Abstract
This paper discusses the potential of deterministic wave prediction as one basic module for decision support of offshore operations. Therefore, methods of different complexity—the linear wave solution, the non-linear Schrödinger equation (NLSE) of two different orders and the high-order spectral method (HOSM)—are presented in terms of applicability and limitations of use. For this purpose, irregular sea states with varying parameters are addressed by numerical simulations as well as model tests in the controlled environment of a seakeeping basin. The irregular sea state investigations focuses on JONSWAP spectra with varying wave steepness and enhancement factor. In addition, the influence of the propagation distance as well as the forecast horizon is discussed. For the evaluation of the accuracy of the prediction, the surface similarity parameter is used, allowing an exact, quantitative validation of the results. Based on the results, the pros and cons of the different deterministic wave prediction methods are discussed. In conclusion, this paper shows that the classical NLSE is not applicable for deterministic wave prediction of arbitrary irregular sea states compared to the linear solution. However, the application of the exact linear dispersion operator within the linear dispersive part of the NLSE increased the accuracy of the prediction for small wave steepness significantly. In addition, it is shown that non-linear deterministic wave prediction based on second-order NLSE as well as HOSM leads to a substantial improvement of the prediction quality for moderate and steep irregular wave trains in terms of individual waves and prediction distance, with the HOSM providing a high accuracy over a wider range of applications.
Highlights
Ships and offshore structures are exposed to the sea, limiting the scope of application of offshore operations, from efficient and economic offshore operations in moderate sea states to reliability as well as survival in extreme wave conditions
The following study comprises investigation of irregular sea states based on JONSWAP spectra evaluating the influence of wave steepness, the shape of the underlying spectrum and the wave propagation over large distances
The focus lies on the investigation of the prediction accuracy for varying parameters such as wave steepness, enhancement factor of the JONSWAP spectrum and wave propagation distance
Summary
Ships and offshore structures are exposed to the sea, limiting the scope of application of offshore operations, from efficient and economic offshore operations in moderate sea states to reliability as well as survival in extreme wave conditions. Different measurement methods are available, from point measurements (e.g., wave buoy) in time domain to surface elevation snapshots taken from the ship’s X-band radar in the space domain. HOSM without time-consuming reconstruction of the wave field in space domain They generally showed that long-time and large-space simulations of non-linear sea state evolutions can be performed accurately and efficiently with the HOSM. Desmar et al [37] applied the HOSM for the generation of reference wave snapshots in order to evaluate reconstruction methods of different complexity and the consequences on the prediction accuracy This investigation showed that non-linear methods for wave reconstruction (based on spatio-temporal optical measurements) are improving the accuracy of the reconstructed initial wave field and prediction. The accuracy of the predictions is evaluated with the surface similarity parameter allowing an exact, quantitative evaluation
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