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Collaborating Authors
International Journal of Offshore and Polar Engineering
Hydrodynamic Interaction Between a Fixed And a Floating Cylinder
Kazi, Imam H. (Department of Mechanical Engineering, The University of Hong Kong, Hong Kong) | Chwang, Allen T. (Department of Mechanical Engineering, The University of Hong Kong, Hong Kong) | Yates, George T. (Department of Mechanical Engineering, The University of Hong Kong, Hong Kong)
ABSTRACT Hydrodynamic interaction between a fixed circular cylinder and freely floating cylinders is studied experimentally in the present paper. Six different moving cylinders are used with circular, square and rectangular cross-sectional shapes of various dimensions. Investigation is concentrated on the force acting on the fixed cylinder and motion of the floating cylinder. Comparison of the measured velocity of the moving cylinder shows reasonable agreement with the results of the potential flow model. However, the force on the fixed cylinder is somewhat below the predicted values. Reasons for the over prediction of the theoretical model are attributed to viscous effects and are discussed. INTRODUCTION Two solid bodies approaching each other in fluid are not very rare in nature. Such a case can be found in military application when a torpedo is launched toward another ship or submarine in the sea. A very common example which is of grave concern is the flow of an ice mass in an arctic or subarctic region where oil platforms are located. As a large floating ice floe approaches an offshore structure, the water is deflected around the structure. Because the trajectory of the ice floe is not necessarily the same as the pathline especially when the latter is curved, there is no assurance that the ice floe will not impact the structure. Even though the fluid may be deflected around the structure, the ice floe will be influenced by wind, viscous and inertial forces, including the Coriolis force that needs be taken into account in the case of large-scale motions over great distances. Such ice masses are a direct threat to a platform structure and act as a catalytic motivator to study the two-body interaction problem. Such a problem was studied theoretically by Yamamoto (1976), Isaacson and Cheung (1988) and later by Landweber, Chwangand Guo (1991).
ABSTRACT An oriented ice thickness distribution is presented. It describes sea ice as an oriented material and provides the description of ice conditions for an anisotropic plasticity model. This model describes the formation and evolution of individual lead and ridge systems. We can resolve ice behavior on scales as small as a few kilometers. Ship navigation and other offshore operations should benefit from the added information provided in simulations and forecasts of lead and ridge orientations. INTRODUCTION Ice observations throughout the polar regions provide fractions of area covered by ice of different stages of development (thickness). Ice charts produced by the U.S. Joint Ice Center classify ice into the categories: Open water and new ice (0-10 cm), Young ice (10-30 cm), First year thin (30-70 cm), First year medium (70- 120 cm), First year thick (120-200 cm), and Old ice (survived at least one summer's melt). The Russian ice observation system is similar, but adds a separate category for open water. The Canadian Ice Centre is similar, but divides New ice into New and Nilas ice, and divides Young ice into Grey (10-15 cm) and Grey-white (15-30 cm). Additional information on the size of ice floes can be added to an ice chart, but it will not be discussed here because sea ice dynamics models cannot now provide such information. Thorndike et al. (1975) introduced the ice thickness distribution to describe ice condition. Hibler (1979) used thickness distribution in a viscous plastic model, and Pritchard (1981) used it in a modified and extended Aidjex model. Most of the recent large-scale sea ice behavior models use a reduced thickness distribution (two categories: open water and thick ice). A few exceptions appear: e.g., Pritchard et al. (1990) described a coupled ice ocean model developed to forecast ice edge behavior.
Viscous Effects In Wave-Body Interaction
Landrini, M. (INSEAN, Rome, Italy) | Ranucci, M. (Dipartimento di Meccanica e Aeronautica, Università di Roma La Sapienza, Rome, Italy) | Casciola, C.M. (Dipartimento di Meccanica e Aeronautica, Università di Roma La Sapienza, Rome, Italy) | Graziani, G. (Dipartimento di Meccanica e Aeronautica, Università di Roma La Sapienza, Rome, Italy)
ABSTRACT The laminar flow about a circular cylinder beneath a Stokes waves train is numerically investigated. An efficient grid-free algorithm is developed by coupling an accurate boundary integral equation method for computing the velocity field with a viscous vortex method for solving the Navier-Stokes equations near the body. The problem of generating the incident wave system is effectively circumvented by means of a perturbation formulation which assumes the Stokes wave solution as the base flow. For β≈500 and Kc = O (1) the systematic comparison with the available experimental values for the Fourier components of the loading is presented. An overall good agreement is observed, even for the added inertia coefficient which is known to be largely affected by viscous effects. INTRODUCTION The flow of an incompressible viscous fluid about a circular cylinder beneath a regular Stokes wave train is a typical problem of marine structures' hydrodynamics. When the relevant wavelengths are comparable with the characteristic dimension of the body, the hydrodynamic loads are quantitatively dominated by the momentum exchange between the wave and the body. Consistently, it is widely accepted that the nonlinear potential flow theory effectively describes the force on the structure. In fact, experimental analysis confirms that the vertical mean value of the hydrodynamic force and of both the second and third harmonics of the fluctuating components are well explained in terms of the inviscid wave diffraction in agreement with the predictions of weakly or fully nonlinear models (Ogilvie, 1963; Vada, 1987; and Liu et al., 1992). However, the potential theory fails to exhaustively capture the entire phenomenon. Actually, a significant reduction in the amplitude of the fundamental harmonic of the loading is emphasized by Chaplin (1984), with respect to purely inviscid models. This correction exhibits a marked dependance on the Keulegan - Carpenter number Kc.
ABSTRACT The loads on an Arctic offshore structure may be governed by the force necessary to fail the ice at the structure or the environmental forces available to drive the ice past the structure. The factors controlling limit-force loads were investigated, and the likely magnitudes of these loads were established. Limit-force and limit-stress loads were compared for a scenario in which an Extreme Ice Feature interacts with an offshore structure. Probabilistic descriptions were produced for all key parameters, and Monte Carlo analyses were carried out. The analyses showed that, at exceedence probability levels of interest for design, extreme ice loads on the structure are likely to be reduced significantly by the available driving forces, in comparison to the case where only limit-stress loads are considered. However, the analyses can be considered only preliminary as more data are required to define the essential variables, and other load limits, such as limit momentum, need to be considered. INTRODUCTION Ice loads on offshore structures in the southern Beaufort Sea may be limited by the force to fail the ice at the structure (typically termed the limit stress), the available environmental driving forces and the capability of the pack ice to transmit them (typically termed the limit force), the momentum of the ice, and the force necessary to split floes contacting the structure. This paper summarizes analyses made by Comfort, Dinovitzer and Gong (1994) and by Comfort and Singh (1996) to quantify limit-force loadings, and to compare them to limit-stress loads. APPROACH AND SCENARIO CONSIDERED The distribution of limit-stress and limit-force loadings was predicted using Monte Carlo analyses for the interaction of an Extreme Ice Feature (EIF) with a structure. This assumed scenario commences with EIFs being generated north of the southernBeaufort Sea, and then drifting towards the area of interest
ABSTRACT A combined BEM-FEM model has been developed to simulate the dynamic interaction between nonlinear waves and an impermeable seawall with a submerged rubble toe over a sand seabed. Fully nonlinear potential equations are used to solve the wave domain and modified Navier-Stokes equations for the porous media. A poro-elastic finite element model has also been developed and used under the nonlinear surface pressure computed by the BEM-FEM model. The wave deformation and its induced pore-water flow are analysed for different hydraulic properties and sizes of the toe. The effects of the seabed shear modulus and thickness on the toe deformation and stability are investigated. INTRODUCTION Seawalls are sometimes provided with a submerged rubble toe. The toe causes early breaking of high waves and protects the sand seabed in front of the seawall from liquefaction/scour. However, the influence of the permeable toe on the wave deformation and the porous flow has not been investigated. The deformation and stability of the toe and seabed were not also evaluated in addition to the scour and liquefaction in the seabed offshore the toe. The existence of a permeable structure over a permeable seabed under the attack of nonlinear water waves presents great difficulties for the solution of the problem. Therefore, simplified approximate solutions are common practice. These solutions may define either the wave field around the structure, assuming it to be impermeable with a rigid seabed, or may provide the waveinduced pore-water pressures and velocities inside the structure and seabed using the linear water wave theory. The wave-induced pore-water pressure and stresses in a poroelastic seabed were computed either analytically (Yamamoto, 1977; Magda, 1994) or numerically using finite element method (Gatmiri, 1990). In their studies, they aimed at testing the possibility of seabed liquefaction and/or shear failure under severe wave conditions.
Assessment of the Wave-Iceberg Load Combination Factor
Foschi, Ricardo (Department of Civil Engineering, University of British Columbia, Vancouver, B.C., Canada) | Isaacson, Michael (Department of Civil Engineering, University of British Columbia, Vancouver, B.C., Canada) | Allyn, Norman (Westmar Consultants Inc., North Vancouver, B.C., Canada) | Saudy, Ibrahim (Westmar Consultants Inc., North Vancouver, B.C., Canada)
ABSTRACT The present paper describes an extension to a recent study (Foschi et al., 1996), which was undertaken to determine the appropriateness of the recommended value of the load combination factor relating to the combined effects of wave and iceberg loads, as described in the Canadian Standards Association (1992) code for the design and construction of fixed offshore structures. The study examines the sensitivity of the load combination factor to various iceberg and wave parameters typical of three sites off the East Coast of Newfoundland. The methodology is based on a numerical analysis in which loads due to waves alone, an iceberg alone, and an iceberg and waves in combination, have been calculated for a range of iceberg and wave parameters, with the results applied to a first-order reliability analysis to study force levels corresponding to specified annual exceedence probabilities. The results indicate that the load combination factor is most sensitive to the wave angle relative to the current direction, and to the wave climate during the iceberg season — therefore, the load combination factor is site dependent. A load combination factor has been calculated conservatively as 0.20, applicable throughout the range of situations considered. This compares with the Code values of 0.8 or 0.4, for icebergs and waves which are taken to be stochastically dependent or independent, respectively. INTRODUCTION The selection of suitable environmental loads and load events is of critical importance in the design of offshore structures intended for operation in extreme environments. Such loads may include those due to wind, waves, earthquakes, ice floes and iceberg collisions. The CSA Offshore Structures Code CAN/CSAS471-92 (S471) (Canadian Standards Association, 1992) describes the use of such loads in offshore design, and indicates the use of probabilistic methods on which the selection of loadevents and design loads should be based.
- Data Science & Engineering Analytics > Information Management and Systems (0.66)
- Facilities Design, Construction and Operation > Offshore Facilities and Subsea Systems (0.48)
- Well Drilling > Drilling Operations > Drilling operation management (0.34)
- Reservoir Description and Dynamics > Reserves Evaluation > Probabilistic methods (0.34)
ABSTRACT The amount of natural gas within the gas hydrate accumulations of the world is believed to greatly exceed the volume of known conventional natural gas reserves. The hydrocarbon production history of the Russian Messoyakha field, located in the West Siberian Basin, has been used as evidence that gas hydrates are an immediate source of natural gas that can be produced by conventional means. Re-examination of available geologic, geochemical and hydrocarbon production data suggests, however, that gas hydrates may not have contributed to gas production in the Messoyakha field. More field and laboratory studies are needed to assess the historical contribution of gas hydrate production in the Messoyakha field. INTRODUCTION The Messoyakha gas field in the northern part of the West Siberian Basin is often used as an example of a hydrocarbon accumulation from which gas has been produced from in-situ natural gas hydrates. Production data and other pertinent geologic information have been used to document the presence of gas hydrates in the Messoyakha field (Makogon et al., 1972; Makogon, 1981, 1988; Cherskiy et al., 1985; Krason and Ciesnik, 1985; Krason and Finley, 1993). It has also been suggested that the production history of the Messoyakha field demonstrates that gas hydrates are an immediate producible source of natural gas, and that production can be started and maintained by conventional methods. Recently, however, several studies suggest that gas hydrates may not be contributing to gas production in the Messoyakha field, and that the potential resource significance of gas hydrates may have been overestimated (Verkhovsky et al., 1988; Ginsburg et al., 1990; Ginsburg, 1993). In this paper we re-examine the evidence for gas hydrates in the Messoyakha field and critically review the available geologic data in order to determine if gas hydrates have contributed to gas production in the Messoyakha field.
- Phanerozoic > Mesozoic > Cretaceous > Lower Cretaceous (0.93)
- Phanerozoic > Mesozoic > Cretaceous > Upper Cretaceous (0.68)
- Geology > Sedimentary Basin > Cratonic Basin (0.82)
- Geology > Geological Subdiscipline > Geochemistry (0.66)
- North America > United States > Oregon > Washington > North Pacific Ocean > Cascadia Basin (0.99)
- North America > United States > Oregon > North Pacific Ocean > North Pacific Ocean > Cascadia Basin (0.99)
- North America > United States > California > Eel River Basin (0.99)
- (4 more...)
ABSTRACT On the basis of our recent studies, various important phenomena relating to swell are discussed. First, the change in swell by turbulent winds is considered. It is shown that the magnitude of the decay rate of swell by adverse wind is almost the same as that of the growth rate of swell by following wind. Second, the effects of swell on various sea surface phenomena are discussed. As expected, the effects of swell are negligible unless the swell steepness is large. However, with the increase in swell steepness, swell shows an interesting effect on sea surface phenomena. Swell propagating against the wind intensifies the growth of wind waves, if the swell steepness is large. This is quite contradictory to the well-known phenomenon that steep swell propagating in the direction of the wind suppresses wind waves. The former phenomenon cannot be explained by the mechanism proposed by Phillips and Banner (1974); it explains only the latter case, even though the mechanism should be applicable to both phenomena. The similarly contradictory effect of swell is seen in the effect of the swell upon wind-induced drift current. Swell propagating against the wind intensifies the drift current, while swell propagating in the direction of the wind shows little effect upon the drift current. In spite of the drastic changes of wind waves by swell, the microwave intensity backscattered from the sea surface is not much affected by swell. This is probably due to the fact that swell changes only the dominant part of the wind wave spectrum and does not affect the high-frequency region of the spectrum, which mainly governs the Bragg scattering mechanism of the microwaves at the wavy surface. INTRODUCTION It is a ubiquitous phenomenon in the ocean that wind waves insome particular wind areas coexist with swell
- North America > United States (0.47)
- Asia (0.30)
Field Experiments And Numerical Prediction On Dynamics of a Light Floating Structure Moored In Deep Ocean
Koterayama, Wataru (Research Institute for Applied Mechanics, Kyushu University, Kasuga, Japan) | Mizuoka, Hiroo (Research Institute for Applied Mechanics, Kyushu University, Kasuga, Japan) | Takatsu, Naoyuki (Kawasaki Heavy Industries, Kobe, Japan) | Ikebuchi, Tetsuro (Kawasaki Heavy Industries, Kobe, Japan)
ABSTRACT A floating structure for fish-gathering has been developed and moored in the deep sea south of Okinawa Island. The depth of the sea in this area is 1300 m. The structure is moored with a composite mooring line of wire rope and chain; its displacement is about 94 tons. To establish a design for the small floating structure, a series of field experiments was carried out in which motions of the structure, current velocity and wave height were measured with motion sensors, a current meter and a wave rider buoy. Motions of the structure obtained from the field experiments were compared with numerical simulations. The dynamics of the mooring line were calculated using a lumped mass technique. Hydrodynamic coefficients used in the numerical simulations were obtained from theoretical calculation and model experiments in a wave tank. The design method for a small floating structure moored in deepsea was confirmed through the field experiments. INTRODUCTION Designing a floating offshore structure requires theoretical and experimental studies to determine the anticipated performance and safety. In the final stage of the design, performance under working conditions and safety under survival conditions should be confirmed using numerical simulation programs developed on the basis of these studies. Establishment of a numerical simulation method for analyzing the dynamics of a small floating structure for fishery or ocean observation is necessary, yet the difficulties confronted are much more serious than for a large oil rig: The incident wave height is relatively large in comparison with the scale of the floating structure, which leads to strong nonlinear effects. The computer program developed for the numerical simulation of the motions of a small floating structure must therefore be validated by experiments, as has been done by many researchers over the past 20years for a large offshore structure
- North America > United States (0.68)
- Asia > Japan (0.47)
- Research Report > New Finding (0.49)
- Research Report > Experimental Study (0.34)
ABSTRACT In the present study, we examine the consistency and validity of the Boussinesq and KdV equations in describing nonlinear water waves generated by submerged disturbances moving with near critical speed in a rectangular channel. The study is focused on investigating the effect of disturbance length L on wave generation, and whether the two long wave models, which in theory require L to be much greater than water depth H, can actually be applied to cases where L/H = O(1). The numerical results based on the two wave models show that, if L is sufficiently long, the dominant forcing factor affecting wave amplitude and period is the “blockage coeffcient” defined as the ratio of the maximum submerged cross-sectional area of the disturbance over the wetted channel cross-sectional area, while L has little effect. This confirms Ertekin's (1984) and Mei's (1986) earlier results. However, when L is of the same order as H, the present results show that, as L decreases, it weakens the forcing strength significantly. This indicates that for short disturbances, both the blockage coefficient and the disturbance shape are important. Results from our towing tank experiments with Froude number ranging from 0.8 to 1.07 showed good agreement with the numerical results even when L is slightly shorter than the water depth. INTRODUCTION In 1982, Wu and Wu discovered from numerical simulations based on Wu's (1981) generalized Boussinesq equations that upstream-advancing solitary waves (also called runaway solitons) can be generated periodically by a steadily moving pressure distribution moving with near critical speed on the free surface. Despite earlier observations of runaway solitons in several experimental studies, Wu and Wu's study provided the first theoretical understanding of the phenomenon. Later, more theoretical analysis based on mass conservation and energy principles was given in Wu (1987).
- Research Report > New Finding (0.87)
- Research Report > Experimental Study (0.54)