0.2 0.4 0.6 0.8 1


0 0 5 10 15 November 2009 20 25


calculated measured


30


motions increases from 12.3s to 13.5s. This 1.2 second change in roll period due to draft change is small, hence the roll RAO, used to calculate the roll response in historic weather, is considered independent of the draught.


Realistic estimate With a stochastic linearisation of the quadratic roll damping from the model tests, the standard deviation of the roll motions can be evaluated in the hindcast weather. The figure compares these calculated roll motions with the measurements for the month of November 2009. The method gives a qualitative idea about the roll damping available in the field but makes it difficult to quantify since the ratio between measurement and calculation varies over time.


Instead of comparing calculated and estimated roll of the same time periods, a novel method was proposed to study the distribution of roll against probability of exceedance. This distribution is also driving the fatigue life consumption. The measured and calculated distributions are compared in the second figure, which also shows the results with the model scale damping in calm water are conservative.


100


calculated measured


101


102


103


0


0.2


0.4


0.6


0.8


1 roll st.dev [deg]


1.2


1.4


1.6


1.8


Both comparison methods indicate that the predicted roll motion is conservative due to the conservatism built into the roll damping estimate. By comparing occurrence probability and exceedance probability of the calculated and measured roll motions, the probabilistic


method can be used to quantify the in- field roll damping as well. This probabilistic method provides a realistic estimate of the roll damping for full-scale results and demonstrates the effect of the bilge keel modifications.


report 15


Roll st.dev. [deg]


prob of exceedance []


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