Wind resistances and moment acting on a ship model measured in a wind tunnel are non-dimensionalized using the air density, the projected area of the model and the representative wind velocity. However, there is no common definition of the representative wind velocity in wind tunnel tests for ships, thus wind velocities which are obtained by an anemometer fixed in a certain place in a wind tunnel or measured at a height equivalent to 10 m from the sea surface are often used. In principle, there is no problem as long as the wind velocity profile under the same condition is assumed and the wind velocity at the same position is employed when the wind resistance coefficients are converted into forces of different scales. Nevertheless, if the wind velocity distributions between in the wind tunnel test and the assumed distribution by the user are different, the wind resistance coefficients cannot be used in the first place. And the existence of various definitions of non-dimensionalization makes it difficult to simply compare wind resistance coefficients themselves.

Since it is meaningful to clarify the definition of the representative wind velocity which can provide wind resistances in a realistic and reasonable manner using the wind resistance coefficients without being influenced by the assumed wind velocity distribution, it was derived from the results of wind tunnel tests under the conditions of various wind velocity profiles and using various types of ship models. Furthermore, the influence of the wind velocity profile on the wind resistance measurement was evaluated with the results of the wind resistance measurement in different wind velocity profiles. Several findings on the wind velocity profile required for wind resistance measurements are reported in this paper.

Numerical methods based on CFD (Computational Fluid Dynamics) are rapidly expanding their application range, especially with recent remarkable developments in computer performance. Currently, the RANS (Reynolds average Navier-Stokes equation) model, which is a time-averaged model of turbulent flow, is generally used for ship performance estimation. The estimation accuracy of a turbulence model using numerical methods can be improved by adjusting the model with more accurate and detailed flow field data. However, it is still difficult to obtain detailed flow field data through experiments on actual ships and scale models. In a LES (Large Eddy Simulation) model, which is used to spatially filter turbulent flow, if the grid resolution (filter size) is fine, the limitations imposed by the model itself are reduced. LES simulations performed with a sufficiently grid resolution are known to achieve the same estimation accuracy as DNS (Direct Numerical Simulation) but with a lower computational load.

Based on these considerations, as the first attempt at practical estimation of ship performance by numerical calculation considering turbulence, large-scale LES simulations were carried out using “K computer" for a marine propeller in open water. In this paper, the authors show how the grid resolution affects the estimation of propeller performance and flow around the propeller, and the influence of the Reynolds number on the calculation results.

Analytical considerations given to the effect of blockage and inclination of water surface of circulating water channel (CWC) show the necessity of a correction to the resistance test by CWC. Through the comparison of resistance test results of the towing tank (TT) and CWC, a correction method for the result of CWC is proposed by correcting effects due to the blockage and inclination of water surface. The proposed correction method covers practical type of ships ranging from thin to blunt. Through the comparison of the results of TT, the corrected results show the potential of CWC resistance test using a small model ship. Furthermore, it is clarified how the shape factor and the wave-making resistance coefficient obtained in CWC differ from the towing tank test results. Finally, as an application, a method is proposed that can be corrected even if the data of the type ship is not available. The estimation accuracy of this method is quantitatively evaluated, and it is proved that it can be corrected to the level of the towing tank using a large model.

In order to rotate a rudder smoothly, the maximum rudder torque during a navigation is important for the capacity of a steering gear. However, compared to a lot of studies on rudder normal force, there has been still a few studies and open data on rudder torque. A rudder is commonly positioned behind a hull and exposed in the complex flow where the wake flow shed from the hull is accelerated by the propeller. Moreover, in maneuvering, an oblique flow comes into the rudder position. Therefore, it is difficult to understand the characteristic of rudder torque, and more investigation is required.

In this study, free-running model tests with a 1/61.2 bulk carrier model installing a mariner-type rudder were conducted. They were carried out under full and ballast load conditions, and the rudder torque performance in turning maneuver was investigated. Captive model tests were also conducted with a variety of propeller loads and the influence of the propeller load on the rudder torque was studied. CFD analysis was used then to visualize the pressure field on the rudder surface, and explain the mechanism of the influence. Eventually, a simple polynomial equation related to a propeller load was presented to estimate an average application point of rudder normal force. Incorporating this equation to a standard maneuvering mathematical model (MMG model), we could roughly simulate the rudder torque measured in turning maneuvers.

A roll response model during maneuvering, which can express a transient motion after steering, is proposed. The transient motion, namely the transition process from initial inward heel to outward heel, is used in the rudder–roll stabilization system. Nowadays, the model reference control is well used as an autopilot system of the actual ship. As to the model, the yaw response model such as the K–T model proposed by Nomoto is used because of the convenient adjustment of parameters. However, there is not the roll response model to be able to use for the rudder–roll stabilization. In this study, the 4th order roll response model proposed by Yasukawa and Yoshimura which was obtained from the 4 degrees of freedom (surge-swayroll- yaw) mathematical maneuvering model is focused on. And it is called as the Y–Y model in this study. Firstly, assume that the change of the rolling against steering is gradual, the Y–Y model is approximated by the 2nd order. Moreover, the modeling is done by considering the fact that the rolling due to steering is a non–minimum phase system. In order to verify the effectiveness of the proposed model, the numerical experiments were carried out. The KCS container ship was selected as the sample, since there is the information of maneuvering derivatives, which was obtained by the captive model experiments, including the effect of rolling. As the result, it can be confirmed that the proposed model can be expressed rolling in the transient situation after steering well under the assumption used in the modeling of Y–Y model. Furthermore, the procedure to estimate the parameters is also proposed by using the measured onboard monitoring data, and the effectiveness is confirmed based on the numerical experiment.

In 2012, a mooring line of the semi-submersible“ Deepsea Atlantic "that had been operating at Gullfaks South Field in the North Sea was broken. A JIP team investigated causes of the accident in Norway and reported that the accident occurred due to the combination of viscous wave drift force and anomalous wave. Until now, potential wave drift force has been used in the mooring design for semi-submersible but viscous wave drift force has not been taken into consideration. Generally, in the frequency band where the wave length is long, the ratio of potential wave drift force is small among wave drift forces acting on the floating body, while viscous wave drift force becomes dominant. In a structure of which splash zone has a short diameter, such as a column of semi-submersible, the wave length is long compared to the column, and the viscous wave drift force acts remarkably.

In this paper, we demonstrated a method for a moored semi-submersible to estimate the viscous wave drift force in irregular waves. Then, the validity of a simulation model was verified by comparing the result of numerical simulation and the result of model test. In addition, with regard to the safety evaluation of mooring force, we repeated Monte Carlo simulations using the same simulation model, and investigated the influence of number of samples and duration on the prediction of the extreme value of mooring force using different methods for extreme value statistical analysis.

In this study, the author has developed the mathematical hull form, “generalized Wigley model," based on the modified Wigley model. The proposed hull form presents well the actual ship and it is expressed as explicit function determined by principal particulars of the ship; length L, breadth B, draft d, block coefficient C_b, waterplane area coefficient C_w, and cross-sectional area coefficient amidship C_m. Therefore, the generalized Wigley model is suitable for series calculation when investigating the effects of principal particulars on ship response. To validate the applicability of the proposed form, the author has carried out the calculation of ship motion and vertical vending moment in waves by the actual hull forms as well as generalized Wigley models that have the same ship parameters as the actual ships. As a result, it has been found that the proposed model could imitate actual hull form well regarding enlarged ships. On the other hand, in terms of container vessels, it has been found that the vertical bending moment is over predicted in head sea condition because the proposed form cannot express the large bulbous bow.

Lazy Wave shape and similar shapes is often adopted for riser cables of floating offshore wind turbine (FOWT). Buoyancy is used to form and maintain the arch shape of Lazy Wave part while keeping tension in the cable low to avoid increase of tension fluctuation caused by motion of FOWT. Drawback of this shape is that it is sensitive to weight changes due to biofouling. In this study, the influence of biofouling on the static equilibrium shape, shape in current and dynamic response caused by FOWT motion was investigated. In addition, attaching of vertical spring consisting of a buoy and a chain to riser cable is proposed to reduce the sink of the cable due to biofouling. The effects of the spring on the static equilibrium shape, shape in current and dynamic response were also investigated using analysis models. It became clear that the spring has enough ability to suppress deformation of the shape of riser cable due to biofouling, and also has ability to suppress the deformation in currents and the increase of maximum tension due to the motion of FOWT.

A classical study of the transverse static stability of the plane side formula ship whose side is not necessarily perpendicular to a water surface is carried out. In this paper, the metacenter radius is shown by only one parameter related to the angle between the side and a water surface. The derived metacenter radius coincides with the existing results including Scribanti's metacenter radius when the side is perpendicular to the water surface. Symmetric relationship of the metacenter radius for the angle of the side from the perpendicular line is found. Furthermore, it is shown that the righting lever takes a minimum value when the side is perpendicular to the water surface.

The occurrence of the Mathieu-type instability was investigated theoretically and experimentally. The main aim of this paper is utilization of auto-parametrically excited oscillation to increase efficiency of wave energy converters. In this paper, the subject is the auto-parametrically excited oscillation of a spar-buoy type point absorber with the coupled motions of pitch and heave. A spar-buoy model with innovative ballast control system was made and the model experiments were carried out to realize the large oscillating motion based on the Mathieu-type instability. The ballast control system installed in the buoy model can make the ballast move in the vertical direction to realize a certain change of the pitch natural period. Free-decay experiments were performed to determine the natural frequencies and damping ratios with different ballast positions. During the experiments in regular waves, it was observed that the pitch motion became unstable and the large pitch occurred suddenly. Based on time histories of experimental results, the points (peaks and troughs) are extracted near the large pitch occurring and the periods of transient states were calculated to derive the parametrical values of stability. Considering these values with plotted stability chart of Mathieu equation, the occurrence of the large pitch motion is discussed.

Accurate estimation of the wave load acting on a ship hull structure is one of the most important factors in ship structural design. The present authors reported in the previous report about the sea condition which actual ship encountered using AIS (Automatic Identification System) data and Hindcast wave data. However, the quantitative evaluation regarding to the maximum wave load considering with operational conditions could not be carried out.

In this research, for container ships with the length 280 m to 400 m, the vertical bending moment (VBM) were estimated utilizing the joint probability consisting of ship operational conditions and its sea states. Joint probability density function p(Hs,Tz, _X,V),which consists of the significant wave height Hs, the zero-crossing wave period Tz, the encounter wave direction X and the ship speed V, were obtained from the AIS data and the Hindcast wave data. We developed a method to obtain the maximum load corresponding to the exceedance probability Q = 10^-8, which is required for ultimate strength evaluation, considering with the effect of the conditional probability of operations given sea states. Using the developed method, it was found that the maximum load is reduced by about 10% by applying the joint probability density function p(Hs,Tz,_X,V) compared with the conventional method based on independent probability density functions p(X) and p(V) .

In addition, it was found that the worst sea state in which the maximum load acts, does not coincide with the sea condition and operation condition in the class rules specified for strength evaluation. Container ships with the length 280 m to 310 m tend to avoid head seas under rough seas, and it was indicated that the maximum load acted in following seas condition. And it was indicated that the maximum load acts on container ships with the length 310 m to 340 m in the head seas condition as assumed in the class rules, but the ship speed slightly higher than those assumed in the class rules.

Most of in-service structures are subjected to many types of cyclic loading and these cyclic loadings have different axial components with different phase. In-addition, most of the fatigue cracks occurring in the structures are so-called surface cracks which can be approximate elliptical shape. However, the fatigue strength of structures is evaluated according to design standards established by theoretical and experimental investigations under uniaxial cyclic loading condition. On the other hand, authors proposed the numerical simulation procedure of through thickness fatigue crack propagation histories of a cracked plate subjected in-plane biaxial loadings with phase differences of each loading component from a viewpoint of fracture mechanics approach. In this study, fatigue surface crack growth behavior under in-plane biaxial loading with different phase conditions are investigated. The phase difference effect for fatigue surface crack shape evolution under in-plane biaxial loading is confirmed by measured ones. Besides, the numerical simulation method of fatigue crack growth is proposed and confirmed this method by comparisons of measured crack evolution with the numerical simulation.

In this research, the contour method was applied to measure the residual stress distribution of a bead-on-plate specimen. The measured residual stress distribution was compared with those measured by neutron diffraction method and calculated by thermal elastic plastic finite element analysis. As a result, it was found that the residual stress distribution obtained by these three methods are in good agreement. In addition, the equilibrium of reaction force on cut plane was considered in the contour method to measure residual stresses in asymmetric cutting which is assumed in the measurement of real structures. The proposed method was applied to the measurement of the residual stress in the multi-pass welded joint. The measured results indicated that the proposed method can measure the residual stress distribution in multi-pass welded joint in asymmetric test specimen.

It is necessary to develop quantitative wear estimation method of mooring chains for floating structures such as floating offshore wind turbine. In our previous research, a numerical analysis combining 1) wear analysis between links by finite element method and 2) response analysis of mooring system has been proposed and validated qualitatively in the case of a single moored buoy. In this paper, the proposed method is applied to a spread moored floating spar with three mooring lines for further validation. Here, the effect of wind in addition to waves on the wear of chains is examined. As a result of comparison, it is found that the wear amounts estimated by the proposed method fairly agree with the measured values. However, it is also found that the estimated wear amounts around the touchdown point and those next to a clump weight considerably overestimate the measured values.

The importance of deep ocean surveying and monitoring is increasing because of the change of situation of energy and oceanic resources. We propose a deep tow system which consists of a launcher and a towed vehicle to achieve high speed towing and good depth and motion control of the vehicle. In this research, motion simulations of a full-scale system were carried out. The influence of the towing cable's length, towing speed and the launcher's weight on the submersion depth of the towed vehicle, the influence of motions of the mother-ship on motions of the towed vehicle, the influence of the mother-ship's turning radius on the towed vehicle's drop and motion control performances of the towed vehicle are shown.

In recent years, predictions of environmental changes on Earth and studies of ecodevelopment have become increasingly important. Although oceanographic data in deep sea had attracted attention until now, these days, the importance of data in shallow water is increasing. Then, we aim at development of an underwater glider for virtual mooring that can be used in shallow water. In order to improve a motion control performance, a vertical tail wing and thruster are attached to the disk-type underwater glider already developed. In this paper, results of model tank test for motion control performance verification are shown.

International Maritime Organization Maritime Safety Committee, at its 91 session on November 2012, adopted draft amendment to SOLAS regulation II-1/3-12, which changes the Code on noise levels on board ships from non-mandatory to mandatory. This paper provides optimizing process for ship noise in the accommodation space on the basis of results of noise analysis using SEA models of a certain oil tanker of 6,500 kiloliters actually constructed by a shipyard. In this regard, the authors identified how hull structure should be arranged and how noise control measures should be applied in order to reduce the target ship's noise levels efficiently by using generally useful optimizing process.

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 analysis, it's fundamental to predict wave-induced coupled dynamics between vessel and suspended load. The coupled dynamics makes the each mechanics more complex. In the case of the multi purpose offshore supply vessel, deep understanding about the mechanics is required because it offers various crane operations, Unfortunately it's not easy to understand how the parameters related to the suspended load (e.g. sling point) change the motion characteristics for the vessel. Therefore the objective of this paper is to understand the effect of the parameters on the coupled dynamics by sensitivity analysis using numerical simulation. The mathematical model used in the numerical simulation is explained and the accuracy of the numerical simulation is validated through scaled model experiments in this paper. The results of the sensitivity analysis show, especially in the case of irregular wave with peaks in long period range, the coupled dynamics plays significant role even if the suspended load is lightweight.

The aim of the present study is to obtain the ultimate strength and load response of a single side shell panel of a bulk carrier under longitudinal thrust and out-of-plane pressure. Time-domain analysis using nonlinear strip method and a whole ship finite element analysis are performed to calculate the pressure and stress responses of the side shell panel. It is shown that the most critical sea condition in terms of buckling strength is an oblique sea that causes the maximum longitudinal stress due to horizontal bending moment, and the longitudinal thrust and out-of-plane pressure work on the panel almost in the same phase. The elastic buckling analysis and elastoplastic large deflection analysis of the side shell panel are carried out to investigate the collapse behavior. It is found that the collapse mode of the panel is a local buckling mode and a whole hull structure has a sufficient strength reserve even after the ultimate strength of the panel is attained. This suggests that the scantling of side shell structure under the current rule may be reduced. FORM (First Order Reliability Method) is adopted to identify the critical combined load condition using the limit state function based on the results of the ultimate strength analysis. Safety indices obtained by FORM show that the oblique sea of 120° heading angle is the worst condition similar to the result of time-domain analysis of load responses in short-term irregular waves. Integrating the findings of load and strength analyses, a rational method of ultimate strength evaluation of a side shell panel of a bulk carrier is discussed.