Viscous Effect on the Roll Motion of a Rectangular Structure

Experimental study on viscous effect in roll and heave motions of a rectangular structure

Experimental studies of a damaged ship section in beam sea waves

Nowadays, deep-waters are the new frontier of the latest huge oil discovers. The most common technology for oil exploration in deep waters is based on the FPSO concept. Therefore, a key element for a successful exploration of deepwater oil fields is the accurate prediction of the unit motions in order to minimize the stop production due to excessive motions and maximum accelerations to which the process plant equipment and supports are subjected. The excessive motions of a FPSO can be caused not only due to rough weather but also due to mild weather at beam sea conditions. Beam sea conditions are faced by FPSOs in a spreading mooring configuration or even in single point mooring configuration but moored in regions where the swell sea is important, for instance, in Campos Basin offshore Brazil. In such cases, the accurate evaluation of roll motions is of prime importance. Moreover, in order to reach valuable results, a critical step is the proper evaluation of the different damping effects in particular viscous damping induced by the bilge keel, or those of risers. The state of the art assumption is that the roll damping is composed of two parts: the linear one and the quadratic one. The former accounts for the potential wave radiation and partly to some frictional effects, while the latter accounts for drag forces, flow separation, and other non-linear effects. In practice, the linear part can usually be neglected and the only remaining damping is the quadratic part. These fact strengths the role played by the non-potential damping in a reliable evaluation of the roll motion amplitude. In this paper, we will show the implementation of a practical procedure for the evaluation of the roll motions including the non-potential damping of a FPSO moored in a spreading configuration at Campos Basin and its validation through the comparison of the numerical results to the model tests. Introduction Compared to the other modes of motion, the evaluation of the roll motion appears to be much more complicated and the corresponding results more uncertain. The reasons for that are mainly related to the evaluation of the roll damping coefficient, which is usually dominated by the non-potential sources of damping. The usual procedure for calculation of the ship motions is the use of the so-called diffraction-radiation software based on the potential flow theory. The damping calculated by this kind of codes is linear and related to the wave making. As far as the linear theory is concerned the horizontal modes of motion (surge, sway, yaw) are usually out of resonance in the common sea conditions (periods of 5 to 25 seconds). In those cases the role of damping is not very important so that the potential flow theory evaluate correctly the (linear) amplitudes of these motions. The only modes of motion that may enter in the (linear) resonance are vertical modes (heave, pitch and roll) and for these motions the role of the damping is essential.

This chapter reviews three recent full-scale events with parametric rolling for Ro–Ro Large Car and Truck Carriers (LCTC). The events represent three principally different modes of parametric rolling: principal parametric resonance where the period of encounter is half of the roll natural period in following seas (I) and in head seas (II), and fundamental parametric resonance where the period of encounter coincides with the roll natural period in following seas (III). Roll motion, course, and speed recorded during the events are presented and analyzed together with the present weather situation based on recordings, forecasts, and re-analysis. Different aspects of on-board operational guidance for assisting crews in avoiding parametric rolling are discussed in relation to the presented events. Involved complexities and considerations that are normally not included in well defined model tests or numerical simulations are exposed.

The performance of offshore work from floating platforms dictates the desirability of "Minimum platforms dictates the desirability of "Minimum motions". One of the most troublesome motions, especially in the case of ship-shape or barge-shape platforms, is the roll motion because of the large platforms, is the roll motion because of the large amplitude and relatively large acceleration forces that could arise from such motion. Different devices have been employed to minimize The roll motion of ships bilge keels, gyroscopic stabilizers, solid moss transfer, stabilizing fins, U-tanks, flume tanks, active ballast transfer tanks, etc. The systems utilizing fins are effective only when the ship is under way and are not suitable for zero-speed offshore operations. The barge keels are generally very effective in damping the roll motions, and they prevent the angle of roll from becoming too large, but their reduction of the roll is limited to angles that are too great for the satisfactory performance of offshore operations, i.e., oil drilling. performance of offshore operations, i.e., oil drilling. The gyroscopic stabilizers generally are not used because of their high cost and complexity. The U-tanks, flume tanks, and active stabilizing tanks work by transferring ballast horizontally and by creating a stabilizing moment out of phase with the exciting force. This paper details use of the detuning tank. This system is different from others in the sense that it does not try to compensate the action of the forces imparted by the sea to the floating body with properly phased compensating forces, but tries to properly phased compensating forces, but tries to prevent the sea from imparting the forces to the prevent the sea from imparting the forces to the body. The effectiveness of the detuning tanks bas been experimentally verified in model basin motion tests and studies. Introduction Safety at sea is the first consideration of marine designers. Ships of conventional form or floating platforms for offshore operations must, under all platforms for offshore operations must, under all expected circumstances, float and be stable-hence, the application of suitable criteria for stability is one of the cornerstones of naval architectural design. The ability to maintain stability under design weather conditions, even after sustaining a certain amount of damage, has dictated maximum allowable heights of the center of gravity (KG) or minimum metacentric heights (GM). This limitation is certainly necessary in order to meet the demands of safety, but it is necessary only during certain extreme weather conditions or after flooding caused by damage. But this occurs, if it ever does, only during a very small percentage of the life of the ship, and to be prepared for this eventuality we may be forced to select some "safe parameter", like minimum GM, that may affect the parameter", like minimum GM, that may affect the motion performance of the ship during the major part of her useful working life. part of her useful working life. The motion performance may be of certain importance for ships engaged in ocean trade from the standpoint of crew comfort and the ability to maintain sea speed both of which have a certain economic value. But ships and platforms that are engaged An offshore work are more vitally affected by the motion performance, and their very effectiveness as offshore tools depends on how many days of the year they can perform their functions and the weather conditions that would force operations to be suspended. Roll motions have been one of the reasons for discontinuing offshore operations for ship-shape and barge-shape platforms not only because the amplitude of the motions but because of the high acceleration forces in the work area that can be originated by relative small amplitudes at short motion periods. The inclination due to wave action appears to be composed of two periodic functions, the period of one being the wave period T and the other period of one being the wave period T and the other the natural rolling period of the ship T . Rolling in still water is a free oscillation and the ship will roll in its own natural period. Among waves, the impulses producing the roll are periodic and tend to set up a forced oscillation of the ship in the period of the wave. If waves of constant period act for a sufficient time upon the ship, it will roll in the period of the waves, but if the period of the waves period of the waves, but if the period of the waves is not constant, the ship roll will not follow exactly that of the waves because of the tendency of the ship to revert to roll in its own natural period.

<p indent=0mm>Coral reefs are widely distributed in the South China Sea, and its topography is different from the gentle slope of the coastal continental shelf, which has a great impact on wave propagation and deformation. At the same time, reefs are located in a harsh marine environment, so these factors having a profound effect on the stability of the backfilled sand island and the effective operation of other ancillary facilities. Seawall is an important protective structure in the protection works of reefs, and the wave overtopping of seawall plays a decisive role in the engineering safety of reefs. At present, there are few studies on the wave overtopping of reefs around the world, and the existing formula for calculating wave overtopping discharges over seawall on gentle slope terrain is not applicable to reefs terrain, so detailed research on wave overtopping of a vertical seawall on coral reefs is needed. A series of experiments are carried out in the Large Wave Flume (LWF) of Tianjin Research Institute of Water Transport Engineering (TIWTE), which is <sc>456 m</sc> long, <sc>12 m</sc> deep and <sc>5 m</sc> wide. The maximum wave height generated by LWF is 3.5 m, and the wave period range is from 2 to <sc>10 s.</sc> The model layout includes 3 parts: Deep-water zone, steep slope zone and platform zone. The depth of the deep-water zone is <sc>5 m.</sc> The front slope is divided into 3 sections with slope 1:1, 1:4 and 1:15. The steep slope toe is <sc>330 m</sc> away from the wave generator. The strongly non-linear physical process of wave transmission and breaking can be reproduced by this physical model. Based on Froude similarity criterion and a 1:15 scale model, the propagation and deformation process of regular waves on reef-top with seawall is analyzed. The effects of wave height, wave period, water depth and the distance between seawall and reef-edge on the mean overtopping discharge are studied. The results show that the mean overtopping discharge increases significantly with increasing of wave height and period, while it decreases with decreasing of water depth. However, the mean overtopping discharge is not the monotone function with distance. Mostly the mean overtopping discharge decreases with increasing distance. When wave period is longer than <sc>13.56 s,</sc> the mean overtopping discharge decreases firstly, then increases a little, finally decreases with increasing distance. The dimensionless mean wave overtopping discharge increases as the wave steepness decreases for given wave height. The dimensionless mean wave overtopping discharge is found to be functions of relative freeboard height, the dimensionless mean wave overtopping discharge decreases as relative freeboard height increases for given wave conditions. On the basis of the experimental data, a formula considering the effects of wave height, wave period, water depth and the distance between seawall and reef-edge on the wave overtopping, is presented to estimate the mean wave overtopping discharge of a vertical seawall on coral reefs.

Study on wave-induced motions of a turning ship in regular and long-crest irregular waves

CFD modelling of a small–scale fixed multi–chamber OWC device

The roll damping coefficient is essential when considering the viscous effect in the potential-based hydrodynamic analysis of fishing vessels; it is an important factor in the roll motion response. The present study performs free roll decay simulations, altering weight variables using Computational Fluid Dynamics (CFD) to investigate the correlation between the roll damping coefficient and the weight variation of a fishing vessel. The time series of roll amplitude and roll damping coefficient are compared, for varying vertical and longitudinal centres of gravity and radii of gyration in roll motion. As the vertical centre of gravity increases, both the roll decay period and the roll damping coefficient also increase. The roll decay period tends to increase with the increase in the radius of gyration during roll motion, while the roll damping coefficient exhibits a decrease. A longitudinal centre of gravity has a limited effect on free roll decay characteristics. The roll damping coefficients between the maximum and minimum combinations of weight variables show significant differences. The findings of the present study could enhance the understanding of the safety of fishing vessels based on their loading conditions. Consequently, future research could further improve the results obtained in the present study by considering various hull shapes and speeds.

Systematic laboratory measurements were performed to determine the steady drift of isolated, tabular ice-floe models in regular waves. A relatively simple expression for the steady wave-induced drift of two-dimensional floes of uniform thickness and density in deep water was developed. This empirical wave-drift formula indicates that the drift velocity depends primarily on the ratio of wave period to ice-floe roll period, wave steepness and length of the floe. The highest drift velocity is produced by waves with period somewhat greater than the ice-floe roll period, with fundamentally different drift behavior evident on either side of this maximum (long-wave and short-wave drift response). The dependence on wave height was found not to be linear but increased with wave length (from approximately H1/2 to H3/2). A simplified equation for the roll period of a prismatic body of uniform density and rectangular cross-section was developed and found to be of practical value. INTRODUCTION An object floating freely at the surface is generally exposed to the combined action of winds, waves and currents, and will normally undergo oscillatory motions and also experience a net drift with respect to the seafloor. When ocean currents and wind forcing can be neglected, the body will drift in the approximate direction of wave advance while simultaneously undergoing horizontal oscillations at the wave frequency. Drift rates are typically small so that the net drift is often only of interest over time periods much longer than the wave period. Horizontal motions diminish as the wave length decreases, so that the associated oscillatory velocities can often be neglected compared to the drift velocity, i.e., the steady drift velocity and maximum instantaneous velocity are practically identical for short waves. By time-averaging the horizontal motions of the floating body (over periods large compared to the wave period), the so-called steady drift velocity is obtained. Although the steady drift excludes motions at wave frequency, it is still of practical interest because (a) it permits time-of-travel estimates to be made (when will an abandoned barge or an ice floe reach a given location); (b) it provides information about the rate of dispersal to be expected for a group of floating objects of different size (wave-induced separation of ice floes); and (c) it can be used to estimate the maximum kinetic energy with which drifting objects can strike a fixed or floating structure (at least for the case of relatively short waves). We report here the results of laboratory measurements of the steady (mean) drift of ice-floe models in regular waves. This is the first part of an ongoing effort to define the wave-induced motions (both mean and instantaneous) of ice-floes nearing the point of impact with fixed structures or sand and gravel beaches. The present investigation was performed with regular progressive waves as the only driving force. In the absence of winds and large-scale ocean currents, the body is propelled by wave action alone, and a separation into a wave-induced surface current Vs and a wave-thrust velocity may be conceptually helpful.

ABSTRACTIn the study of the motions of vessels floating side by side, it is necessary to employ accurate methods of analysis other than potential theory to overcome inaccurate solutions due to viscous effects. In this study, computational fluid dynamics (CFD) simulations for the analysis of single- and side-by-side vessels are performed. The CFD codes are formulated for solving the Navier–Stokes equations using the finite-volume method with OpenFOAM, an open-source CFD software program. Taking into consideration viscous damping and vortex shedding effects, advantage of viscous damping and vortex shedding, three-dimensional numerical simulations for six-degree-of-freedom motions of a single vessel and side-by-side vessels are conducted. The simulation results for a single vessel are validated by comparison with results obtained from ANSYS AQWA software and from model testing. Simulation results for side-by-side vessels are compared with the AQWA results in order to show that the CFD code can be used effectively in predicting the motions of side-by-side vessels in regular waves.

The double-row and double-chamfered aquaculture tank is a special tank structure of the aquaculture ship. The tank sloshing of this structure is coupled with the hull motion, which has an important impact on the safety of the hull motion. In the present study, research on the tank sloshing and hull motion response of aquaculture ships was conducted based on the model seakeeping and tank sloshing tests in regular waves. The test results were compared with the numerical simulation results of solid loading without sloshing. The results showed that the numerical simulation of the pitch motion was consistent with the amplitude-frequency response curve of the experimental results. Under certain transverse wave conditions, a large discrepancy existed between the amplitude-frequency response curve of the heave motion by the numerical simulation and the test results, and the roll motion differed most from the experimental result. Severe roll resonance occurred when the wave length-ship length ratio was 0.6. The roll motion amplitude was increased by 183.2%. Therefore, compared with aquaculture ships without sloshing, the sloshing of the tank has little effect on the pitch but has a great impact on the roll and heave motions, with the most significant effect on the roll motion.

Application of particle image velocimetry on velocity distribution in 5 × 5 rod bundle channel under rolling motion

Cho, Y.H.; Zhang, C.; Nakamura, T., and Mizutani, N., 2018. Failure mechanism of armor blocks of submerged breakwater. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 1166–1170. Coconut Creek (Florida), ISSN 0749-0208.This paper describes the failure mechanism of armor blocks of a submerged breakwater under regular and long period waves. Laboratory experiments with the scale of 1/50 were conducted in a wave flume. The measured normal and tangential wave forces on the damaged armor blocks were compared with 31 tests for a submerged breakwater with 3 submerged depths and 3 respective wave periods of regular and long period waves. The experimental results described the failure of armor blocks in the threshold conditions occurred mainly at the seaward edge of the crown by normal wave forces under regular waves, and the landward edge of the crown by normal and tangential wave forces under long period waves. Two-dimensional numerical analysis was carried out to investigate the effect of temporal and spatial variation of wave fields around the submerged breakwater on wave force acting on armor blocks. The results demonstrated that the dominant wave forces on armor blocks were induced by the decrease of the pressure near the damaged blocks resulting from the wave breaking for regular waves and the increase in the flow velocity for long period waves. It was confirmed the other blocks on the crown tends to be stable under long period waves.

The paper investigates the motion response of a floating body under the action of small amplitude regular waves and determines the influence of uncoupled as well as coupled systems through an analytical approach. The governing equations of motion describing the overall balance between the wave-exciting force with the linear form of inertial, damping and restoring forces are transformed into frequency domain by applying Laplace transform technique. The closed-form solution is sought for three specific cases: (i) Case I: when only the roll motion is considered, (ii) Case II: when coupled roll and yaw motions are considered and (iii) Case III: when coupled sway, roll and yaw are considered. To get an insight, we consider a floating body of displacement 19,190 tonnes under the action of beam waves with wave frequencies 0.56 and 0.74 rad/s. The analytical result indicates that roll amplitude increases with the increase of vessel speed for the case of single degree of freedom. All three motions show harmonic behaviour and attain dynamic equilibrium for time t > 100 s. Model result indicates that the reduction in degrees of freedom enhances phase lag and exhibits artificial increase in amplitude for roll. The importance of this study lies in terms of obtaining closed-form solution for a linear system while evaluating damping factor and critical encountering frequency for uncoupled and coupled motions.