Sea-keeping and added resistance in waves

Simulation illustration: MARINTEK

When vessel performance is being evaluated, behaviour in a seaway is as important as calm-water resistance. To evaluate ship responses and added resistance, MARINTEK performs unsteady CFD computations of ships advancing on a straight course in regular waves.

Hull resistance is traditionally estimated under calm-water conditions. This gives valuable information about the ship’s performance at the early design stage. However, calm water is the exception at sea. Maintaining a desired heading in a seaway may induce large ship motions, increase resistance and reduce propulsive efficiency. Vessel responses and motions are very important from a safety point of view, while added resistance and loss of speed are economically important. The study of the behaviour and performance of vessels in waves is therefore an essential aspect of ship design.

Since estimates of ship responses and added resistance are critical, it is essential to employ accurate prediction methods in the design process. Traditionally, the prediction of added resistance was done by analytical methods based on potential flow theory. Since these methods are useful for making gross estimates of added resistance, they have been widely used as practical design tools. However, a clear disadvantage of these methods is that they cannot account for nonlinear flow features such as waves breaking in the near-field of a vessel. Because these nonlinear features contribute significantly to the added resistance, analytical methods are often not accurate enough for the quantitative prediction of added resistance. CFD simulation methods offer the advantage of being able to deal directly with nonlinear flow phenomena without explicit approximations. This makes them capable of tackling problems with strong nonlinearity, such as the prediction of added resistance.

At MARINTEK, unsteady CFD computations can be performed for a vessel advancing on a straight course in regular waves in order to evaluate its performance, including ship response and added resistance. Both free and fixed heave and pitch motions can be simulated for a wide range of important wavelengths. Such simulations require larger computing resources than calm-water resistance simulations because the grid must be very fine throughout the region occupied by the wave train in order to ensure good propagation of incident waves with no noticeable damping. However, very acceptable calculation times can now be obtained by running parallel computations on our cluster. Typical outputs from such simulations include time series of the hull resistance and ship motions. Movies can also be recorded over the duration of one period.


To see the figures in this article, please open the pdf version of the article in MARINTEK Review No. 2-12

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