Page 284 - Practical Design Ships and Floating Structures

long run. However, the risk measure presented herein does not state anything about the frequency of
occurrence of cargo shifts. It may be likely that a vessel will experience at least one cargo shift during
its lifetime, but there is a difference between experiencing one or for example one hundred. An
alternate risk measure, e.g. quantifying the most probable number of cargo shifts, may be of interest as
a complement to the present measure, i.e. the risk of initial cargo shift.
An implementation of the methodology where consideration is taken to operational factors, such as
speed reduction in severe weather, will give a more realistic, and probably lower, risk level. This could
be done by simply reducing speed in waves with significant height above a specified limit. A more
refined method would be to connect the speed reduction or alteration of course to appropriate comfort
criteria and added resistance in waves. Statistical methods to account more realistically for differences
in cargo and lashing equipment, as well as improved cargo models would also result in more realistic
risk levels. An improved cargo model could comprise e.g. deformation of cargo and lashings,
improved criteria for shifting, three dimensional models, and accounting for dynamical effects.
Another area of further research is alternative methods for estimating the probability of cargo shift for
a specific cargo unit in a specific condition, e.g. by the use of probability distribution curves. This
would enable a reduction of the computational time, which is rather large due to extensive simulations.
Since cargo shifting is most probable in severe sea conditions improved ship motion calculations,
especially roll motion with rather strong non-linearity, is of interest.
References

Anderson, P., Koch, M. and Sjobris, A. (1986). Optimum Safety Factors for Securing of Cargo On
Board Ships. Research Report, MariTenn AB, Gothenburg, Sweden
Cram&, H. & Leadbetter, M. R. (1967). Stationary and Related Stochastic Processes. John Wiley &
Sons, New York, US
Ericson, A. (2000). Ship and Cargo Models for Studying the Risk of Cargo Shifting in Waves.
Licentiate Thesis, TlUTA-FKT, ISSN 1 103-470X, ISRN KTWFKT/SKP/L.--00/20--SE, Department
of Naval Architecture, KTH, Sweden
Ericson, A., Kitllstam, K., Rutgersson, O., ThedCen, T. (1 999). Probabilistic Models for Ship Safety in
Rough Seas -Final report forphase I. KFB-Report 1999:34, Stockholm, Sweden
Hogben, N., Dacha, N. M. C. & Olliver, G. F. (1986). Global Wave Statistics. British Maritime
Technology Limited, FeItham, UK
McTaggart, K. (1998). Ongoing work examining capsize risk of intact frigates using time domain
simulation. 4Ih International Stability Workshop. St. John’s 27-29 September 1998
Lloyd’s Register. (1999). World Casualty Statistics 1998. Lloyd’s Register, London, UK
Salvesen, N., Tuck, E. 0. and Faltinsen, 0. (1970). Ship Motions and Sea Loads. Trans. SNAME Vol
78,250-279.
St. Denis, M. and Pierson, W. J. (1953). On the Motions of Ships in Confused Seas; Tram. SNAME
VOI 61,280-357.
Swedish Maritime Administration. (1 994). sjiifrtsverkets forfattningssamling 1994:27 9J. Swedish
Maritime Administration, Sweden (in Swedish)