Dispersion of spilled oil by a cavitating jet at sea

This article presents the possibility of using a high-speed water jet for the dispersion of spilled oil at sea. Laboratory experiments showed that a high-speed water jet is very effective in dispersing heavy oil. Two methods were examined and compared using freshwater. The first method was to disperse an oil layer on the water surface using a cavitating jet ejected upward at an angle of 45°. A target plate was placed at the interface of the water and oil layer to enhance the collapse of cavitating bubbles. The second method was to eject a high-speed water jet vertically downward from the air. A guide plate with a guide hole was placed under the oil layer with no target plate as such. The second method showed an improved dispersion ability and thus its effectiveness was examined using an oil layer on seawater. The dispersion of oil was better in seawater than in freshwater, probably because seawater contains natural surfactants.     

Study on the preciseness of hot spot stress of web-stiffened cruciform welded joints derived from shell finite element analyses

A new simple and accurate shell FE-based structural Hot Spot Stress (HSS) determination method for web-stiffened cruciform joints has been proposed. Local stress of welded joints in full-scale bulk carrier (BC)’s lower stool models subject to bending and pressure loadings are examined. HSSs determined by the proposed method are compared with those derived by Lotsberg’s method and the conventional 0.5t-1.5t extrapolation. As results, following are found:

(1)

The local stress of full-scale BC lower stool models with various stool angle and plate thickness can be calculated accurately solely from shell FE results by means of the proposed method.

(2)

HSSs of welded joints in full-scale BC lower stool models subject to bending and pressure loadings derived by the proposed method show good agreement with the target HSSs determined from fine solid models. This demonstrates the validity of the proposed method for actual ship structures under the real load.

(3)

The excessive safety allowance of HSS determined by the conventional 0.5t-1.5t extrapolation can be reduced substantially by adopting Lotsberg’s method or our proposal. The proposed HSS determination method gives more accurate estimates compared to Lotsberg’s method under the conditions chosen, and the application range of the proposed method is equal to or wider than Lotsberg’s method.

     

The effects of free surface and end cell on flow around a finite circular cylinder with low aspect ratio

Vortex-induced motion is an oscillatory phenomenon which occurs to a floating body with low aspect ratio. The basic phenomenological study about the effects of free surface and end cell on flow around a finite fixed circular cylinder was investigated in this study using particle image velocimetry and hydrodynamic force measurement. It was found from the former experiment that the wake of the cylinder is influenced by the both end cell and free surface. Blowup and back flow are generated from the end cell, and their effects are suspended by free surface. The result of hydrodynamic force measurement showed the effect of Reynolds number, Froude number, and the aspect ratio of the floating body on the hydrodynamic force. Fluctuating components of hydrodynamic coefficients decrease for increasing Reynolds number, Froude number, and the aspect ratio. On the other hand, the mean drag coefficient increases as Froude number increases and decreases as the aspect ratio increases. The interpretation to these results was discussed in comparison with flow structures observed in the experiment. In addition, it was found that the effect of Reynolds number on the mean drag coefficient changes at different aspect ratios. A possible interpretation to this phenomenon was proposed.     

Visualization of a hole and accident preventive measures based on the Swiss cheese model developed by risk management and process approach

It is important to find the characteristics of holes theorized in the Swiss cheese model created by Reason when preventive measures are taken. Reason does not explain where the holes open and how the holes move and shut. The objectives of this research are to find the characteristics of a hole, visualize the development of the hole and propose accident preventive measures to shut the hole. I investigated four real cases of three types of marine accidents using risk management and process approach. The focus of the study was on the processes both of the Safety Management System and risk management imbedded in the procedures at the time of the accidents. I found the following results. A hole in a defensive layer opened during the process of plan or check imbedded in the Safety Management System and moved through the Plan-Do-Check-Act (PDCA) cycle. The hole in the defensive layer moved or affected the development of the hole in another defensive layer at the local workplace. The hole in another defensive layer at the local workplace opened during the process of risk identification or risk analysis and moved through risk management process. Shutting the hole was drawn in a structured manner by the method of risk reduction. With a focus on process, the development of holes was visualized. These findings indicate that the characteristics of holes and their visualization can show the location where efforts toward preventive measures should be directed effectively and efficiently.     

Relationship between latent conditions and the characteristics of holes in marine accidents based on the Swiss cheese model

In his Swiss cheese model, Reason states that no one can foresee all possible accident scenarios. If holes can be visualized and the relationship between holes and latent conditions can become clear, then it is possible to control the occurrence of holes. The objective of this research is to determine the relationship between latent conditions and the characteristics of holes. In this study, 84 serious marine accidents, divided into six types, were analyzed. Furthermore, the safety management system (SMS) in organizations and risk management at local workplaces were considered as defensive layers, and 10 latent conditions were defined by modifying the software–hardware–environment–liveware (SHEL) model. The following results were found. Holes in the SMS defensive layer tend to arise during the early stages of the plan–do–check–act (PDCA) cycle, except for cases involving sinking. Holes in the defensive layer of risk management tend to arise during the early stages of the risk management process in cases involving collisions, occupational casualties, fire, or explosion. The most frequent latent condition was an inadequate condition of operators, but was not necessarily the same for different types of accidents. These findings indicate that the locations of and reasons for the opening of holes can be determined. By applying a method for closing holes in combination with the findings of this study, accidents can be systematically prevented.     

Fatigue strength improvement for ship structures by Ultrasonic Peening

Ultrasonic Peening has attracted recent attention as a fatigue strength improvement method. The main features of Ultrasonic Peening are generation of compressive residual stress and smoothing of weld toe radius. However, for ship structures, various loads act on the hull structure over a ship??s life, including the construction period, and the compressive stress field of structural members generated by Ultrasonic Peening may change. In order to verify the technique's benefits to ship structures, the influence of load history on the improvement effects on fatigue strength by Ultrasonic Peening needs to be clarified. In this paper, the improvement effects on fatigue strength by Ultrasonic Peening for welding joints were confirmed by experiments with several joint type specimens. In addition, fatigue tests modeling launching, when the stress conditions of a ship's structure changes significantly, were carried out in order to clarify the influence of load history in a ship??s life. Consequently, some cases that have the possibility of decreasing or increasing the improvement effects on fatigue strength by Ultrasonic Peening were clarified, and some efficient methods of Ultrasonic Peening for ship structures were suggested.