Distribution characteristics of wave occurrence probability influence long-term prediction results that have estimated by using short-term ship responses and a wave scattering table. Because the observation volume of data and methods of analyzing wave data in each scattering table of significant wave height and mean wave period are different, long-term prediction results depend on the scattering table to apply. Especially, the difference of the frequency count of higher wave height in the scattering table strongly affects the results.
When a ship encounters a severe wave condition, the ship speed decreases (nominal speed loss). When the wave condition becomes more severe, the captain judges the deceleration and the change of the course to avoid the danger of the damage of the ship. A long-term prediction method that has modeled the nominal speed loss and the ship handling is necessary to obtain a realistic prediction value.
In this paper, the characteristics of a higher wave height for variety of scattering tables are investigated. Additionally, this paper proposes a long-term prediction method that models a situation when a ship is operated in such a way so as to avoid higher waves. Finally, this paper investigates the influence on a long-term prediction when the occurrence probability of a higher wave height is changed.

View full abstract

Tank tests are conducted to capture the ship maneuverability using ship models with three different block coefficients C_b =0.81, 0.84 and 0.87 of the same principal dimension ratio such as L/B and B/d. First, free-running model tests are carried out to capture the maneuvering motions in a square tank, and next captive model tests are conducted to capture the maneuvering force characteristics using the same ship models. With increase of C_b, turning advance (A_D) and tactical diameter (D_T) become small in both full and ballast load conditions, and the values of A_D and D_T satisfy the IMO regulations with sufficient margin. On the other hand, overshoot angles (OSAs) of 10/10 zig-zag maneuvers become large and the course stability becomes worse with increase of C_b. The value of 1st OSA for a ship with C_b =0.87 is on the critical line of the IMO regulation. Reason why the course stability becomes worse with increase of C_b is mainly due to increase of the absolute values of N′_υ and Y′_γ - m′ - m′_χ terms. We investigate the reason why the absolute value of N′_υ typically increases with C_b by CFD computations. With increase of C_b, hull lateral force per unit ship length (ΔY) in oblique towing condition becomes large at fore part and just behind the midship position, but small at stern part. As the result, the hull lateral forces (Y) which is obtained by the integration of ΔY become almost the same for three ships due to the cancellation of plus and minus ΔY at fore and stern parts. On the contrary, the yaw moment (N), which is obtained by the integration of product of ΔY and the longitudinal position χ increases with increase of C_b. This feature comes from the effect of negative pressure on the hull surface at face side of the stern part.

View full abstract

Hull girder stresses caused by the whipping vibration increases as ship size becomes bigger, and it is therefore concerned that design fatigue life of such large ship might be unconservative. In this study, in order to clarify effects of superposed elastic vibrations on linear wave loads in the fatigue life estimation. Using created time history of stress, numerical simulations for fatigue strength of a post-panamax container ship has been conducted. Random stress time history is created using a storm-model, and a high frequency stress model is proposed to estimate effects of the whipping vibration. Two kinds of evaluation methods are adopted. One is fatigue crack propagation analysis using fracture mechanics taking account of the nonlinear retardation phenomena. The other is fatigue damage factor based on Miner's Law.
Results of the fatigue strength analysis in a period of the maximum storm condition for a ship's life in the North Atlantic show that the fatigue crack length obtained by the fatigue crack propagation analysis has a similar trend as the damage factor by Miner’s Law. With regard to the effect on the fatigue strength of the whipping, different treatments of high frequency stress to extract low frequency time history give different results. If the low frequency time-history (LF) is adopted, the difference is remarkable, the fatigue damage of RAW become approximately 50%-100% greater than LF. If the envelope time-history (ENV) is adopted, the difference is relatively small, e.g. 0.1-2.0%.

View full abstract

This paper presents a numerical simulation for fatigue crack propagation based on improved strip yield model, which enables the strain hardening effect of materials to consider. One-dimensional bar elements, which consist of the strain hardening materials, are plugged up the gap corresponding to the fictitious crack opening displacement in the plastic zone to describe the role of crack wake generated over fatigue crack surfaces. Validity of this numerical model under monotonic loading, unloading and reloading conditions was verified by comparison of the COD profile and plastic zones size with elastic-plastic FE analysis. Proposed model was implemented into the numerical simulation of fatigue crack propagation considering the crack opening / closing. Proposed simulation of fatigue crack propagation, which enables the strain hardening effect of materials, was validated by comparing fatigue crack propagation histories obtained by the numerical simulations with measured ones.

View full abstract

In order to investigate the influence of strength mismatching on brittle fracture strength for large heat-input welded joints, a series of tension tests was conducted using surface notched specimens. Furthermore, notched round-bar tension tests were carried out for simulated bond material (homogeneous material) which was base metal heat-treated on the same condition as the weld bond. Thereby, cleavage fracture strength of the bond material without the influence of strength mismatching was obtained. The point of fracture initiation was identified by SEM observation and, the critical maximum principal stress at the point (σ1_c) was calculated by FE analysis for each specimen. σ1_c values of surface notched specimens were found to be distributed in the range of 900-1,400N/mm². This result indicates that the strength mismatching does not significantly affect σ1_c. In addition, distribution of σ1_c for the notched round-bar specimens is in good agreement with that for surface notched specimens. This indicates that the cleavage fracture of the weld bond is a dominant fracture on the welded joints. Peak values of maximum principal stress σ1^p and equivalent plastic strain ε_p^p increase with increase in the strength mismatching ratio defined as the ratio of yield strength of weld metal to that of HAZ (σ_Y^WM/σ_Y^HAZ). The increase in σ1^p causes the decrease in fracture strength of the weld joints. ε_p concentration at notch tip of HAZ side is due to the lower yield strength of HAZ. Except for small-scale and large-scale yielding conditions, the influence of ε_p distribution caused by strength mismatching on fracture strength also needs to be considered.

View full abstract

Ring stiffened cylinder is generally used as main structural members of offshore and submergible structures. In designing of such structures, it is very important to estimate the buckling strength of ring stiffened cylinder under external pressure accurately. The buckling mode of ring stiffened cylinder under external pressure can be divided into the following four modes. The first is the general buckling mode which occurs over the entire length of the cylinder involving ring stiffeners. The second is the shell buckling mode which is the buckling of cylindrical shell between the ring stiffeners. The third is the frame tripping mode of ring stiffener. The fourth is the local buckling mode of ring stiffener’s web or face. The purpose of this research is to propose the simplified formula for predicting the elastic shell buckling strength included the influence of stress ratio of shell and ring stiffener, and to propose the simplified formula for predicting the elastic ring frame tripping strength combined the proposed formula of the elastic shell buckling strength. The elastic shell buckling strength and the elastic ring frame tripping strength predicted by these simplified formulas are compared with the result of finite element analysis by changing various parameters, such as the scantling of ring stiffener and shell thickness. From the comparison of these results, accuracy of these simplified formulas is examined.

View full abstract

In the previous report (report1)¹⁾, it became clear that introduction of the concept of dissipated fracture surface energy (G_c) during each crack propagation step is needed for good correspondence between numerical crack propagation analysis (3D-FEM dynamic analysis using local stress criterion²⁾) and experiment of small CCA (Compact Crack Arrest) specimen. In this paper, G_c concept is applied to ultra wide tensile problem which is representative for much major structure. Hypothetical G_c are arranged as the calculated crack propagation rate shows experimentally obtained range in three stress level conditions. As a result, Stress field around the running crack tip is increased as crack propagates and saturated at the point of K_nom/σY･√ t=4. This saturation tendency is good correspondence with famous K_eff concept³⁾ Next, in order to understand the intrinsic meaning of G_c and the change of G_c subjected to driving force, brittle fracture appearance were observed by small specimens fractures under various stress field. It is revealed that under high driving force condition roughness of fracture surface tends to increase and many sub-cracks are observed. These observations are qualitatively supporting the appropriateness of hypothesis of G_c.

View full abstract

This paper proposes a high-precision real-time correction sounding system using Micro Electro Mechanical Systems (MEMS) inertial measurement units, which can correct the effective depth from the sea surface to the seabed in real-time. The advantages of the proposed system are as follows: (1) The system outperforms a single-beam sounder available on the market in that the range of depth errors caused by the swing of a research vessel is about 2%: (2) Since the proposed system is small compared to the existing attitude and heading reference system, it can be employed for unmanned surface vehicle, remotely operated underwater vehicle, autonomous underwater vehicle, and so on. Experimental results show that an attitude, heading and depth measured by the developed system are within the acceptable range, and the system can give a stabilized attitude, heading and depth information for a long time.

View full abstract

This paper aims to develop further our previous work, which is concerned with a just-in-time scheduling problem of block-assembly lines lines conveyorized in a shipyard. The blocks fabricated are stocked for pre-fitting and painting works and they are provided to pre-election area. We proposed a new method called as Max-Plus Method to solve the problem. In some cases, however, this method can’t satisfy a synchronization constraint that pre-subblocks must be fabricated before the specific subblocks are fabricated. Also it can’t manage the stockyard very well except the utilization of round-robin type. In the paper, another new method called as Heap Method is proposed, which is inspired from the Heap Model in the field of Max-Plus algebra. It is shown that the above unsolved problems can be solved.

View full abstract

Effectiveness of fluorescent ferric ion indicator for under film corrosion detection of coated steel panels in water ballast tank is examined. (1S)-3'-(diethylamino)-N,N-diethyl-3-oxo-3',9a'-dihydro-3H-spiro[isobenzofuran-1,9'-xanthen]-6'-amine oxide “RhoNox-1” synthesized by Hirayama et al. (2013) is chosen as Fe2+ indicator. Under film corrosion sensing semi-transparent modified epoxy films which contains RhoNox-1 are prepared. SS400 steel test panels are blasted and the prepared semi-transparent Fe2+ sensing paints are applied. Some panels are scribed after coating, and NaCl solution is dropped onto the steel surface before coating in other panels. These panels are subject to immersion tests in NaCl solution and cyclic corrosion tests by using artificial seawater. Changes in fluorescence are observed during these tests. As results, followings are found: 1) RhoNox-1 is not responsive to amine-based hardener, and "prematurely fluorescent” can be prevented; 2) RhoNox-1 is functioning as an early detection indicator for steel corrosion; 3) Steel's anode reactions in underfilm corrosion that progressed during the cyclic corrosion tests had been detected clearly by using the developed Fe2+ sensing paint. 4) It is enough to apply 1wt% RhoNox-1 containing paint as a mist coat for nondestructive monitoring of anode reaction when semi-transparent paint used in this study is applied as a top coat.

View full abstract