In recent years, ship owners and shipyards need more energy-saving vessels by the regulation of SOx and EEDI (Energy Efficiency Design Index). Therefore, main shipyards are already equipped with ESDs (Energy Saving Devices) in front and behind the propeller, such as a stern duct and a rudder bulb. However, there are almost no research examples of the interaction of propellers with ESDs, and there is a possibility of improvement of propulsion performance by consideration of the interaction.
The self-propulsion test results with the stern duct and rudder bulb showed almost the same effect as adding the energy-saving effect of each ESD, and the highest energy-saving effect compared with bare hull was 15.0%. Also, due to the propeller position and pitch distribution, BHP (Brake Horse Power) difference of a maximum of 4.9% occurred. The PIV (Particle Image Velocimetry) measurement results confirmed that the flow field in front and behind the propeller changed depending on the propeller position and pitch distribution, and the interaction of propeller with ESDs also changed.
In this paper, the authors described model test results to investigate the interaction of propellers with ESDs for achievement of highest efficiency of propulsion devices.
In this study, the authors have developed the simple and accurate formulae of the Froude-Krylov forces of 6-DOFs based on the linear theory, as a fundamental study to develop a closed formula of ship motion in waves. The proposed formulae have been developed by approximating the hull-form under waterline by the function uniquely determined by the principal particulars of the ship; length L, breadth B, draft d, block coefficient Cb, waterplane area coefficient Cw, midship section area coefficient Cm, height of center of gravity KG, longitudinal center of floatation LCF. Therefore, the proposed formulae are expressed as the explicit function determined by only principal particulars as well as the wave condition parameters. It was confirmed that the proposed formulae have high accuracy for all merchant ship types in any wave condition (wave angle and wave length) through validation compared with numerical calculation by using actual hull-forms of 77 ships × 2 conditions.
In this study, the author newly developed a mathematical hull-form called “generalized Wigley α (G.W α) hull-form” based on the generalized Wigley (G.W.) hull-form which has been proposed in the 1st report to estimate the ship response in waves rationally and easily. It was found in the 1st report that ship responses of full ship can be estimated accurately with the use of the G.W. hull-form except for slender ship. The main reason was found in subsequent investigations to be the difference of 2nd moment of waterplane area between the G.W. hull-form and real ship. In this study, therefore, the author developed an enhanced mathematical hull-form, the G.W α hull-form, with which the 2nd moment of waterplane area can be varied independently of waterplane area. Moreover, the longitudinal asymmetry parameters, longitudinal center of buoyancy LCB and longitudinal center of floatation LCF, were also adopted as the hull-form parameter, which strongly affect the ship response in waves. The proposed G.W α hull-form is expressed in the form of explicit function determined by only 10 principal hull-form parameters. Hence it is suitable for sensitivity calculation of hull-form parameters for ship response. To validate and clarify the applicability domain of proposed hull-form, the author carried out the calculation of ship response in waves for 12 actual ships as well as G.W alpha; hull-forms of the same hull-form parameters as the actual ships. As a result, it was confirmed that the heave, pitch motion and vertical bending moment of proposed hull-forms have good agreement with the actual ships regardless of ship types and wave conditions, and not G.W. α hull-form but G.W. hull-form should be applied for estimation of horizontal bending moment, horizontal shear force and torsional moment.
If an unexpected load collapse occurs, a ship may heel. The ship must navigate safely in the heeled condition until the destination port is reached. In this paper, the turning performance of a heeled ship is investigated. Firstly, a free-running test is conducted using a pure car carrier model to capture the turning characteristics with a rudder angle ±35° of the heeled ship. In starboard turning of a ship with port side heel, the turning radius decreases with an increase in the absolute value of the initial heel angle (φ0). In comparison, there is no significant difference in the port turning trajectory. The effect of the initial heel angle on the turning performance differs, depending on the port and starboard. We then propose the maneuvering simulation method for the heeled ship. To confirm the validity of the proposed method, the turning simulation results were compared with the free-running model test results. The simulation results are roughly in agreement with the model test results, although the simulation accuracy deteriorates in the case of large heel angles, such as φ0=±20°. Based on the linear theory for maneuverability of the heeled ship (Yasukawa and Hirata, 2013), the heel effects on the turning performance are discussed and summarized as follows: a turning motion appears due to the effect of the heel-induced lateral force and yaw moment which are in proportion to the magnitude of the heel angle, and the course instability increases with the heel angle. Both effects can explain the turning behavior of a heeled ship that is observed in free-running tests.
The tandem offloading operation is widely used in offshore oil and gas production industry. This system can ensure higher allowable weather condition than the side by side operation. But the slewing motion of the shuttle tanker and increasing of hawser tension induced by the motion affect availability of the operation.
Previous studies about the slewing motion mainly treated about the dynamic stability analysis of the single point mooring system. However, specific values concerning the motion and hawser tension are needed when evaluating the availability of the offloading operation. The purpose of this study is to construct simulation model that can estimate range of the slewing motion and the value of hawser tension under complex environment such as wind, wave and current. In addition, understanding on the behavior of the shuttle tanker under this complex environment is also the purpose of this study.
Model test and simulation were conducted to achieve the above purposes. A membrane type LNG carrier was assumed as the shuttle tanker. Production and offloading vessel like a FPSO or FLNG was omitted to simplify the problem. The effect of tug boat’s assistance was simply modeled by using the thruster attached at stern of the carrier.
As the result of the comparison between experimental and numerical results, good coincidence was confirmed in the situation that without astern force and all environmental forces come from head of the carrier.
Parametric studies were conducted using the constructed numerical simulation model. All environmental forces came from head of the LNG carrier and astern force was not generated. The result shows that increasing of wind velocity impact on the hawser tension. And it also shows that increasing of current velocity and hawser length impact on the slewing motion of the LNG carrier. A noteworthy result is that the motion was also increased at no current velocity. The maximum value of the motion exceeded the operational threshold at this condition. These information will be useful when evaluating availability and improving the offloading operation.
The collision between a floating offshore wind turbine (FOWT) and a ship is one of the most significant events that will be appeared in lifecycle of FOWTs, and it is especially important to correctly evaluate it. Although the numerical simulation like the FEM is assumed as the general analysis method of the collision phenomenon, there are several conditions which affect the collision phenomenon between them for example floater type, mooring and so on. In this study, a tank test was carried out to analyze the collision from the viewpoint of the energy balance and quantitatively evaluated the effects of various conditions. As a result of the tank test, it was found that floater type and ship speed have a large effect on the dynamic characteristics, which were likely to affect the energy contributing to deformation. On the other hand, it was clear that catenary mooring and wave have relatively little effect on the collision under certain conditions.
The purpose of this study is to investigate the feasibility of catenary chain mooring with sinker in the shallow waters of 50 to 100 meters depth facing the open ocean. Although the effect of the sinker can be expected to reduce the hardening nonlinearity of the catenary characteristics statically, it is necessary to consider the dynamic effects of sinkers ascending or sitting off the sea floor. As the procedure, we set the environmental design conditions assuming the coastal area of Japan and the semi-submersible type floating offshore wind turbine model, and perform static analysis to investigate the effect of parameters on the mooring characteristics, and then dynamic analysis to judge the strength feasibility of the mooring chains in storm conditions. Catenary theory is applied to the static analysis, and OrcaFlex is used for dynamic analysis to consider the interaction between the floating body, mooring system and sea floor in time domain. The pros and cons of using sinker in “Catenary chain with intermediate sinker type” and “Catenary with Hinged Arm Sinker Mooring type” are discussed to clarify the design considerations.
This paper addresses the issue of a control subject to physical constraints, involving an energy maximization for wave energy converters with a linear generator. Previous approaches to this problem including model predictive control (MPC) have currently some challenges. The conventional control methods, on the other hand, to be able to estimate the performance of wave energy converters are required in practice. In this study, casual constrained optimal control based on the short-term prediction method is proposed. A novel short-term prediction method of both satisfying with maximum output-power and limited heave amplitudes using the energy spectrum method is shown in this paper. Maximizing estimated output power subject to making the estimated amplitude smaller than motion limitation realizes maximizing output power under the heave-motion limitation. To confirm the accuracy of the short-term prediction method and the performance of the proposed control method, the output power is compared with the output power obtained from other control methods through the frequency and time domain simulations in irregular waves. The results show that the short-term prediction method is useful, and the proposed method can maximize output power under the heave-motion limitation.
The onshore bench test system consisting of two linear shaft motors is produced to emulate the wave energy converter (WEC) motion in waves, control force, and power generation under different control strategies. Two conventional control strategies, the resistive load control (RL)and the approximate complex-conjugate control with considering the copper loss (ACL), and an optimal control strategy, that is, the nonlinear model predictive control (NMPC), were implemented to the onshore bench test system. The measured motion of the test system, control force, and power generation are good agreement with the calculated ones in the simulated regular waves conditions. In the simulated irregular waves conditions, the measured motion of the system and control force are reasonably consistent with the calculated ones. The NMPC is able to finish the optimal calculations less than 50 ms in the tested cases in this paper. It is confirmed from the onshore bench tests that the calculation iteration number of the NMPC increased as the control input was approaching to the input constraints.
This paper describes corrosion fatigue properties of KA40 which is high tensile steel for a ship hull approved by Class NK. The structure of a ship hull is in a severe corrosive environment and subjected to cyclic loads due to waves. It is important to grasp the corrosion fatigue properties of materials for the structure of a ship hull. However, there are few reports of corrosion fatigue test results for hull structural materials. Therefore, further data accumulation is required to understand the corrosion fatigue properties of hull structure composed of materials with various mechanical properties. In this study, fatigue tests and fatigue crack propagation tests by using specimens for KA40 were carried out both in air and in synthetic seawater. Furthermore, in comparison of the results of KA40 with the results of KA32, the relationship between the difference in mechanical properties and the corrosion fatigue properties of each material is discussed.
Buckling strength of a non-spherical tank in the partially filled condition, which is one of the severest loading conditions in terms of the buckling design of Moss-type LNG tanks, is investigated both experimentally and theoretically. As the first part of the study, an axisymmetric loading condition for an upright tank position is considered. The buckling collapse test of scaled models is performed under the loading condition similar to the partially filled condition. The non-linear FE analyses of the test models are carried out and the results are compared with the test results to verify the accuracy of the imperfection sensitivity obtained by the FE analysis. A method to estimate the reduction factor of the elastic buckling strength due to initial shape imperfection is proposed by introducing a concept of an equivalent toroidal shell analogy and applying the Hutchinson’s solution to the toroidal shell under combined circumferential compression and meridional tension based on the Koiter’s initial post-buckling theory. A simplified method to estimate the elastoplastic buckling strength is also proposed by applying the Mises’s type plastic correction to the estimated elastic buckling strength. The reasonable accuracy of the estimated elastic and elastoplastic buckling strengths is demonstrated through a comparison with the results of scaled model tests and FE analyses.
This paper is the second of the two companion papers dealing with the buckling strength of a non-spherical tank in the partially filled condition, which is one of the severest loading conditions in terms of the buckling design of Moss-type LNG tanks. A method to estimate the reduction factor of the elastic buckling strength due to initial shape imperfection is proposed based on the Koiter’s initial post-buckling theory, and a simplified method to estimate the elastoplastic buckling strength is proposed in the first report. The reasonable accuracy of the estimated elastic and elastoplastic buckling strengths was demonstrated through a comparison with the results of scaled model tests and FE analyses of model-scale tanks.
In this paper, the applicability of the proposed estimation method is verified for actual scale tanks under axisymmetric load condition in the upright position and also under non-axisymmetric one in the heeled condition. For three types of actual scale tanks with different flatness, the characteristics of buckling mode and strength are examined. It is found that the concept of an equivalent toroidal shell analogy is applicable to the non-axisymmetric load conditions as well as to the axisymmetric cases. The reasonable accuracy of the estimated elastic and elastoplastic buckling strengths is demonstrated through a comparison with the FE analyses of actual scaled tanks.
A series of fatigue tests has been carried out for steel T-shap ed tubular joint specimens with different weld toe radius by Yagi et al.. Through the tests, interesting fracture patterns were found at the chord/brace intersection. The patterns were characterized by the toe radii. In the experimental observation, the surface crack was curved along the crack width and depth. In addition, a twisted fracture surface can be found in a joint with relatively large toe radius. To investigate the fracture behaviors, stress analysis and crack propagation simulation are carried out employing advanced numerical metho ds, i.e., finite element metho d (FEM) and eXtended FEM (X-FEM). Through the experimental and numerical studies, mechanisms of the fatigue fracture phenomena are carefully examined. It is found that change of the deformation around the crack before/after the wall penetration affects the twisted surface generation.
The objective of this work is to obtain the supper long fatigue life of boxing fillet joints through simple repair welding. Firstly, based on the numerical simulation technology, an elongated bead method was developed and practically all position welding conditions suitable in ship yards were tested. Then, a low transformation temperature (LTT) welding wire having optimal phase transformation characteristics for generation of large compressive residual stresses was designed and manufactured, which is capable to all position welding using the low cost CO2 shield gas. Compared with the conventional fatigue life of boxing fillet joints, the new elongated bead method using the newly manufactured LTT flux cored welding wire extended the fatigue life to more than 10 times at the joint by flat welding and 3-8 times at the joints by overhead welding, horizontal welding and vertical welding, respectively.
Aiming at developing underwater battery recharging system, we have been researching on automatic docking of an underwater robot using stereo-vision-based visual servoing and 3D marker. The docking function deems to be an important role not only for battery recharging but also for other advanced applications, such as information transmissions. The authors have proposed a optical docking system and conducted real sea experiments to verify the practicability of the proposed docking system composed of stereo-vision-based 3D pose (position and orientation) realtime measurement system. The docking experiments have forced laboratory members to endure heavy burdens of preparing, conducting, and dismantling the experimental devices at sea, which hinders the efficiency of experiments at real sea. To improve the efficacy, firstly, the authors report that permanent stage for underwater robot experiments has been constructed on a shallow sea. Secondly, we propose a docking station that can adapt and change its docking direction to the current direction, through which the burden of controlling the underwater robot’s heading can be reduced. Thirdly, the effectiveness of the docking station adaptive to the changing current direction has been proven by successful repeated docking experiments in the environment with fluctuating current and turbidity disturbances in real sea. This also has shown that the combined system of the stereo-vision based 3D pose estimation and the current-adaptive docking station can improve the adaptive abilities against current changing disturbances, having shown the practicality of the combined system has been enhanced.
Aiming at developing underwater battery recharging system, we have been researching on automatic docking of an underwater robot using stereo-vision-based visual servoing and 3D marker. The docking function deems to be an important role not only for battery recharging but also for other advanced applications, such as information transmissions. The authors have proposed a optical docking system and conducted real sea experiments to verifythe practicabilityof the proposed system composed of stereo-vision-based 3D pose (position and orientation) realtime measurement system. However, our proposed system sometimes failed the docking operation in the dusk and turbid environment since our recognition method lost the 3D marker in the natural lighting environment that changes every moment. In this paper, therefore, we proposed a new fitness function for improving the robustness against the lighting change. To improve the robustness, firstly, we propose a fitness function composed of color (HSV) and brightness evaluation for overcoming the difficulties to estimate in realtime 3D pose of the underwater vehicle in lighting and turbid varieties. Secondly, we modify the fitness function to improve sensitivity by adding a heuristic rule that increases the evaluation value when the all color balls of the model overlap real 3D marker in the camera images. This approach enables our docking system to apply to the natural lighting environment that changes every moment for increasing success rate of the docking operations. Thirdly, the effectiveness of the proposed fitness function adaptive to the changing lighting environment has been confirmed in the outdoor pool environment. Finally, our proposed docking system has been verified to be robust against lighting environment varieties, by successful repeated docking experiments in turbid environment with lighting condition changes from daytime to sunset in real sea.
An Ocean Thermal Energy Conversion (OTEC) floating plant which is converted from a pre-owned ship may be able to reduce the cost, and thus such a concept has been developed targeting for 100MW-NET power plant. The distinction of the plantship is the attachment of the Cold Water Pipe (CWP) which has 800m length and a diameter of 12m. For discussion of the position keeping system and the CWP, coupled behavior between the plantship and the CWP is analyzed in this study. An analysis model of the plantship is designed from KVLCC2M and the CWP is assumed as made of FRP. The environmental conditions for Indonesia seas are assumed for the extreme analysis. A spread mooring system is considered preferable as a position keeping system. Preliminary designs by several combinations of flexible joint, clump weight, taut mooring system and catenary mooring system are compared on their dynamic behavior by using OrcaFlex. Two kinds of models which are calculated by direct coupled system and only CWP under the forced oscillation obtained by the moored ship without CWP are compared in order to examine those interactions. In addition, a simplified model is proposed, in such a way that comprehend the character of the coupled behavior. As a result of the comparison of these models, the simplified model is generally consistent with the numerical simulation. Also it is found that the interaction is significant and thus should not be ignored around the resonant frequency of the CWP and the slowly-varying motion of the plantship.
Marine container transport is now an indispensable mode of transport for the global economy. For Japan, the import and export of goods by marine containers is extremely important for social and economic activities, too. To improve the transportation effectively it is very useful to develop a methods which enables detailed and quantitative analysis of the impact of time, cost, and other diverse transport attributes on the choice of transport route. On the other hand, research on artificial intelligence (AI) has been progressing rapidly in recent years. There are some fields in which cognition ability by AI exceeds that by human. In this research, we take up the problem of estimating the transportation route of imported and exported marine container cargo which originates and arrives in Japan. We try to develop the route selection method using the Deep Learning which is one of the main technologies in AI research field. This approach takes into account nonlinear and complex relationships among various transport attributes. It is expected that detailed examination of route performance will become possible based on the appropriateness of route for individual cargo.
The probability of Nankai Trough earthquake occurring within 30 years is estimated to be 70% to 80%. Its damage would be extremely extensive and devastating. It is necessary to quickly transport a large amount of relief supplies after the disaster. Japanese government and minicipality are planning transporting disaster relief supplies. The transportation by ship is supposed to be carried out mainly when the truck transportation does not function sufficiently. On the other hand, there is a high possibility that truck transportation capacity will be insufficient because of road network damage and shortage of available number of trucks and so on. It is meaningful to evaluate the transport potential of a ship as a redundant system. In this study, we evaluate the availability potential of coastal ferry/RORO vessels for transporting relief supplies after the possible largest scale of damage in the Nankai Trough earthquake by using AIS and vessel attribution data. As a result, it was found that the ship has high transportation potential even after the Nankai Trough earthquake.
The application of advanced welding technologies is important for improving the efficiency of hull construction processes and reducing production costs. Laser-arc hybrid welding (LAHW) technology, which combines the advantages of laser and normal arc welding, is of high quality and is expected to improve productivity. However, there are some challenges in applying LAHW to the hull construction stage of general merchant ships, because the thickness of the main structural members is thicker than that of passenger ships and high-speed ships.
The present study focused on investigating the following key aspects for involving LAHW in construction stage of commercial vessels, and useful knowledge was obtained. (1) Suitable welding conditions to fabricate the full-penetration tee joint by one-side single-pass welding, (2) Influence of the weld groove cutting process method, (3) Influence of retained primer and oxide film on joint quality, (4) Confirmation in the production by prototyping with long welding length, and (5) Improvement of welding quality by applying the rare earth metal bearing weld wire.