Wake losses are perceived as one of the largest uncertainties in energy production estimates (EPEs) for new offshore wind projects. In recent years, significant effort has been invested to improve the accuracy of wake models. However, it is still common for a standard wake loss uncertainty of 50% to be assumed in EPEs for new offshore wind farms. This paper presents a body of evidence to support reducing that assumed uncertainty. It benchmarks the performance of four commonly used wake models against production data from five offshore wind farms. Three levels of evidence are presented to substantiate the performance of the models:• Case studies, i.e. efficiencies of specific turbines under specific wind conditions; • Array efficiencies for the wind farm as a whole for relatively large bins of wind speed and direction; and • Validation wake loss, which corresponds to the overall wake loss within the proportion of the annual energy production where validation is possible.The most important result for predicting annual energy production is the validation wake loss. The other levels of evidence demonstrate that this result is not unduly reliant on cancellation of errors between wind speed and/or wind direction bins.All of the root-mean-squared errors in validation wake loss are substantially lower than the 50% uncertainty commonly assumed in EPEs; indeed, even the maximum errors are below 25%. It is therefore concluded that there is a good body of evidence to support reducing this assumed uncertainty substantially, to a proposed level of 25%.

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A wider use of composites in larger, commercial vessels has been limited by initial costs and fire regulations, but both of these obstacles are diminishing. Increasing fuel costs and more stringent emission requirements have heightened the value of lightweight structures. Due to the higher acquisition costs and other entry barriers, composite designs must be as cost efficient as possible in order to compete with traditional steel or aluminium designs. The purpose of this article is to investigate which fibre-reinforced polymer materials and types of structures are most suitable for different parts of a ship design in order to minimize weight or cost. This is done by designing and comparing individual composite panels while varying a wide range of input parameters and strictly following the 'Det Norske Veritas (DNV) Rules for Classification of High Speed, Light Craft and Naval Surface Craft'. The results are presented as weight and cost comparisons between materials and structures and also degree of utilization for the different design criteria; carbon fibre structures are on the average 20%-30% lighter than glass fibre structures but are consistently more expensive. The results also indicate that sandwich panels in most cases are lighter than single-skin panels, and that for sandwich structures, the mechanical properties of the core material are commonly the critical design criterion. The minimum amount of reinforcement stipulated by the rules is also found to be a critical factor.

SUMMARYParametrization of vertical diffusion tends to cause excessive smoothing of free-troposphere frontal-zone gradients in several forecast models. It has been observed that these models produce frontal structures that are diffuse, and that the dynamic stability of the frontal zones is higher in forecasts than in the corresponding analyses. Because of their inherent characteristics, frontal zones are sensitive to the way turbulent diffusion is parametrized in the model calculations. In several models, the estimation of vertical diffusion is based on the theory for mixing within boundary layers, assuming that the free-tropospheric turbulent exchanges may be determined by an extension of boundary-layer theory. This is an unrealistic assumption and consequently the observed model de ciency in describing upper frontal zones may to some signi cant degree be attributed to the crudeness of the vertical-diffusion scheme. To address the problem, and in an effort to achieve further understanding of instabilities and turbulent uxes associated with frontal dynamics, a two-dimensional, semi-geostrophic, nearly adiabatic model describing a vertical cross-section through an idealized frontal zone is presented. The model utilizes the same kind of vertical-diffusion scheme as numerous operational forecast models. Different types of instability generated in the model's interpretation of the frontogenesis process are presented and discussed in the context of vertical diffusion. Results show that turbulent diffusion as described by the parametrization scheme plays an important role in smoothing the frontal gradients, especially within the shear zone below the jet stream. Furthermore, the scheme shows sensitivity to the choice of parameters. Extreme values of wind shear and temperature gradient in frontal zones are likely to be weakened by the parametrized turbulent uxes. These effects impact on short-range forecasts but may also affect the results when modelling future climatological scenarios.