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Collaborating Authors
The Seventeenth International Offshore and Polar Engineering Conference
Spudcan footprint left by former jack-up activity can be hazardous to new spudcan installation. A centrifuge model study was carried out to investigate the changes in the shear strength profiles within a footprint at two different times after its formation. By attaching a T-bar on a moving platform, the shear strength profiles at different locations within a footprint can be determined. The test results reveal that the interaction between a new spudcan installation and an existing footprint interaction is a time dependent problem. INTRODUCTION Mobile jack-up rigs are commonly deployed for offshore oil and gas explorations. In some instances, a jack-up rig may return to an old site to drill additional wells or maintain old wells. The existence of old footprints due to former jack-up activities may be hazardous for the new installation as an existing footprint can exceed 10 m in width and depth in soft soil (Stewart & Finnie, 2001). The new installation of jack-up spudcan foundation at a certain distance from the footprint center may induce excessive stress on the jack-up legs or slewing of the rig into the old footprint. The uneven soil bearing resistance at and around a footprint during a new spudcan installation is believed to be a significant factor for the problem of interaction between the spudcan and an existing footprint. Knowledge on the pre and post shear strength profiles within a footprint would certainly be useful to offshore engineers for the assessment of the footprint interaction problem. In view of this, a centrifuge model study has been initiated at the National University of Singapore to investigate the shear strength changes within a spudcan footprint. Details of the study are presented in this paper. EXPERIMENTAL SET-UP AND PROCEDURE Centrifuge Model Set-up Fig. 1a shows a photograph of the centrifuge model set-up for the present study.
We extend the work of Longuet-Higgins and Phillips (1962) to present the expression for changes in phase speed of one wavetrain in the presence of another in water of finite depts. It is shown that the change in phase speed is more pronounced when the directions of two wavetrains are opposite. The increase or decrease in phase velocity of parallel or antiparallel wavetrains is investigated from the temporal surface elevations simulated using Zahkarov's equation for various water depths. INTRODUCTION Longuet-Higgins and Phillips (1962) studied the resonant interactions between two trains of gravity waves, and indicated that the phase speed of one wavetrain on the surface of water is modified in the presence of another, and vice versa. This change is of third order and is different from, but the same order as the increase predicted by Stokes for a single wave train. The phase velocity effect of these nonlinear interactions can be traced if we consider the resonant interaction in sets of four waves. Longuet-Higgins and Phillips (1962) obtained an analytical expression for the change of phase velocity when the wavenumbers are equal in pairs and hence the corresponding wave frequencies are also equal in pairs. Hogan et al. (1988) generalized the results of Longuet-Higgins and Phillips (1962) for gravity-capillary waves, based on Zakharov's integral equation. They derived the correct form of Zakharov kernel function for gravity-capillary waves and used the form to calculate the change in phase speed for both gravity-only motion and motion under the combined effects of gravity and surface tension in deep water. More recently, Tanaka et al. (2002) presented three different expressions of the change in phase velocity due to nonlinear interaction between two wavetrains in deep water, and indicated that all these expressions derived from Zakharov-type equations are equivalent to each other, as well as with the result of Longuet-Higgins and Phillips (1962).
Soil removed by dredging has generally ken disposed of by dumping at sea in an unplanned manner. However, the London Convention 1972 and the 1996 Protocol call for assessments of the environmental impact of dredged soil disposal at dumping sites. Compliance with the Protocol will require techniques for predicting the deposition configuration of dredging sludge dumped from barges. As part of the present study, a quantitative model for estimating the spatial deposition of earth and sand dumped from barges is developed. The accuracy of this model is then verified through laboratory and field experiments. Based on these results, this study proposes a simple calculation diagram capable of predicting the shape of the deposit that results when dredged soil is dumped at sea. INTRODUCTION Sediment must be dredged from ports, harbors, marinas, and inland waterways to keep shipping lanes clear. Although much of the material removed is disposed of at sea, both the dredging and the disposal of the dredged material pose environmental impact risks. The London Convention 1972 and the 1996 Protocol call for assessments of the disposal of dredged material to prevent ocean pollution resulting from waste dumping. "The guidelines for assessment of waste or other matters that may be associated with dumping" are intended for use by national authorities responsible for regulating waste dumping and embody mechanisms for guiding national authorities in evaluating methods of waste dumping in a manner consistent with the provisions. Hopper and split barges are often used in such disposal. One advantage offered by barges is speed: They are capable of carrying large volumes of dredged material. To enable effective planning for assessments of the disposal of dredged material to prevent ocean pollution resulting from waste dumping, there must be some wav to forecast the shape of the deposits of earth and sand dumped from the barges before actual dumping occurs.
Flaring Shaped Seawall has an excellent checking effect of the wave overtopping due to its deeply curved cross section. On the other hand, an impulsive breaking wave pressure tends to occur on its upper curved face depending on incident wave conditions as well as mound configurations. In this study, a slit type wave dissipating structure, which consists of a row of circular columns, was employed to reduce the impulsive wave pressures on the Flaring Shaped Seawall with maintaining its excellent checking effect of the wave overtopping. A series of hydraulic experiments were carried out to investigate the efficiency of the cylindrical slit wall on reducing the impulsive wave pressures. The wave overtopping rate and the wave reflection coefficient were also measured to confirm the influences of the cylindrical slit on these hydraulic quantities. INTRODUCTION Flaring Shaped Seawall as shown in Fig.1 has an excellent checking effect of the wave overtopping due to its deeply curved cross section (Murakami, et al. 1996). Authors have been improving the seawall cross section to increase its stability against wave actions (Kamikubo, et al. 2000). Depending on a sea bottom configuration or a seawall depth, a rubble mound structure is required under the Flaring Shaped Seawall to keep an appropriate construction space. The mound structure causes a forward tilting of incoming waves on its slope. The waves, that have a steep profile due to mound structures, often bring an impulsive wave pressure, and an appropriate countermeasure has to be applied under certain circumstances to reduce this wave pressure. Kataoka, et al. (1999) proposed a composite wave dissipating works, which consists of both a slit structure inside the curved section and wave dissipating blocks piled up in front of the mound with a low crest elevation, to reduce the impulsive wave pressures acting on the Flaring Shaped Seawall.
A Study On A Semi-Submersible Floating Offshore Wind Energy Conversion System
Shimada, K. (Shimizu Corporation) | Ohyama, T. (Shimizu Corporation) | Miyakawa, M. (Shimizu Corporation) | Ishihara, T. (The University of Tokyo) | Phuc, P.V. (The University of Tokyo) | Sukegawa, H. (Tokyo Electric Power Company)
A new semi-submersible floating structure is proposed on which three wind turbine towers are installed. This paper presents a basic characteristic of the wave-induced motion of this semi-submersible floating structure via. numerical computations and 1/150 scaled rigid model experiments in a wave tank. In the numerical computations, nonlinear damping effect due to drag forces modeled by the Morison's formula is considered in the equation of motion, where the linear hydrodynamic forces are obtained from the Green's function model. As a result, the response characteristics around the resonant frequency region were successfully improved. In addition to such basic examination, major results of feasibility studies, including the structural stability for severe wave conditions and the long-term fatigue limit state, are presented for a realistic situation. INTRODUCTION At the end of 2005 in Japan, total amount of installed wind power capacity reached to 1,078MW. However, a large portion of wind energy potential in Japan is located at rural area, where demand is low, the grids are weak, and the limitation in integrating wind energy to the grids exists. Onshore wind resource is almost developed and little land is left for large-scale wind farm. Thus if offshore energy potential near urban area is high, it will help the penetration of wind energy in Japan. In the European countries where wind conversion systems have been developed for a long time, Offshore Wind Energy Conversion systems (OWEC) have already been operated. In Japan where totaling 3,000 MW installed wind capacity is officially planned to reach by 2010, realization of an OWEC is strongly expected in order to achieve the target. Fig.1 displays a sea area where the offshore wind farm is supposed to constructed in this study and spatial distribution of annual mean wind (Sukegawa et al. 2006). Wind speeds depends on the region, however, there are some regions where wind speed is expected over 7m/s only at 10km off from coast line.
We have developed a propulsion mechanism using a variable-bending-stiffness fin with a variable-effective-length spring of which stiffness can be changed dynamically. The apparent bending stiffness of the fin can be changed dynamically by changing effective length of spring. We described the flow around the mechanism with changing effective length of the spring, and discussed the effect of flow on the thrust force of the mechanism. INTRODUCTION The conventional screw propeller is the general propulsion system for ships or underwater vehicles. As an alternative propulsion mechanism for higher propulsion efficiency and better safety than the screw propeller, propulsion by oscillating an elastic fin resembling a caudal fin or pectoral fin of fish has been proposed, and a basic studies on the oscillating fin and their developments as propulsion methods for ships or underwater vehicles/robots have been carried out (Morikawa, 1980; Nakashima, 2000; Watanabe, 2002). The optimum elasticity of a fin is not constant and changes according to the movement task and environment, such as swimming speed and oscillating frequency (Nakashima, 2000; Watanabe, 2002). However, it is very difficult to exchange fins of different bending stiffnesses while moving. We aimed to develop a propulsion mechanism using a variable-bending- stiffness fin of which stiffness can be changed dynamically. For the development of a propulsion mechanism using an oscillating fin, we have made a variable-bending-stiffness fin with a variable-effective-length spring. The apparent bending stiffness of the fin can be changed dynamically. We have also made the flow visualization system. In this paper, we described the structure of the propulsion mechanism in fluid using a fin with a variable-effective-length spring, and the flow around the mechanism with changing effective length of the spring. Furthermore, we discussed the effect of flow on the thrust force of the mechanism.
The purpose of this study is to develop a navigation and control system for a biomimetic-autonomous underwater vehicle (BAUV) to track a target. A Bayesian method, using an extended Kalman filter, combining localization and environmental mapping by a BAUV is implemented. This strategy selects the best sensor measurement by choosing one of several forward-looking directions. BAUVs' body moves in a cyclic pattern, so a cheap echo sounder may be installed on the head of the BAUV to detect environmental features, without the need to use expensive scanning devices. The localization and environmental mapping problem is then transformed into a nonlinear two-point boundary value problem. The optimal policy is to maintain the accuracy of the predicted states and to approach minimal cost of observation by solving the control problem. A line-of-sight guidance law which drives the BAUV to the target is used. A method that controls the motion of the body/caudal fin and pectoral fins of the BAUV is proposed for the target tracking. The estimation, measurement, and control process are integrated to form a working system. Experiments performed using a testbed BAUV confirm the effectiveness of the proposed method. INTRODUCTION Undersea vehicles, including autonomous underwater vehicles (AUVs), have become important in undersea inspecting and surveying. However, poor propulsive efficiency and maneuverability while hovering remain challenges to AUV designers. Biomimetic AUVs (BAUVs) mimic natural fish that have been evolving for thousands of years. Fish have a remarkable ability to remain very stationary and turn tightly and quickly. The shapes of fish are appropriate for swimming in water. The BAUV as a fast and highly maneuverable vehicle is promising for the design of future underwater vehicles. The swimming skill of fish is a valuable reference for designers of future underwater vehicles. For example, fish can rapidly, with the radius of 10% to 30% of body length, turn and vary the advancing direction[Colgate,2004].
- Information Technology > Artificial Intelligence > Robots (0.87)
- Information Technology > Communications > Networks > Sensor Networks (0.60)
- Information Technology > Artificial Intelligence > Representation & Reasoning > Uncertainty > Bayesian Inference (0.34)
- Information Technology > Artificial Intelligence > Machine Learning > Learning Graphical Models > Directed Networks > Bayesian Learning (0.34)
Greater control of movements gives actively swimming aquatic animals greater abilities to move as necessary. Control mechanisms affect posture, swimming trajectory, static and dynamic stability, and maneuverability. Control mechanisms are both passive and powered. This paper focuses on an unrecognized set of passive mechanisms of control of posture and trajectory that occur in a wide variety of actively swimming aquatic animals. We discuss insights concerning passive mechanisms of control deriving from our earlier work with tetraodontiform fishes that we think also apply much more widely. Detailed consideration is given to possible roles of lateral keels and rows of scutes located on and near the caudal peduncles in many different elasmobranchs (sharks) and bony fishes, also to the peduncular keels found in all cetaceans. Similar structures in some fossil aquatic forms are also discussed. INTRODUCTION The purpose of this paper is to bring to the attention of the engineering community interested in biomimetics and bioinspired design some hitherto unrecognized potentially interesting features of many kinds of actively swimming fishes (both sharks and bony fishes), also of all cetaceans (whales, porpoises, dolphins), We believe that these features produce specific kinds of hydrodynamic effects that can play significant passive roles in increasing the stability and agility of the animals having them while they swim and maneuver. Incorporating similar features into the designs of engineered underwater or aerial vehicles might have beneficial effects (Bar-Cohen, 2006). BACKGROUND Robust mechanisms for both posture and trajectory control are essential for actively swimming aquatic animals moving and maneuvering at varying speeds through variably turbulent waters. The better these animals can control their body postures, trajectories, stability and maneuverability the more easily they can move as they need to. Mechanisms contributing to posture and trajectory control are either passive or powered.
The purpose of this study is to understand a propulsion mechanism of a jellyfish for applying its mechanism to a soft-matter micro robot. We observed the motion of a jellyfish (Aurelia aurita) by a motion-capture camera, and measured the vector field of flow around a jellyfish by using a PIV (Particle Image Velocimetory) measurement. A jellyfish is principally propelled by a jet at the contracting phase of a jellyfish motion. If that is true, it is interesting that a jellyfish never stops traveling even at the expanding phase. We found that a vortex ring with the opposite vorticity to shed vortex ring was inside a jellyfish body in the expanding phase. We discussed a cause of an increase in thrust force and keeping constant speed in the expanding phase. INTRODUCTION The swimming motion of a jellyfish is elegant and complex in spite of simple components of body. The main component of the jellyfish is 98% of water (Azuma, 1980) (Sakata, 1994) (Azuma, 1997). The swimming motion of the jellyfish consists of expanding motion and contracting motion of the skirt (Matthew, 2003) (Dabiri, 2005). The motion is called scissors kick motion. We measured change in speed of the body by using a motion-capture camera. When we compared the change in speed of the body with edge motion, we found that the acceleration occurred in the expanding phase. This is not commonsense because it is popular to think that the acceleration occurs in the contracting phase like as jet propulsion of a squid. We tried to understand how a jellyfish gets propulsion to take account of change in momentum. The flow around a jellyfish during its swimming was observed by flow visualization and measured velocity field by PIV method. EXPERIMENTAL SETUP AND METHOD We used a popular jellyfish that called moon jellyfish (Aurelia aurita) with 46.6mm in diameter.
High Performance Concrete (HPC) is a composite mixture containing cementitious material and aggregate. The cementitious material is blended with Pozzolans such as silica fume particles and fly ash to enhance its binding and durability properties. This paper studies the effect of pozzolanic materials on the compressive strength and modulus of elasticity of HPC. An experimental program is conducted to evaluate the effect of silica fume, fly ash, and combination of the two used as cement supplemental materials on the modulus of elasticity. Results show that adding silica fume to HPC increases both the compressive strength and the modulus of elasticity at early ages. However, the increase subsides at later ages (< 28 days). New equation is proposed to accurately predict the modulus of elasticity of HPC. INTRODUCTION As higher emphasis is made on long-term durability, high-performance concrete (HPC) is becoming a standard for transportation structures in the United States of America (USA) [1-8]. HPC is created to reduce the porosity by adding pozzolanic materials (i.e., silica fume and fly ash) that with the presence of water react with the calcium hydroxide released by Portland cement hydration to form a cementitious compound. As a result, HPC becomes denser with lower capillary pores and prevent chloride ion penetration reducing the corrosion potential of the steel reinforcement. The addition of pozzolanic materials does not only affect the HPC durability but also the mechanical properties, namely the compressive strength and modulus of elasticity. Both the compressive strength and modulus of elasticity are very important properties for structural engineers to design and evaluate the structure, especially for deflection and creep calculations [9, 10]. Thus, the effect of pozzolanic materials on modulus of elasticity needs to be investigated. The objective of this paper is to evaluate the effect of pozzolanic materials, namely silica fume particles and fly ash, on the modulus of elasticity of HPC.
- Research Report > Experimental Study (0.68)
- Research Report > New Finding (0.49)