ABSTRACT

A stochastic method has been developed and validated to estimate non-linearities of added resistance with respect to wave height in short-term oblique sea conditions using the joint probability distribution of wave height and period and non-linear added resistance response in the frequency region which is obtained by CFD numerical simulations for RIOS containership. The non-linearity with respect to wave height squared is empirically expressed as a "correction coefficient" using CFD simulations in oblique regular waves for the RIOS containership model and incorporated this non-linear wave height effect into linear added wave resistance transfer function. CFD simulations were performed for the incoming oblique bow quartering regular waves of 120° heading angle and results were validated using ship vertical motions in linear calculation reproduced using a potential flow-based method. Short term predictions of added resistance in oblique seas were estimated by proposed non-linear PDF method which showed the fair results in comparison to spectrum method and in an acceptable range of accuracy with CFD numerical results in irregular oblique waves. Further improvement in the method is needed.

INTRODUCTION

Recently, the importance of ship operation efficiency and safety at sea has increased and Energy Efficiency Design Index (EEDI) regulations for the powering performance of commercial ships are being actively pursued. Considering the recent tightening of regulations, it is more important to develop a hull form that takes into account the non-linearities of added resistance in real sea performance. In the case of an actual seaway, ships rarely travel in head sea waves as this condition encounter highest added resistance in waves and bow slamming. In most of the practices, ship hydrodynamics is simulated in head sea waves to predict added resistance and make assumptions for oblique wave cases based on head wave results. Also, in severe seas with large wave heights, the design of the ship hull above the water line become particularly important, which is mostly ignored in potential theory-based simulations. Thus, short term predictions of added resistance in oblique waves with taking into account the wave steepness non-linearity are important to properly evaluate a hull form design and to get a realistic idea of its behavior during the actual sailing.

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