Optimizing hull form design is playing a more important role in the shipbuilding industry, especially when regulations on the environmental protection and improvement of ship energy efficiency become more stringent than ever. Though the studies on design optimization of ship hull have a long history and achieved some significant results, studies on the stern shape optimization of a ship hull have been a few due to the complexity of flow field behind a transom stern. In this paper, a numerical method for optimizing the stern shape of a container ship based on a nonlinear programming method and a Navier-Stokes analysis is proposed. A CFD solver which solves the three-dimensional Reynolds Averaged Navier-Stokes (RANS) equations for incompressible flows is used for evaluation of an objective function and analysis of flow fields around ship hulls. The Sequential Quadratic Programming (SQP) method is utilized as an optimizer which automatically determines values of the design variables in such a way that the objective function is minimized subject to the given constraints. Design variables are selected so that the modified transom shapes are efficiently created during the optimization process. To demonstrate the applicability of the present method, transom stern of a container ship is optimized to minimize the pressure resistance coefficient at a model scale. In addition, the effects of the initial designs and of the frameline modification functions to the optimized results are examined in this paper. The optimized results show that the present optimization system is able to create a stern shape that decreases the pressure resistance coefficient; however, it turns out that special cares should be taken in selection of the initial designs and of the hull form modification functions since the optimization results strongly depend on these parameters.
This is the second paper of preliminary hull form planning for Non-Ballast Water Ship (NOBS) and Minimal Ballast Water Ship (MIBS). In the previous paper, the author proposed some simple procedures to estimate propulsive performance for NOBS/MIBS. In this paper, the author shows usability of these procedures to investigate preliminary hull form planning for NOBS/MIBS through carrying out some trial designs for VLCC. The trial designs reveal a relationship between principal dimensions and propulsive performance of NOBS/MIBS. In addition, CFD technique is used to estimate propulsive performance of Conventional Ship and NOBS/MIBS in the full load condition. The accuracy of CFD results is confirmed by comparing CFD results with results of towin g tank tests. At the stage of running CFD simulation, an initial hull form is required to generate grids. The author proposes a practical method for making an initial hull form of NOBS/MIBS. The initial hull form of NOBS/MIBS also can be used at the further desig n stage.
An experimental methodology of tank test, which enables us to directly obtain speed of actual ship under waves just by experimental result of model test under corresponding same wave condition, is newly developed. One of features of the methodology are controlling propeller rotational speed, managed by a self-propulsion motor set on a model ship, as power plant system of actual ship rotates its shaft speed against fluctuations of load torque. We call the intelligent motor system as “Marine Diesel Engine Simulator.” The other feature is, under using the MDES and the Auxiliary Thruster System, making total external forces acting on model ship, similar to those of actual ship under corresponding wave conditions. In this study, three fundamental components of the developed methodology are introduced, mainly focusing on similarity laws between model and actual ship. Thus, experimental results by the methodology are discussed by comparing with the conventional experimental methodology and numerical predictions. Finally, significances of the developed methodology are referred.
Tsunami consecutively brings about several types of disasters, which is described as “disaster chain.” In the case that vessels are moored to the quay, the breaking of the mooring line possibly causes drift or collision with other vessels. There is also a case that a mooring tether of the vessel moored to the floating pier is not broken in the damage of tsunami, which was confirmed at Shiogama port in Miyagi when the East Japan Great Earthquake occurred on March 11, 2011. In this paper, the objectives are to clarify the effect of reducing mooring tether force in case of that vessel is moored to floating pier by using MPS method and also the advantages of floating pier for mooring quay. In addition, in this paper the authors are discussed the following items on the basis of Shiogama port: 1) Results of field investigation at Shiogama port in Miyagi, and accurate to reproducibility simulation of MPS method. 2) Response characteristics due to the difference in the Wave height in tsunami. 3) Response characteristics in various incidence angles. From the research results, the authors clarify following things: 1) Mooring vessel with floating pier has an advantage over mooring with quay. 2) It was possible to confirm the effect of reducing the mooring line tension, especially in the case where the incident angle of the tsunami is close to 90 degrees. 3) Reproducibility simulation on basis of the Shiogama port confirmed that the MPS method is suitable to the response of a moored floating body.
A wave-power generation is rich in type of method; the methods of various sizes have been devised so far. For example, Salter’s Duck type and Pelamis type are famous as the device using the motion of the ﬂoating body. Studies on these shapes have been done a lot. And there are a lot of indicators for evaluation of the optimal shape. So, we examined the difference among the optimal shapes obtained by various evaluation factors such as. And recently some studies are attempting to increase the energy absorption efficiency by arranging the multiple units. In such studies, the multi-column array often consists of a ﬂoating body which is obtained as an optimal shape in solo use. The shape which is suitable for multiple units, however, does not necessarily coincide with the shape which is suitable for single unit. Therefore, we also examined the difference of the optimal shape between multi-unit and single-unit. As a result, we have conﬁrmed that the optimal shape changes in case of changing number of ﬂoating body and the evaluation method.
To design floating body such as a floating wind power generation, it is necessary to develop the calculation method to estimate the effect of the fin installed in order to reduce the motions of the body and the behavior of the floating body in resonance frequency. The purpose of our study is to develop the numerical simulation method with high accuracy to represent the behavior of floating body in nonlinear wave by using vortex method. In this paper, motions of moored rectangular and triangular floating bodies in waves are calculated by the vortex method based on the Core-Spreading method. To calculate the vorticity generation from the body surface, the vorticity layer and the vorticity shedding models are used. And total pressure acting on the body is calculated by using the boundary integral equation represented by the velocity and the vorticity, deduced from fundamental equations of viscous fluid. The calculated motion amplitudes of the moored body in waves are compared with the experimental results and the calculated results by the vortex method agree well with the experimental results.
The behavior of hydrodynamic forces acting on a ship at lateral motion was investigated. Force measurement in towing tank and numerical simulation on viscous flow field were made, and correlations of hydrodynamic forces with flow field around ships were examined. A large flow pattern change at extreme shallow water case such as H/d<1.20 was observed in model scale, and it causes serious characteristics change of lateral force. Present characteristics change could be caused by the pre ssure loss at the gap between sea bottom and ship bottom, which may be referred by ‘retaining effect’, and that generates the upstream flow pattern and separation pattern change.
Vortex-Induced Motion (VIM) of a floating body is the result of the exciting force by vortex shedding on the hull of a bluff body. VIM motions result in additional oscillating mooring line tensions and fatigue loading. In this paper, it was observed how the behavior of VIM changes due to changing the shape of a floating body in Lattice Boltzmann Method. In terms of the oscillatory displacement, square column is with least influence of VIM. As a result of using four columns in square configuration, there is not a proportional relationship between gap of columns and oscillatory displacement. Additionally, it was examined how vortexes occurred from upstream columns affect downstream columns.
In recent years, the use of natural gas as an oil alternative energy has been increasingly expected. To ensure the stable supply of natural gas, the construction of ﬂoating LNG (FLNG) is now under investigation around the world to develop small or middle scale gas ﬁelds, so-called stranded gas ﬁeld. In most cases, the FLNG under investigation is equipped with a turret mooring system to keep its own position at each installed gas ﬁeld. However, detailed methods to design the mooring system are not indicated in guidelines published by the typical classiﬁcation society, or designers have to decide by themselves which of the guidelines they should adopt at the time of beginning design because of different design requirements for every guideline. In this paper, we investigated combinations of severe direction of environmental forces, such as wind, current, wave and swell, selected from items indicated in the different design requirements. This paper also presents the results of investigation on the method of statistics analysis to evaluate the extreme value of line tension.
Efficient shallow water mooring system was investigated for deployment of a floating offshore wind turbine. Static mooring performance analysis of a mooring system with buoy and/or clamp weight was formulated and incorporated into optimization program. Optimal design parameters were investigated which realize an efficient mooring design equivalent to a mooring system with chain of larger nominal diameter designed for deeper water. In the formulation of static analysis of a mooring system, whole system is divided into several regions which can be assumed uniform within a region. Catenary solution is applied to the regions and whole system is solved by obtaining continuity of displacement and equilibrium of forces at the connecting points between the regions by Newton’s iterative method. This static analysis program was incorporated into optimization algorithm and the design parameters of weight of clamp weight, position and size of chain which minimize mooring spring constant under large drifting force typical for FOWT were obtained. It was understood that 1) a mooring system with clamp weight is desirable to restrain the motion of FOWT and minimize mooring spring constant, 2) effect of water depth, weight and position of clamp weight on mooring spring constant under large drift force was investigated and it was understood that there are optimal values for the parameters to minimize the mooring spring constant, 3) It was shown that mooring spring constant of a mooring system with chain of 132mm nominal diameter installed at water depth of 100m can be realized by a mooring line with clamp weight and chain of nominal diameter 68mm at water depth of 50m.
We investigated the nonlinear effect and forward speed effect of hydrodynamic forces for the longitudinal motions of trimarans. To discuss the nonlinear effect and forward speed effect of hydrodynamic forces, we employed the nonlinear strip method and linear Rankin panel method respectively. Results show the nonlinear strip method is more effective than linear strip method to predict the longitudinal ship motions in the case of Fn=0.2, and the results of the longitudinal ship motions by using Rankin panel method give better agreement with the experimental ones in the case of Fn=0.4 and 0.6. It is confirmed that the phase difference of restoring forces acting on the main hull and side hulls affects the restoring force of trimarans.
This paper describes ship response by “zigzag maneuver with equal time-alternative steering”. This maneuver runs as follows: firstly the helm is turned to a certain angle during a certain time, secondly it is turned to the opposite side to the same angle during the same time, and thirdly it is returned again to the original position. In this maneuver, the analytical solutions of response, that is, angular velocity, heading angle and trajectory, etc., are obtained using first order model of Nomoto. These equations clarify the relationship between the steering quality indices and the ship response. The response of actual ships from published existing data is also checked out using the equations. The obtained charts summarized here will be available for design review of maneuverability. Finally, the trajectories are analyzed under some conditions that for instance the ships have the same non-dimensional steering quality indices and the different ship lengths. The significant difference of them indicates it is effective to consider trajectory in the early stage of design.
A Resonance-Free SWATH (RFS), which has negative restoring forces, is introduced to minimize the motion responses in waves because the strict time-punctuality and the high value of the cargo require the superior seaworthiness such as no speed reduction and no slamming. Accordingly, the attitude of RFS should be adjusted by small underwater control fins. The latent ability of RFS for seaworthiness is not completely made use in the case of PD control because P gain in PD control acts as the large restoring force. In this study, a nonlinear sliding mode control has been applied to make up for deficiencies in PD control. The simulation using the time domain simulator for the sliding mode control and the experiments using the model have been carried out. As a result, the effectiveness of the sliding mode control has been confirmed. The pitch motion response of RFS under the sliding mode control is decreased by half compared with PD control.
The optimization of the ship structure with design variables of the plate thickness and the shape of stiffener is a popular study ﬁled. But, the optimization of the structure with the design valuables of the number and the position of stiffners are necessary for obtaining a better optimal solution. Therefore, the optimization with theses design valuables is performed in this paper. The optimization with theses design variables needs a lengthy calculation time because recreating FEM models and re-analyzing must be performed. In order to overcome this problem, the calculation method for evaluation of the strength of structures without recreating FEM models and the method of reducing calculation amount of FEM are proposed. There is another problem in optimization part. Since the number of stiffeners is changed the number of design variables may vary during an optimization process. In order to solve this problem, an effective optimization method for this problem is proposed. The structure which imitate a double hull part of oil tanker is selected for target of optimization. These proposed methods are applied to this optimization model and their effectiveness an veriﬁed.
In order to examine the effect of initial imperfections and lateral loads on the hull girder ultimate strength of intact and damaged hull structures, a series of nonlinear calculation is performed using FE analysis code, LS-DYNA. In addition, the hull girder ultimate strength of intact and damaged hull structures is calculated utilizing the simplified calculation program, which is developed by authors and based on the Smith’s method, and the reduction ratio of hull girder ultimate strength is also investigated. Furthermore, to examine the accuracy of this simplified calculation program, calculation results are compared with results obtained by FE analyses.
Ultrasonic impact treatment (UIT) is a remarkable post-weld technique, which applies mechanical impacts in combination with ultrasound into welded joints and plastically deforms the weld toe, consequently introducing beneficial compressive residual stress near the treated area. Recent experimental studies demonstrate that acoustic softening of metals during ultrasonic impact (UI) may lead to increased plastic deformation and play an important role in the mechanism of UIT. In this study, a 3D simulation approach including thermo-mechanical welding simulation and dynamic elastic-plastic FE analysis of UIT process of welded joints has been proposed. In the FE model, the actual process parameters and ultrasonic -induced material softening, which is appropriately adjusted to fit experimental results, are considered. The predicted residual stress distributions of non-load-carrying cruciform joints before and after UIT are compared with experimental results, showing a fairly good agreement with each other. In addition, an evaluation of fatigue streng th of welded joints based on fracture mechanics has been carried out. The results not only clearly distinguish the difference of fatigue strengths of as -weld and UIT welds, but also show the influence of preload and stress ratio on the improvement, so that the proposed model may provide an effective tool to simulate UIT-process in engineering structures.
Enlargement of containerships during this decade has brought about imminent concerns on ship’s dynamic elastic response and its influence on ultimate and fatigue strength of hull structures. The authors reviewed literatures on this topic, identified problems to be solved, and firstly developed a dynamic elastic response analysis system comprising ship motion analysis based on Rankine source method and 3D finite element analysis based on newmark- β method, enabling simulation of hull stresses including dynamic elastic components. Comparison of calculated results and model test results was carried out both in regular and irregular waves. It was confirmed that this system can simulate ship motions, hydrodynamic pressure and vertical bending moment including dynamic elastic response with sufficient accuracy. Based on the achievements, future tasks were identified as expansion of the applicable scope of the system, further verification of calculation accuracy including comparison with full scale measurement, parametric studies of the effect of various parameters on dynamic elastic response, and rational generation of design irregular waves.
Plate-Bending-Vibration (PBV) type fatigue testing machines, which can carry out fast and low-cost fatigue tests of welded joints subject to wave loads with high frequency vibration, have been developed. These machines are designed for plate bending type fatigue tests, and wave load is applied by using motors with eccentric mass. Springing vibration is superimposed by attaching an additional vibrator to the test specimen, and whipping vibration is superimposed by hammering. PBV machines can drastically reduce the testing time and the electricity to be used for high frequency superimposed fatigue tests. It is found that the fatigue life under constantly high frequency superimposed loads can be predicted with acceptable accuracy by the modified Miner rule when rainflow stress counting is performed and S-N curve is modified so that it fits the equivalent stress range's Ps=50% curve. It has also been found that the fatigue lives are prolonged substantially than estimated when the high frequency components are superimposed intermittently. This means that the application of Miner rule to rainflow counting might lead to over conservative estimates under real stress sequences in which springing and whipping occur intermittently. The validity of rainflow counting method should be further examined by carrying out fatigue tests with realistic stress histories which emulate intermittent occurrence of springing and whipping in ship structure.
A new method of structural analysis is proposed for the solution of problems of response uncertainty for the case that involves uncertainty in shape. The method makes use of a Hermite polynomial chaos expansion of the process. The proposed method involves a mathematical formulation which is a natural extension of the deterministic finite element concept to the space of random functions. In this paper, two example problems are investigated by the new method, i.e., a plate with a circular hole at the center and a cruciform weld joint. Then accuracy of the analysis will be discussed for different size of a plate with a circular hole. The validity of a new method of structural analysis is discussed using a result of uncertainty analysis through a Monte-Carlo simulation by solution of the deterministic problems.
The effect of whipping vibration on the fatigue life was estimated for larger ship safety. Miner’s law and Fatigue crack propagation analysis were applied for the fatigue life estimation. The delay phenomenon occurring fatigue crack growth according to over loading caused by whipping were simulated by using non-linear fatigue crack propagation program. The simulated stress acting on deck structure of the actual post-panamax container ship were applied to numerical program, the whipping effect was evaluated by comparing with the superimposing vibration stress (RAW) and without vibration stress (LF). It was found that the effect is about 30% in Miner’s law and 10-55% in Fatigue crack propagation analysis assuming a long-term storm of 25 years in North Atlantic. The effect became smaller than those reported in the previous report (abt.50%), where a maximum storm was assumed. And, it was estimated that the effect of delay phenomenon is same level of the whipping effect. Furthermore, as an example of probabilistic approaches for fatigue life evaluation, structural reliability analysis in Monte Carlo simulation was performed in the parameters with the ship speed, wave direction, and the rank of storms. The effect of operating condition was quantitatively indicated by the reliability index and the failure probability. It was indicated that the effect of whipping on fatigue life can be canceled by operational effect of ship speed reduction in severe sea.
Renewable energy has been an attention of scientists for last few decades around the world. Ocean current is also expected to be a new source of renewable energy and some types of Ocean Current Turbine is developed. For optimum ocean current turbine design, utilizing current velocity information is essential but the fact is that there is little information about current velocity and it is not organized well. Thus, in this study, a method to organize and utilize ocean current information based on power spectral model is proposed. Moreover, observation system for lack of data is considered and in-situ observation is conducted around the Kuroshio watershed area. As a result, fluctuation intensity range is quantified at the observed location and by applying current velocity reproduced from proposed spectral model, numerical simulation for design assessment gets easier. The load on the blade in time domain is estimated as an example and it would be used for the design.
A prototype of a 3000m-class underwater shuttle glider for virtual mooring is being developed by JAMSTEC (Japan Agency for Marine-Earth Science and Technology) with RIAM (Research Institute for Applied Mechanics) of Kyushu University. The vehicle houses an observation equipment and glides back and forth between the sea surface and the seabed collecting ocean data in a specific area (virtual mooring area). In this paper simulated results of the gliding are compared with the field experimental results, and the accuracy of the motion simulator is discussed.
This report deals with the stability of ships having comparatively large projected lateral area above water line and shallow draft. Such ships usually have wide breadth because they have tendency to raise their center of gravity. Large GM made by the wide breadth brings strong stability. But it gets weaker if the ship has higher center of gravity because GM/(OG+T/2) becomes smaller. A research challenge of this report consists of the procedure to clarify the acceptable center of gravity and how breadth and draft to be examined and determined. These procedures enable the optimization method in the initial design. In the analysis, the metacenter is estimated by Ohgushi’s and Morish’s formula and the accuracy of the estimation is confirmed using merchant ships’ data released for public. The present method obtained by the analysis shows effectiveness to find out reasonable dimensions and acceptable height of center of gravity in the early stage of ship design.
The fact that the crest-line-like lines can be seen in the PPI image of ship radars was known from relatively old days. Many trials obtaining wave information as wave directional spectrum, significant wave height and wave mean directions were also done. Recently, products giving such information and supporting navigation are also developed and becoming in use. Considering such a situation, wave radar will become a necessary item as a standard equipment for ships, in not a so long future. On the other hands, Hirayama et al. Showed, that by the PPI image, individual waves are recognized and very short term predictions, for example, a few minutes after, will be possible. If the prediction of individual waves is possible, then ship motion on them become possible, including dangerous conditions, and finally contribute to the safety navigation of ships. This paper reports about the results of the demonstrative examination of individual waves done in the actual sea and this will be the first in the world.
We investigated the wave loads acting on the cross deck of trimaran in oblique waves. We developed linear strip method to prediction method for wave loads acting on cross deck of trimaran in oblique waves. And, we carried out the experiment measured wave loads acting on cross deck of trimaran in oblique waves. Results show the numerical results of ship motions in oblique waves by the present method agree with experiment, and the roll component of the inertial force affect the wave loads acting on the cross deck in head and bow sea.