An Approach to Calculate the Probability of Wave Impact on an FPSO Bow

This work aims at characterizing the probability of wave impact and expected impact load on the bow geometry of an FPSO. In order to determine the instants when impact occurs, an experimental program was performed on a specific bow shape. The bow was instrumented with pressure transducers and the test program, also making use of video recordings, was designed such that it was possible to determine the correlation between undisturbed wave shape and the impact pressure time traces. It has been found that wave impact at the bow is highly correlated with the local wave steepness, which for very high waves has at least second order effects. A comparison between the probability distributions of local wave steepness of the experimental undisturbed wave time trace and numerical simulations of second order wave theory is provided and it confirmed that the latter is very adequate for calculations. The experimental results were further used to determine how the probability of impact varies with free surface vertical velocity. It was found that the significant wave height of the sea state itself does not have significant influence on the result and a regression model is derived for that type of bow. The proposed model for determining the probability of impact load is based on combining both models. The analytical nature makes it fast and easy to expand to other cases of interest and some example calculations are shown to demonstrate the relative ease of the procedure proposed. The position of the impact is determined by the non-linear wave crests and the ship motions. The ship motions can be determined based on a linear response to the non-linear waves considered.