Experimental and Numerical Study of the Slow-Drift Motion of a Rectangular Barge Moored at an Inclined Beach

An experimental campaign is reported on the slow-drift motion of a rectangular barge moored in irregular beam seas. The 24 m long false bottom of the basin is raised and inclined at a slope of 5%, from 1.05 m below the free surface to 0.15 m above. The barge is moored successively at 4 different locations, in water-depths ranging from 54 to 21 cm. The measured slow-drift component of the sway motion is compared with state-of-the-art calculations based on Newman approximation. At 54 cm depth good agreement is obtained between calculations and measurements. At 21 cm depth the Newman calculation exceeds the measured value. When the flat bottom setdown contribution is added up, the calculated value is 2 to 3 times larger than the measured one. A second-order model is proposed to account for the shoaling of a bichromatic sea-state propagating in decreasing water-depth. Application of this numerical model to the scale-model tests shows that in shoaling conditions the setdown contribution to the slow-drift excitation can counteract and not necessarily add up to the Newman component.