Uncertainties related to predictions of loads and responses for ocean and offshore structures

This paper discusses uncertainties related to the prediction of loads and responses for ocean and offshore structures in accordance with the findings by the Ocean Engineering Committee of the International Towing Tank Conference (ITTC). The parameters that may cause uncertainties in ocean engineering model tests, full-scale tests and numerical simulations are presented in terms of physical properties of the fluid, initial conditions, model definition, environment, scaling, instrumentation and human factors. Emphasis is given to the uncertainty sources in model tests involving deepwater mooring lines, risers and dynamic positioning systems and the need for quantifying them. A methodology for uncertainty analysis is described according to the International Organization for Standardization (ISO) Guidance for Uncertainties in Measurement (GUM). As an example of application, the combined and expanded uncertainties in the model tests of a moored semi-submersible platform were assessed and quantified in terms of motion responses, air gap and mooring line tensions. It is concluded that the quantification of uncertainties may be challenging in model tests and numerical simulations of ocean and offshore structures. It is particularly challenging in extrapolating model test results to full scale and utilizing complex numerical models, especially if the effects of hydrodynamic nonlinearities are significant.