Airgap on Semi Submersibles: A Practical Guide on the Implementation of a Stochastic Approach

Several articles in the literature cover methods for the prediction of semi submersible airgap. In general, using these methods, the reader will have to take care of the implementation, because in most cases not all factors that influence the final airgap are covered. This paper will describe the practical implementation of a calculation methodology covering all environmental aspects that affect the airgap for semi-submersibles. The proposed method utilizes a frequency domain stochastic approach (spectral method) to combine all known factors that can be calculated and that affect the airgap. These effects include the first order vessel motions under an undisturbed wave field, diffracted wave elevation along the free surface under the platform, slow drift quadratic transfer function (QTF), vessel set down and heel effects due to mooring stiffness into a single airgap prediction. Wave run up is not accounted in this procedure due to its high non-linearity, its difficult prediction and measurement. This is a local effect that is considered in the design phase by assuming that wave run up will hit the structures near columns and these components should be designed accordingly. The method proposes the combination of these statistical processes to be utilized for both the initial and final stages of the design. Model test results are also presented which show good correlation to the proposed method. A discussion of the limitations of this stochastic method, time simulations and model tests is also presented.