The viscous flow simulation is carried out around the KVLCC2 tanker model by the simple and effective way using a new body-force distribution model for the propeller-hull and propeller-hull-rudder interaction. The simple body-force model based on quasi-steady blade element theory is coupled with the Reynolds averaged Navier-Stokes (RANS) code CFDSHIP-IOWA. The captive tests in the trimmed condition and even-keel condition are computed with rudder and without rudder. The computational condition is set up according to the experiments in Osaka University towing tank and the output flow fields are analyzed in details especially in the wake field around the rudder and propeller. The computational results are not only validated with the experiments but also compared with the real propeller computation and other body-force models in order to find out the advantage of the current method. Summarizing the results, the present study could provide the complicated wake field patterns behind the tanker hull form which are as close as the experiment by simply using the new body-force distribution model and the current method can predict the wake field superior to the other body-force models.
In this 2nd report, unsteady flow field around a 2-D oscillating wing has been calculated for the pitching mode. On the other hand, in our 1st report was calculated for the heaving mode. Mathematical treatment is boundary element method which distributes the continuous vortex layer on the wing camber line. Calculated unsteady lift and moment were good agreement to Theodorsen's analytical solution for the thin wing.
Flow simulation at the time-dependent is performed, and flow vectors are visualized in the case of pitching mode. As well as the previous heaving mode, flow field of the induced velocity is also separated by bound vortex and trailing vortex. As a result, we showed the dependence of the frequency and pitching amplitude to flow vector around oscillating wing. And, the generation process of trailing vortex has been elucidated by visual flow vectors.
Hull form design is one of the most important stages for developing economical ships. Usually, hull form design is conducted by deforming manually the hull form and then the performance of the deformed form is estimated. This procedure is repeated until new forms with less fuel consumption are obtained. Recently, instead of these manual-operation based procedures, hull form optimization methods have been developed and many research works have been reported. In most of them, Sequential Quadratic Programming (SQP) technique is employed. However the results obtained from SQP method tend to result in the local optimization if the objective function possesses multiple peaks.
In the present paper, two hull form optimization techniques for minimizing Brake Horse Power (BHP) are compared and these efficiencies are discussed. The first one is SQP technique. The second one is Surrogate-Based Optimization (SBO) technique. By employing SBO technique, global optimum results can be obtained in shorter time. The result of VLCC hull form optimization by SQP is presented and verified by comparing with the tank test results. Then the optimum results obtained by both SBO and SQP are compared and the performances of the two techniques are discussed in terms of hull form optimization efficiency.
Pod-driven ships have high performance both for energy saving and for utility as stern thruster, but they also have the difficulty in designing maneuverability. It is accordingly important to estimate the steering quality indices as accurately as possible in the early stage of design. This report deals with the steering quality indices by analyzing zigzag experiments of several pod-driven ships presented in the paper by M. D. Woodward, et al. The indices obtained are summarized in the graphs using the key parameter made of strut area and the graphs indicated here will be useful in the practical design of a new pod-driven ship. As the result of the consideration, it is found that pod-driven ships have small damping moment similar to that of conventionally propelled cargo ships and tankers with rudder. Through this study, it is also revealed that increase of strut area enables improvement of course-keeping quality.
This paper proposes a practical control system for track-keeping. The control system consists of a heading control loop (HCL) and a tracking control loop (TCL). Both have a state feedback and a state estimator. These control gains calculate analytically by pole placement method. The control system inhibits the closed-loop stability deterioration caused by hull motion parameter uncertainty. HCL takes explicitly the uncertainty into account in the control object and satisfies a specification (a damping factor) of the closed-loop stability. In TCL, the feedback chooses a configuration not to be affected by the uncertainty and the estimator chooses a configuration not to contain the hull parameter. The control system prevents occurring cross-tracking error. Disturbance components consist of a yaw angular velocity bias, a sway velocity bias and a tidal current component, are estimated by the estimator as offsets except for the sway velocity bias. Correction amount of the cross-tracking error is determined by the relationship between the estimated values and the disturbance components. The relationship is based on the fact that the disturbance components can not estimate all and the estimated values have error. Finally, the proposed technique verifies the validity by simulations.
The wave height effect on the added resistance in waves is investigated by carrying out the resistance test in regular head waves with changing the incident wave heights for three full hull ships with different block coefficient from 0.81 to 0.87. Using the response amplitude of ship motions and the added resistance coefficient obtained in the regular wave tests, the short-term prediction is conducted and the prediction results are compared with the results obtained in irregular wave tests. As a result, the followings are obtained. Strictly, added resistance in regular waves is not proportional to square of incident wave amplitude, although the exponent should be two theoretically. (The exponent is called `exponent of wave height effect' in this paper.) Particularly, in the range of λ/L ＜ 0.7 where λ/L means the wave length-ship length ratio, the exponent of wave height effect is significantly smaller than 2.0. On the other hand, the exponent becomes close to 2.0 in the range of λ/L ＞ 0.7. The averaged values of the exponent for three full hull ships are 1.77 for full load condition and 1.91 for the ballast load condition. The wave height effect of the regular waves for the response amplitude on the short-term prediction of heave and pitch is negligible. Generally, there is a tendency that the short-term prediction of the added resistance becomes larger when the regular wave test results with lower incident wave height are used. The prediction accuracy of the added resistance in irregular waves of a larger wave height case is improved by use of the regular wave test results with the corresponding larger wave height.
In order to develop safe navigation environment, it is important to know the characteristics of ship traffic flow. Although there are some correlations between parameters of ship traffic data such as position, velocity, and ship size, they have been dealt individually in sea traffic simulation in past researches. To carry out more realistic ship traffic simulation, it might be necessary to generate ship traffic data taking into account the correlations.
In this study, authors developed a method of characterization and generation of ship traffic flow by using Principal Component Analysis. By applying the method to AIS data in the Kanmon Passage, the trend of the generated ship traffic data which is same as characteristics of AIS data was shown. Authors also developed a method of simulation of evaluating encounter probability between ships. Encounter conditions of ships with collision risks could be extracted, so that the encounter probability could be calculated. To show the validity of the method, the computed probability was compared to an existing estimated collision probability.
Recently, the maximum speed of small passenger craft is increasing. In case of the craft, the encounter wave period becomes shorter with increase of forward speed, and very large upward vertical acceleration is caused when its bow goes into the water surface. It is known that it causes not only bad ride comfort but also failure of hull or injury of passengers in some cases.
For development hull form, it is necessary to estimate the response of vertical acceleration for its forward speed and sea condition. For safety operation, it is important to estimate statistical short-term or long-term prediction of occurrence of un-desired large vertical acceleration.
The purpose of this study is to investigate the above-mentioned characteristics of vertical acceleration of small passenger craft and to propose an estimation method based on measured data. First, in order to measure accurate vertical acceleration by partly captive model test, data sampling and data analysis methods are discussed. Next, the vertical acceleration on a hull is measured in irregular waves, and the characteristics of the vertical acceleration for wave height, wave period and forward speed are investigated, and its probability distribution for it amplitude is also investigated for the results in irregular waves. Moreover, the same measurements for two additional different hulls are carried out, and the effects of hull form is investigated. Finally, an estimation method based on measured data of the passenger craft whose some particulars are informed is discussed.
The near ﬁeld method is a numerical method for calculation of the wave drift forces and moment, by the way of integrating the pressure over the wetted body surface. Therefore, this method is available not only for the single ﬂoating body but also for the each ﬂoating body in multiple. Computing the velocity potential on the ﬂoating body, the boundary element method (BEM) is used to solve the boundary integral equation which is derived by Green function and the linearized boundary conditions. The higher order BEM and the constant panel method are available to discretize the boundary integral equation. The constant panel method is relatively easy to compute the velocity potential. However, when we obtain the velocity potential by means of the constant panel, near ﬁeld method cannot evaluate the wave drift forces and moment accurately.
In the present paper, a new numerical technic of the constant panel method is proposed, in order to take an advantage of the near ﬁeld method to calculate the wave drift forces and moment. In the new technic, we introduce the extrapolation function and the isoparametric elements. The velocity potential calculated by constant panel method on the ﬂoating body can be extrapolated to the new evaluated points. These new evaluated points can be used for near ﬁeld method to calculate the wave drift forces and moment. As one example of the proposed technic results, we use an ellipsoid model. The results of the wave exciting forces and the wave drift forces between following three methods will be compared. One is the new numerical method, the others are higher order boundary element method and the conventional constant panel method.
In order to guarantee the safety of vessels, it is very important to estimate roll motion adequately. However, it is very complicated to calculate roll motion theoretically, because of significant viscous effects on roll damping. It is well known that there is a prediction method of the roll damping proposed by Ikeda et al.1)2). It is developed with theoretical and experimental backgrounds for periodical roll motion. Therefore, it is difficult to apply it to estimation of transitional and non-periodical roll motions (i.e. roll motion in irregular waves, broaching to capsize etc.). In the previous studies2)3)4), it is pointed out that the flow-memory effects and the effects of transient motion on the bilge keel component of roll damping are necessary to consider for time domain simulations of non-periodical roll motion.
In this study, the bilge-keel component of roll damping is focused, because the component is generally most part of total roll damping. In order to estimate bilge-keel component of non-periodic roll damping for time domain, the flow-memory effects are investigated by numerically to understand its mechanism. Moreover, based on the result, the roll damping time domain prediction method including the effects of transient motion, which is proposed by Katayama et al.5) based on Ikeda's method1) is improved.
In this paper it is reported the possibility of the maneuvering test of the model ship in Circulating Water Channel (CWC). The wall effect on the model ship with the drift angle in CWC is researched by using model test and CFD analysis. Then a condition/criterion is studied for drift angle of the model and the distance between ship model and the side wall of CWC. Hydrodynamic force acting on the turning hull is measured in Yawing test by Planar Motion Mechanism (PMM). The frequency dependency is shown in the test data result from the setting condition (period and r'). To avoid the frequency dependency, reasonable period range is proposed. Oblique towing test and Yawing test are carried out with keeping the above criterion. The analyzed data agrees with the results by towing tank. Then the maneuvering test in CWC has enough accuracy for course stability criterion of the ship practically.
In this research, the effect of geometrical variation on the efficiency of an oscillating water column (OWC) wave energy converter was investigated by a water tank test. An inclined cylindrical OWC model was used for the test. The cylinder can be inclined from the horizontal level by 90°, 45°, and 18.4°. The top of the OWC model was sealed with an orifice plate in order to test the effect of power take off damping on the efficiency of OWC device. It is showed that the inclination of cylinder affected the OWC's resonance frequency more than that predicted by increased water plane area because of reduced gravity restoring force caused by inclination. Moreover, the volume of air chamber does not showed significant effect on the efficiency of OWC compared with the effect of inclination of OWC. Optimum nozzle area for the efficiency of OWC is increased proportional to square root of wave height with both inclined and not inclined conditions. Finally, supposed energy production is calculated using real sea state and inclination of water column has significant impact on the annual energy production.
In this research, atmospheric and underwater small-scale pipe rupture disk tests were conducted. Tested pipes with rupture disk were pressurized by pure nitrogen, in which internal pressure changes and external pressure changes were measured by pressure transducers. By comparing results of the underwater tests with results of the atmospheric tests, water effects were clarified: (1) gas decompression in the underwater tests delays due to constrained gas leak by bubble generation and growth as compared to gas decompression in the atmospheric tests, and (2) larger external pressure changes were measured in the underwater tests. In addition, 1D finite-difference based coupled model for gas decompression, in which gas flow inside a pipe and bubble growth are considered, was developed. In the model, governing equations for gas flow inside a pipe and a bubble are derived from mass conservation law and momentum conservation law assuming isentropic changes of gas. Bubble growth is formulated based on Rayleigh-Plesset equation. The calculated results for the rupture disk tests using the model showed good agreements with the experimental results, which analytically clarified the above stated water effects. This research reveals an importance of considering the water effects in evaluating fractures in offshore pipelines. In the future, it is expected that the model is extended to incorporate crack propagation and pipe deformation together with gas decompression and bubble growth.
In recent year, the extremely thick plate becomes used in the deck plate of the large ships. To prevent the brittle fracture it is important to detect and measure the inner defects. Ultrasonic testing is used to find defects in structures. However, the precise estimation of defect shape and size is difficult through ultrasound testing of welds; currently, such high precision estimation depends on the experience and skill of the inspecting technician. This study aims to develop a method to quantitatively identify the shape of defects easily, quickly, and with high accuracy from the wave reflected from a defect during an ultrasonic test; in addition, such a method should not require the knowledge and experience of an expert. For this purpose, a neural network that uses numerical simulation results as training data was developed, and a defect identification system based on ultrasonic wave information was investigated. To confirm the usability and estimation accuracy of this system, measurement data from actual ultrasonic tests were used as the input in the defect identification system.
An underfilm corrosion simulation method for epoxy coated steel panels in ship’s water ballast tanks (WBT) has been developed. The incubation and extension of coating failure are simulated by using 2-dimensional cellular automaton method, and the steel diminution is simulated by modifying the corrosion model proposed by Kawamura et al. (2015). Analysis parameters are determined by using the results of onboard exposure and cyclic corrosion tests performed by Shiotani et al. (2014). The change in corroded surface shape of epoxy coated and scribed steel panels made of conventional steel and corrosion resistant steel (CRS) exposed in an ore carrier’s WBT for 4.8 years is simulated. The simulated coating deterioration (blister) area and the corroded surface profile agree well with those measured. This demonstrates the effectiveness of the developed simulation method and the determined parameters. The differences in analysis parameters between conventional steel and CRS suggest that CRS can reduce the harmful effect of the active corrosion region on the remaining coating life at the blister’s frontline and the corrosion under the blister.
The characteristics of Fatigue Damage Sensors (FDS) that are in use for fatigue life estimation of monitoring structural welding members in ship structures are discussed in order to improve prediction accuracy of estimated fatigue life exposed to random wave loads such as storms under various loading conditions peculiar to ship structures. In our past research, coefficients, which named load effect factors (LEFs) affected by loading histories against constant amplitude loading were introduced for the estimation procedure using FDSs and confirmed by several series of fatigue testing under simplified storm-type loading patterns. In this paper, it is ensured that the abovementioned LEFs are suitable for application to actual random wave load conditions on oceans by numerical simulation using load histories based on a storm model in consideration of variable of sea areas and seasons. Additionally, it is found that the characteristics of FDS under random wave load superimposed whipping vibration show the suitability for the modified Miner rule.
The characteristics of Fatigue Damage Sensors (FDS) that are in use for fatigue life estimation of monitoring structural welding members in ship structures are discussed in order to improve prediction accuracy of estimated fatigue life exposed to random wave loads such as storms under various loading conditions peculiar to ship structures. In our past research, coefficients, which named load effect factors (LEFs) affected by loading histories against constant amplitude loading were introduced for the estimation procedure using FDSs and confirmed by numerical simulations and fatigue tests under simplified storm-type loading patterns or load histories based on a storm model in consideration of variable of sea areas and seasons. In this paper, we proposed the estimation method of the abovementioned LEFs using several frequency distribution data such as significant wave height and stress obtained from a storm model simulated actual sea condition.
Elastic-plastic behavior analysis methodology for a flexible FOWT is developed adopting a coupled simulation code which considers the flexibility of the floater and malfunction of blade pitch control. This paper proposes a simulation method for predicting the structural response of FOWTs when subjected to blade pitch control malfunction. The structural load and motion characteristics are evaluated in elastic and plastic regions to understand the collapse behavior at the base of the tower. A dynamic aero-hydro-structural coupled analysis method developed by some of the present authors is utilized to evaluate the elastic-plastic response of FOWTs under combined effect of wind and wave. It is found that there is significant increase of rotor thrust due to blade pitch malfunction thereby increasing tower base vertical bending moment. Further, efforts are made to understand structural collapse behavior of FOWT due to increased bending moment.
Resource assessment is an essential step in the reconnaissance to feasibility study stages of marine renewable energy development. However, minimization of uncertainties associated with the estimation requires that data be provided at a sufficiently high resolution and duration long enough to include effects of climate variation. This paper describes a recently completed dataset of wave power, ocean and tidal current power, and ocean temperature power. The estimate is based on a 21-year hindcast of waves at a 1 km resolution along the coast of Japan and a 10-year hindcast of ocean and tidal current at a 3 km resolution around Japan. Power is estimated as a long-time mean at various scales and uncertainties are quantified as well.
Firstly, risk analysis for loss of life was carried out based on IHS Fairplay Casualty and Ship databases. Passenger/Cruise ships and RoPax ships 1,000 GT or above were focused in this study. The risk analysis was conducted for ships built before 1990 and for ships built in or after 1990. The results show that the risk of Passenger/Cruise ships and RoPax ships is reduced. Next, risk evaluation was conducted by using ALARP borders on F-N diagram. The results show that F-N curves of Passenger/Cruise built before 1990 and built in or after 1990 ships and RoPax built in or after 1990 ships locate within the ALARP regions. On the other hand, F-N curve of RoPax built before 1990 ships locates within Intolerable region. In this paper, sub-groups have priorities in considering risk control options are considered and the reason that F-N curve of RoPax built before 1990 ships locates within Intolerable region is discussed.
To promote maritime coastal container feeder and modal shift we developed an evaluation method of measures and policies for unit load land and sea intermodal transportation. The method uses generalized cost model expanded with correction terms (residual generalized cost terms) to estimate cargo transportation route. Its origin-destination mesh of domestic cargo has 207 livelihood spheres and its transportation model doesn't abbreviate any coastal sea routes and ports which appear in targeted cargo history data. By calibrating the correction terms semi-automatically the system achieved highly accurate transportation reproduction although the simulation model is precise. This paper describes the above method and shows the analysis result of coastal cargo incentive policy as an example problem.
The authors made the analytical solution of ship trajectory by simple zig-zag maneuver with equal time-alternative steering in their previous study. It is found, in this study, that 1) Lateral deviation remains even after the helm returns to midship, 2) The bigger the T(index of stability on course and quickness in responding to steering ) becomes, the longer the maximum lateral deviation is delayed. These facts indicate it is hard to predict ship trajectory even by the simple steering. It is more difficult, consequently, to search the steering of passing through some waypoints. This report describes how to compute the optimal steering for passing through several waypoints by linear theory with constant velocity. The steering function is expressed as Fourier sine series with coefficients which are fixed by calculus of variations under the restriction on passing through all waypoints. A frigate and a container, as examples, are analyzed along with the target course represented by three waypoints. The analysis shows that ship length and Nomoto’s steering quality indices have strong influence on both capability of passing through waypoints and lateral deviation from the target course.
In shipbuilding, typically the design objective is based on technical performance of the system at a given operational profile. This paper proposes a methodology to improve life cycle value of target ships by introducing a real option approach. The proposed methodology can consider options of repairs and retrofits in a dockyard very early, during conceptual ship design. A proposed flexible design is evaluated by using a ship performance simulator and a wide range of market fluctuation scenarios generated by the Monte-Carlo method. This methodology was applied to the case study of Very Large Crude Carrier (VLCC) life cycle value evaluation. The result shows that flexible design including real options can significantly aid in consideration of market fluctuation scenarios in comparison with recent typical design optimization approaches.
In this study, stiffened plates under biaxial compression are analyzed by using the commercial FEM software, LS-DYNA, and the collapse behavior is investigated. Firstly, the decrease of effective width of the ship bottom plate is observed and compared with the formula of CSR2). Considering the results of FEM analysis, a new formula to estimate ultimate strength of stiffened plates considering biaxial compression is developed. Secondly, the method to estimate ultimate strength of hull girder considering the biaxial compression of ship bottom is proposed by considering the new formula and modifying the conventional formula of the hard corner element. Finally, hull girder strength considering biaxial compression that is estimated by the proposed method is compared with the FEM analysis of mid ship model to show the validity of the proposed method.