Scholarly record

ENVIRONMENTAL ASPECTS OF SHIP MANEUVERING IN GUSTY WIND

Magdalena Bratkova, Dobrin Efremov, Evgeni Milanov

Abstract

In real navigation, a ship is subjected to the influence of wind, which for ships with a large windage area results in impaired maneuverability, especially in presence of strong and sudden gusts. In such conditions, environmentally critical situation may arise, such as a ship’s inability to perform an adequate maneuver when entering a restricted area, posing a risk of collision with another object or grounding in shallow water. This requires the study of the parameters of non-stationary wind impact using indicators to assess this influence. In present paper, these indicators are analyzed as a function of the ratio of the effective wind speed to the ship’s speed. For the purpose of the study, a suitable object was selected – a research vessel with a high ratio of windage to underwater area, which is intended to operate in diverse navigational conditions. The characteristics of the wind above sea surface were defined as a were applied in processing data from experimental measurements of wind speed and direction in the Varna water area. Methods for modeling steady wind and wind with periodic gusts were analyzed. Wind time series harmonic components were generated based on Harris spectrum superimposing the gust realization. The aerodynamic characteristics of the ship’s above-water part were determined using published regression models, with one suitable for the parameters of the object’s superstructure being selected. To simulate ship maneuvers in presence of external influences, the established Abkowitz simulation model was chosen, with the unknown coefficients in the model identified based on hydrodynamic experiments in a towing tank. The programming of all algorithms used was carried out in the Matlab environment. The numerical research scenario is based on simulating a turning circle maneuver up to 180 degrees course change for two cases – without and with gusts at different wind speeds and directions, varying the ratio of wind speed and to ship speed. For comparative assessment of the wind impact, two indicators were used – the degree of trajectory deformation and the direction of the ship’s drift in the direction of the wind, related to the IMO turning ability criteria. For practical application, both indicators are presented graphically as a function of the ratio of the effective wind speed to the ship’s speed, which can serve for environmentally safe decisions given known dimensions and depth of the water area. Decisions for further research on the problem are outlined.

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