Beach nourishment, maintaining excess channels to harbors, dredging trenches for pipe lines and covering pipelines with stone, trailing suction hopper dredges, cutter suction dredges and stone dumping vessels, are used. All vessels are operating in the open sea exposed to wind, waves and current and often hampered by the weather conditions. In the last decade the offshore industry has made a considerable progress to predict downtime. The mentioned offshore techniques to determine the downtime were applied to the existing MARIN computer programs for cutter suction dredges (DREDSIM), DT trailing suction hopper dredges (DPSIM) and DP stone dumping vessels (DPSIM). For the determination of downtime of a dredge at a certain location and during a certain season, wave scatter diagrams combined with current and wind data are often used. The determination of the operating limits in these sea states (downtime points) is of prime importance. If these downtime points or lines are known, the downtime can be calculated as an average downtime by using scatter diagrams. Although it may give valuable information, it does not give the real downtime during a project. A more realistic approach to predict the downtime during a project is the method of the long-term time-domain simulation. Key items in the long-term simulations are: • the long-term time-domain simulation of the weather conditions and • both pre-and post processing of downtime lines and • anticipated scenarios. In this paper the tools as developed at MARIN to predict the operational limits will be presented. The status on the developments at MARIN on the prediction methods to generate swell and sea waves based on available long-term wind data are discussed. These two highlighted key items together with applied scenarios make it possible to predict in a reliable statistical manner the realistic duration of a project to be carried out in the open sea.