A Design Procedure for Evaluating the Air Gap on a Large-Volume Semi-Submersible Platform

This paper presents a design procedure for the evaluation of the air-gap response on semi-submersible platforms subjected to irregular sea conditions. The suggested procedure takes into account both first and second order (low frequency) effects on hull motions when evaluating the air gap. As a first step of the procedure, a large range of sea conditions with different returning periods and directions of incidence are simulated using a frequency domain model. This first step is intended to determine the critical sea conditions regarding the air gap response of that particular floating unit. For those conditions, it is suggested to be performed a more complete analysis of the problem, including time-domain CFD simulations, in order to improve the results, especially for areas that may be susceptible to intense wave run-up effects. Experimental results for some typical sea states of Campos Basin have been employed to validate the procedure using as an example a large displacement four column semi-submersible platform operating at Campos Basin, Brazil. Results have confirmed that the sea state with the highest significant wave height, or peak period, may not lead to the worst air-gap situation. It’s also shown that, although for the critical sea conditions the first order effects were dominant in the air gap response, at many non-critical sea states the second order effects presented magnitudes comparable to those of first order, indicating that the resonant response of the unit should not be disregarded a priori when dimensioning the air-gap of similar deep-draft semi-submersibles.