The Statistical Properties of Low-Frequency Motions of Nonlinearly Moored Tankers

The motions and mooring loads of storage/production tankers are dominated by large amplitude. low frequency components related to wave drift forces. In the design stage use is often made of time domain simulation and/or model tests to determine those quantities. In view of the relatively long periods associated with low frequency motions and forces long simulation or model test times are required in order to reduce statistical variance of the results.
In this paper a theoretical expression is given for the statistical variance of these quantities for the case of a linearly moored tanker in irregular head seas. This expression is compared with results of model tests and time domain simulations. For a non-linearly moored vessel model tests and time domain simulations are used to quantify the statistical variance. The results indicate that the main elements governing the statistical variance of the data are the natural period of the moored vessel. the system damping and the simulation/model test duration.

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