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Reliable Verification of Design Parameters at Prototype and Serial Turbines
For reasons of cost reduction, modern large wind turbines and blades are designed to use less material and to make use of new materials. They are designed using new simulation tools and smaller safety factors, going beyond established knowledge. Therefore, turbine type certification requires simulation validation by real-life prototype measurements to prove that design dimensions (e.g. clearance between blade tip and tower), parameters and assumptions assure safe operation during the planned service life. For safe and reliable operation, every serial turbine also has to comply with certified design parameters. This keeps O&M costs and lifetime consumption low, despite unmanned 24/7 operation in remote areas. Hence, highly accurate but also cost efficient and safe measuring methods are needed to avoid excess fatigue loads, such as those from resonance issues related to the tower’s natural frequencies or intolerably high rotor imbalance and blade angle deviation. For these applications, video-based analysis is a suitable method to measure motion and vibration.
By Anke Grunwald, Christoph Heilmann and Michael Melsheimer, BerlinWind GmbH, Germany
For reliable, long-lasting and safe operation, as well as low O&M costs and avoidance of damage-related standstill, it is essential to validate design simulation by prototype measurements. In addition, serial turbines have to comply with design parameters relevant for fatigue and lifetime consumption. This requires accurate and effective field measurements.
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Assessing Long-Term Hub Height Wind Specifications
Numerous wind energy yield assessments are based on measurements carried out with a met mast located at a lower height than the hub of the planned wind turbine. This article deals with the issue of vertical extrapolation of measured wind characteristics. It benchmarks four distinct empirical extrapolation methods based on a simple measurement set-up – two heights of anemometers, a wind vane, a short-term measurement of a remote sensing device (lidar, sodar) and different assumptions about the wind shear (α). These methods were tested on nine of Maïa Eolis’ 80-metre met masts in France and benchmarked to a WAsP calculation.
By Olivier Coupiac, Maïa Eolis, France
The first three methods, alpha-series, alpha1 (α1)-sampling and alpha-correlation assume the consistency of the wind shear along and above the met mast, which makes them quite sensitive to near-ground turbulence and to the height of the first anemometer. The last method, alpha2 (α2)-sampling, investigates the wind shear dependency on wind speed, direction and atmospheric stability.
A permanent challenge for industrial wind assessment is to select the most efficient way of assessing the long-term hub height wind specifications, though keeping the uncertainties at minimum. Performing long-term measurement campaigns at great heights can be incompatible to project timescale and budget. However, the vertical extrapolation can be a major source of uncertainty [refs 1, 2, 3].
Methods
The tested methods rely on a middle-term (typically one year) wind measurement campaign carried out with a met mast at a lower height than the planned wind turbine's hub (hh). We assume here a basic equipment set-up with one wind vane and anemometers at two different heights h1 and h2 (with h2>h1) in order to calculate the vertical wind shear – see Figure 1.
Alpha-series
We assume here that the vertical wind shear α2 between the mast and the hub equals at any time the wind shear along the mast α1 calculated from the two wind speeds u2 and u1 at heights h2 and h1. The hub height wind speed uhh time-series is then calculated from the highest wind speed u2 and α1.
Alpha1-sampling
Because of the near-ground turbulences, the calculation of α1 can rely on a noisy signal and result in non-realistic values for the hub height wind speed uhh. The α1 sampling method aims to bypass this drawback by averaging α1 in wind speed u2 and direction bins. Another version of this method uses atmospheric stability bins, which try to account for the strong time and seasonal dependency of the wind shear, as shown in Figure 2. The new assumption will then be that the two vertical wind shears α1 and α2 are on average equal for each direction, wind speed and stability bin.
Estimating atmospheric stability
As the measurement set-up of the met mast often does not allow a reliable estimation of atmospheric stability, this information is extracted from reanalysis data [ref. 4]. In this article we will only deal with the Monin-Obukhov length from MERRA [ref. 5] data. Monin-Obukhov length values (MOL) are sorted into three different stability classes, as detailed in Table 1.
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First Reference to Help Ensure Decisions are Rational and Unbiased
The Top 30 Wind Turbine Faults chart is a database of the most significant failure mechanisms, identified through a failure modes and effects analysis (FMEA), which is validated and regularly updated using information available in the public domain and Lloyd’s Register’s experience of working with wind farm operators. This provides a basis for intelligent decision-making during new product design, sensor selection, configuration of condition monitoring software, SCADA specification, O&M management specification and O&M task prioritisation.
By Mark Spring, Senior Wind Loading Specialist, Lloyd’s Register, UK
A risk-based approach ensures a logical, balanced treatment of failure mechanisms. Quantitative and qualitative inputs are combined, relating both to common and rare failure scenarios. The data is converted into knowledge and stored in a consistent manner, using advanced software which is readily interrogated to present clear health and performance indices to be used for decision-making.
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The Multiple Facets of this Subject, Challenges and Latest Advances
Noise is one of the environmental impacts of a wind farm that requires attention, and can in some cases represent a key constraint on the farm’s operation. Regulation and therefore control of this noise has tended to focus on the level or ‘loudness’ of the noise, which is challenging in itself to measure. But recently increasing attention has been given to features in the noise (in other words its character). Objective measures of these aspects have been developed but they still require careful and time-consuming analysis. Advances in the capabilities of sound measuring equipment are, however, helping practitioners and wind farm operators to obtain results more directly.
By Matthew Cand, Executive Acoustic Engineer, Hoare Lea Acoustics, UK
Although wind turbines are not in themselves very noisy when compared, for example, to large industrial plant or transportation sources, they still produce a certain level of noise which needs to be adequately controlled. This noise is produced by the rotation of the blades as well as mechanical components in the nacelle.
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A Pathway to a New Offshore Wind Business Model
Numerous organisations and agencies are focused on offshore wind cost reduction initiatives throughout the life cycle and across the supply chain but few are focused on the issue of installation. Capacity will have to increase three-fold to meet even the most conservative of 2050 estimates for offshore wind energy needs. While significant cost reduction is a big prize, it and the problem of lack of construction capacity will not be solved without major change, unless there is massive expenditure on installation vessels. However, as projects are risk averse and resistant to change and the industry is capital intensive, the risk of major change is not one that projects are keen to bear. For them, minor change is preferable. This article sets out how a pathway to major change can help the industry mature and reach its potential.
By Matt Bleasdale, Director, OWLC, UK
The offshore wind industry is still in its infancy and suffering from growing pains. Currently about 11GW are installed worldwide and industry forecasts for 2050 range between 150 and 350GW. To achieve even the low estimate, costs need to be reduced and installation rates increased.
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The Blade Way Concept
During the past few years the market for servicing existing wind turbines has grown rapidly. One of the more significant market segments is for exchanging blades. Commonly, large mobile cranes are used, together with a blade yoke. The costs associated with the use of a crane are high, and so cheaper solutions are being sought.
By Per Fenger and Ruben Tjell Lambertsen, Liftra, Denmark
The already high costs of using cranes are often increased significantly by the groundworks necessary to improve the approach roads to the wind farm and allow access across the site itself. Availability is another issue when using large mobile cranes. It can take months to get the needed crane to the desired spot, due to the limited number of these cranes and the distance to the wind farm. One solution to the above challenges is to do away with the need for cranes by attaching pulley blocks to the upper blades and suspending two wires between these and two ground-placed winches. Two yokes are attached onto the wires, one for the blade root end and one for the blade tip end. The blade is clamped and secured by the two yokes which then transport the blade along the wires using the principle of a cableway – which is why the product is named Blade Way.
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Advanced Modelling Makes This Possible Without Compromising Safety
Unlike wind turbine towers and rotors, which are fabricated under controlled conditions, in general foundations must be tailor-made. This is because soil conditions (hard or soft) and available space dictate the solutions for the foundation. However, the perception is that optimisation of foundation designs leads to higher risk. Maybe due to this the average foundation designer takes a simple, conservative and conventional design approach. But a client should look at things in a different way. Saving money is one thing, saving materials and reducing CO2 emissions is another. A client should aim to have an optimised design. The skills and experience are available and have shown that substantial design optimisations are possible, without increasing risks.
By Axel Jacobs, Civil Consultant Wind Energy, ABT, The Netherlands
Figure 1 shows an example of a project were something went wrong with regards to the foundation. You can look at it in two ways:
