The motion behaviour of a Dual Draft vessel was calculated and experimentaly validated by means of model tests. Both the tests and computations showed that the roll behaviour of a Dual Draft vessel in pipelay / transit mode does not correspond to the usual observed response amplitude operator (RAO), where the roll RAO decreases with increasing wave height due to non-linear damping. When the wider top section of the Dual Draft vessel enters the water due to a roll motion, the buoyancy increases at that side and results in an increase of the excitation moment and roll RAO. Non-linear damping is of significant importance on the ship motions and is particulary noticeable in roll motions. This paper describes the effect of a Dual Draft hull on the motion behaviour of a pipelay / heavy-lift vessel in beam seas. The non-linear damping of a Dual Draft vessel will be discussed and calculations of the motion behaviour will be presented and compared with results of model tests.

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