A propulsion performance test using a model for estimating the horsepower of a general cargo ship is usually performed to grasp the performance in a straight running condition. However, it is expected that understanding of the propulsion performance at low speeds and oblique conditions will be important for slow blunt ships and wind-assisted propulsion ships that will be developed to achieve zero GHG emissions in the future.

In this study, the propulsion performance was measured using a slow blunt ship model at low speed and oblique conditions, which was not performed in a normal tank test. The effects on the wake and propulsion performance due to the change of the oblique angle and the effects in the low speed were experimentally understood.

In order to estimate an ice resistance in level ice considering the underwater hull geometry, the calculation method was developed. This method uses the Ice-Covered hull Model (ICHM) which estimates ice pieces distribution on hull surface considering that ice pieces' rotation (around water line), concentration and clearing. In this method, an ice resistance due to bending failure is calculated based on Ionov's method. In addition, an ice resistance due to ice pieces is calculated on the basis of ice pieces distribution and movement in ICHM. As the results of comparing with ice tank tests, this method was able to estimate the ice resistance well.

Evaluation of ship performance in actual seas is important for reduction of greenhouse gas emissions from the shipping sector. Recently, activities to investigate ship performance by means of onboard monitoring have become increasingly widespread. While onboard monitoring can collect a vast amount of data, it is necessary to follow a proper procedure for an accurate evaluation of ship performance.

Prior to this study, the authors proposed a filtering method called the Resistance Criteria Method (RCM) with the aim of extracting onboard monitoring data collected in a calm sea, in which the rate of added resistance is a key parameter. This study addresses validation of RCM using onboard monitoring data of a panamax container ship and a medium range tanker. The validation was carried out by comparing the ship speed and fuel consumption in the onboard monitoring data and a voyage simulation using the evaluated performance based on the conventional method and RCM as an input. The results showed that application of RCM is effective for evaluating ship performance based on onboard monitoring data.

Onboard monitoring for ship performance evaluation in actual seas is widely conducted in the shipping sector. Onboard monitoring usually involves a large number of instruments for measuring ship performance and the sea conditions in which the ship navigates, and such instruments are subject to some amount of error, which means that the accuracy of the instruments influences the evaluation. In this study, an uncertainty assessment was carried out to investigate the influence of instrument accuracy on two subjects by Monte Carlo Simulation. One subject is ship performance in calm seas evaluated by correction of performance for the effects of waves and winds, and the other is voyage simulation. As a result, this study clarified the accuracy of the instruments used in onboard monitoring required for achieving a reliable evaluation of ship performance in actual seas.

Container ships are becoming larger in order to reduce fuel consumption per unit transportation volume. As the ship size grows, the number of bays in which containers are loaded will increase, and the number of containers per bay will also increase. The loading order of a large number of containers is usually optimized by a computer-aided planning system called a Loading Computer that can comprehensively consider ship stability and cargo handling efficiency. However, due to the increase in the size of ships, the number of ports of call in one voyage and the number of empty containers to be transported also increase, and the container loading patterns are more diverse than before. There is more than one solution.

On the other hand, the loading condition of the containers on deck affects the wind forces during the voyage. The above-mentioned Loading Computer does not consider the wind forces acting on a ship, but from the viewpoint of improving energy efficiency, it is desired to consider the wind forces that change depending on the loading patterns as one parameter.

In this paper, the relationship between the loading patterns of containers on deck and wind forces was clarified by means of wind tunnel tests, focusing on the number of gaps between container bays.

This paper reports an experimental study on application of pendulum theory to determine the possibility of overturning of a rectangular hull model. The model was not a hull of a self-propelling ship but was a pontoon used as a non-self-navigating work vessel. The model begins to list when an unbalanced load is applied. It can then either stabilize in an inclined state or continue to overturn, throwing the unbalanced load midway, and finally stabilizing in the inverted position. In a mid-air experiment, this correlation was observed for two motion scenarios: pendulum and rotational motion. The two motions were reproduced by changing the mass of the load. The experimental results closely match those of analysis performed using the Runge-Kutta method. In an experiment in a water tank, the change in the mass of the load applied to the model hull and the effect of buoyancy were considered for analysis. Both experimental results and analytic results exhibited the same tendency for overturning. The validity of our analytical method using pendulum theory was confirmed by the consistency between these relationships.

In this study, we propose the righting moment rate (Rr) as a new stability index for rectangular hulls; the hulls of such a shape have both a lower height and larger width than ships, and they are used for work vessels without a propulsion unit. In naval architecture and marine engineering, the metacentric height (GM) is one of the most basic stability indexes. The numerical range of GM has been historically shown for typical classes of ships at their corresponding times. However, the GM for work vessels with rectangular hulls is substantially larger than that for ships; this is problematic because work vessels can therefore incorrectly be deemed stable. Thus, we propose Rr as a new stability index for rectangular hulls to avoid faulty stability evaluations. In this study, we use examples of work vessels (e.g., crane barges) and ships (e.g., freight vessels) to clarify the differences between their respective stabilities. Hull depth, draft, center of gravity height, center of buoyancy height, and GM were modeled for rectangular hulls and ship-shaped hulls based on these examples. We determined the characteristics of the buoyancy, gravity, and righting moments, as well as the difference between the buoyancy moment and the gravity moment acting on each respective hull. In this manner, an adequate correlation between Rr and GM as stability indexes was both confirmed and determined theoretically.

Free-running model tests and captive model tests were conducted using a ship model under full- and trial-load conditions to capture the maneuverability of a 498-GT cargo ship ''Jinmon-maru'' equipped with rudders on both sides of the propeller. In this paper, the rudder is called a side rudder (SD-rudder). Similar model tests were conducted using an another identical model, in which only the rudder was changed from a side rudder to a fishtail rudder, to evaluate the model test results. Thus, a quantitative evaluation of the difference in the performance of a ship with a side rudder (SD-rudder ship) and that with a fishtail rudder (FT-rudder ship) was realized. The model tests showed that the turning performance of the SD-rudder ship was inferior to that of the FT-rudder ship. It was revealed that the deterioration of the turning performance of the SD-rudder ship can be attributed to a rudder-angle limitation applied to prevent the hull from touching the rudder.

Owing to the compelling demand for CO₂ reduction in recent years, various attempts have been expended to improve the resistance performance of ships in actual seas. In this study, the effect of the pitch moment of inertia on the added resistance in waves is investigated via tank tests and strip-method-based calculations for an S-175 container ship measuring a length (L) of 175 m. Consequently, when the pitch radius of gyration is reduced from 0.27L to 0.23L, it is discovered that the added resistance in waves reduced by approximately 30% for a ship moving with Froude number 0.15 in irregular head waves with a significant wave height of 4 m. In addition, the effect on the added resistance is significant in the head wave direction from 0° to 75° but insignificant in the beam and following wave direction from 90° to 180°. Hence, it is effective to reduce the pitch moment of inertia to improve the propulsive performance of a ship in waves.

The external turret system is widely used for mooring of Floating Production, Storage and Offloading system (FPSO). Slamming is one of important issues which should be considered for designing FPSOs because FPSOs must stay in the same area even if those are under harsh conditions such as a storm. In response to cases of accidents caused by slamming, such as Schiehallion, a series of research projects were conducted in Europe, which proposed that the simplified estimation method of maximum slam force; however, it was too simplified to be used for detailed design.

In this paper, we present the extreme value estimation of maximum bending moment including the slamming effect at the connection between the FPSO bow and the external turret lever. The foundation of the estimation method is based on the time-domain simulation by using the simulation model which was validated by comparing the results of the simulations with those of model tests. The simulation model was validated through comparison between model test results and the simulations. Two estimation methods for the expected value of 3 hour-maximum bending moment using the validated simulation model were investigated; one was the way of averaging maxima in multiple simulations, and another was due to extreme value analyses by four kinds of methods. As a result, the first method required more than 30 simulations; for the second method, the generalized Pareto distribution was the most suitable for extreme value analysis.

Recently, the strength of the hull structure including fatigue strength has seen significant improvement because of application of IACS CSR (Common Structural Rules). Various methods which can increase the fatigue strength at welded joints are suggested, since the damage morphology on the recently welded structures was observed to be primarily fatigue failure. In Shipbuilding industry, smoothening of the welding bead toe by grinding is the method that is customarily adopted to improve the fatigue performance. Recently, many peening treatment methods at the welding bead has also been proposed. These peening processes produce a compressive residual stress field near the peening location. In addition, the peening process smoothes the toe shape of the weld bead and reduces stress concentration. In case of generating compressive residual stress by impact, the same effect could be expected by shot blasting, which is the steel surface preparation for painting.

In this study, the condition of the residual stress generated at welding toe by shot blasting was investigated, and then from results of the fatigue test, the increase in fatigue strength due to residual stress is verified. In addition, referring to the S-N curve based on the fatigue test results, the effect of the fatigue strength improvement effect due to blasting on the fatigue design of the hull structure was investigated, and the possibility of hull weight reduction was confirmed.

In recent years, the adoption of fiber reinforced plastic (FRP) sandwich structure has increased not only for small but also for large structures, since the VaRTM (Vacuum assisted Resin Transfer Molding) has been developed to practical use ¹⁾. In the structural approval inspection of small ships ^2,3), GFRP sandwich structures are allowed the same strength evaluation as single GFRP plate structures. However, not only the inclusion ratio to the thickness of core material when using material other than the specified wood materials but also the thickness ratio of the inner and outer FRP plates are regulated for the sandwich structures ³⁾. In the present study, in order to examine those influences on the bending strength of sandwich structures, tensile tests for various materials and bending tests for single GFRP plates and sandwich plates were performed. It was revealed from the test results that the inclusion rate may be considered even when a material other than the specified wood materials is used for the sandwich core. In addition, it became clear that the influence of the thickness ratio of inner/outer plates on the bending strength is relatively small compared to the variation in the test results.

Although the strength characteristics of the tested materials could be obtained from the various test results, they had large variations, so their average values were used as the reference values of the strength. By using the reference values for the tensile and compressive strengths of the test materials, the results of finite element method (FEM) analysis which are executed by the commercial FEM program LS-DYNA for the three-point bending tests of the sandwich plates show a good correlation with the test results. In the forthcoming second report, based on the reference values, FEM analysis of sandwich panels and hull models will be executed, and detailed verification of the inclusion rate of sandwich core and the thickness ratio of inner/outer plates will be performed.

A hatch cover is not a primary strength member in ship structures. However, if the hatch cover collapses in rough sea, water ingress may happen into the cargo hold, which may result in severe casualty. From this point of view, it is very important to assess the strength of a hatch cover. In CSR (Common Structural Rules) specified by IACS (International Association of Classification Societies), formulas are given to estimate the buckling/ultimate strength of a top panel of a hatch cover. However, these formulas look not adequate. Therefore, in this paper, the CSR formulas regarding the hatch cover strength are assessed on the basis of eigenvalue and nonlinear FEM analyses. At the same time, alternative simple formulas are newly proposed modifying the existing simple formulas, and their applicability is demonstrated.

A fillet welded joint has been used for ship and offshore structures. During the welding process, unwelded parts remain in the structure. When a crack propagates from the web plate to the joint across the unwelded part, interesting crack propagation (CP) phenomena, i.e., crack front separation and crack coalescing, were observed. In the present study, threedimensional CP simulation is carried out to carefully investigate the fracture behaviors. X-FEM, a powerful computational fracture mechanics analysis method, is adopted. A crack option in an open source CAE software Code Aster is chosen. It is found that a modification is needed to analyze the complicated fracture behaviors appropriately. The analysis control file and source code have been modified to improve accuracy for the evaluation of fracture mechanics parameter and crack path. With the present approach, the complicated fracture phenomena can be solved effectively. The fracture behavior is discussed well with the crack trajectories and history of the fracture mechanics parameters for the propagating crack.

In the cargo tank structure of the chemical tanker, fatigue cracking at the cruciform welded joint between the transverse bulkhead and the double bottom, which is the most important part under severe stress conditions, can lead to a serious accident.

In this study, the shapes of the weld toes of cruciform welded joint specimens, which were assumed to be the weld part between the transverse bulkhead and double bottom, were measured using a weld bead laser scanner, which was a handy measuring device, to estimate the stress concentration factors. These shapes were also measured using conventional silicon mold. It was shown that the stress concentration factors for each measurement were almost the same.

In addition, fatigue tests using the cruciform welded joint specimens were carried out and compared with the results obtained in previous studies. Each fatigue strength for full-penetration cruciform welded joint of duplex stainless steel plate and that of austenitic stainless steel plate was determined by summarizing all these results. Furthermore, from the comparison between the fatigue strength obtained from the fatigue tests and the fatigue design diagram for ship hull structure provided by the classification society Nippon Kaiji Kyokai (ClassNK), The maximum allowable stress concentration factor of the examined full-penetration cruciform welded joint was determined.

As a result of them, we have put into practical use a technique for measuring of the shape of welded joints in cargo tank structure of chemical tanker under construction at shipbuilding site using the weld bead laser scanner and estimating the stress concentration factor.

If the estimated stress concentration factor is less than the maximum allowable stress concentration factor, it can be immediately determined that fatigue cracks will not initiate. It is a breakthrough in terms of quality control and assurance in shipbuilding.

The estimation of buckling strength of plates and stiffeners as a basic component of ship structures is very important to evaluate her safety. As for plates, recent classification rules consider the compressive ultimate strength, which is beyond elastic buckling strength, as their buckling criteria because the elastic bucking strength of plates likely gives over-conservative strength especially for thin plates.

The previous research developed many kinds of ultimate strength evaluation methods, which often adopted so called “curve fitting" technique to reflect the results of model experiments and/or non-linear FEM. Therefore, the methods might be affected by specific boundary conditions, initial deflection and residual stresses where the experiments and FEM were performed. To seek for the approaches with clearer physical backgrounds, the authors have developed a simplified method to evaluate ultimate strength of a rectangular plate subjected to combined loads. The method employs the elastic large deflection theory and Mises's yield criteria without using any “curve fitting" techniques. This paper discusses how to develop the elastic large deflection analysis to represent actual buckling behavior of plates using closed formula and how to judge the plate collapse with using yield functions, and then shows the accuracy of the developed method through a comparison with the results of non-linear FEM.

As crane lifting operation in ocean is vital to construct offshore structures and install subsea facilities on the seafloor, evaluation of safety and availability for the crane lifting operation is needed before the construction. For these analyses, it's fundamental to predict wave-induced coupled dynamics between a vessel and a suspended load. The coupled dynamics makes the each mechanics more complex. In the case of using the multi purpose offshore supply vessel to the crane lifting operation, deep understanding about the mechanics is required because it offers various crane operations. In our previous paper, we propsed the numerical method to estimate the swirling of the suspended load in air and considered the impact of the suspended load's location on the coupled dynamics using the both numerical calculation and estimation method. To expand our knowledge and numerical method for the crane operation, we inestigated the coupled dynamics right after the suspended load entry into the sea surface. In such the situation, it's possible that the hydrodynamics on the suspended load change the coupled dynamics and produce unexpected risk for the crane lifting operation. In this paper both experiments and numerical simulation we conducted are shown to consider the coupled dynamics between the vessel and the suspended load. Furthermore we indicated the results of sensitivity analysis by numerical simulation to put the point of focus on the natural period of the vessel's roll motion. As results, we made clear the inversion of the natural period could be occured depending on where the location of the suspended load is.

In recent years, global warming has become more and more serious, and the practical application of renewable energy that does not rely on fossil fuels has become an urgent issue. The 17 Sustainable Development Goals (SDGs) adopted by the United Nations as goals to be achieved by 2030 include "take urgent action to combat climate change and its impacts" and "conserve and sustainably use the oceans, seas and marine resources for sustainable development", and countries around the world are beginning to work on it. The ocean, which occupies about 70% of the earth's surface, has a great influence on the global climate change, and it is extremely important to understand the ocean environment correctly. Recently, autonomous underwater vehicles (AUVs) are playing an active role in oceanographic surveys and seafloor exploration. Especially, autonomous underwater gliders that can be operated for a long period of time are attracting attention for global marine environment surveys. The underwater glider with independently controllable main wings developed at Osaka Prefecture University realises high kinetic performance by controlling the left and right main wings independently. In this research, the authors attempted to develop an autonomous control system and verify it by automatic navigation simulation aiming at the establishment of a control system to be implemented on a testbed vehicle of the underwater glider with independently controllable main wings, "SOARER".

Japan depends on imports for almost all resources, which are supported by maritime trade. Since the shipping industry is a single global market and highly competitive, it is important to anticipate future fluctuations for stable transportation. On the other hand, existing research on time-series forecasting uses only past observations, making it difficult to predict future fluctuations more accurately. In this paper, we propose a training method of percolative learning model. We apply this method to test problems of predicting future shipping market. The results indicate that the proposed method is more accurate and effective than the conventional method.

An automatic nesting algorithm based on deep reinforcement learning for shipbuilding is proposed. Automatic nesting method is desired because of the large number of parts to be placed. The nesting problem in shipbuilding is very difficult because of the relatively small area where they can be placed and the variety of shapes that can rotate freely. Since it is very difficult to manually create effective placement rules, this paper proposes an algorithm to generate rules autonomously based on reinforcement learning. To apply reinforcement learning to the nesting problem, we organized the problem as a Markov chain process. Based on deep q network, a type of reinforcement learning, we used the components of a real ship's block to learn the network parameters. The parts are represented in pixel format. It is confirmed that the present method was superior to the conventional method, and the results were comparable to those of a skilled person. However, for unlearned members, the results are inferior to those of the conventional method. This problem can be solved by relearning including unlearned components.

For high accuracy of theoretical prediction of propeller cavitation, it is indispensable to investigate the behavior of unsteady cavitation on propeller blades. Various methods of measuring cavity shapes have been proposed such as using stereo photography, pin gauge and laser beam scattering. Recently, authors have developed a method of measuring the three-dimensional shape using a combination line CCD camera measurement method which can measure faster and more accurate than previous conventional methods. To verify effectiveness of the developed system, model experiments on a highly skewed propeller of a training ship “SEIUN-MARUI" has been carried out in the large cavitation tunnel of NMRI. Through the experiments, the authors show the measurement results of model propeller blades shape and cavity shapes on the blades. Furthermore, the authors have verified the accuracy through a comparison of the pressure fluctuation estimated by the cavity volume and measured by the pressure sensor. In the estimation, since the unsteady cavity is replaced by a spherical bubble moving with varying radius, the pressure fluctuation induced by the sphere is calculated. The pressure fluctuation is estimated by applying the proposed method to experimental data of the cavity shape. The pressure fluctuation estimated by the proposed method is in good agreement with the pressure fluctuation measured by pressure sensors. This result also shows that the developed measurement system can measure the cavity shape with sufficient accuracy.

The main topics of this paper are the motion characteristics of a spar-buoy. Motion measurements for the spar-buoy with and without a ring-fin motion stabilizer in currents and waves were carried out at the circular water channel and the towing tank of Osaka Prefecture University, to investigate the characteristics of its motion. Motion amplitude of the spar-buoy with the stabilizer is much smaller than the spar-buoy without the stabilizer, however, the spar-buoy with the stabilizer causes the inclination that means a time average value of motion and the inclination becomes larger according to increase in wave height. On the other hand, from the results in currents, the inclination of the spar-buoy with the stabilizer is much smaller than the results of the spar-buoy without the stabilizer, however, it is found that there is clear possibility that it has multi-balanced-inclinations for higher current velocity than the designed current velocity. The mechanisms of these unexpected phenomena were investigated, and it is indicated that they can be avoided to optimize cross-section shape of the stabilizer to change its characteristics of hydrodynamics.

Real-time implementation of a nonlinear model predictive control (NMPC) for a point absorber type wave energy converter (PAWEC) with a tubular air cored permanent magnet generator is experimentally validated through tank model tests. The control inputs of the NMPC were updated by the differential equation to trace the solution of an associated two-point boundary-value problem. Two conventional control strategies, the resistive load control and the approximate complex-conjugate control with considering the copper loss (ACL), and the NMPC were implemented to the PAWEC model. The floater heave motion, control force of the generator, and power generation characteristics were compared in these control strategies. The power generation performance of the NNPC was the same level of that of the ACL in regular and irregular waves. The sensitivities to the linear viscous damping estimation error and to the wave excitation force estimation error in the NMPC were experimentally evaluated by the tank model tests.