Estimation of added resistance of an LNG carrier in oblique waves

Abstract

Despite recent moderate oil prices, the reduction in fuel oil consumption of voyaging ships has been of great interest. Ships experience additional resistance from wind and waves in actual seas during their voyages, and those added resistance contributes the increase of fuel consumption. The accurate prediction of the added resistance is, therefore, crucial for developing highly energy efficient and low fuel-consuming ships.Ships routinely navigate along pre-determined routes between ports and thus relative heading angle is typically not controlled. Therefore, ships can encounter wind and waves coming from any direction. However, added resistance in oblique direction has less been studied by researchers than that in head seas. To predict the operating performance of ships accurately, more focus should also be given to added resistance in oblique direction.In the present study, the motions and added resistance of an LNG carrier are estimated by experimental investigations and numerical computations. The scaled model ship is towed at a constant forward speed in a square basin and the resistance increase is measured in regular oblique waves. Potential flow method and Reynolds-Averaged Navier-Stokes (RANS) based computational fluid dynamics method are employed to compare the magnitudes of motion and added resistance to those obtained from model test. The global trend of motion responses agrees reasonably at all heading angles between the results from experiments and numerical computations. In regard to the added resistance, however the level of accuracy is less favorable for both numerical methods. The discrepancy is further investigated by checking the relevant parameters in potential method, roll damping and relative phases. The detailed results are discussed and key finding is addressed.In the experimental and numerical investigations, the magnitude in the transfer function of added resistance due to waves is significant at bow quartering seas. In addition, the resistance increase appears to be not negligible even at the following and stern-quartering seas. Therefore, operating efficiency of a ship can be evaluated appropriately when contribution of all heading angles is properly taken into account to the evaluation of added resistance.