Citation

Boonstra H, Blok J, van Daalen E. Harbin Gongcheng Daxue Xuebao 2006; 27(Suppl 2): 50-58.

Affiliation

Marine Technology, Delft University of Technology, Delft, Netherlands

Copyright

(Copyright © 2006, Gai da xue)

DOI

unavailable

PMID

unavailable

Abstract

Traditionally the intact stability of ships is judged against regulations or criteria based on IMO recommendations, which originate from the famous Rahola (1939!) approach. The criteria do consider the ship in a flat sea (no waves) and consider only initial metacentric height and static and dynamic arms of stability at certain (ranges of) heeling angles. The advantage is that the criteria are simple to use, but disadvantages are a) effects of waves are not taken into account, b) the level of safety is unknown and c) the criteria are based on statistics derived from ship types and ship shapes of 50 or 60 years ago. With present-day computer tools however it has become possible to actually simulate in the time-domain the behaviour of a ship in a seaway, taking into account the encounter of sea-states and non-linearities in vessel motions. In the 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 analysis was done through a combination of two existing computer programmes: GULLIVER and FREDYN. 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. The primary goal is to find a way to combine these two tools into a method for calculating the long term capsizing 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. With FREDYN we are able to identify the conditions in which the ship is prone to capsize. Combining the two gives a good indication of the average short term capsize risk. The computer tools provide us some means to quantify the effect of the captain's decisions based on the ship behaviour, through changing sailing speed and to mimic course-deviations a posteriori, in order to estimate the effect of the captain's decisions on the capsize risk. The final step is to translate this short term capsize risk into a long term capsizing risk. We believe that the method described can in the future be used to judge the stability of vessels in a more realistic way than using present day criteria. Two options are envisaged: a) direct calculation for each individual ship or b) simulations for a group of ships or ship types, from which more easy-to-use guidelines are derived, still taking into account the absolute safety levels.