Efficient estimation of extreme long-term stresses by considering a combination of longitudinal bending stresses

Longitudinal stresses due to combined horizontal and vertical bending moments in ships, corresponding to a return period of 20 years, are estimated by linear response analysis. In principle, the stress should be obtained by combining the stress in all sea states that can occur over a long-term period. A method to determine the desired long-term extreme stress by considering only a few short-term sea states is presented. The sea states have a certain probability of occurrence, and are each identified by a contour line in the (H _s, T _p)-plane. This approach makes it possible to estimate the extreme loads on the vessel in a practical and accurate manner. Moreover, it is shown that the long-term stress can be estimated by combining the individual long-term extreme stresses due to vertical and horizontal bending moments by using the sum-of-squares approach and accounting for the correlation between stresses. It was found that the correlation coefficient can be taken as the largest of the ones calculated along the contour line. It is shown that this correlation coefficient can even be approximated by the normalized phase angle at the wave length where the dominant response has its peak value. A comparison with the results obtained using well-known combination rules is presented. While linear analysis has been used here, it is believed that the approach can be generalized to stresses with nonlinear behavior, and hence represent a significant improvement in calculation efficiency. Received: September 18, 2001 / Accepted: December 18, 2001     

Prediction method of hydrodynamic forces acting on the hull of a blunt-body ship in the even keel condition

The mathematical modeling group (MMG) model is well known and is widely used in the field of ship maneuverability. However, the MMG model can be applied only after determination of the hydrodynamic coefficients either from comprehensive captive model tests or from general empirical data. Around the cruising speed, when a ship's drift angle is relatively small, several methods have been developed to predict hydrodynamic coefficients from the ship's principal particulars, e.g., Kijima's method. Kijima's method is efficient in predicting the ship's maneuverability at the initial design stage and is even able to assess the effect of changes in stern design. Similarly, for the low speed range when a ship's drift angle is relatively large, several methods for predicting the ship's hydrodynamic coefficients have been proposed, based on captive model tests, such as those by Kose, Kobayashi, and Yumuro. However, most of the methods developed for low speeds cannot be applied to general ship types without additional experiments being performed. In contrast, Karasuno's method uses theoretical and empirical approaches to predict the hydrodynamic forces, even for large drift motions. Although Karasuno's model utilizes the ship's principal particulars and is applicable to a general vessel, it has not been widely used. This is because the form of Karasuno's model is relatively complicated and its accuracy around the cruising speed is less than that for other methods that have been specifically developed for the cruising speed range. A practical method for predicting hydrodynamic forces for the entire operating speed range of blunt-body ships is proposed in this article. It is based on the MMG model and predicts hydrodynamic coefficients based on a ship's principal particulars. A regression model for the proposed method has also been proposed by analyzing 21 different blunt-body ships. Finally, simulations of a very large 4-m crude carrier (VLCC) model using the proposed method were carried out and the results compared with free-running experiments (both at the cruising speed and at low speeds) to validate the efficacy of the model.     

A review of the effect of a／W ratio on fracture toughness （Ⅰ）—experimental investigation in EPFM

Many experimental investigations have previously been performed and recently done on different shipbuilding structural steels where the specimens size and crack depth/specimen width (a/W) were varied. A series of interesting results have been gained. It is worthwhile to have a review on the effect of a/W ratio on fracture toughness, and further theoretical analysis is necessary. In this paper, experimental work in elasticplastic fracture mechanics (EPFM) was discussed. Tests had been carried out on 10 kinds of strength steels. Results showed that J _i and δ₁ values increased with decreasing a/W when a/W<0.3 for three-point bend specimens and that shallow crack specimens which have less constrained flow field give markedly higher values of toughness than deeply notched specimens. However, for a/W>0.3, the toughness was found to be independent of a/W. Slip line field analysis shows that for shallow cracks, the hydrostatic stress is lower than that from standard deeply cracked bend specimen which develops a high level of crack tip constraint, provides a lower bound estimate of toughness, and will ensure an unduly conservative approach when applied to structure defects especially if initiation values of COD/J-integral are used.     

Geometric modelling of bulbous bows with the use of non-uniform rational B-spline surfaces

Nowadays, bulbous bow forms have become a common design feature in most conventional ship designs. The design of a bulbous bow is usually attempted with the use of certain form parameters that are imposed using the designer's experience or regression values that provide the optimum parameters based on experimental tests or computational fluid dynamics (CFD) calculations. In this article, the geometric modelling of a bulbous bow form that complies with a series of parameters is presented. First, a wire model was constructed with cubic B-spline curves that hold certain form parameters that the designer wants to impose. Second, a B-spline surface that fits these splines was mathematically constructed. A review of the influence of the design parameters used on the bulbous bow properties was made. This method could be used prior to numerical optimization of a bulbous bow because different variations of the design can be easily generated. These variations can be tested numerically using CFD software. A practical application example of the method is shown.     

New Model and Simulation of Condenser for Ship Nuclear Power Plant

A new reasonably perfect dynamic mathematic model has been established for condenser used in ship nuclear powerplant according to its structural features and Operating principle. The model has been solved by the Runge-Kutta method. Andan analysis program has been developed for dynamic numerical simulation under steady operation condition, disturbance condi-tion, and accident condition. The dynamic characteristics of condenser has been calculated and analyzed under several kinds of     

Calculating the return value using a mathematical model of significant wave height

This study presents a new method for calculating return values. The essence of the method is that it utilizes nonstationary data to calculate the return value for a region in the Northeast Pacific. The nonstationary data was obtained from a model which was previously developed for the behavior of the significant wave height as a function of time in the region. The method is illustrated by convolving two generalized Pareto distribution functions fitted to two parts of the model, computing a suitable extreme value from the new distribution function, and calculating the return value using this extreme value.     

A new marine propulsion system

A new marine propulsion system is proposed . A small liquid sodium cooled reactor acts as prime mover; alkali-metal thermal-to-electric conversion (AMTEC) cells are employed to convert the heat energy to electricity; superconducting magneto-hydrodynamic thruster combined with spray-water thruster works as propulsion. The configuration and characteristics of this system are described. Such a nuclear-powered propulsion system is not only free of noise, but also has high reliability and efficiency. It would be a preferable propulsion system for ships in the future。