The appearance and propagation of uncertainties in propulsive performance tests in towing tank were investigated. A concept of higher-order uncertainty was introduced, and the idea was applied to the evaluation of uncertainties originated from the model propeller accuracies. A series of performance tests using five stock propellers were carried out, and the influences of manufacturing accuracy and deterioration due to the long-term operation were examined. The details of uncertainty propagations were analyzed, and the indirect propagation mechanisms of higher-order factors were clarified. The evaluated results show the possibility of violence on final result quality by inappropriate quality managements. Utilizing the established evaluation system, an advanced quality control procedure was proposed. By tracing the uncertainty propagations in opposite direction from the ordinary evaluation actions, proposed procedure enables us to feed back the demands on final results to the source factors
The KVLCC2 tanker in fully-loaded condition free to heave and pitch at Froude number 0.142 in three head wave-ship length ratios (0.6, 1.1 and 1.6) is simulated by CFDSHIP-IOWA V4.5. The detailed phenomena of nominal wake behaving in waves are studied by analyzing the velocity and vorticity distribution, and vector field on the propeller plane. The periodic change with phase lag for the volume average axial velocity and circulation are observed. The axial velocity distribution is also decomposed by Fourier analysis. Two vortex systems appear: bilge vortex shedding from the hull body and secondary vortex shedding around the shaft. Based on the results, the propulsion design with higher efficiency, more proper engine margin and lower gas emission are expected.
A submergible fish cage will be useful for avoiding the damage from high waves, attack of polluted water, etc. The current submergible fish cages have the framework for the base of flotation and submersion in the mooring system. However, the vertical position of the fish cage is fixed at certain depth. In the present paper, a fish cage installed at variable depths was proposed for fish to be cultured in suitable environment. The fish cage is moored by four ropes from anchors, and a float is inserted between the fish cage and each anchor. The vertical position of the fish cage can be varied by adjusting the buoyancy of floats. In order to examine the safety of the fish cage, the drag of the fish cage was examined by a towing test in a small water tank, and the mooring system of the fish cage installed at variable depths was tested under various current velocities in Ocean Engineering Basin, Institute of Industrial Science, The University of Tokyo. The model of the fish cage was made according to the similarity law. The scale ratio was 1/100, assuming the actual fish cage for farming tuna with the diameter of 50 m and the depth of 20m. As a result, the drag of the fish cage was proportional to the 1.8th power of the current velocity, which is smaller than 2nd power. This will be attributed to the effects of Reynolds number, which was less than 80 based on the diameter of the twine. In the mooring experiments, the tension of the rope between the float and the anchor increased with increasing current velocity more rapidly than the 1.8th power of the current velocity. The inclination of the fish cage is likely to increase the area which face water current directly. The position of the downstream float went up at the beginning, and then went down with increasing current velocity since the downstream float was pulled by the sinking fish cage. However, the fish cage did not touch to water surface or water bottom in a range of the current experimental conditions. These experimental results provide the benchmark for the validation of a numerical model, which will be developed to design the mooring system as future works.
Ship's speed through water is one of the most important factors in the energy efficiency evaluations of ships in operation. Recently ship performance monitoring systems are being installed in the wide variety of ships for efficient operations and ship's speed through water is usually measured using the Doppler speed log system. The measuring accuracy of the Doppler speed log systems, however, may not be sufficient for the evaluations of various energy-saving measures since the measuring locations are 3 to 20 meters below the bow bottom of ships where the water speed may be affected by the local disturbances of a ship hull. In this study, in order to investigate the properties of velocity distributions under bow bottom of ships, the flow fields of full scale ships are simulated using CFD approach. The computed results for a blunt ship and a slender ship in laden and ballast conditions with various ship speeds are presented. It is shown that the properties of the disturbance velocity due to a ship hull depend on ship's draft and ship speeds differently with a ship type.
The flow under bow bottom of ships was calculated using CFD methods and characteristics of flow-speed distribution were analysed. In the study, it was confirmed that flow speed under the bow bottom of a ship changes according to water depth by disturbance of the sailing ship and a range of the field where the disturbance effects depends on types of ships, draft and ship speed. In this report, a developed measuring system named "multi-layered relative velocity meter" is introduced to measure distributions of relative flow speed along water depth. After testing a function of the measuring system on a test boat, the measuring system was installed on a PCC and the relative flow speed at each layer was measured during a voyage. The obtained data were evaluated by comparing with the result of CFD analysis and it was confirmed that relative flow speed measured on the PCC had a similar distribution with the result of CFD analysis. Considering both the CFD analysis result and the onboard measured data, finally a new method to improve measurement accuracy of ship's speed through water is proposed.
This study takes up the Wing-In-Ground effect (WIG) flying over the still water surface and addresses its steady aerodynamic properties and pressure fields focusing on the free-surface interaction effect and the influence of wake deformation. The boundary element methods both in the steady state and in the time domain are applied to two different wings in the ground effect proposing a new method which can take the viscous drag into account. The wind tunnel test and the towing test in the water are also carried out in order to validate the numerical calculations. Especially the measurements on pressure distributions over the ground are the first trial in the research field on WIG and experimental data itself is valuable. Comparisons between computed and measured results show good agreement not only on the aerodynamics but also on the pressure fields.
The WIG (Wing-In-Ground effect) is an unconventional aircraft and /or a ship which is operated in a seaway utilizing the high lift-to-drag ratio. When the WIG flies over the waves, unsteady aerodynamic forces induced by the waves act on the airframe and cause the unsteady change of the pitching moment with the movement of the center of pressure. They have a possibility to cause the instability of the motion of the airframe. Therefore, the characteristics of unsteady aerodynamic forces become an important topic related to WIG's flight stability. This paper takes up the WIG flying with heave motion in the ground effect and makes clear the property of unsteady aerodynamic forces and moment. The Time-domain boundary element method is employed for analyzing the problem and the validation of the obtained results are also carried out by comparing with experimental results. In the experiment, not only the measurement of unsteady forces and moment but also the measurement of the unsteady pressure field over the ground are attempted by applying the technique to measure the unsteady waves of ships. The later one is the first trial in this research field and obtained experimental data itself is valuable from the academic point of view.
When the wings fly near the ground/water surface, a phenomenon so called ‘ground effect’ appears in which the liftto-drag ratio drastically improved. Although a numerous studies focused on the ground effect have been presented so far, there are less experimental results of pressure distribution on the surface of 3-D wings in the ground effect. In this study, authors develop a measurement system using the wind tunnel to measure both pressure distributions on the wing surface and aerodynamic forces at the same time, and carry out the wind tunnel test for a rectangular wing and a main wing with end-plates which is a part of a canard-configuration WIG designed by the authors. Wing models are accurately manufactured by using the 3-D printer and the NC machine so that the pressures at 126 points on the wing surface are measured together with aerodynamic forces. Experiments are performed not only for the cruising condition but also for the heeling condition and measured results are compared with the theoretical calculation results. It is confirmed that the theoretical calculations in cruising condition show good agreements with measured pressure and aerodynamic forces. Additionally, it is suggested that the measured asymmetric aerodynamic forces in heeling condition may contain the forces acting on the strut, which is induced by the aerodynamic interaction effect between the wing models and the strut.
The frequency-domain Rankine panel method is one of the practical tool to estimate ship seakeeping. Although several numerical infinite conditions which are essential in the method have been proposed by many researchers around 1990, there still remains an unsettled problem on how to satisfy the infinite condition accurately in low-speed and/or low-frequency range where τ = Uωe/g takes small values. Recently Das et al.10) and Yuan et al.11) proposed a Sommerfeld-type radiation condition for that problem from a concept of the Doppler shift on ring waves. In this paper, we originally derive a general form of the Sommerfeld-type radiation condition applying the asymptotic wave theory. The result indicates irrational points of their method and leads us to a more accurate method. The proposed method is validated by solving the wave field generated by a point source and by comparing with other methods with different numerical infinite conditions. Then we confirm that the present method is the most accurate and also effective for the actual ship seakeeping problem.
The frequency-domain Rankine panel methods, which were developed by many researchers around 1990, have been applied to the seakeeping estimation of actual ships. However there still remains an unsettled problem on the numerical treatment of the infinite condition in the low-speed/low-frequency range and it prevents us using the method in the wide frequency region such as in oblique waves. In this paper, we employ two methods which are expected to settle the problem and make the Rankine panel method more useful. One is to introduce the Rayleigh’s artificial friction coefficient and another is to impose the juncture condition on the artificial control surface far from the ship. The former is originated in Yasukawa’s method2) and the later is presented by one of the authors8) extending the method proposed by Das et al.6) and Yuan et al.7) Experiments are also carried out in following wave condition and for zero forward-speed case, and compared with numerical results obtained by two methods. It is confirmed that both methods give accurate estimations in the low-speed/low-frequency range and the later method is especially better in accuracy.
In this study, the proposal of Floating Large size Tsunami shelter (FLTS) and the analysis of motion of FLTS when the shelter is attacked tsunami flow using the experiment and the numerical simulation by moving particle semi-implicit method (MPS).FLTS is an evacuation facility without seeking refuge to a hill and the tower. Because an evacuation place surface by itself buoyancy.
FLTS will solve some critical problems. For example, improve to seek for refuge of people of requiring assistance during a disaster, protect daily and medical goods and improve quality of life level after the disaster. To realize such services, we evaluated the validity and safety of a floating large size tsunami shelter using the mooring dolphin by water tank experiment and numerical simulation. As a result, enough performance was recognized. However, it became clear about the numerical simulation that improvement was more necessary.
Twin-propeller ships are widely used for from high-speed coastguard ships to low-speed wide beam cargo ships. One of the authors1) firstly proposed the simulation method for twin propeller and twin rudder ships by means of applying MMG’s mathematical model. After then, some experimental researches2)～4) have been done. However, the manoeuvring prediction for these ships is not easy because propeller and rudder forces are different between port and starboard sides, which introduce the complicated yaw moment. Moreover, its mechanism is very complicated and the hydrodynamic data are also limited comparing with conventional single propeller and single rudder ships.
In this paper, the authors have investigated again with the experimental data and propose the equivalent single propeller and single rudder model for twin propeller and twin rudder ships. However, the characteristic of thrust coefficient as well as propeller load factor should not be changed by the use of above equivalent model since they strongly affect on the characteristic of rudder force. In order to realize this, the propeller diameter is assumed to be √2 times larger, propeller rps 1√2 times larger, and rudder area 2 times larger, respectively. This assumption can be also applied to the twin propeller and single rudder ship.
This procedure makes very simple and easier prediction since the usual mathematical model as well as the hydrodynamic database for single propeller and single rudder ships can be used. In order to validate the above method, simulated results are compared with those of the original mathematical model as well as the free-running model tests. As the result, it is found that this simple mathematical model becomes very useful for the simulation of twin-propeller ships.
Ship manoeuvrability has been generally predicted based on 3-DOF mathematical model. However, medium-high speed ships such as containerships and passenger ferries make sometimes a large roll motion by the steering, which more or less affects on the ship manoeuvrability. Although a lot of researches have been done about the effect of roll motion, the mathematical models of hydrodynamic forces have been proposed individually putting the additional terms of roll angle into the conventional 3-DOF mathematical model. This makes the difficulty in the prediction of manoeuvrability.
In this paper, the captive model tests have been performed with a container ship and a passenger ferry. In order to grasp the characteristics of the effect of roll angle on the conventional 3-DOF mathematical model, CMT(Circular Motion Test) have been carried out with setting a constant roll angles. And then, the effects of roll angle on the hydrodynamic derivatives have been extracted. From this analysis, the suitable 4-DOF mathematical model has been introduced by means of eliminating the less contributing terms of the roll effect. For the validation of the proposed mathematical model, free running model tests were carried out with several GM and ship’s speed since the roll angle during a manoeuvring is affected by them. After comparing simulated results by the proposed 4-DOF mathematical model and the results of free running model test, it is found that the effect of GM and ship’s speed on turning trajectories or the overshoot of Z-test can be well predicted by the proposed 4-DOF mathematical model.
Added resistance due to waves is one of the major components for the evaluation of ship performance in actual seas, and it is important to be calculated accurately. Ocean waves are characterized as short crested irregular waves which consist of waves with various frequency and directional distribution. Added resistance in short crested irregular waves is usually evaluated by the superposition of directional wave spectrum and frequency response of added resistance in regular waves. Various evaluation method for added resistance in regular waves has been confirmed by tank tests so far. On the other hand, estimation method for added resistance in irregular waves has rarely been verified by tank tests since a given restoring force to a ship model affects to the long-period variation in irregular waves. In this paper, tank tests in long crested irregular waves for VLCC model and PCC model have been conducted, and added resistance in irregular waves has been measured and compared with the estimated results by the Newman's approximation. As a result, it has been shown that the measured long-period variation of added resistance in long-crested irregular waves can be expressed by the Newman's approximation and that the estimated results of averaged added resistance in long-crested irregular waves by the Newman's approximation with the consideration of wave height effect explained the experimental results better than that without the consideration of wave height effect.
This paper is the first of two companion papers dealing with ultimate longitudinal strength analysis of a container ship considering local loads effects. The purpose of this paper is to clarify the effect of bottom local loads on the longitudinal bending collapse behavior of a large container ship. An important problem is a bending deformation of the double bottom due to upward bottom local loads that may reduce the hull girder ultimate strength.
A 1/2+1+1/2 hold model of a container ship is analyzed using nonlinear implicit finite element method. The analysis shows two major effects of the bottom local loads on the ultimate hull girder strength; i.e. the buckling of the outer bottom plating is accelerated due to the increased longitudinal compressive strain, while the effectiveness of the inner bottom plating that is in the tension side of local bottom bending is significantly decreased.
This paper is the second of the two companion papers dealing with ultimate longitudinal strength analysis of container ships considering the effects of bottom local loads. The major causes of reduction of ultimate hull girder strength due to local loads were discussed based on the finite element analysis of a hold model in the Part 1. The objective of this paper is to develop a practical method of progressive collapse analysis of a hull girder subjected to combined longitudinal bending and bottom local loads.
Smith’s method is widely used to estimate the hull girder ultimate strength under pure bending. It however cannot consider the local deformations such as double-bottom bending because it assumes that a hull-girder cross section remains plane. A new methodology is therefore proposed, which idealizes the double bottom structures as a plane grillage consisting of longitudinal and transverse beams and extending over a hold length in the longitudinal direction. The rest part of a hull-girder cross section, such as a ship side and a bilge, is modeled as a unit beam and connected with the grillage model along the bilge parts using multi-point constraints. The calculation of the stiffness of longitudinal beam elements is based on the original Smith’s method, including the definition of plate and stiffened-panel elements and the application of a concept of average stress-average strain relationship for each element. The proposed model may be called an “extended Smith’s method”. The progressive collapse behaviors and hull girder ultimate strength predicted by the extended Smith’s method are compared with the result of nonlinear finite element analysis.
Coating deterioration behavior of water ballast tanks in a very large ore carrier using a corrosion resistant steel (CRS) was investigated by analyzing the photographs which the same parts were taken in 4.8 and 7.3 years after shipbuilding. The investigated components were upper decks and edges of upper deck longitudinal members. Coating deterioration from 4.8 to 7.3 years was classified into the growth of single deterioration, combination and new generation. The ratio of new generation on the edge in 7.3 years was less than that on the upper deck, and decreased very much compared to 4.8 years. Deterioration area on the upper deck and deterioration length on the edge of the longitudinal member using the CRS were suppressed to about 40% and 70% of those using a conventional steel, respectively. One dimensional coating deterioration rates of the CRS were reduced to about 70% of those of the conventional steel on the edge part of the longitudinal member and the upper deck. This ratio of the CRS to the conventional steel in the hull components agrees with those in a laboratory corrosion test and an onboard doubling plate test. From the change of coating deterioration length on the edge until 7.3 years, the average recoating life with the CRS is considered to be 25 years or more.
Fatigue strength of a cruciform joint subjected to bending stress is studied under various combinations of main plate thicknesses and attached plate thicknesses, using FE analysis and Siebel diagram. Conventional approach to deal with the fatigue strength under bending used to be to multiply the stress range by the bending correction factor of 0.8, and apply the typical S-N curve obtained from axial fatigue test. However, in this study, it was revealed that axial force and bending have different sensitivity of main plate thickness and attached plate thickness to the fatigue strength, and therefore, actual bending correction factor depends on the combination of the thicknesses. As a result, the authors proposed an approximate formula of bending correction coefficient covering all the combinations with an appropriate safety margin.
The importance of deep ocean surveying and monitoring is increasing because of the change of situation of energy, oceanicresources and CCS (Carbon Capture and Storage) under the seabed. In recent years, AUVs (Autonomous Underwater Vehicles) have been operated for deep ocean scientific research. However, their long cruising is limited because of their battery performance. Although deep tow systems will solve this problem, the towing speed of current systems is not fast, and sizable motions are induced by the mother ship. We therefore 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. Model experiments were carried out in a towing tank and simulated results were compared with the experimental ones. Interesting results of depth and motion control are shown in this paper.
In recent years it has become evident that ocean data in time and space is required to make predictions of environmental changes on earth. As one method of acquiring data, we propose a virtual mooring system using an underwater glider, and have developed a prototype vehicle called “BOOMERANG” which can glide back and forth between the sea surface and the seabed collecting ocean data. In order to carry out oceanographic observations, the performance of the vehicle was re-improved. This paper presents details of re-modifications of the vehicle, and oceanographic observations carried out at the west sea-area of the Goto islands in Nagasaki Prefecture and in the East China Sea.
This paper presents the results on R&D of a bottom-mounted tidal current turbine system including concept and preliminary designs and element technology developments. On the design of the turbine blade, CFD simulation and tank experiment are carried out to evaluate the hydrodynamic performance. A full scale blade test is performed to prove that the strength of the blade is sufficient. On the design of the drivetrain, appropriate devices are selected in order to be operated safely. Considering the work vessels that can be used in Japan, special effort is paid to reduce the weight of the nacelle by increasing the voltage of the generator and moving the converter from the nacelle to the land. Performance and endurance tests are carried out on the seals for main shaft, pitch shaft and yaw shaft and it is confirmed that the leakage rate and the material durability satisfy the design criteria. The effect of biofouling on the performance of the heat exchanger is experimentally investigated, showing that the cooling capacity drops to 60% for a bare device, and 30% for that with anti-fouling coating.
The lack of nearby offshore field activities has led to disadvantages for the Japanese offshore technology R&D. Offshore technology is a field based technology comprised of many discipline and phase. Thus, the consideration of both deep specialization and the broad system integration are necessary. We define an important aspect of offshore technology as “Integrated Ocean Engineering”, and analyzed elements that are essential to achieve its R&D. We define six elements that need to be implemented in the research activity. From the system science perspective, needs identification, connection of basic science and applied research, and integration of technology for implementation must be considered. Considering the unique features of offshore technology, basic science in ocean engineering, demonstration in actual ocean and expectation to future market are also important elements. Existing R&D activities in Japan do not incorporate these elements together, mainly due to the lack of strong driving force. We propose a new structure of organizing research, namely co-creation platform.
Remotely Operated vehicle, so called ROV is widely used for the seafloor explorations. The most of the existing ROVs are the hover type ones which is suspended in the sea. However, the hover type ROV is not suitable for the operations such as moving along the seafloor, stopping at a certain location on the seafloor and doing the heavy works like digging the seafloor. The crawler driven ROV is considered to be an applicable system for those operations. It is known that the crawler driven ROV is easy to run in unsteady attitude due to the buoyancy and the hydrodynamic force acting on the ROV and sometimes causes a turn over. Therefore, we have to know the precise moving capability to design the crawler ROV. Additionally, as the ROV is under the restrictions of the cable, it is necessary to know the movable area of the crawler ROV considering the cable tension. The authors have developed a simple method to estimate the movable area of the ROV based on the simple statics. The cable of the crawler driven ROV sometimes touches the seafloor. Therefore, the cable tension should be calculated considering the contact of the cable with the seafloor. However, in the previous research, the effects of the cable contact with the seafloor was not taken into accounted. The present study shows a numerical procedure to estimate the cable tension considering the contact of the cable with the seafloor and also the bending stiffness of the cable. Adapting this procedure, a method to estimate the movable area of the crawler ROV is shown. The model experiments are conducted to validate this method.
Preventing collision accidents is essential issue in safety navigation at sea. However, collision accidents still account for high proportion of all marine accidents. It is effective to alert risk of collision to operators. However, the timing of alert needs to match the operators’ sense, otherwise they may feel it somewhat strange or doubtful.
The purpose of this study is to evaluate the collision risk quantitatively between two ships in order to recommend initiating the collision avoidance manoeuvre to operators. The authors have analyzed the actual manoeuvres extracted from measured AIS data. Then, a new index to quantify the collision risk between two ships in a coastal sea area is proposed.
Installation of new production facilities in a shipyard can trigger product manufacturing error, causing rework as well as decreased manufacturing speed and increased cost. Features of the shipyard and the workers’ skill levels combine to impact the manufacturing error. In this study, a method to evaluate installation effect of production facilities in the shipbuilding process considering both the shipyard’s features and the worker’s skill is proposed. This method evaluates the production process defined by integrated product, workflow and organization models using a simulation with rework including manufacturing error of the target product. In the case study on a production process of curved blocks, it is quantitatively clarified that installation of an accuracy evaluation system for curved shell plates by laser scanner to heat bending process can reduce the duration and cost of the process. Under the condition where production space is limited in a shipyard, the proposed system installation leads to greater effectiveness than under the condition where the space is readily available. Furthermore, it is suggested that the proposed system is more effective if the skill level of the workers is low.
Generally, understanding shape deformation and the precise amount of three dimensional deformation at manufacturing processes is important in accuracy control. Since ships are very large products, three dimensional deformation measurements are very difficult to conduct during ship manufacturing process, and for that reason only few cases have been reported. In this study, a hatch cover is selected as a target structure and the three dimensional welding deformations observed in each manufacturing process.
This study examined the measurement accuracy of the total station in the actual shipyard and impact factor to the measurement accuracy are examined. Moreover, we conducted tracking investigations on the deformation of the hatch cover due to the progress of the process, and the accuracy improvement methods are examined.
For the cruciform welded joints between corrugated bulkhead of Duplex stainless steel plate and inner bottom of stainless (SUS) clad steel plate under biaxial tensile loading condition, numerical analysis of finite element method were performed with changing the loading condition, fillet size and welding condition of penetration. The stress concentration on each weld toe of the cruciform welded joints was evaluated. As results, the geometrical symmetry between upper and lower side welded joint shape on inner bottom plate of cruciform welded joint is important to prevent high stress concentration under biaxial tensile loading condition and partial penetration weld is effective to suppress stress concentration from comparison with fillet weld.