Exploring the gap between ‘best knowledge’ and ‘best practice’ in boundary layer meteorology for offshore wind energy

Abstract

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...

Wave Influenced Wind and the Effect on Offshore Wind Turbine Performance

“…MOST is valid in the constant flux layer (where the fluxes are assumed to vary little with height). Under neutral atmospheric stability conditions MOST theory leads to the logarithmic wind profile [5]: (1) where k = 0.4 is the von Kármán's constant, z0 is the roughness length (defined as the height where the wind speed equals zero) and is the friction velocity. The friction velocity is defined as [5], ,…”

“…There are however, no considerations of how the sea state itself can affect the wind profile and the turbulence in the MABL and the following possible indirect effect of waves on the rotor-nacelle assembly [1].…”

“…Momentum from the air is transported to the ocean during the wave development, but the waves themselves will also influence the wind field. This process is often ignored, both in weather forecasting and in the field of offshore wind energy [1]. Different sea states affect the wind field in various ways.…”

“…Occasionally, swells will oppose the wind field, and this is known to give rise to interesting situations with increased turbulence levels over the sea surface [3]. Although it is known that fast moving waves in a low wind regime will influence the whole depth of the MABL, it is still uncertain to what extent the wave induced wind field will affect an offshore wind turbine or a wind farm [1].…”

On Offshore wind turbine fatigue caused by wave influenced wind

“…All though it is known that fast moving waves in a low wind regime will influence the whole depth of the marine atmospheric boundary layer (MABL) [1], it is still uncertain to what extent the wave influenced wind field will affect an offshore wind turbine or a wind farm [2]. The aim of this study is to investigate if a wave influenced wind field will affect the wind turbine loads and fatigue.…”