The operation of dredgers at sea may be seriously affected by the marine environment. This can lead to downtime during operations. The question whether a system is able to operate in specific seastates is defined as 'workability'. In recent decades improvements have been made in the hydrodynamic modelling of marine structures. However, the coupling with the actual workability for a certain project and location is less developed. This paper presents Dredsim2000 as the result of a Joint Industry Project to develop an integrated tool to determine the workability. The paper further focuses on two different methods to determine the workability (or downtime) in an accurate manner, using the results of (hydro-)dynamic analysis as input. The first method is widely used in the industry: prediction of the downtime on the basis of wave scatter diagrams. The second method is less common but results in a more reliable downtime prediction: determination of the 'job duration' on basis of scenario simulations. The analysis based on wave scatter diagrams simply checks which entries of the diagram satisfy the operational limits. The combined probability of all acceptable entries results in the workability. The workability is used to correct the required nett productive hours to determine the gross project duration. This approach can be used for each operational mode individually, however applying this approach for a combination of modes is principally impossible In the determination of the gross project duration on the basis of scenario simulations, long term seastate time records are analysed by checking for each subsequent time step which operational mode is applicable. Past events and weather forecast are taken into account. The gross project duration is defined by the consumed time between start of the operation and the moment when the nett dredging hours are achieved. In a case study the two methods are compared and discussed for a realistic dredging project. The clear differences between the methods will be presented and suggestions for further applications in offshore dredging operations are given.