Chang-Kyu Rheem

Distributed mass/discrete floe model for pack ice rheology computation

A new model, called the distributed mass/discrete floe model, is proposed for performing practical computations of mesoscale pack ice rheology. This model possesses the advantages of both the continuum and the discrete element models: it can express the discrete nature of pack ice, for which it is difficult to use a continuum model, and requires a much shorter computation time than a discrete element model. The pack ice is divided into ice bunches in which the floes, assumed to be distributed uniformly, are modeled as inelastic disks or rectangles floating on the water. The ice interaction forces are formulated from the relationship between the impulse on the bunch and the variation of momentum in the bunch. The ocean flow is calculated simultaneously with the floe movement using a multilayer model. In a circulating water channel, drift tests of physical model floes were performed in order to investigate the characteristics of their motion and interaction with ocean structure models. Near the structure, the floe motion depends on the floe shape. Disk floes show a lateral motion in front of the structure. They flow out around both sides of the structure and the number of floes in front of the structure decreases with the lapse of time. On the other hand, rectangular floes scarcely flow laterally. The number of floes in front of the structure remains constant over time. These experiments indicate that when the motion of pack ice around a structure is simulated, it is important to consider the floe shape. The disk floe motion and the rectangular floe motion can be regarded as extreme cases of pack ice motion. Actual pack ice motion may be between these two extremes. Computations were carried out using the distributed mass/discrete floe (DMDF) model. Simulation results were compared with the circulating water channel experiment results and sea ice motion in the southern part of the Sea of Okhotsk. The DMDF model predicted the circulating water channel drift test results quite closely. The DMDF model results also compared quite well with the sea ice motion.

Time-domain analyses of elastic response and second-order mooring force on a very large floating structure in irregular waves

Real sea areas where very large floating structures are installed are random wave fields. Then, it is necessary to analyze the responses in the time domain due to directional waves. There exist hydroelastic deflections and slow drift motions in the responses of VLFSs in ocean waves. However, it is very time consuming to solve the equations of motion in the time domain and obtain these responses due to directional waves. It is unnecessary to solve such equations in the time domain, if mooring forces can be turned into on equivalent linear system. In this paper, we analyze the time-series responses without solving the equations of motion in the time domain. And, the corresponding model tests in uni- and two-directional irregular waves are carried out. The present method is validated by comparisons between the analytical and measured results of time histories, and moreover, the analytical method of a slowly varying wave drift force is also validated.

Hydroelastic Behavior of Air-Supported Flexible Floating Structures

A pontoon type very large floating structure has elastic deformations in ocean waves. The deformation is larger than that of a semi-submergible type one. Thus, a pontoon type one will be installed to tranquil shallow water field enclosed by breakwaters. Moreover, a semi-submergible one will be applicable to development at offshore field. The authors has developed a pontoon type VLFS with an OWC (oscillating water column) type wave energy absorption system. This can be install to offshore field being deep water relatively. Such VLFS can reduce not only the elastic deformation but also the wave drifting forces. However, it is very difficult to reduce the wave drifting forces effectively because an effect of the reduction depends on the wave energy absorption. Therefore, the authors propose an air supported type VLFS. This idea has been already proposed. However, it wasn’t handled a flexible structure. Such an air-supported structure makes to transmit many waves. Therefore, the wave drifting forces may not increase. In addition, the elastic deformation may decrease because pressure distribution due to the incident waves becomes constant at the bottom of the structure, i.e. the pressure is constant in a same air chamber. We develop the program code for the analysis of the hydrodynamic forces on the VLFS with the air cushion. The potential flow theory is applied and the pressure distribution method is used to the analysis of the wave pressures. The zero-draft is assumed in this method. The pressure and volume change of the air cushion are linearized. In this paper, basic characteristics of the elastic deformations of the air-supported flexible floating structures are investigated. We confirm the effectiveness, and discuss behaviors of the water waves in air chamber areas.Copyright

Slowly varying wave drifting force on a very large floating structure in short crested waves

Hydroelastic responses of very large floating structures (VLFS) have been studied activity in Japan. Most targets of them are hydroelastic behavior and an estimation method of linear hydrodynamic forces for the prediction of it. However a design of mooring system is very important to install a VLFS in a real sea area. Then, it is advisable that slowly varying wave drifting forces, which are the 2nd-order wave excitation, are predicted accurately. The authors have validated the theoretical method of the wave drifting force on the elastic floating bodies in long crested and crossing irregular waves by comparing with results of the model tests. It seems that irregularity and multi-directional quality greatly affect to the wave drifting force on a VLFS comparing to general scale floating structures. In this paper, characteristics of the slowly varying wave drifting forces on a VLFS are investigated systematically by numerical calculations with elastic behavior. A wave drifting yaw moment increases due to geometrical asymmetry of elastic behavior in short crested waves. Slowly varying wave drifting forces occurs due to interactions of wave frequencies in component waves. In case of the multi-directional irregular waves, there are effects due to the frequencys interaction of component waves in not only a same direction but also different direction. The terms of the latter effect are investigated in this paper.

Hydroelastic Motion of Aircushion Type Large Floating Structures With Several Aircushions Using a Three-Dimensional Theory

This paper describes hydroelastic motion and effect of motion reduction of aircushion supported large floating structures. Motion reduction effects due to presence of aircushions have been confirmed from theoretical calculations with the zero-draft assumption. A three-dimensional prediction method has been developed for considering draft influence of division walls of aircushions. It is investigated that hydroelastic motion reduction is possible or not by using the three-dimensional theoretical calculations. In addition, the aircushion types are supported by many aircushions which are small related to wavelengths. The Green’s function method is applied to the prediction method with the linear potential theory in which effect of free water surfaces within aircushions are considered. Hydroelastic responses are estimated as not only elastic motion but also a vertical bending moment. From the results, the response reduction is confirmed, in particular, to the vertical bending moment in wide wavelength range and in whole structure area.Copyright

A Prediction Method of Hydroelastic Motion of Aircushion Type Floating Structures Considering With Draft Effect Into Hydrodynamic Forces

An aircushion type floating structure can prevent to enlarge the wave drifting force restraining the hydroelastic response of it in water waves. The floating structure should be large scale to incident waves in order to make the best use of such advantages, i.e. it is a very large floating structure. The linear potential theory is useful to easily handle the wave force etc. on the aircushion type floating structure theoretically because it is predicted that its theory can give good results of behaviors of water elevation within aircushions and pressure and of wave loads on the structure qualitatively. The authors have confirmed from our past model experiments that non-linear effect does not always increase but for some exceptions. A prediction method of hydroelastic responses for the aircushion type very large floating structure by using the three-dimensional linear potential theory is shown in this paper. The validity of the method is proven and the application of the method is investigated by comparing the theoretical results with the results of the past model experiments.Copyright

Performance and Characteristics of Take-Off Power of a Vertical Axis Marine Turbine With Variable-Pitch Blades

This paper describes performances and characteristics of a vertical axis marine turbine (VAT) on which variable-pitch blades are equipped. Then, model experiments in a current channel and CFD calculations were carried out in this study. In the model testing, a variable-pitch type turbine model was used. Accuracy of the CFD computation is validated by comparing with experimental results of torque and number of rotation of the turbine model. The CFD calculation is three-dimensional as well as two-dimensional. The corresponding turbine model has three straight blades, which diameter is 0.3 m. Three kinds of tests were performed for model testing in the channel and CFD calculations as well. They were the starting-driving test, the braking test and the forced-rotation test in currents. The forced-rotation test was performed in still water as well in order to know a simple thrust and to estimate ideal efficiency of the turbine.Form results of model experiments and numerical experiments, the CFD calculation was satisfactory to estimate performance of the turbine. The starting performance could be definitely improved by applying the variable-pitch system. For driving performance, it was very useful to control the pitch angle a little bit even in very fast velocity currents.Copyright

Response Reduction of Motion and Steady Wave Drifting Forces of Floating Bodies Supported by Aircushions in Regular Waves: The 2nd Report—Response Characteristics in Oblique Waves

This paper describes aircushion effect for response reductions of floating bodies and thereby the model experiment in a wave tank is carried out and their details are explained. The floating body model has three aircushions and is supported by the aircushions. The response reduction due to the aircushion is considered and confirmed using the model experiment in regular waves and oblique waves. A pontoon type floating body model is also used to the experiment for comparisons of responses with the aircushion type model. From results of the experiment, vertical motion can be reduced due to the aircushion effect in not only head sea waves but also oblique waves. And then, it is confirmed that the second-order tension of tethers which moor the experimental model can be reduced at the same time. The response reduction due to the aircushion effect is useful in oblique waves and even then the wave angle approaches beam sea conditions.Copyright

Effects of Aircushion Division to Hydroelastic Responses of an Aircushion Type Very Large Floating Structure

A target offshore structure in this study is an aircushion supported very large floating structure. The aircushion type VLFSs behave elastically in water waves. Corresponding aircushions are very large or relatively small size. The VLFSs considered in this study are supported by a large aircushion, two aircushions, or several module aircushions. The zero-draft theory is applied to the prediction of the hydrodynamic forces. The zero-draft theory is based on the pressure distribution method. The elastic deflection predicted by the zero-draft method is compared with that by another three-dimensional method in order to confirm the validity of it. In addition, the steady wave drifting forces on VLFSs with the aircushion are shown and their characteristics are examined. Then, the momentum theory is applied to the prediction. In the final section, effects of aircushion division to the elastic deflection and the wave drifting force are investigated. From the results, it is confirmed that the elastic deflection is can be reduced in the specification relation between the wavelength and the length of a module aircushion. In addition, it is possible to ajust the aircushion setting in order to simultaneously reduce the elastic deflection and the steady wave drifting force of the aircushion type VLFS on the case.Copyright

A Prediction Method for Horizontal Plane Behavior of FPSO Under the Single Point Mooring

It is well known that in single point mooring or anchoring the slowly varying oscillation of a ship is caused by action of current and wind. During the slowly varying oscillation, extraordinary tension occurs in the mooring line when the ship’s yaw angle becomes nearly maximum, and incurs, as the case may be, in breakdown of mooring lines or unforeseen drift of anchors. Floating Production, Storage and Offloading (FPSO) systems are often moored as Single Point Mooring (SPM) systems. SPM systems can be Catenary Anchor Leg Mooring (CALM) systems or Single Anchor Leg Mooring (SALM) systems. It has been required to predict and evaluate performance of horizontal plane behavior of FPSO in current, wind and waves, since the workability and safety of FPSO become important from the stand point of the Life Cycle Engineering. Numerical simulation is one of the practical methods for prediction of FPSO performance and it needs quite accurate values of hydrodynamic coefficients in the mathematical model. Recently some attempts on improvement of accuracy in prediction of the hydrodynamic coefficients were made and approximate formulae for hydrodynamic derivatives including the interaction effect of main hull form and appendages were also proposed. Recently extensive studies for numerical models which describe components of hull, propeller, rudder, thruster, wind and waves separately, and these interactions have been made successively. In this paper, first, the basic equations of maneuvering motion are explained. And, an estimation method of slender body theory for hydrodynamic force acting on the hull is outlined. The authors explain numerical models to obtain FPSO coefficients for the horizontal plane behavior from mathematical model of ship maneuverability. And, numerical test of FPSO under the slowly varying oscillation is carried out. Finally, a new mathematical model is proposed to describe the current forces acting on FPSO under the slowly varying oscillation.Copyright