Enhanced Unified Theory with Forward-Speed Effect Taken into Account in the Inner Free-Surface Condition

Abstract

The enhanced unified theory (EUT) has been used as a core theory in the integrated system developed at the Research Initiative on Oceangoing Ships (RIOS) of Osaka University for predicting the propulsion and seakeeping performance of a ship in actual seas. In this study, the EUT is modified by adopting partially the solution method in the rational strip theory of Ogilvie and Tuck as a particular solution in the inner problem, thereby a forward-speed effect in the convection term of the free-surface condition is incorporated in the inner solution. This forward-speed effect is analytically shown to contribute only to the cross-coupling radiation forces. Some other forward-speed and 3D effects important in a low-frequency range are also included in the homogeneous component of the inner solution through matching with the outer solution in a similar manner to the unified theory of Newman. Numerical computations are implemented for a slender modified Wigley model and the RIOS bulk carrier model. Good agreement is confirmed in a comparison with experimental data for the cross-coupling added mass and damping coefficients between heave and pitch and also for the resulting ship motions, particularly in heave near the resonant frequency. The added resistance around the motion-resonant wavelength is found to be improved but sensitive to a slight change in heave and pitch motions. Thus, it is stressed that accurate prediction of the ship motions and resultant Kochin function is critical for more accurate prediction of the added resistance in waves. Introduction Although the design of the ship hull form has been based mainly on the propulsion performance in still water, recently, prediction and onboard data analysis for the propulsion and seakeeping performance of a ship in actual irregular waves have been attracting attention of the researchers (Orihara & Tsujimoto 2018; Minoura et al. 2019). In fact, real ships navigate mostly in rough seas, and thus, the so-called short-term and long-term predictions of ship response in actual seas must be made to guarantee the performance and safety of a ship. This trend to study the seakeeping performance of a ship is partly because the Energy Efficiency Design Index regulation was introduced by International Maritime Organization (IMO) to reduce greenhouse gas emission from the ships in operation. Thus, it becomes important to predict with sufficient accuracy the wave-induced ship motions, the added resistance, and the resultant speed loss of a ship in irregular waves represented by a directional wave spectrum (Kashiwagi 2009; Kim et al. 2017) even in the initial stage of ship design, necessitating computations for various profiles of a candidate ship.