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

Abstract

This work aims at characterizing the probability of wave impact and determining the position of impact on an FPSO (floating production storage and offloading platform) bow geometry. 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 the 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 was derived for the bow type in the experiments. The proposed model for determining the probability of having an impact is based on combining distributions, adjusted a priori to the numerically generated second-order free surface vertical velocity, and the experimental probability of impact of a known certain seastate and free surface velocity. The analytical description 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 nonlinear wave crests and the ship motions. The ship motions can be determined based on a linear response to the nonlinear waves considered.

Slamming and green water loads on a ship sailing in regular waves predicted by a coupled CFD–FEA approach

“…Ships can experience water impacts when sailing at high speed or in severe sea states, or both. According to Xu and Duan (2009), the hydrodynamic impact can be classified into four types, which have also been identified in experimental work: bow flare slamming (Hermundstad and Moan, 2005), bottom slamming (Shin et al, 2018), green water on deck (Fonseca and Guedes Soares, 2005) and wave slap (Guedes Soares et al, 2007). Slamming is a strongly nonlinear response, which can result in critical structural failure and onboard facilities damage.…”

Joint distributions of wave steepness in narrow band sea states

“…Ocean wave steepness is investigated most often because of the danger a steep wave constitutes for ships and floating structures (Kjeldsen, 1997;Guedes Soares et al, 2007. Costs of damage caused by a wave of large height and steepness make of wave steepness a parameter of great importance.…”

Verification of Quasi-Determinism theory against Baltic Sea Data

“…These events initiated a series of studies on the steepness and asymmetry of deep water waves in Norway (Myrhaug, Kjeldsen 1984, 1986. The probability of marine structure damage connected to extreme local steepness was studied by, among others, Guedes Soares et al (2007). Investigations on wave profiles led to the formulation of many definitions describing individual wave profiles.…”