Estimation of full-scale ship manoeuvrability in adverse weather using free-running model test

Abstract

The ship manoeuvrability in actual seas with both wind and waves, especially in adverse weather conditions, for a full-scale tanker with different maximum available power of the engine is studied. It is directly evaluated based on the result of a free-running model test (FRMT) under a course-keeping manoeuvre in long-crested and short-crested irregular waves of Beaufort scale force (BF) 7–10. The FRMT combines rudder effectiveness and speed correction considering the operational limit of an engine, and a wind load simulator, both of which were previously proposed by the authors. Therefore, not only the effect of waves but also effect of wind, rudder effectiveness of the full-scale ship, and the engine limit for continuous operation are considered in the FRMT. To validate the reliability of the direct evaluation method in FRMT and the measured data in irregular waves of large amplitude, the time-averaged manoeuvring motion of the FRMT at BF 9 is compared with ship manoeuvring motion under steady equilibrium conditions estimated using numerical simulation based on a modular mathematical model. The wave drift forces and moment in the simulation are determined on the basis of the results of the captive model test in regular waves, which is carried out using the same tanker model before the FRMT, so that we can eliminate the estimation errors in the manoeuvring motion caused by the imprecision in the wave drift forces as much as possible. The comparison shows the FRMT results acceptably agree with the simulation results, although a discrepancy is observed in the rudder angle in beam seas for the tanker with a low engine output. Further, FRMT data themselves show that full-scale rudder effectiveness, operational limit of an engine, and the engine with lower power cause the ship speed to decrease and the drift angle and the rudder angle to increase in head, bow, or beam seas because of the difference in the propeller rotational speed in each model control condition. In addition, it was clarified that operational limit of an engine also affects the significant single amplitudes of the fluctuating components of them and the oscillatory ship motion. Next, the authors assess the manoeuvring limit in adverse weather by comparing the FRMT data of the time-averaged values of the longitudinal ship speed, as well as the drift and rudder angles in short-crested irregular waves at BF 7–10 with their thresholds. The assessment clarifies that the tanker becomes unmanoeuvrable at BF 9–10 and in head, bow, or beam seas owing to a lack of sufficient advancing speed. However, the limit is not attributable to the drift or rudder angle, although the lower-power engine worsens the course-keeping ability owing to the decrease in the ship speed and the increase in the drift and rudder angles. Furthermore, the tanker is manoeuvrable in any wind and waves directions at BF 8, which is defined as adverse weather conditions for this subject ship in the interim guideline (IMO, 2013). It indicates that the tanker has sufficient margin of the engine power to keep manoeuvrability in adverse weather conditions. In summary, these findings reflect the possibility that FRMT can replace numerical simulation as a direct evaluation method of full-scale ship manoeuvring motion and limits in adverse weather.