Durability and Fatigue Advances in Wind, Wave, and Tidal Energy

Engineering Integrity Society

4 November 2008

BAWA, Bristol

ABSTRACTS

The following abstracts are available and further ones will be added shortly:

"Fatigue life prediction of Teeter bearing springs used in wind turbine"
R. K. Luo, W. J. Mortel, X. P. Wu - Trelleborg IAVS & School of Civil Engineering & Architecture, Central South University, China

Rubber-to-metal bonded springs are widely used in industry as anti-vibration components giving many years of service. The Teeter bearing rubber springs are important components of a wind turbine. The springs, known as a conical rubber bush, consist of metal plates and bonded with rubber through a moulding process. Based on the requirements from the engineering applications several suitable finite element models were generated. Since the component has two materials it is necessary to use two different criteria to estimate the service life. A British Standard BS7608 (Fatigue design and assessment of steel structures) was employed to assess the effects of the fatigue loads on the metal parts of the spring. For the rubber materials there are several hyperelastic material models that are commonly used to describe rubber and other elastomeric materials based on strain energy potential or strain energy density. Here a Mooney-Rivlin model was used to represent the elasticity of the rubber. The fatigue life estimation for the rubber parts of the spring was based on previously-obtained data for the rubber material used and on an effective stress. The effective stress was a function of the principal Cauchy stress ranges taking multi-axial loading effect. Three loading spectra, which consisted of Teeter angle loads (27 cases), axial loads (23 cases) and radial loads (21 cases), were analysed. The results have shown that the service life of the conical bush can meet the application requirement.

In addition to the service life evaluation heat generation and natural frequency analyses were also carried out. The information can also be used at the stage of the product design.

"NDT and Strain/Deformation measurement technologies for Green Energy Applications"
R. Wood - Dantec Dynamics Ltd

The high demand for green energy and especially the electrical power generated by wind turbines is driving an increasing need for fast and reliable inspection methods during production and also in-service. These requirements can be perfectly matched by the latest optical measurement technologies including Digital Image Correlation (DIC) and the NDT technique of Laser Shearography. Each method can be applied to various stages of the production process or the in-service inspection. DIC is perfect for optimising the blade structure during the development process whilst Shearography can be used to detect anomalies inside the material. Both technologies are currently used in the wind turbine industry for amongst other things wrinkle detection and quantification.

Digital Image Correlation provides 3-dimensional, full-field strain and deformation data using a stereoscopic camera setup. The object must be prepared with a stochastic pattern which will provide the basic information for the image correlation process. The object is then loaded either statically or dynamically and images are acquired either manually or automatically. The hardware setup is facilitated by a fully automatic calibration and evaluation procedure which makes the system easy to use for the operator. The results are displayed graphically showing all available data with on an icon and this includes all information about the object’s response (contour, displacement and strain). Therefore, unexpected principal strain directions or regions of high strain can be easily identified. Special gauge tools including a line, polygon and point tool are available to evaluate the results in more detail. All relevant data from displacements and strain in all directions are directly available after the measurement. This allows a quick overview of the object behaviour under loading and previous FEM data can be validated. This offers the operator a unique opportunity to actually see how certain areas of the sample respond and if they comply with the expectations based on the FEM results and to optimise the use of other technologies.

Non-destructive testing with Shearography offers a unique view inside the material under loading. This laser based interferometric measurement principle provides surface displacement information which allows a detailed examination of the area of interest. The active inspection technology uses the response of different stiffness levels inside the structure based on mechanical, pressure, vacuum, temperature or vibration loading. Shearography can display defects like wrinkles, delaminations, stringer separations, “zero volume” bondings, debondings, and many other defects by subtracting two images under different loading levels from each other. The result is the stiffness change between both images where potential defects will be easily detectable. The measurement takes only a few seconds and results will be displayed in a live real-time mode. The hardware set-up time is reduced to a minimum and therefore a fast and easy inspection is accomplished.

This presentation will describe the technologies and the available hardware as well as describing relevant measurement examples from different application areas including specific current Wind Turbine applications.

"Advanced Composite Materials in Wind Turbine Applications: Recent R&D Activities"
P.Hansen, Dr S. Giannis and Dr R. Martin - Materials Engineering Research Laboratory (MERL) Ltd.

The UK Government has set targets of 20% of the UK electricity to be generated by renewable sources by 2020. Experts predict that a significant majority (>75%) of this will come from onshore and offshore wind energy fields. Europe-wide, the wind energy market has grown significantly over the last decade and is predicted to continue to grow. The challenge for the wind energy industry is to further reduce through-life costs for wind farms to enable them to better compete with other energy sources. Cost reductions can be achieved in the complete life cycle of a wind turbine including manufacturing, installation and operation. In particular, operational costs can be reduced either through more efficient wind turbine designs or by reducing maintenance costs of both the turbines and blades. The wind turbine industry in the UK is moving towards large offshore wind farms where the cost of inspections will be exorbitant and will need to be kept to essential only.

Wind turbine blades, which are complex structures, often comprise of glass fibre reinforced polymer composites (initial designs were using glass fibre/polyester systems), wood and significant amounts of adhesives. As blades become larger the need for more lightweight materials leads to the extensive use of high performance carbon and glass reinforced epoxy composites. The exact material selection is heavily dependent on the exact size of the blade, the weather conditions that this will experience while in service, the speeds of the rotor that will be installed, etc. Demand for high performance composites is growing for new generations of >30m wind turbine blades.
For the structural spars thermosets are the materials of choice over thermoplastics since the latter have high processing costs to achieve the desired low void content and the material suppliers have significant investment in developing these materials for these markets.

Throughout their lifetime, wind turbine blades will accumulate damage, during manufacture, installation and in service. Damage can result either from deviations in the manufacturing process or mechanical handling, impact damage from handling tools, lightening strikes and many other damaging events. In addition, these structures are exposed to a hostile environment and are subjected to the unpredictable forces of nature

A recent UK Technology Strategy Board (TSB) project has targeted a significant knowledge gap in developing the damage tolerance methodologies for wind turbine spars and in the NDT inspection of these structures. The project sought to provide guidance in structural integrity management by using Damage Tolerance (DT) methodologies within design and manufacturing.

The project objective was to use applied research to produce validated technologies to improve the SI management of long, structurally loaded composite components, specifically wind turbine blades. The project has developed rapid ultrasonic methods, calibrated Structural Health Monitoring (SHM) methods with inspections and developed relevant DT test and numerical analysis methods for expected damage threats.

This presentation will aim to provide an overview of the main elements of the project and to present data from the testing and analysis performed.

"Type testing of high power wind turbine gearboxes – advances in modern instrumentation"
Prof. J.ROSINSKI and D. SMURTHWAITE - Transmission Dynamics, Northern Power Transmission Research Laboratories

The global increase in energy generated from the natural resource of wind power is growing at an astonishing rate. As the current trend to develop wind turbines of increasing power capacity continues, the requirement to validate the performance and reliability of new designs is more important than ever.

A fundamental consideration before launching a new design of power transmission system is the establishment of appropriate procedures for type testing. Typically, type testing should include full functionality test, overload tests, cold start, operation under emergency conditions, verifying gear mesh load intensity distribution (KHb) and for planetary gearboxes with more than three planets verifying load sharing distribution (Kg). Recent experience of a significant amount of unexplained bearing failures calls for additional requirements of type testing which should target the dynamic behavior of cylindrical rolling element bearings. In recent years Transmission Dynamics have
elaborated a comprehensive range of ultra-miniature instrumentation, which includes electronic gear mesh alignment modules deployed in multiple stages of complex designs of epicyclic stages, planet load sharing distribution, digital multi-channel torque and shaft bending measuring systems and planet carrier deflection measurements. The system can be further enhanced by including instrumentation targeting the dynamic behaviour of individual rolling elements and shaft movement as a result of complex interactions of a complete power transmission system, including blades and power electronics.

The configuration, installation and operation of such instrumentation systems is described in this paper. The paper concludes that type testing of wind turbines should include detailed study of the dynamic response of a complete transmission system deployed in fully operational wind turbines, followed by an extended period of in-service load measurements.

"Effects of wind and wave loads on stress intensity factors of a cracked offshore structure"
M. R. Ayatollahi, Karo Sedighiani, M.Sc. student - Fatigue and Fracture Lab., Department of Mechanical Engineering, Iran University of Science and Technology,
Offshore wind energy has received much attention in the recent decades as one of the new alternatives for traditional power generation techniques. However, there are still concerns about reliability and safety analysis of structures that support the wind turbines in the harsh conditions related to the wind and sea wave loading. In comparison with onshore applications, even greater focus is expected for a reliable lifetime analysis of the offshore wind turbine structures due to their much higher maintenance and replacement expenses. Meanwhile, the cyclic nature of wave and wind loads together with the corrosive effects from the sea water are major factors for creation and growth of flaws and cracks in offshore structures. These cracks can be the cause of instantaneous failure in marine structures. Therefore, the cracks that are found during the service life of offshore structures should be studied carefully.

Offshore wind turbine structures are generally subjected to complex loading conditions. Therefore, the cracks that are generated in the support structures often experience mixed mode loading. The mode I and mode II stress intensity factors KI and KII are two important parameters for fatigue life estimation and fracture resistance analysis of cracked structures subjected to mixed mode I and II loading.In this paper, a crack is considered in a support structure that can be typically used in the offshore wind turbine constructions. The finite element analysis is employed to determine the related stress intensity factors for different loads.

The Tripod model is one of the favorite support structures which are used for offshore wind turbines particularly in deeper waters. In this structure, a huge central column is welded to three lateral braces. Due to the stress concentration and welding effects, the heat affected zone situated next to the welding area is one of the most likely locations for initiation and growth of fatigue cracks. In the present research, a crack was considered at this critical point and the stress intensity factors KI and KII were calculated from finite element analysis for different crack lengths. A refined mesh was introduced around the crack where high stress gradients are anticipated. Also singular elements, which are highly recommended for crack modeling, were used for the first ring of elements around the crack tip. Three types of loading were considered separately on the tube: a bending moment M, a shear force S and an axial compressive load P. The bending moment and shear force represent the effect of wind and wave loads and the compressive load corresponds to the weight of wind turbine and the tube itself.

It is shown that the variations of stress intensity factors with crack length differ significantly for the three loading types. Also the mode II stress intensity factor is not negligible compared to the mode I stress intensity factor. Therefore, for analyzing the fatigue life or the fracture load of similar cracked offshore structures, appropriate mixed mode crack growth criteria should be employed together with the curves derived in this research for stress intensity factors.

Improved Materials and Design Techniques for Tidal Turbines
Paul Harper, Stephen Hallett (University of Bristol)
Angus Fleming, Matthew Dawson (Aviation Enterprises Ltd.)

Tidal turbine blades are generally manufactured from a combination of glass and carbon fibre reinforced composite materials, which offer increased fatigue and corrosion resistance relative to an equivalent metallic component. However, there remains much potential for improved fatigue performance and design methods due to the following factors:
There is currently no composite resin system specifically designed for tidal turbine applications.
Fatigue design margins are excessively high due to the lack of an integrated test and modelling methodology, which can accurately predict crack initiation and propagation within the blade structure.

A collaborative Technology Strategy Board project, ‘New Materials and Methods for Energy Efficient Tidal Turbines (NEW-MMEETT),’ involving Aviation Enterprises Ltd. (blade manufacturer), Advanced Composites Group (ACG) Ltd. (resin manufacturer), Materials Engineering Research Laboratory (MERL) Ltd. and the University of Bristol has recently been initiated to address these issues. This will enable a reduction in the mass of material required for blade manufacture, essential for reduced lifecycle costs, whilst also ensuring required in-service lifetimes are met with minimum maintenance requirements. The outcomes of the programme, with respect to both more fatigue resistant resin systems and improved design methodologies, capable of accurately predicting the nature of fatigue damage progression, will have strong potential for knowledge transfer to the wind turbine industry.

This presentation will outline the limitations of current design/manufacturing methodologies applied to tidal turbine blades, before providing an overview of how these will be addressed by the NEW-MEET project. Specific focus will then be placed on the fatigue modelling technique which will be applied within the programme, which has been developed by the University of Bristol.

Using finite element software, essential for analysing stress states in complex 3D structures, the model enables fatigue crack propagation to be simulated both between composite plies (delamination) and along adhesive bond-lines. Due to lack of through-thickness reinforcement, these are common fatigue failure mechanisms in composite laminates. Once calibrated using experimental results from standard fracture toughness specimens, the model can be applied to critical failure regions in the full-scale blade structure, such as ply-drops and bond-line edges. Use of this technique allows for a damage tolerant design approach, where some crack growth is allowed provided this does not compromise structural integrity over the required in-service lifetime. Such an approach is essential for enabling reduced fatigue design margins in both tidal and wind turbine blades. The ability to model specific damage mechanisms can also provide guidance on improving the design of geometric details and composite lay-up sequence for enhanced fatigue life.

Simply complicated; fatigue design and analysis of bottom hinged wave energy converters
Donald Naylor – Aquamarine Power Ltd.

There are several similar concepts under development of flap type wave energy devices hinged at the seabed. Aquamarines Oyster® is one of the most advanced; a full scale prototype having been manufactured and ready for installation at the EMEC test site in 2009.
On the face of it, flap type devices are very simple and offer the advantages of prolonged maintenance free operation. However, like the majority of wave energy devices, they are subjected to large, fully reversing loads with every wave cycle. Their design has been shown to be driven by fatigue resistance rather than ultimate strength.
Aquamarine have used a combination of wave tank test data, hydrodynamic models, and Power Take Off (PTO) system simulations to derive time domain data for the hydrodynamic and cylinder loading on Oyster®. This load data has been counted and combined with stress results from detailed finite element models to generate fatigue life predictions.
Interestingly, fatigue life is dominated not by the biggest, most powerful waves, but by the design of the PTO and the control of the system in the commonly occurring small and medium seas.