HYDRODYNAMIC PROPERTIES OF A SUCTION CAN OSCILLATING NEAR THE FREE SURFACE

Offshore operations often require heavy subsea equipment, such as suction piles or cans, to be lowered by a support vessel into the sea. A lifting device must have adequate capacity to withstand the dynamic loads generated by the motions of the vessel and the heave response of the structure. The objective of this study is to determine the added mass and damping of a suction can oscillating in heave near the free surface; knowledge of these hydrodynamic properties is required for the accurate prediction of the dynamic lift forces during the deployment. This project is a logical progression following two similar studies, which investigated these hydrodynamic properties for the suction can in the mid-water position and when approaching the seabed. All three studies involved the conduct of model tests to determine the hydrodynamic properties. Free decay tests were conducted at several heave frequencies, and the added mass, linear and quadratic damping components were determined. In addition, the effect of varying the percentage of open hatch area has been investigated. Test data demonstrates that the heave added mass is strongly dependent on the frequency of motion, and its values are significantly smaller than those measured in the unrestricted flow. From observations, there was no dependency on the motion amplitude, nor did the size of open hatches have notable effect on the added mass. It was observed that when the top plate of the structure was in contact with the free surface a mean "pull down" force appeared. This force is caused by the suction underneath the top plate when the can moves upwards. As opposed to the mid-water position and near the sea floor, the study indicates that the area of open hatches has no noticeable effect on the heave damping when the suction can is oscillating near the free surface.