The Japanese government has established the target for the proportion of renewable energy in the electricity supply to 36–38 % by 2030. One of the ocean renewable energy sources is wave energy. This study focuses on a point-absorber wave energy converter (PA-WEC). Let irregular waves be expressed as the sum of regular wave components whose periodic components are “known”, then the control force required to maximize the average generated power can be solved theoretically. However, in real sea, the optimal control force cannot be identified instantaneously. This is because the periodic components of the irregular waves are “unknown”. Therefore, we have investigated that the optimal control force could be estimated instantaneously using artificial intelligence (AI) with only instantaneous input data. In this study, time-domain simulation is conducted while the control force estimated by AI is included in the motion equation each time step. We investigated the appropriate explanatory variables for training to increase the average generated power in regular and irregular waves. The average generated power was less likely to be negative using the heave velocity of the float as the explanatory variable, and the heave displacement made a large contribution in wave periods of 7.0–10.0 [s].

　The Japanese government has set a target for renewable energy to account for 36–38 % of the electricity supply by 2030. Wave energy is one of the ocean renewable energy sources. This study aims to enhance the workability of a point-absorber wave energy converter (PA-WEC). A PAWEC float has a physical limitation in its heave motion, and exceeding this limitation necessitates operation suspension to avoid damage. This study proposes a control strategy designed to reduce the non-operating time of a PA-WEC by deriving control force coefficients to keep heave motion within the limitation. Additionally, investigation on the effectiveness of switching control scenarios is also conducted. The results indicate that the workability of the PA-WEC is improved, with validation performed through case studies involving the heave limitation up to 3 meters.

　In this study, characteristics of maneuvering hydrodynamic forces acting on a small boat at low speed and minimum required number and range of experimental conditions of captive model tests to obtain adequate maneuvering motion simulation based on a mathematical model are investigated. Range of experimental conditions in drift angle, yaw angular velocity and running attitudes that are heel, trim angle and draught are determined according to measured maneuvering motions of berthing and turning for a real boat. From the measured results by oblique towing tests, it is found that the hydrodynamic forces acting on hull are significantly changed by flow separation from hull caused by large drift angle, on the other hand, the effects of towing speed on the forces are almost disappeared by dividing the forces by the square of towing speed. It is also found that hydrodynamic forces obtained from PMM test with yawing and without towing speed are almost the same magnitude as hydrodynamic forces obtained by PMM test with pure yawing and towing speed due to large longitudinal asymmetry of hull. From the results of PMM tests with pure swaying and pure yawing, it is confirmed that these tests should be conducted in the conditions which include the range of maneuvering motion of real boat. On the other hand, one combination of towing speed and yaw angular velocity is sufficient for PMM test with yawing with constant drift angle since the hydrodynamic forces at same yaw angular velocity and different towing speed divided by towing speeds are almost same.

　This study proposes a new concept for floating offshore wind power generation facilities: a single-point mooring with twin vertical-axis turbines with a variable pitch mechanism. The goal of this concept is to control the direction of thrust by pitch control and to function as an assist to the wind tracking effect of the floating structure. As a first step, a numerical model of the gyration motion of a single-point mooring with pitch control of a variable pitch vertical axis wind turbine was developed, and the effect of pitch control on the gyration motion of the floating structure was examined in conjunction with the results of wind-alone experiments. In the numerical model, it was confirmed that periodic motion, which did not appear in the experimental results, occurred. It was confirmed that this phenomenon can be reduced by introducing a damping force coefficient that depends on the angular rate of gyration, as has been shown in existing studies. It is necessary to develop an estimation method for these fluid forces in the future. Although changes in the direction and magnitude of the thrust can be calculated by modeling the pitch control in the two-stage multiple flow control theory, it is necessary to calculate the thrust as an external force that varies from time to time depending on the blade arrangement. As inferred from the experimental results, the direction of thrust is changed by the pitch control, and it is confirmed that the thrust can be used as an assist mechanism for the floating body to follow the wind.

　The purpose of this study is to simulate the performance of real-scale high-speed planing craft with outboard engine running on the water surface as a gas-liquid surface in a two-layer fluid using the "ITO" supercomputer and reveal the effect of propulsion on hydrodynamic forces acting on hull and lower-hull of outboard engine. In the studies of prismatic planing surface, it is shown that high resolution of partial mesh around stagnation line and application of surface tension model are significant to obtain highly accurate calculation results. The same procedure is applied for a planing craft with outboard engine, and the hydrodynamic forces acting on the planing craft with outboard engine are calculated and the calculated results are compared with measured results. The calculations for the planing craft with outboard engine excluding the propeller are also performed to investigate the effects of propulsion on the hydrodynamic forces acting on the hull and lower hull of the outboard engine and on the flow field. As a result, the force parallel to the keel line increases by 10% and the force normal to the keel line decreases by 5% due to propeller action, and the lower-hull of the outboard engine is significantly affected.

　Verification through tank model tests and actual sea trials is indispensable to improve the accuracy of seakeeping performance estimation, including wave analysis. Results of ship performance estimation in actual sea conditions must be verified by measurements in irregular waves. However, it is not easy to extract accurate information from experiments in irregular waves, which require more attention and time than experiments in regular waves in terms of measurement time length that can ensure a sufficient steady state and a sufficient number of measurements to average the analysis results. The irregular wave time series data generated using the wave spectrum for the tank model experiment is a single sample, and the results obtained by navigating through the irregular waves are only one solution. The time required for measurements in tank model tests is limited by the length of the tank and the dimensions of the model ship, which also makes it difficult to obtain accurate information from experiments in irregular waves. If the measurement time length can be increased, the variance values of the wave and hull response will converge to the true value, at which point the variance of the variance values will asymptotically approach zero. Therefore, the variance of the variance values is an important statistic in considering a reasonable measurement time length.

　In order to investigate the problem of measurement accuracy in irregular waves, we analyze the relationship between variance of variance values and measurement time length using variance value distributions obtained from multiple irregular wave time series, and the relationship between measurement error and measurement time length using an autocorrelation function.

　A novel safety monitoring method for ship structure in waves by using real-time response measurements is proposed. In the method, the structural responses, such as the vertical and horizontal bending moments, at unmeasured points can be estimated by the Wave profile Kalman Filter (WKF) method using the ship response measurements and the transfer functions. The state value of the combined loads is defined by the vertical and horizontal bending moments. The extreme values of the state values are fitted to the Weibull distribution, the probability density function, to estimate the failure probability. To the best of the author's knowledge, this study is the first attempt at a method that estimates unmeasured structural responses using ship response measurements and monitors the failure probability in real time. The monitoring method is validated by applying it to the results of tank tests using an acrylic ship model. The factors affecting the accuracy of the failure probability estimation are also discussed.

　In recent years, the transportation of crude phosphoric acid has increased as a type of liquid cargo for chemical tankers. And, crude phosphoric acid contains many impurities and is more corrosive than refined phosphoric acid.

　In this study, we investigated the corrosion effect of crude phosphoric acid on the stainless steel plates and these welded joints applied in the cargo tank of chemical tankers. Two types of stainless steel plates such as an austenitic stainless steel plate (SUS316L-clad steel plate) and duplex stainless steel plates (SUS329J3L-clad steel plates), and five kinds of reagent of crude phosphoric acid, which were simulated as produced in different countries (Jordan, Senegal, South Africa-1, 2 and Tunisia), were tested in the corrosion test of crude phosphoric acid. The following results were obtained.

➀ It is difficult to transport crude phosphoric acid by the chemical tankers with cargo tanks made of SUS316L steel plate or SUS316L-clad steel plate.

② The chemical tankers constructed with SUS329J3L steel plate or SUS329J3L-clad steel plate have the capability to transport crude phosphoric acid produced in Jordan, S.Africa-1 and S.Africa-2.

③ The results of the pitting corrosion test and corrosion test in crude phosphoric acid show that SUS329J3L-clad steel plate have superior corrosion resistance than SUS316L-clad steel plates tested.

　In pursuit of a carbon-neutral society by the year 2050, floating offshore wind turbines (FOWTs) garner heightened expectations. Mooring chains are commonly employed for the station-keeping of FOWTs; however, long-term use can cause failure attributable to wear and corrosion. Among these, interlink wear caused by floating body motion can be estimated by dynamic analysis, but it is important to accurately determine the wear characteristics in order to properly estimate the wear allowance. In this paper, an additional 11 wear tests were conducted following the previously reported by using the heat-treated surfaces under fully submerged conditions to investigate the effect on the wear amount by the characteristics of the wear test machine such as sliding range and sliding direction. Since these were found to have little effect on the wear amount, a statistical reassessment of the wear coefficient was performed including the previously reported results. As a result, it was found that the obtained wear coefficients conform to a log-normal distribution, and a formula for wear estimation was proposed considering the observed variations.

　In order to properly evaluate the fatigue strength of hull structures, it is necessary to consider the effects of whipping on fatigue crack propagation in more detail. However, so far, the relationship between the frequency of whipping and sea conditions has not been well studied, making it difficult to predict the frequency of whipping under different sea conditions and to evaluate its effects. In particular, it is difficult to reproduce whipping wave equivalent to those that actually occur from the probabilistic models reported to date on the occurrence of whipping. Therefore, it is difficult to investigate the effects of variable wave with whipping on fatigue using fatigue tests or fatigue crack propagation analysis.

　In this study, longitudinal bending stresses in the center of the hull of an 8600 TEU container ship were measured in headwaters, and the waveforms that could be considered as whipping were extracted. The probability of whipping occurring and the magnitude of whipping were defined, and a statistical model was constructed to determine the probability of whipping occurrence and its magnitude based on the sea conditions using the MCMC method. This enables the generation of wave including whipping under arbitrary sea conditions at the time of sailing in head-waves, and enables fatigue tests and various studies to be conducted using the wave that more closely resembles actual phenomena.

　Full-scale hull monitoring is a direct way to ensure the structural integrity of the ship during actual operation. Typically, multiple strain sensors are installed on structural members of several cross-sections. The greater the number of sensors, the more accurate the monitoring is, but the more costly to install and maintain them. This paper proposes a pseudo-strain sensor for hull monitoring using an artificial neural network (ANN). It means that the ANN learns the relationships of structural responses among the multiple sensors for a certain period, and after the learning is completed, the representative sensors predict the structural responses at the remaining sensors. First, the proposed method is applied to a full-scale measurement on an 8,600 TEU container ship to investigate the teacher data needed to predict the response of the ship's bottom from sensors on deck. Next, the predicted results by the ANN are compared to a data assimilation method to predict the hull-girder response using the Kalman filter (RKF), and the pros and cons of the ANN and RKF are discussed. Finally, the learned ANN is used to estimate fatigue damages of the ship's bottom members based on Miner's rule. The estimated results show a good agreement with the measured ones and that the ANN is applicable to the development of pseudostrain sensors for hull monitoring.

　To extend the fatigue life of the boxing fillet joint widely existing in ship structures, the newly developed 16Cr-8Ni low transformation temperature (LTT) flux cored welding wire available to all welding positions and LTT elongated bead method were applied to this joint. The repair procedures of LTT elongated bead method using semi-automatic CO₂ welding practically suitable at ship yard were explored for weld defect prevention. Based on the results of fatigue test on 38 pieces of large size specimens welded in all welding positions, the welding operation guideline was derived. The experimental results showed that, the fatigue life of both the flat and horizontally welded specimens was extended more than 6 times compared with those of conventional boxing fillet joint; The fatigue life of vertically welded specimens reached about 4 times; that of overhead welded specimens was between 2.5 with welding defects and more than 6 times without welding defects.

　The paper proposes a method, SWSE-LNPCA, to assess uncertainty in wave parameters for container ship design and safety. It introduces Lognormal Principal Component Analysis (LNPCA) to handle non-normal distributions in transfer functions, addressing the limitations of previous techniques. Furthermore, by introducing a directional distribution function, the method was extended for application to short wave crest irregular waves, and a new approach for assessing the uncertainty of stress response at arbitrary locations on the ship hull was proposed. By applying this method to real ship loading uncertainties, the study validates its effectiveness in estimating wave parameters' uncertainty, crucial for enhancing vessel safety and performance amid increasing container ship sizes and maritime cargo movement.

　This paper proposes a model for estimating total installation time of FOWF (Floating Offshore Wind Farm) by incorporating the meteorological and oceanographic characteristics of the operation area, and this model is applicable to various sea areas in Japan. The total installation time of FOWF can be defined as the sum of the time of seven main processes. The installation time for main processes can be described by aggregating time of sub-processes and is formulated by using eleven unit-installation time corresponding to the repetitive works during the actual installation and operation window of ten sub-processes, which vary depending on the operation areas. The unit-installation time is detected and compared through the analysis of actual installation data from Fukushima Floating Offshore Wind Farm Demonstration Project, hereinafter Fukushima FORWARD. The operation window for sub-processes in five different areas are derived based on the installation plan of Fukushima FORWARD and statistical data concerning winds and waves in the offshore of Japan. By comparing the results of operation window derived from Fukushima FORWARD data, the methodology of operation window is useful when reproducing the actual data. Finally, this paper concluded that the total installation time of FOWF can be estimated more accurately by using the updated model that takes into account meteorological and oceanographic characteristics.

　This study explores the automation of welding inspection processes in shipyards by leveraging deep learning technologies to develop a system that aims to improve both efficiency and accuracy. By optimizing deep learning models in a laboratory setting, we improved the accuracy of estimating these parameters. In addition, we conducted field tests using a simple welding carriage commonly used in shipyards to validate the applicability of our system in real-world environments. The results demonstrate the potential of our approach to improve production efficiency through automated weld quality inspection. However, the study also identifies future challenges, including the need for more comprehensive training data, the incorporation of environmental data, and improvements in the estimation capabilities for various weld appearance features. This research serves as a step towards the automation of welding processes in the shipbuilding industry and provides directions for future research.

　The effects of wave overtopping and sloshing on the second-order wave exciting forces of a barge-type floating structure with a moonpool and the applicability of weak nonlinear potential theory are investigated. The weak nonlinear potential theory can be applied to estimate the second-order sum frequency wave exciting forces under the condition that no wave overtopping over the floating structure occurs. On the other hand, the second-order sum frequency wave exciting forces increase due to wave overtopping. As a result, it is confirmed that the total wave exciting forces may greatly exceed the first-order component. It is also confirmed that the weak nonlinear potential theory can roughly estimate the steady wave drifting forces under conditions that no wave overtopping over the floating structure occurs. When wave overtopping occurs, the steady wave drifting forces increase in some frequency bands, but in other frequency bands, the steady wave drifting forces tend to decrease with the increase in wave height.