A methodology for assessing the effect of different sources of uncertainty on the calculation of load effect on offshore structures is presented. A consistent classification of uncertainties was adopted and used as a basis to develop models to estimate the effect of different uncertainties on specified design loads. It is shown that distribution parameter uncertainties arising from limitations on the quantity of statistical data are not likely to have a significant effect on design loads. By contrast, model uncertainties can greatly increase the design loads, and the increase is sensitive to the probabilistic models used to describe model error. The methodology and results can be used by design engineers to take model uncertainties into account in estimating specified loads. They also form the basis for developing and calibrating a new information-sensitive code format.