Forecast of Critical Situations in Short-Crested Seas

Abstract

The impact of single extreme waves or wave groups on marine structures and ships causes enormous forces often leading to critical situations or even loss of ship, cargo and crew. One approach to avoid dangerous situations is to adjust heading and cruise speed. To identify critical situations well in advance the forecast of the incoming wave train is essential. Concerning the method to predict the wave train a ship will encounter within the near future — some minutes ahead — the so far unidirectional WAVE FORECAST method, pre-calculating an encountering wave train from surface elevation snapshots of the surrounding sea — taken by radar — has been improved. This paper presents a method to predict the entire sea state within the surrounding area of the vessel considering multidirectional waves. Thus the evolution of critical waves coming from various directions can be predicted. In addition the SHIP MOTION FORECAST method — pre-calculating the vessel response — has also been enhanced. Taking into account the encounter angle of the incoming wave components, depending on time and course angle of the vessel, the ship-fixed compass rose is divided into a number of sectors. The corresponding encountering wave train for every sector is derived by superimposing all wave components coming from certain directions. With a set of directional Response Amplitude Operators (RAOs) for the six degrees of freedom the sector-wise vessel responses can be calculated as well. The response spectra are derived in frequency domain and transferred into time domain by the use of Inverse Fast Fourier Transformation (IFFT). Thus the overall vessel response is obtained by superimposing the time domain responses for every sector and degree of freedom, delivering a comprehensive data base for the analysis of critical situations in advance.