Onshore wind turbine technology is moving offshore, and the offshore wind industry tends to use larger turbines than those used over land. This calls for an improved understanding of the marine boundary layer. The standards used in the design of offshore wind turbines, particularly the rotor-nacelle assembly, are similar to those used for onshore wind turbines. As a result, simplifications regarding the marine boundary layer are made. Atmospheric stability considerations and wave effects, including the dynamic sea surface roughness, are two major factors affecting flow over sea versus land. Neutral stratification and a flat, smooth sea surface are routinely used as assumptions in wind energy calculations. Newly published literature in the field reveals that the assumption of a neutral stratification is not necessarily a conservative approach. Design tests based on neutral stratification give the lowest fatigue damage on the rotors. Turbulence, heat exchange and momentum transfer depend on the sea state, but this is usually ignored, and the sea surface is thought of as level and smooth. Field experiments and numerical simulations show that during swell conditions, the wind profile will no longer exhibit a logarithmic shape, and the surface drag relies on the sea state. Stratification and sea state are parameters that can be accounted for, and they should therefore be considered in design calculations, energy assessments and power output predictions.161 weather-sensitive installations or decommission operations. Also, day-to-day metocean conditions need to be forecast with high accuracy, and services must be tailored to specific operations, i.e. tuning the daily operation of the farm, planning maintenance work and reporting expected power output to the market.The offshore wind industry therefore relies on accurate assessments of the site-specific metocean climate and also on forecasts of the same metocean conditions under installation and decommission as well as in the operational modus. The different stages of the wind industry are closely linked to each other. Forecast methods are sometimes used in the assessment and design phase (such as hindcast models), and the forecasting requirements are again highly influenced by the assessments and the design guidelines. As stated in the latest NEK IEC 61400-3 standard (Wind Turbines-Part 3: Design Requirements for Offshore Wind Turbines, hereafter named IEC 61400-3), 3 issued in 2009, it is also important to assess the wave climate and other oceanographic features. Hence, extensive knowledge of the geophysical processes in the marine atmospheric boundary layer (MABL) is essential in all five stages.The aims of this paper are to highlight the major simplifications made with regard to the MABL in the governing standards for the offshore wind industry and to some extent indicate the implications of these simplifications. In doing so, the paper explores the gap between 'best knowledge' (science) and 'best practice' (codes, standards). A successful outcome from the major inves...

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In this paper the effect of wave influenced wind on offshore wind turbines is studied numerically.The wave is seen as a dynamical roughness that influences the wind flow and hence the wind turbine performance. An actuator line representation of the NREL's 5 MW offshore baseline wind turbine is placed in a simulation domain with a moving mesh that resolves the ocean waves. These wave influenced wind turbine simulations, WIWiTS, show that the wave will influence the wind field at the turbine rotor height. Both the produced power and the tangential forces on the rotor blades will vary according to the three different cases studied: wind aligned with a swell, wind opposing the swell and wind over a surface with low roughness (no waves).