Tuning the Air Pressure of Aircushion Supported Structures at Model Scale

摘要

To allow transportation of bottom-founded offshore structures from a shallow building dock to deeper water, the draft of some structures is temporarily decreased by pumping compressed air underneath the construction. At the final location, the air is released and the structure is installed on the seabed. Motion behaviour and stability change when air escapes from the cushion underneath the structure. This paper describes the change in motion behaviour of an aircushion supported structure at different drafts. Calculations are performed based on linear three-dimensional potential theory using a linear adiabatic law to describe the air pressures inside the cushions. The water surface within the aircushion and the mean wetted surface are modelled by means of panel distributions representing oscillating sources. Results of model tests of an aircushion supported structure at a constant draft are included and serve as a validation of the computational method. The description of the motion behaviour also includes a discussion of the heave added mass, damping, cushion pressure variations and structural loads.