Uncertainty in forecasted environmental conditions for reliability analyses of marine operations

Marine operations, e.g., the sea transport of heavy objects and the installation of offshore units and equipment, need to be planned and executed with proper consideration for environmental conditions and operational limits with respect to vessel motions and structural loads. Marine operations with a limited duration, usually less than 72 h, are typically designed as weather-restricted operations. The environmental design criteria are thus predefined, and the actual weather conditions are confirmed by weather forecasts issued immediately prior to the start of such an operation. Marine operations of longer duration are typically designed as weather-unrestricted operations, and the environmental conditions are calculated based on long-term statistics, possibly depending on the season. More detailed information about uncertainties in weather forecasts could increase the feasible duration of weather-restricted operations. The uncertainty inherent in weather forecasts, notably that in the significant wave height, is studied. Further, a method to assess the reliability of weather forecasts is described. Data from the Norwegian Sea are used to quantify the uncertainty in forecasted data. The probability of exceeding the design criteria used in the planning of a weather-unrestricted marine operation can be estimated based on forecast statistics. The corresponding uncertainty can be incorporated into structural reliability analyses.