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Collaborating Authors
The Twelfth International Offshore and Polar Engineering Conference
ABSTRACT A large buoy has been developed to obtain the sea truth data on ocean waves and the current for use in evaluating the accuracy of an ocean radar system. Displacement of the buoy is 70 tons and it has a diameter of 9m, it is now moored near the Okinawa islands (Southwestern part of Japan) in an area of the sea 1450m deep. This area is world famous because it is suffered from many strong typhoons every year and it is the best location to study waves. With this buoy system waves are reproduced using the measured heave of the buoy and its response function obtained from theoretical calculations, and evaluation is made by model experiments in the wave tank. This report presents results of the theoretical calculations, model and field experiments and discusses their accuracies. INTRODUCTION There is a variety of remote sensing tools housed in artificial satellites, which are very useful for monitoring the global environments. Sensors and software for measuring water temperature, wind velocity, current velocity, wave height and chlorophyll on the sea surface have been developed and installed on these satellites. Remote sensing using the artificial satellite is most effective in its breadth and simultaneity but it must be calibrated by direct measurements. Ocean engineers have been charged with developing these vehicles and buoys for the direct measurements (Koterayama, et al., 2000). The wave radar system is very useful tool to measure waves over a wide area with real time observation, but it still lacks a method to check its accuracy. This area is noticed for the serious typhoons which hits these islands each year. It has been very difficult to measure waves in the open ocean during a typhoon and there are not many accurate measurements. LROR is looked forward to giving new findings.
ABSTRACT For the global and fatigue structural strength analysis of a semisubmersible platform, the wave loads of design conditions are calculated by using the three-dimensional boundary element method. The maximum of vertical bending moment, torsion moment and horizontal split force are determined from a serious of design waves with limited wave periods and positions of incident wave crest, indicated by the wave phase from 0° to 360° in one wave period. The extreme wave loads under the combination of wave parameters and the transfer functions of wave loads are used as the input of hydrodynamic pressure in the three-dimensional FEM analysis process. INTRODUCTION In the practice of engineering design for ships and ocean structures the direct calculation procedure is wildly applied in structure verification, in which the three-dimensional FEM analysis is used more and more commonly. The hydrodynamic pressure distribution on hull surface induced by waves is the dominant component of the input loads of the FEM analysis. Ordinary three methods are used in the calculation of hydrodynamic pressure: the Morison formula method, the strip method and the three-dimensional panel method. Considering complex structure and large scale for the semi-submersible platform, the panel method is used in this computation. To assume the fluid is irrotational, the problem of prediction for the motions and loads of floating body in waves can be expressed in the form of Laplace's equation with propriety boundary conditions. For the frequency domain, the evaluation of Green function without forward speed is well discussed by Newman(1985) and Stergios(1992). The evaluation of Green function of the translating and pulsating source is much more complex because of a double integral. Zong and Huang(199l) give a form of single integral for this function but the kernel function is oscillated in some parts of integral field.
ABSTRACT To standardize and simplify the design process of offshore structures, the designer wave load is developed. The load can accurately predict the extreme response of offshore structures by the static analysis under the realistic wave conditions. The designer wave load can be calculated by multiplying modification factors and the wave force based on Morrison's equation with the mean wave height and mean wave period. INTRODUCTION The dynamic response of offshore structures subjected to random wave forces can be computed by the random vibration analysis (Malhotra and Penzien 1970a), and its maximum expected value and probability of the first passage of a barrier level during its lifetime can be examined (Cartwright and Longuet-Higgins, 1956, Vanmarcke, 1975). However, it is obviously inefficient to carry out dynamic analyses repeatedly during design process and also troublesome to combine dynamic responses with other design responses due to design loads. Since it implies the necessity of the simple design methods to evaluate extreme responses of the offshore structure, this paper describes the fundamental theory of the designer wave load, which evaluates the maximum expected responses of an offshore structure by the static analysis (Taniguchi and Kawano, 1998). By knowing the wave height and the wave period, the Morrison's equation enables to calculate wave force on the vertical column immersed in the sea. Therefore, the maximum expected wave force on the vertical column can be determined by the probability analysis based on the spectral form of wave force. To calculate the maximum expected wave force t&n the known parameters such as the mean height and the mean wave period, the modification factors for them are defined. At the time, since the drag and inertia forces do not reach their maxima at the same time and the same frequency, the modification factors for them are defined respectively.
ABSTRACT We measured the diffraction force acting on three types of models in the AMOEBA. The AMOEBA, developed by the authors, is a compact circular wave basin (diameter 1.6m) enclosed by segmented absorbing wave makers. The experimental models were simple models, ocean structure models and a ship model. For the ship model, we conducted experiments with the model both moving forward and stationary. We observed that the measurement results in the AMOEBA were the same as those in a large basin, although the scaling down factor for AMOEBA is much larger than that in a large basin. This indicates that we can carry out some experiments with good accuracy in the AMOEBA instead of a large basin. INTRODUCTION A wave basin enclosed by segmented absorbing wave makers has some advantages. For instance, it is possible to generate multi-directional wave fields, to make a broader expanse of a wave field without a wave-absorbing zone, and to carry out experiments without undesirable waves being reflected from the wave basin wall or the experimental model. Many researchers have studied the absorbing wave maker (Bessho,1973; Falnes and Budal,1978; Bessho and Yamamoto, 1980; Naito and Nakamura, 1985; Naito et al,1987). However, the above wave basin has not yet been built because of the technical difficulties and the prohibitive cost. The authors have continued to study the absorbing wave maker to build such a wave basin, and we succeeded in developing a plunger-type absorbing wave maker using a voice coil motor (VCM) system as the driving equipment (1990). This wave maker is small and mobile because of the simple mechanism of the VCM. Fifty units of the wave maker have been placed along the periphery of a basin of diameter 1.6 m. It is called AMOEBA (Advanced Multiple Organized Experimental BAsin).
Shipping Water Load Due to Deck Wetness
Ogawa, Yoshitaka (Ship dynamics division, National Maritime Research Institute Japan) | Minami, Makiko (Ship dynamics division, National Maritime Research Institute Japan) | Tanizawa, Katsuji (Ship dynamics division, National Maritime Research Institute Japan) | Kumano, Atsushi (Nippon Kaiji Kyokai) | Matsunami, Ryoju (Nippon Kaiji Kyokai) | Hayashi, Tutsuyu (Nippon Kaiji Kyokai)
ABSTRACT A series of model tests in waves were conducted to measure the shipping water loads that act on deck and hatch covers due to deck wetness. A model of bulk carrier was used. The tests were carried out in irregular waves of which significant height is 10.6 meters and peak period is 14 seconds. In order to discuss the effects of wave heading and ship forward speed on the shipping water loads, the model tests were made in several combinations of wave heading and ship speed conditions. It was confirmed that the deck wetness and shipping water loads will be reduced considerably if the wave heading is altered to the quarter or beam seas or the ship speed is reduced. In order to verify the results of experiment quantitatively, shipping water loads on fore deck and hatch cover were estimated by improving estimation methods that were developed by the author (Ogawa, 1997). Measured shipping water loads were also compared with present rule and requirement for hatch cover. Although it is difficult to directly correlate measured values with the rule and requirement, mean values of measured results tend to be larger than the ones of the rule. It is also found that the shipping water loads defined in the requirement is ranked to somewhere between l/l0 and 113 significant values in relation to the measured results. INTRODUCTION A Revision work of International Convention on Load Lines (ICLL66) is carrying out in the International Maritime Organization (IMO) in recent years. Rational revision is needed by the use of the seakeeping theory, which has been progressed after the establishment of ICLL66. Revision work is carrying out gradually. At the present time, revision of regulation especially about minimum bow height and hatch cover is carrying out. (Watanabe et. al., 2000)
- Transportation > Marine (0.87)
- Transportation > Freight & Logistics Services > Shipping (0.55)
ABSTRACT In this paper, wave drift speed of a floating body is discussed. Measurements of wave drift speed were conducted on both twodimensional floating body and three-dimensional floating body. Then, an estimation method of wave drift speed was derived from the analysis of measurement results. The mechanism of wave drift speed varies with wavelength. In short wave range, wave drift force due to wave scattering pushes the floating body. Therefore, drift speed is decided by the equilibrium of wave drift force and fluid drag and it is proportional to the wave slope. In long wave range, on the other hand, wave drift force hardly acts on the floating body, because wave almost transmits the floating body. Therefore, the drift speed is decided by the wave-current speed and it is proportional to the square of the wave slope. Taking these wave drift mechanisms into consideration, an estimation method of wave drift speed, which covers entire wave range, is proposed. INTRODUCTION A joint research project named "On the drifting prevention of disabled ships in rough waves" is conducted by National Maritime Research Institute, the Maritime Safety Agency of Japan, Osaka University, Kyushu University, rope manufactures and a salvage company. In this project, development of towing support system for rescue ships and accuracy improvement of drift course prediction system are expected as final output. The authors are in charge of the latter system. The drift course prediction system was developed by the Maritime Safety Agency of Japan and had been put into practice. Accurate estimation of the drift speed is the key technology for the drift course prediction. In the total drift speed, the ocean current speed and tidal current speed are leading terms and drift speed due to wind and waves are correction terms.
- Energy > Oil & Gas (1.00)
- Government > Regional Government > Asia Government > Japan Government (0.45)
ABSTRACT The paper demonstrates how the findings of a series of model tests and diffraction calculations, performed to determine the motion behavior of a new semi-submersible design were used to influence the final design. From the model tests at different drafts, wave headings and current speeds it was observed that a semi could take on a steady list angle in regular head waves. It was observed that this steady list angle increases for smaller vessel drafts and higher current speeds. This mean list angle appeared to be induced by shallow water wave effects on top of the pontoons and resulting set-down. Increasing the stability of the semi by changing the vertical centre of gravity reduced the steady list angle, and in some cases removed the list completely. As a result the design of the stability columns was changed to provide a substantial increase in initial stability at both operating and survival drafts. The model test results were confirmed by a diffraction analysis that also provided a good assessment of the forces causing the steady list angle. INTRODUCTION The majority of today's deep-water developments are located in the Gulf of Mexico, Brazil, and West Africa. These are all areas with moderate environments characterized by much lower wave heights than are found in areas subject to harsh environments such as the North Sea and Eastern Canada. A semi-submersible designed specifically to operate only in moderate environments will have acceptable motion characteristics even if the height of its stability columns is less than the column height of a rig designed to maintain sufficient air gap in storm waves and to have acceptable motion characteristics in larger operating waves. The capital cost of such a moderate environment semi-submersible will be substantially less than the cost of its harsh environment cousin.
- North America > Canada (0.54)
- North America > United States > Texas (0.28)
- Europe > United Kingdom > North Sea (0.24)
- (3 more...)
- North America > Canada (0.89)
- Europe > United Kingdom > North Sea (0.89)
- Europe > Norway > North Sea (0.89)
- (2 more...)
Wave Drift Added Mass Of Floating Bodies Measured From a Free Decay Test Or a Slowly Forced Oscillation Test In Waves
Kinoshita, Takeshi (Institute of Industrial Science, University of Tokyo) | Bao, Weiguang J. (Institute of Industrial Science, University of Tokyo) | Yoshida, Motoki (Institute of Industrial Science, University of Tokyo) | Ishibashi, Kazuko (Institute of Industrial Science, University of Tokyo)
ABSTRACT When a body is slowly oscillating in waves, there exists another source of added mass and damping, i.e. the so-called wave drift added mass and wave drift damping. They are resulted from the nonlinear interaction between the slow oscillations and the waves. Different from the conventional added mass and wave-radiating damping, they are quadratic forces in wave amplitude. The wave drift added mass is measured in the present work from a free decay test or a forced slow oscillation test. The model is either a circular cylinder or an array of four circular cylinders. Experimental parameters are systematically changed to examine their effects on the wave drift added mass. A calculation based on the potential theory is also carried out. INTRODUCTION Ocean structures are usually constrained by mooring systems, which supply relatively weak restoring forces in horizontal plane. Under the slowly varying drift forces exerted by ocean waves, these structures may undergo low-frequency resonant oscillations in the horizontal motion modes, i.e. surge, sway and yaw. Conventional added mass and damping can be obtained by solving a linear radiation problem in which the body of the structures is forced to oscillate in the calm water. In the case when the frequency of the resonant motion is very small, the wave-radiation damping is negligibly small, while the added mass is the same order of the displaced water mass. However, with the presence of the incident waves, there exists another kind of added mass and damping that is caused by the nonlinear interaction between waves and slow oscillations. As part of the nonlinear wave loads, their magnitude is proportional to the square of the wave amplitude, which is different from the conventional added mass and damping, and they are called wave drift added mass and wave drift damping respectively.
ABSTRACT A method that large fluid bags are allocated in the cargo hold is proposed for safe and efficient transportation of massive water with general cargo ships. Pressure by sloshing occurs on the side wall of the hull by the liquid bags and the strength seems to vary by the distance between the rotation center and the liquid bag center of gravity. The experiment with the l/10 scale model of GT499-type cargo ship is conducted to confirm the safety and the feasibility of the bag system. From these results, it is clarified that the method of water transportation with hold size liquid bag can decrease the sloshing pressure drastically. Furthermore, the sloshing pressure becomes big according to the increase of the distance between the liquid bag and the center of rotation and the tendency becomes predominant in the rolling period under 10 seconds. INTRODUCTION A special ship is used for the liquid transported by the vessel. And as for the present liquid transport by the cargo ship, it is limited to the transport with miniature container. However, many cargo handling times are necessary in such a way of transportation. And, the efficiency becomes bad when a large quantity of liquid is transported. It should be especially necessary to transport a large quantity of water at once when the disasters or shortage of water happen. The technology that the water bag of large quantity is towed on the water surface is: partly put into practice in Norway. However, there are some problems such as regulations on the transport and the sea condition to apply that technology in Japan. So, we propose a method as shown in Fig. & and we carry out research to transport a large quantity of water by the general cargo ship without using a special ship.
- Transportation > Marine (1.00)
- Transportation > Freight & Logistics Services > Shipping (0.97)
ABSTRACT The installation design of small structures using an AHTS (anchor handling and towing ship) is addressed, through the coupled motion analysis of the two structures connected by an installation cable, in the frequency domain, using the WAMIT program. This problem is sometimes analyzed by imposing the AHTS stern motions on the top of the installation cable, an uncoupled analysis. We will show that in some situations the presence of the small structure will influence the AHTS motions, mainly the pitch motion, and should be taken into account. The influence of some parameters on the solution will be addressed, like the depth of the submerged small box structure, and the cable stiffness. The converged results will be post-processed in order to get short term statistical responses of the line tensions as a function of the sea-states, using a Jonswap spectrum to represent the sea environment. Varying the significant wave height and peak period, we will be able to define a window in this domain where the marine operation will be safe with regard to the line tensions. The suspended structures weight around 100tf, while the installation cables have an operational maximum tension around 300tf. The most critical design condition will occur when the dynamic maximum tension in the installation line gets close to the structure self weight, as the cable will not be allowed to get slack. INTRODUCTION Offshore marine operations often involve the deployment of small structures, like a manifold, a suction pile or some kind of special pump on the seafloor. These structures have in general a total weight of the order of 1500KN, and in order to carry out this installation, it is common practice to avoid the use of large units, like a semi-submersible crane vessel, and to use an AHTS instead, keeping lower installation costs.