# Incorporating Epistemic Uncertainties in Ship Operability Study
## 4 Discussion
### Correlation with the Experience on the Shipping Route  
The ship analyzed in the present study is similar to the actual ship sailing on the route from Ancona to Split and vice versa. Although the ship is not the same, it is of interest to analyze publicly available heavy weather examples that occurred on the route concerned. There are rare examples of the inconveniences that big waves generated by 'jugo' wind caused to the passenger ships, like the one analyzed herein. 'Jugo' is a southeast wind causing the highest waves in the Adriatic Sea [41].  
Figure 6: Polar plots representing the effect of ship speed on probabilities of exceeding limit states (Left: Ship speed 5 knots; Right: Ship speed 17 knots.)The marine accident that happened on 1 November 2012, just after midnight, was described in [42]. Seventy vehicles were damaged in a ferry on the voyage from Ancona to Split because of the large roll amplitudes. During a severe storm, vehicles began to slide and collide with other vehicles and sides of the ferry. The maximum significant wave height along the route on the night of the accident was 5.31 m, as may be seen from the Mediterranean Sea Wave Reanalysis database [43]. This significant wave height corresponds to Sea State 6 according to the Douglas scale (Table 4). The significant wave height at the time of the accident is shown in Figure 7.  
Another accidental event was recorded recently, on 23 December, when a delay of 6 h occurred because of the heavy weather. The reconstruction of sea states in the time of the delay can be carried out using Mediterranean Sea Waves Analysis and Forecast [45]. The sea states along the route are shown in Figure 8. The maximum significant wave height along the route was only about 3.13 m. The significant wave height corresponds to Sea State 5 according to the Douglas scale.  
It is interesting to mention that the maximum recorded significant wave height along the route in the Mediterranean Sea Wave Reanalysis database, since January 1985, was 6.12 m, which occurred on 1 February 1986. The severity of sea states along the route from 1 January 1985 to 31 May 2013 is presented in Figure 9. One may notice that rough Sea State 5 (2.5 m \(<H_{S}<\) 4 m), according to the Douglas scale, is not rare along the shipping route. Very rough seas, corresponding to 6 on the Douglas scale, with a significant wave height above 4 m occur on average once per year.  
Figure 7: Significant wave height in the Adriatic Sea on 1 November 2012 (generated using E.U. Copernicus Marine Service Information [44]).  
The examples presented from the sailing route concerned may be compared to the operability diagrams in Figure 5, where special attention should be paid to the heading angles of 45\({}^{\circ}\) and 135\({}^{\circ}\). One may see that diagrams with a 5% exceeding probability are rather pessimistic, as even Sea State 4 is considered critical regarding the criterion of vertical acceleration at FP. However, for the 50% and 95% exceeding probabilities, the operability diagrams look credible, as the ship operability in Sea States 5 and 6 is questionable. For Sea State 5, the most critical criteria are the vertical acceleration at FP and propeller emergence, while for Sea State 6, most of the criteria are unsatisfactory.