A probabilistic method to estimate non-linearities of added resistance with respect to wave height in short-term sea conditions is proposed 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 KCS hull type. The non-linearity with respect to wave height is empirically expressed as a "correction coefficient" for linear calculations using CFD simulations and correction is added to the theoretical linear calculation result. The present systematic study demonstrates that the numerical results have a reasonable validation with the linear-based methods and emphasizes the non-linearity in the prediction of the added resistance with the varying wave height in the irregular waves.
There is a growing demand for a highly accurate evaluation of the actual sea performance of ships from the viewpoint of the operational economy and environmental protection. Recently, Energy Efficiency Design Index (EEDI) regulations for the powering performance of commercial ships have made it impossible to operate ships that do not meet specific standards. It is more important to estimate ship propulsion performance accurately in actual sea conditions. When a ship travels in waves, the drag on the ship increases by 15-30% compared to when it travels in calm water. One of the factors that affect ship performance in the actual seas is an increase in resistance during waves and it greatly affects the ship's performance in terms of sustainable service speed and fuel consumption in realistic sea conditions. This is because waves are generated by the ship's hull oscillating and reflecting incident waves, and this increase in resistance is called the added resistance in waves.
Traditionally, calculation of added resistance in actual sea conditions has been treated by spectral analysis using added resistance transfer function and wave spectrum that corresponds to statistical values in short term. Meaning using the estimation of frequency response transfer function of added resistance in regular waves, the total amount is predicted by the moment of response spectra criterion for calculating added resistance in short-term sea conditions. Most of the studies focused on calculating the added resistance transfer function accurately in regular waves. Also, potential-based numerical methods are applied to predict the added resistance more accurately. Seo et al. (2014) predict the added resistance using potential-based methods, and Lee et al. (2016) investigated the added resistance using model tests. However, due to its limitations which make it difficult in considering the nonlinearity and viscosity effects. However, all these approaches and methods are incomplete because the concept of estimation of added resistance transfer function does not include the effect of the sea environment and does not reflect the wave-body interactions which is a crucial aspect of considering non-linear hydrodynamic effects.
