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First- And Second-order Hydrodynamic Forces And Moments On Two Offshore Floating Structures In Waves
Ha, Mun K. (Marine System Research Department, Marine Research Institute, Samsung Heavy Industries, Co. Ltd.) | Kim, Mun S. (Marine System Research Department, Marine Research Institute, Samsung Heavy Industries, Co. Ltd.) | Park, Jong J. (Marine System Research Department, Marine Research Institute, Samsung Heavy Industries, Co. Ltd.) | Lee, Jin H. (Marine System Research Department, Marine Research Institute, Samsung Heavy Industries, Co. Ltd.)
ABSTRACT As oil or gas field moves deeper offshore area, offshore offloading operations such as Tandem or Side-by-Side arrangement between two floating structures take place in many locations throughout the world and also have many hydrodynamic problems. Therefore, the researches on the motion response and hydrodynamic force including first and second order between two floating structures are needed to have the more safe offloading operability in waves. In this paper, the hydrodynamic aspects including hydrodynamic interaction effect between two floating structures in waves are studied by using a linearized three-dimensional potential theory together with simplified boundary conditions. Numerical calculations have been carried out for hydrodynamic pressure, coupled motion responses, first and second order wave force between two floating structures in waves using three-dimensional pulsating source distribution techniques. The numerical results give a good correlation with the well-known experimental and theoretical results. The numerical results show the present tool can be used effectively to predict the hydrodynamic force of multiple offshore floating structures in waves. INTRODUCTION Offshore offloading operations take place in many locations around the world as oil or gas field moves deeper area and involve the use of two or more floating structures, which are positioned closely to transfer oil or gas during offloading operation. During offloading, they affect each other''s hydrodynamic aspects due to hydrodynamic interaction between two floating bodies in waves. Therefore, one of the most concerned problems on the operation of two floating structures is undesirable large motion response and hydrodynamic force during offloading. The large motion and hydrodynamic force causes the serious damage of ship hull, offloading system and collision between two structures. Because of these serious problems during offloading operations due to the hydrodynamic interaction effect, it is very important to study the hydrodynamic aspect between two offshore floating structures in waves.
- Transportation > Marine (0.67)
- Energy > Oil & Gas > Upstream (0.54)
Numerical Performance Investigation of an Array of Heaving Wave Power Converters In Front of a Vertical Breakwater
Mavrako, S.A. (Department School of Naval Architecture and Marine Engineering, National Technical University of Athens) | Katsaounis, G.M. (Department School of Naval Architecture and Marine Engineering, National Technical University of Athens) | Nielsen, K. (RAMBOLL A/S) | Lemonis, G. (Center for Renewable Energy Sources)
ABSTRACT The present paper is dealing with the numerical prediction of the performance characteristics of an array of five wave energy heaving converters placed in front of a reflecting vertical breakwater. At a first stage, the appropriate mechanical system modeling is presented. Detailed insight into the system's kinematical and power production characteristics is given. In the second part, the analytical method used for the evaluation of the hydrodynamic characteristics of the cylindrical floats moving in front of the breakwater is outlined, supplemented with representative numerical results concerning the hydrodynamic parameters in frequency and in time domain. Hydrodynamic interactions among the floats and the adjacent breakwater are exactly taken into account using the method of images. The third part of the paper is devoted to the numerical integration algorithm for solving the coupled non-liner equations of motion taking into account both linear and non-linear couplings with the power take-off mechanism. INTRODUCTION The work presented here has been carried out within the framework of the already finished first phase of the LABBUOY research project, supported by the EU, which has dealt with the mathematical and physical modeling of an economically efficient floating device for wave energy conversion into electricity. The operational principle of the LABBUOY concept comprises a row of half-immersed spherical or truncated cylindrical floaters attached on a lever with appropriate power transmission system. The system is mounted on a pier or breakwater above water level, as it is shown in Fig. 1, where some indicative geometrical dimensions of the system are also given. In the context of the present contribution, the numerical system modeling will be presented giving at a first stage insight into the hydrodynamics of the floaters placed in front of the breakwater together with the system's coupled non-linear governing equations of motion.
- Europe (1.00)
- North America > United States (0.28)
ABSTRACT Wave cycle distributions are notoriously difficult to calculate. Available analytic approximations, like those suggested by Longuet-Higgins (1983) and Cavanie, Arhan and Ezraty (1976) make use only of a few spectral moments, and are unreliable for moderate waves. To find distributions under general spectrum shape and parameters one has to use efficient numerical algorithms based on high dimensional integrals. The paper presents a number of examples of wave cycle distributions computed by the software package WAFO developed at Lund Institute of Technology; . All results are compared to simulated or real data. The wave cycles studied are the max-min cycle, the crest-trough wave cycles, the rainflow cycle distribution, and the mean-separated max-min cycle. INTRODUCTION Cycle Definitions Gaussian wave models are the basic model for deep water ocean waves in naval engineering, and for the implied loads on marine structures. In marine safety analysis, the distribution of cycle amplitudes and periods is important. Depending on application, several types of cycles have to be considered; see Figure 1. max-min cycles: Cycles formed by a pair of a local maximum of height M and the following local minimum of height M. The height difference HMm = M - M between the maximum and the minimum is called a max-min amplitude. crest-trough cycles: Cycles formed by the maximum value Ac between an upcrossing of the mean level and the following downcrossings, and the minimal value - At between the downcrossing and the next upcrossing. The height difference HCT = Ac + At is called a crest-trough amplitude or upcrossing amplitude, and the distance between the mean level upcrossings is the upcrossing wave period/wavelength. This type of cycle definition is the common one for wave statistics.
- Europe (0.68)
- North America > United States > Gulf of Mexico > Central GOM (0.25)
Development of a Novel Type of Underwater Micro Biped Robot With Multi DOF
Guo, Shuxiang (Department of Intelligent Mechanical Systems Engineering, Faculty of Engineering, Kagawa University) | Okuda, Yuya (Department of Intelligent Mechanical Systems Engineering, Faculty of Engineering, Kagawa University) | Asaka, Kinji (Kansai Research Institute, AIST)
ABSTRACT In the medical field and in Industry application, a new type of A Novel Type of Micro Biped Robot with Multi DOF (Degree of Freedom) that can swim smoothly in water or aqueous medium has urgently been demanded. The fish-like microrobot is one of the micro and miniature devices, which is installed with sensing and actuating elements. This paper describes the new structure and motion mechanism of an underwater microrobot using ICPF actuator, and discusses the swimming possibility of the microrobot in water. The experimental results indicate that the swimming speed of proposed underwater micro robot can be controlled by changing the frequency of input voltage; the moving direction (upward or downward) can be controlled by changing the amplitude and the frequency of input voltage. INTRODUCTION Intracavity intervention is expected to become increasingly popular in the medical practice, both for diagnosis and for surgery. Recently many microrobots have been developed for various purposes due to the advances of the precise process technology, and further progress in this field is expected. In the medical field and in industry application, a new type of fish-like microrobot that can swim smoothly in water or aqueous medium has urgently been demanded [1]-[2]. The fish-like microrobot is one of the micro and miniature devices, which is installed with sensing and actuating elements. It can swim smoothly in water or aqueous medium such as use for in-pipe inspection and microsurgery of blood vessel. Recently, several types of fish-like microrobot using SMA (Shape Memory Alloy) actuator, GMA (Giant Magnetostrictive Alloy) actuator, PZT Piezo actuator and polymer actuator have been reported so far [3]-[8]. However there are some problems, such as compact structure, low response, leaking electric current, safety in water and so on.
- Asia > Japan (0.47)
- North America > United States (0.46)
- Europe (0.46)
Advanced Deepwater Intervention With MODUS – Latest Results From Model Tests And Full-Scale Operations
Clauss, Günther (Institute of Naval Architecture and Ocean Engineering, Technical University Berlin) | Hoog, Sven (Institute of Naval Architecture and Ocean Engineering, Technical University Berlin) | Stempinski, Florian (Institute of Naval Architecture and Ocean Engineering, Technical University Berlin) | Gerber, Hans (TFH Berlin, University of Applied Sciences - FB VIII)
(figure 1 shown in paper) ABSTRACT The ROV MODUS (MObile Docker for Underwater Sciences) is a specialized inner space shuttle for the deployment, servicing and recovery of benthic stations with maximum payload of 30 kN to the seafloor. As one major part of the EU funded project GEOSTAR (GEophysical and Oceanographic STation for Abyssal Research, contract MAST3- CT98–0183), it is designed to operate down to 4000 m water depth. MODUS is equipped on one hand with off the shelf components such as four units of lighting and cameras, sonar, altimeter, DC-brushless thrusters for horizontal movements etc. On the other hand custom made units like sensors for load and acceleration or a unique latch device for remote coupling and decoupling of versatile scientific rigs are mounted. With its specification it is capable to work as a stand alone vehicle for environmental surveys, as a mobile parent carrier for secondary tethered sampling tools or as a remotely operated ‘crane hook’ for voluminous bottom stations. A growing number of multinational projects (e.g. GEOSTAR, BIODEEP, ORION, ESONET, GNDT, DYALEKT, etc.), funded either by EC or national Italian and German research ministries, are excellent references for the high grade of availability and reliability of this shuttle system. Nevertheless, MODUS and the payload stations are prototype systems, which need continuous research activities dealing for example with non-linear hydrodynamic and hydroelastic aspects of the umbilical tethered ‘workhorse’ in different sea conditions with beam currents etc., affecting with its vertical oscillations the overall performance, the stability during operations and the safety of man and machine. INTRODUCTION With its umbilical tethered mode of operation and its challenging design depth, it fits not only very well in the worldwide structure of underwater research applications, but is also able to reach out for further skills.
- Europe (1.00)
- North America > United States (0.66)
ABSTRACT A linearized 2-D radiation and diffraction problems are considered of a general-shaped floating body in a two-layer fluid of finite depth. A boundary integral-equation method is developed for solving directly the velocity potential on the wetted surface of a body which straddles both of the upper and lower layers as a general case. For this method, appropriate time-harmonic Green's functions are derived, and an efficient numerical method of evaluating those functions is proposed. Based on the Green's theorem, various hydrodynamic relations for the case of finite depth are derived theoretically, and those relations are confirmed to be satisfied very accurately by the results of numerical computation. Experiments are also carried out by use of water and iso-paraffin oil as the two-layer fluids and a Lewis-form body. Measured results of the added mass, the damping coefficient, and the wave-exciting force are compared with computed results, and favorable agreement is found. INTRODUCTION In ship hydrodynamics, the fluid is mostly assumed to be of constant density. However, the density does change in special areas (e. g. a lake or an estuary), due to variation in salinity and/or temperature in the vertical direction of the water. Usually, the change in density is confined within a thin layer called pycnocline, and above and below this pycnocline the fluid density is practically constant. In this case, it is appropriate to model the fluid as a two-layer fluid where the pycnocline is infinitesimally small, and there exists a density discontinuity at the interface between the upper (lighter) and lower (denser) layers. The two-layer fluid model may be used to investigate another type of density changes, such as a large amount of spilled oil covering denser salt water or a thin layer of sludge (muddy water) at the bottom of harbors or channels.
Run-up And Wave Forces On an Array of Vertical Circular Cylinders: Experimental Study On the Second Order Near Trapping
Contento, Giorgio (DINMA - Department of Naval Architecture, Ocean and Environmental Engineering, University of Trieste) | D'Este, Fabrizio (DINMA - Department of Naval Architecture, Ocean and Environmental Engineering, University of Trieste) | Sicchiero, Mauro (DINMA - Department of Naval Architecture, Ocean and Environmental Engineering, University of Trieste) | Codiglia, Riccardo (DINMA - Department of Naval Architecture, Ocean and Environmental Engineering, University of Trieste) | CALZA', Marco (DINMA - Department of Naval Architecture, Ocean and Environmental Engineering, University of Trieste)
ABSTRACT This paper summarizes the results of experimental tests conducted at the Danish Hydraulic Institute (DHI) in the frame of the EU-IHP-ARI project "Wave Forces on an Array of Circular Cylinders: Experimental Investigation on the Higher Order Near Trapping". The aim of this research project was to study thoroughly the so-called "near trapping", and specifically the second order near trapping. This phenomenon regards diffraction effects of TLP like structures in waves, or in general arrays of vertical cylinders that enclose a portion of free surface (inner domain), and it shows itself with a rather large amplification of the oscillations of the mentioned portion of the free surface. These magnifications are particularly large at the fluid-body interface with associated large induced pressures; they show their maximum effect for specific ratios of the column radius a to the incident wavelength λ (diffraction parameter ka) and of the column radius to the distance between the column axis d (a/d parameter). First order near trapping occurs at the incident wave frequency, whereas second order near trapping is a double frequency phenomenon that is expected to occur when double frequency component waves, according to the dispersion relation, exhibit a wavelength comparable to that of linear waves giving first order near trapping. The paper describes the set-up of the experiments, the tests conducted and finally presents the most meaningful results obtained. As far as the in-line and out-of-line forces acting on the cylinders, the wave run-up and the free surface oscillations at selected locations in the inner domain are concerned, their behavior Vs ka shows clearly the occurrence of the second order near trapping phenomenon with impressive quantitative aspects. INTRODUCTION The problem of the linear and non-linear interaction between waves and an array of circular cylinders has been studied by many authors in the last fifteen years.
ABSTRACT In this paper, sufficient conditions for the occurrence of an extreme crest in weakly nonlinear water waves are given. The starting point is the Zakharov equation which governs the dynamics of the spectral components of the wave envelope of the surface displacement η(x, t). It is proven that the optimal spectral components giving an extreme crest at (x = 0, t = 0) are solutions of a well defined constrained optimization problem. A new analytical expression for the probability of exceedance of the wave crest is then derived by means of the theory of quasi-determinism of Boccotti. The analytical results agree well with measurements data at the Draupner field and can be used for the prediction of freak wave events. INTRODUCTION Single waves that are extremely unlikely as judged by the Raleigh distribution are called freak waves. The freak event occurred on January 1 1995 under the Draupner platform in the North Sea (Wist et al., 2002) provides evidence that such waves can occur in the open ocean. During this freak event, an extreme crest with an amplitude of 18.5 m occurred. The maximal wave height of 25.6 m was much more than twice the significant wave height of about 10.8 m. Two linear mechanisms which can cause such a concentration or focusing of wave energy in a small area of the ocean have been proposed: time-space focusing and current focusing. In particular, the first mechanism can be explained by means of the theory of quasi-determinism (Boccotti, 1981,1982,1989,1995,1997,2000). Boccotti showed that, if in a Gaussian sea a very high wave height occurs at some point in space and time, with high probability a well defined quasi-deterministic wave group generates the high crest.
- Europe > North Sea (0.34)
- North America > United States > Vermont (0.28)
- Europe > United Kingdom > North Sea (0.24)
- (3 more...)
Numerical Implementation of Energy Absorbing B.C In Fully Elliptic Modified Mild-Slope Equation Under FEM Computational Environment
Cho, Yong Jun (Dept. of Civil Engineering, University of Seoul) | Cho, Eun Kyung (Dept. of Civil Engineering, University of Seoul) | Kim, Min Soo (Daelim Industrial Co., LTD.)
ABSTRACT Improved method for generating waves in elliptic modified mildslope equation (MMSE) is described. After recasting MMSE into the form of a pair of hyperbolic first order equations, we solved this system with added mass in continuity equation using Green functions, the solution of which reveals the relationship between the source amplitude and surface wave characteristics. We test the model for generation of desired incident waves in an annular entrance channel. We also compare model results with available experiment data. The agreement between numerical and analytical solution is remarkable. INTRODUCTION The proper understanding of the change in the wave field due to the interaction with the seabed plays an important role in resolving coastal engineering problem. The most robust mathematical model of surface water waves among the available, which can explain the combined effects of reflection, refraction and diffraction, might be the Boussinesq type equations of conservation of mass and momentum (Nwogu, 1993; Lee et al, 2003). Despite of its theoretical flawlessness, this nonlinear wave model is rarely deployed in engineering practice to access the wave condition in coastal regions. On the other hand, elliptic mild-slope equation (MSE) derived by Berkoff (1972, 1976) has become very popular even though MSE is derived in the context of linear wave theory. We can explain this popularity by several reasons. First of all, the involved computational works are quite less demanding than the nonlinear counterpart. Second, the application range of MSE has been extended due to the lately added rapidly varying terms, which were neglected by Berkoff (Chamberlain et al, 1995; Chandrasekera et al., 1997; Massel, 1993; Guazzelli et al., 1992.). Elliptic MSE was also modified to time dependent hyperbolic type to reduce the computational efforts (Copeland 1985, Suh et al. 2001).
Energy Dissipation And Transfer In Breaking Waves Generated By Directional And Multi-Frequency Focusing In Deep Water
Hong, Keyyong (Ocean Development System Research Division, Korea Research Institute of Ships and Ocean Engineering, KORDI) | Meza, Eustorgio (Research Center for Applied Science and Advanced Technology, Mexico National Polytechnic Institute) | Liu, Shuxue (State Key Laboratory of Coastal and Offshore Engineering, Dalian University of Technology)
ABSTRACT Wave energy dissipation and energy transfer between wave components during the directional wave breaking are investigated experimentally. Directional breaking waves in deep water were simulated by focusing the multi-frequency and multi-directional wave components at a designed location based on constant wave amplitude and constant wave steepness frequency spectrum. The incipient and plunging breakers with the same spectral characteristics were generated by applying the different scale factors on wave amplitude. The time series of surface wave elevation were measured at 9 different locations around the wave focusing point using a wave gauge array and the horizontal velocity of fluid motion was also measured to examine the variation of directional spreading function. By comparing the incipient and plunging breaking waves, the characteristics of energy dissipation and energy transfer caused by wave breaking are investigated and their dependences on directionality as well as frequency are analyzed. The breaking in deep water significantly dissipates wave energy in the upper region of peak-frequency band while enhance wave energy in the low-frequency band by energy transfer. INTRODUCTION Breaking waves plays important roles in the dynamics of open ocean waters. It dissipates wave energy through the generation of turbulence and the entrainment of air, which limits the heights of wind-driven surface waves, generates currents and enhances air-sea fluxes of heat, mass and momentum (Melville 1996). This research focuses on investigation of wave energy dissipation and energy transfer during the breaking of ocean surface waves for the purpose of predicting the spectra of ocean surface waves. The ability to forecast or predict sea states is crucial to our activities in the ocean such as fishing, recreation, maritime and deep-water oil exploration and production. Wave breaking is one of the three dominant factors affecting ocean wave evolution.