How Draft and Freeboard Affect Green Water: a Probabilistic Analysis of a Large Experimental Dataset

Abstract

Green water is a rare and complex wave impact event that can affect the safety of the ship and those aboard. During the design process, the risks that these events pose have to be mitigated, preferably by minimizing the probability of a high impact pressure. As green water events are rare and complex it is difficult to get a statistically representative dataset and thus previous work has mostly compared design variations through a limited number of events, instead of comparing the probabilities. The present study will use probabilities to compare design variations with different drafts and freeboards at the bow. A large set of green water events in irregular waves with forward velocity was experimentally obtained for six different bow designs. The dataset represents 57 hours of sailing per bow design and 3263 green water events. The data demonstrates that both freeboard and draft at the bow affect the probability of green water in a complex way. Increasing the draft at the bow increases the swell-up, reducing the effective freeboard and in turn, increasing the probability of green water. Increasing the freeboard results in a decrease in the probability of green water, as expected. However, the probability is not reduced equally for different green water impact pressures. The joint probability of green water occurrence and pressures shows that increasing the freeboard only decreases the probability of low-pressure events. Counter-intuitively, increasing the freeboard increases the probability of high-pressure events. These interesting results show the value of using statistically representative datasets when designing for green water.