Effect of Viscous Damping on Hydrodynamic Response of Spar with Heave Plate

In the present paper, effect of viscous damping on the heave response of spar is studied using WAMIT software for spar with single and two heave plates. For spar with single heave plate, a plate is attached at the keel of the spar and the diameter of the heave plate is varied. Diameter ratio (i.e., ratio of diameter of heave plate to diameter of spar - D-p/D-s) of 1.1 to 1.4 is considered here. For spar with two heave plates, one plate is fixed at the keel of the spar and other plate is placed at some distance from the keel. Diameter of the two plates is same and diameter ratio of 1.3 is taken for these plates. Spacing between the plates is varied and relative spacing (i.e., ratio of spacing between the two heave plates to diameter of heave plate - L/D-p) of 0.1 to 0.4 is considered. WAMIT software is based on potential theory so it considers only radiation damping. To account the viscous damping in WAMIT analysis, viscous damping values obtained from the literatures are used as an external damping matrix in WAMIT software. Increase in diameter ratio and relative spacing increases the heave added mass and damping of the spar. Hence, heave response of spar reduces with increase in diameter of the heave plate and spacing between the two heave plates. It is found that heave viscous damping reduces the heave response of spar with heave plate significantly. It is suggested that viscous damping should be included in WAMIT software for the reduction of heave response of spar with heave plate. Diameter ratio of 1.2 to 1.3 and relative spacing of 0.2 to 0.3 are recommended for significant heave motion reduction. (C) 2015 The Authors. Published by Elsevier B.V.