Impact of Drag‐Force Approximations on Spectral Wave‐Force Predictions

This article explores the issue of drag-force approximation through the examination of dimensionless spectral wave-force ratios. These dimensionless ratios were developed in an attempt to clarify when third-order approximations to the spectral drag force could be expected to significantly contribute to the predicted wave force. They represent ratios of the spectral wave force per unit length for rigid cylindrical elements. It was found that for low period values, those around the natural period of fixed offshore platforms, the magnitude of the third-order correction to the drag-force spectrum can be a significant percent of the first-order approximation. At higher periods, those in the range of deepwater or compliant platforms, the drag force dominates the total wave force and the magnitude of the third-order force contribution is significant. This finding is consistent with studies addressing the analysis of offshore platforms presented in the current open literature. Finally, changing the Keulegan-Carpenter number strongly influences the magnitude and the location of the low-period peak associated with the third-order drag-force term. Thus, for high $N_K{}_C$ and low $H_s$, the third-order drag force could become an important consideration in wave-force computations.