Heave and Pitch Response of an Offshore Platform With Air Cushion Support in Shallow Water

Abstract

Model tests were conducted on two 1:100 scaled models of a typical concrete gravity substructure at the University of Western Australia. The two models had dimensions 0.5m length × 0.5m width with the first model being a sealed closed bottom box of height 0.1m and the second model being an open bottom box with skirt length of 0.1m. The open bottom model had the capacity to hold an air cushion with dimensions 0.49m width × 0.49m length × 0.08m height. Each model was floated at a constant draft of 0.1m and tested in water depths ranging between 0.03m (shallow) and 0.8m (deep). The environment comprised of regular waves with periods ranging between 0.6s and 3.5s and amplitude of 0.08m–0.02m. To quantify the dynamic response the heave and pitch motion of each model were measured. The model test results were compared with a theoretical solution based on long wavelength, linear wave assumptions applied to a box shaped floating vessel without an internal free surface. Results show that experimental trends compare reasonably well with analytical solution. Added mass values were predicted from heave and pitch decay tests. The results show that introducing air cushion support into a CGS increases the pitch response, while having little effect of the heave motion. The theory is also used to delineate regions of safe and unsafe tow-out operations of the air cushion structure.