffraction and Froude Krylov) in phase. Therefore, the excitation force (and thus the hydrodynamic damping coefficient) is higher in the case of a submerged volume variation heave oscillating body (like the AWS wave energy converter) when compared to a floating buoy with the same area, and depth, of the active surface (surface responsible for the device radiation capabilities). The paper investigates the possibility of taking advantage of this effect on the device geometry optimization. The methodology is described in this paper as far as hydrodynamics and computation are concerned. First a geometrical optimization is carried out, aiming at finding a shape adapted to predefined wave climate conditions. Then, an absorbed power improvement method based on the regulation of the PTO is detailed. Most of the observations derivate from frequency domain however a time domain simulator had been created too, in order to double check the results.
The behaviour of oscillating bodies in the sea is well known and frequently used in wave energy conversion. Falnes  gives a great overview of the different issues for point absorber devices in which the characteristic dimension is small compared to the wavelength of the incident wave.
Takao, Manabu (Matsue College of Technology Matsue, Shimane, Japan) | Takita, Hiroyuki (Matsue College of Technology Matsue, Shimane, Japan) | Maeda, Takao (Mie University Tsu, Mie, Japan) | Kamada, Yasunari (Mie University Tsu, Mie, Japan)
The objective of this study is to show the effect of guide vane solidity on the performance of straight-bladed vertical axis wind turbine. The experimental study was carried out by a wind tunnel. The wind tunnel with a diameter of 1.8 m is open jet type. The rotor has three straight blades with a profile of NACA4518, a chord length of 100 mm, a diameter of 600 mm and a blade height of 700 mm. The guide vane consists of some arc plates. The wind velocity is constant at 8 m/s.
Floating offshore wind farms are being considered for Japanese waters. In such case, in order to control behavior of the float, it is very important to use a vertical axis wind turbine which has low center of gravity. As such wind turbine, a straight-bladed vertical axis wind turbine (this is named “S-VAWT” in the study) has been developed and investigated so far (Islam et al., 2008; Eriksson et al., 2008). This turbine consists of several aerofoil vertically mounted on a rotating shaft. Although the turbine needs a bearing at the top of the structure and has complicated aerodynamic interaction between the downwind blades and the wakes of the upwind blades, it has some advantages in comparison with a horizontal axis wind turbine (HAWT): (1) The center of gravity is relatively lower because the generator and gearbox can be placed on the ground. (2) A yaw mechanism to turn the rotor against the wind is not necessary for the turbine system. On the other hand, however, there are substantial drawbacks of S-VAWT. That is, the power coefficient and self-starting characteristic of S-VAWT are less than those of HAWT.
Binugroho, Eko Henfri (School of Mechanical Engineering, Pusan National University Busan, South Korea) | Ha, Tae Kyu (School of Mechanical Engineering, Pusan National University Busan, South Korea) | Choi, Jae Weon (School of Mechanical Engineering, Pusan National University Busan, South Korea) | Ko, Nam Gweon (School of Mechanical Engineering, Pusan National University Busan, South Korea) | Seo, Young Bong (Innovation Center for Engineering Education, Pusan National University Busan, South Korea)
This paper discusses the design of an open control platform (OCP) and modeling of hovering type AUV testbed. A publisher/subscriber architecture that use ACE/TAO Real-time Event Service (RTES) as the middleware core is chosen to develop the OCP structure. The designed OCP uses an event-based communication that addresses the real-time transfer of control and data among AUV components which coordinate with each other in a loosely coupled fashion. To perform the real-time performance, several services in RTES are collaborated in the system such as event filtering, correlation, scheduling, and priority setting to meet the quality of service (QoS) requirement of each AUV component. Performance of the software platform is examined with an experiment that simulates data transfer of AUV sensor’s data. On the other hand, AUV testbed that designed using hovering type with two hulls and four thrusters has been modeled. PD controller has been applied as the AUV controller for surge (x-axis positioning), heave (depth) and yaw (steering). Together with the thruster’s model and controller, the AUV model and its controller have been simulated under OCP environment.
During the last decade, the area of information technology has sustained a remarkable level of growth. Processor speed, memory capacity, network bandwidth, and other metrics of progress in computing technology grow in their exponential trends. The advance in the fabrication technology makes computing devices not only have an improvement in performance but also give more benefits such as; smaller in size, lower in price and lower power consumption. In another part, control theory and engineering have a remarkably successful history of enabling automation, and information-centric control. Complex control systems for autonomous vehicles require integrating new control algorithms with a variety of different component technologies and resources. These components are often supported on different types of hardware platforms and operating systems and often must interact in a distributed environment. Due the lacks of standard it is almost impossible to easily use those design into new application.
Lin, Ting-Chieh (Research Center of Ocean Environment and Technology, National Cheng Kung University Tainan, TAIWAN, China) | Hsiao, Shih-Chun (Department of Hydraulic and Ocean Engineering, National Cheng Kung University Tainan, TAIWAN, China) | Hwang, Kao-Shu (Tainan Hydraulics Laboratory, National Cheng Kung University ainan, TAIWAN, China)
This paper describes impulsive wave force due to breaking solitary waves impinging and overtopping an impermeable breakwater upon a sloping beach. A two-dimensional volume of fluid (VOF) type numerical model named COBRAS (COrnell BReaking And Structure), based on the Reynolds-Averaged Navier-Stokes (RANS) equations and the k - ε turbulence closure model, is firstly calibrated against present laboratory experiments with fairly good agreements and then employed to study the impulsive wave force correlated with structure failure mechanisms due to waves interacting against the breakwater. Both the effects of wave nonlinearity and freeboard are addressed. Influence of entrapped air-bubble accompanied breaking waves on reducing wave force is also discussed.
Historical tsunami survey literature points out that the devastating tsunami power is relevant to wave run-up inundation combined with floating debris on shorelines, in which significant wave force of impulsive tsunami wave could destroy inland community and alongshore infrastructure (e.g. Yamamoto et al., 2006). Various impulsive waves would occasionally give rise to different structure failure mechanisms (Kato et al., 2005), and also allow particular wave dynamics to affect alongshore flow fields on structure damage (Tokin et al., 2003). The knowledge of wave-structure interaction is therefore of crucial importance for tsunami hazard mitigation. Over the past decades it can be seen that the subject of wave-structure interaction under solitary-like long waves have been received marvelous attention on tsunami wave research (Synolakis and Bernard, 2006), such as run-up/run-down on a sloping beach (e.g. Lin et al., 1999; Hsiao et al., 2008), disintegration and transmission properties of waves over an abrupt topography (e.g. Losada et al., 1989; Liu and Cheng, 2001), and dynamic vortex shedding and advection around a submerged obstacle or a sub-aerial plate (e.g. Chang et al., 2001). However, the information combined solitary wave impingement with overtopping flow upon a coastal structure is relatively rare.
Chen, Bing (The State Key Laboratory of Coastal and Offshore Engineering, Dalian University of Technology Dalian, Liaoning Province, China) | Teng, Bin (The State Key Laboratory of Coastal and Offshore Engineering, Dalian University of Technology Dalian, Liaoning Province, China)
In this paper, a discrete vortex model based on full vortex cloud method is established to simulate the wave and wave-current flow fields around pile-groups. The variation of calculated grouping effect coefficients versus relative spacing between piles and group-configuration, as well as the flow pattern around the piles, are discussed. Though the combinations of flow field parameters and pile-group configurations are limited in this paper, the results of present model generally agree with that reported by other researchers.
Pile groups or arrays are extensively used in ocean engineering, to calculate the wave and current forces on the piles are crucial in structure design. Due to the shielding effects, the flow pattern around each pile in a group changes dramatically, which makes the hydrodynamic forces on a pile 40% or even higher than those on other piles in a group. Therefore, it is of important practical interest to study the pile grouping effect. Many studies have shown that the grouping effect coefficients, defined as the ratio of hydrodynamic forces on each pile in a group to those on an isolated pile, are affected mainly by the Keulegan-Carpenter number and the relative spacing between the piles. Though there are many literatures about the wave and current loads on isolated pile, the studies about the wave or wave-current loads on pile groups are still quite limited. So far the reported results on this subject are mainly obtained by physical model tests. Chakrabarti (1982a, 1982b) measured the wave forces on the vertical tubes arranged in line with the wave direction. The force coefficients are presented as functions of the KC number and the tube spacing. Chakrabarti reported that the inertia and drag coefficients generally decrease as the spacing decreases due to shielding effect, while the lift coefficient increases as the spacing decreases.
The paper proposes a semi-empirical model for the seawater corrosion of six strand wire rope. The model includes the local effects of water temperature, oxygen concentration, flow velocity, together with the location of the wire within the internal rope structure and the zone location along the rope. The analytical formulation of the model is sustained by physical considerations and calibration of the model parameters is performed against a large set of experimental results available in the literature.
Steel wire rope and chain have been used for mooring floating offshore production systems since their introduction nearly 30 years ago. A recent survey (2006) prepared for the Health and Safety Executive (HSE) of past and presently operating FPS units has shown that serious incidents have occurred in the past, including loss of station. The survey has also shown that even for more up-to-date designs, deterioration of certain areas of the mooring systems may be more rapid than expected. In the majority of systems, unless fully sheathed in plastic, the life of the rope is typically less than that of the installation. This requires a policy of life prediction, inspection and replacement. For long term integrity, it is vital that wear and corrosion are correctly accounted for in the design and selection process of wire rope and chains. The present study focuses attention on corrosion modeling of wire rope.
CORROSION DESIGN GUIDANCE
The American Petroleum Institute (API RP 2SK) and Det Norske Veritas (DNV-OS-E301) give recommendations for life expectancy of different constructions in terms of corrosion resistance. As shown in Table 1,(Refer to the full paper) both require all wires to be galvanized though neither make reference to the different weights of galvanizing available. API, DNV, Bureau Veritas (BV-NI-493), and the International Organization for Standardization (ISO 1990-7) all identify corrosion protection measures (galvanizing, lubricant as blocking compound, sheathing, zinc anode wires) and all draw attention to the need for special protection adjacent to terminations including additional anodes and electrical isolation of the rope, and the socket.
Internal waves in two-layer fluids can be simulated by the Numerical Wave Tank (NWT) technique in the frequency domain. The simulated waves are classified into two different wave modes. The numerical calculation is performed by a two-domain Boundary Element Method (BEM) in the potential fluid using whole-domain matrix scheme. The characteristics of the internal waves and their variations are investigated with incident waves and other computational parameters such as the ratio of fluid density and water depths. The calculated results are compared with available theoretical data. INTRODUCTION Internal waves usually occur in sub surface layers of water that differ in density due to the change of water temperature and salinity. For instance, when a layer of warm water encountered a layer of cold water with a higher salinity, an internal wave may develop at the interface between two density fluids. The amplitude of internal wave ranges from several feet in shallow water to nearly hundreds feet in deep water like a continental shelf. Internal waves have been observed and reported in different parts of the world. Farmer (1978) measured trains of large amplitude, nonlinear internal waves in Knight Inlet in British Columbia. Osborne and Burch (1980) recorded the internal waves occurred in the Andaman Sea, offshore Thailand. Liu et al. (1998) reported many observations of internal waves in China Sea and showed the evidences of strong wave-wave interactions in both East and South China Sea. The propagation of internal waves on the sub surface layer can be predicted by the appropriate dispersion relation. In case of no obstacles, the dispersion relation in two-layer fluids has two solutions (wave numbers) with a given wave frequency. The waves on both fluid layers propagate with two different wave numbers ks and ki corresponding to the surface wave mode and the internal wave mode, respectively.
Liu, Junfei (Design Department, China Offshore Oil Engineering (Qingdao) Co. Ltd.) | Wang, Yong (Design Department, China Offshore Oil Engineering (Qingdao) Co. Ltd.) | Li, Huajun (College of Engineering, Ocean University of China Qingdao, Shandong, China) | Hu, Xingtao (Technique Department, Construction Company, China Offshore Oil Engineering Co. Ltd. Tianjin, China)
The application of Hilbert-Huang Transform (HHT) method in conjunction with the masking signal for modal parameter identification to structures with closely spaced modes of vibration is investigated in this paper, and the masking signal technique is improved. A 2DOF system with closely modes of vibration is first analyzed to demonstrate the strongpoint and basic procedure for identifying modal parameters of the system using the HHT method. Then, a physical model of an offshore platform is analyzed using the FFT method, the scalar ARMA method and the HHT method in conjunction with the improved masking signal technique respectively. The results show that the HHT is able to identify the modal parameters of the structure with closely spaced modes of vibration. The HHT method performed in the timedomain seems to be a very promising tool for system identification of offshore platform structures.
Hilbert-Huang Transform (referred to as HHT), introduced by Huang in 1998, put forward a new time-domain data-processing method which is composed of two parts, Empirical Mode Decomposition (referred to as EMD) and Hilbert transform. Compare with the traditional modal parameter identification methods, EMD can effectively deal with nonstationary data. And the Fourier spectrum, from Hilbert spectrum can be seen not only in amplitude and frequency can be seen with time, which the Fourier spectrum cannot be reflected. In addition, for the non-stationary Time-domain signal, Fourier spectral resolution may be lower, while Hilbert spectrum, because it combines both the frequency and time to coordinate analysis, easy to remove interference and will be beneficial to retrieve signal resolution. First of all, introduce the HHT method, using masking signal technology to solve the problem of mode mixing in EMD, improve the parameter selection methods of masking-signal and prove the validity of the improved masking signal technology through the numerical examples.
Kimura, Sho (The United Graduate School of Agricultural Sciences, Kagoshima University, Kagoshima, Japan) | Gibo, Seiichi (Faculty of Agriculture, University of the Ryukyus, Okinawa, Japan) | Nakamura, Shinya (Faculty of Agriculture, University of the Ryukyus, Okinawa, Japan) | Vithana, Shriwantha Buddhi (Graduate School of Agriculture, University of the Ryukyus, Okinawa, Japan)
The shear strength parameters of Asato landslide, which is a representation of a quasi-first activated slide, was investigated using triaxial and ring-shear tests. The peak strengths of the tested samples were plotted in the range of the strongly weathered-fractured and the moderately/slightly weathered strength zones of the shear strength diagram of Shimajiri-mudstone, while the fully softened and residual strength found their respective places in the middle and the lower group of fully softened and residual strength zones.
The primary cause of landslides is due to the nature of soil and is directly related to the shear strength of the soil at the slip surface. Soil shear strength is not just a uniform characteristic of a particular soil, but it changes from post-peak to fully softened and residual strength according to the soil condition. Although, the main input for a stability analysis of a landslide is the shear strength of the materials that line up along the slip surface at the time of sliding, shear strengths are not measured under laboratory conditions most of the time when postlandslide repair work are designed in Japan. Therefore, in such instances, calculated slope stability values tend to be overestimated or underestimated. As far as landslides in Okinawa, Japan, are considered, many have occurred in the Shimajiri-mudstone area of Okinawa, Japan in the past. The landslides occurrences in this area are classified into four types, namely, the first activated landslides, the quasi-first activated landslides, the re-activated colluvial-soil slides and the other slides (Chen et al., 2007). The major distinctions of the quasi-fist activated mudstone slides include the existence of previous slide blocks within the lower part of the slide mass and sudden and large movement at the time of sliding. The possibility of progressive failure of this type of landslides at the first-slide occurrence is not only due to the bedrock with geological discontinuities, but also due to the old landslides that exist in the lower part of slide mass (Gibo et al., 2008).
Spinneken, Johannes (Imperial College London, Department of Civil and Environmental Engineering London, United Kingdom) | Swan, Chris (Imperial College London, Department of Civil and Environmental Engineering London, United Kingdom)
Recent research on wave generation utilizing absorbing forcecontrolled machines has shown three key factors: (i) a theoretical transfer function between demand signal and surface elevation can be derived; (ii) high absorption efficiency can be achieved over a wide range of frequencies and (iii) little second-order spurious content is introduced when driven with a first-order demand signal. Thus far, both theoretical and experimental work has been limited to the generation of regular wave trains and hinged wave board geometries. The present study extends this work to the generation of irregular waves considering both flapand piston-type wave machines. Whilst limiting the discussion to the effects arising at first-order, specifically addressing the theoretical transfer function and absorption efficiency, an enhanced understanding of the machine’s controller is sought. The implemented absorption strategy is based on controlling the complex relationship between applied force and wave board velocity. The properties of the optimum a-causal controller for this particular strategy are investigated and a causal, hence practical, approximation is derived. A causal controller based on infinite impulse response filters (IIR) and direct optimization in the frequency domain is considered. The relative merits of this approach are compared to methods based on an approximation in the time domain. Experimental evidence is also provided to substantiate the IIR modelling approach. The results are directly relevant to the operation of many installed force-controlled wave machines, provide guidance as to the effective operation of others, contribute to the wave power debate and could be incorporated within advanced numerical wave tanks to provide simultaneous generation and absorption.
Over the last three decades much effort has been made to formulate the problem of optimum control for wave energy devices. The techniques developed in this field are also adopted to describe the ideal controller for absorbing wave makers in laboratory flumes.