Airgap Prediction" Use of Second-Order Diffraction And Multi-Column Models

Sweetman, Bert (Stanford University) | Winterstein, Steven R. (Stanford University) | Meling, Trond Stokka (Statoil) | Birknes, Jorn (Det Norske Veritas)



The airgap of a specific semi-submersible platform subjected to irregular waves is considered. The effects of including second-order diffraction contributions are demonstrated, and the sensitivity of this analysis to numerical model complexity is investigated. Detailed model test results for both motions and airgap time histories are used to verify the analysis results. A new approach is proposed for use in post-processing secondorder hydrodynamic transfer functions. In the new approach, those transfer functions that are unavailable or believed to be unreliable are replaced with those of an undisturbed second-order Stokes wave. Results of detailed hydrodynamic analysis are also compared with those from a simpler numerical method, which uses a multi-column model of the semi-submersible platform to compute the diagonal of the quadratic transfer function matrix. To use the results from this simpler model, another new approach to extrapolating hydrodynamic analysis results is proposed: unknown off-diagonal terms of the quadratic transfer function matrix are estimated from the known on-diagonal terms. The results of each analysis are critically compared. The overall goal is to demonstrate the numerical impact of: 1) performing a very detailed second-order diffraction analysis, 2) performing a simplified secondorder diffraction analysis, or 3) ignoring second-order diffraction entirely.


Airgap modeling is of concern for both fixed and floating structures, but it is particularly challenging in the case of floating structures because of their large volumes and the resulting effects of wave diffraction and radiation. Standard airgap response prediction uses linear theory, which generally does not effectively reproduce measurements from model tests. First-order diffraction is considerably less demanding than second-order, so use of only first-order diffraction merits some consideration. Secondly, order diffraction effects are expected to better reflect observed data. However, these radiation/diffraction panel calculations are very sensitive to the numerical modeling.