Capsize probability analysis for a small container vessel

In this paper we investigate the long term capsize probability for a small container vessel which is operated in a regular service schedule on the North Sea and the north-east part of the Atlantic Ocean. The numerical simulation techniques involved are a ship route scenario simulation method and a time domain simulation method for large amplitude ship motions.
The scenario simulation tool GULLIVER was developed at MARIN as a product for clients interested in the performance of their ship(s) in service conditions, with respect to safety, economy and reliability. The large amplitude ship motion program FREDYN was developed at MARIN within the framework of the Cooperative Research Navies project. Both methods have been applied successfully in many projects and have proven their value in both a research and a commercial context. Our primary goal is to find a way to combine these two tools into a method for calculating the long term capsize probability. With GULLIVER we are able to account for the effect of involuntary speed loss due to waves, wind and current and to quantify the encountered weather conditions in terms of (long term) scatter diagrams. This part of the approach is described in Section 2. With FREDYN we are able to identify the conditions in which the ship is prone to capsize. This part of the work is described in Section 3. Combining the two gives an impression of the average short term capsize risk. The final step is to translate this short term capsize risk into a long term capsize risk, which is described in Section 4.
As for the captain’s influence on the safety of the ship, Gulliver provides us some means to quantify the effect of the captain’s decisions based on the ship behaviour. However, at the time of writing this paper, these measures are restricted to reducing speed whenever a certain criterium is exceeded; they do not (yet) include the possibility to change course during a trip. In Section 5 we will present a method to mimic course-deviations a posteriori, in order to estimate the effect of the captain’s decisions on the capsize risk.