Yoshitaka Ogawa

Water Surface Impact Load Acts on Bulbous Bow of Ships

As a part of the project study concerning on ship structural design for oil spill prevention by collision, optimal design of flexible bulbous bow structure is studied to absorb kinetic energy of the colliding ship. Such a flexible bulbous bow is named as buffer bow. To formulate design criteria of the buffer bow, the hydrodynamic impact forces acts on the bulbous bow was studied theoretically and experimentally. For theoretical study, shape of the bulbous bow was approximated by an ellipsoid and von Karman’s momentum theory was applied to estimate the slamming impact loads with given impact velocity of the emerged bow to free surface. Wagner’s impact theory was also applied to study the effect of free surface swell up. Based on the momentum theory, an estimation method of the impact loads was proposed. To validate the estimation method, a experiment was conducted at the 80m square tank of NMRI. A self-propelling container ship model was used for the experiment. The vertical and lateral shearing forces and bending moments at the root of the bulbous bow were measured. In this paper, the results of the study are presented.Copyright

LONG-TERM PREDICTION METHOD OF SHIPPING WATER LOAD FOR ASSESSMENT OF THE BOW HEIGHT

Publisher Summary This chapter describes model test in regular and irregular waves, which was carried out to develop a long-term prediction method of shipping water load on the bow deck. Model of a tanker for Japanese domestic voyage was used. From the result of test in regular waves, the relation between shipping water load and relative water height at stem was discussed. Shipping water load is found to be proportional to the square of the water elevation over the bow top. Based on this relation, probability density function of shipping water load in short-term prediction was proposed. The proposed function shows a good agreement with the measured distribution in irregular waves, especially the tail of it, better than the conventional function. Using this prediction method, assessment of bow height of Japanese domestic ships from the viewpoint of deck wetness was carried out. The correlation between shipping water load and the exceeded height—the relative water height at stem exceeded the bow height—was seen to be strong. The probability density function of shipping water load was proposed. The method agrees well with experimental results. From the comparison with long-term prediction between present method and conventional method, it is concluded that the present method is more rational than conventional method to predict shipping water load.

Green sea loads on hatch covers of bulk carrier

A series of model tests in waves were conducted to measure the green sea loads that act on deck and hatch covers due to shipping water. A model of bulk carrier was used. The tests were carried out in irregular waves of which significant height is 10.6 meters and peak period is 14 seconds. In order to discuss the effects of wave heading and ship forward speed on the green sea loads, the model tests were made in several combinations of wave heading and ship speed conditions. It was confirmed that the deck wetness and green sea loads will be reduced considerably if the wave heading is altered to the quarter or beam seas or the ship speed is reduced.In order to assess the experimental results quantitatively, green sea loads were estimated by the practical estimation methods that were developed by the one of authors. Having compared with measured results, it was found that the estimated results were in good agreement with measured ones. It is concluded that the experimental results were rational.Comparison of green sea loads between measured results and present rules, ICLL66 and IACS UR-S21, was made. Although it is difficult to directly correlate measured values with the rules, mean values tend to be larger than the ones of ICLL66. It is also found that the green sea loads defined in IACS UR-S21 is ranked to somewhere between 1/10 and 1/3 significant values in relation to the measured results.