durability and fatigue group
Engineering Integrity Society
Durability and fatigue advances in wind, wave and tidal energy
Thursday 30 September 2010
BAWA, Filton, Bristol
Abstracts:
PRIMaRE Marine component testing facility for marine energy converters
Philipp R. Thies & Lars Johanning, University of Exeter.
Currently, several marine energy converters (MECs) are proceeding from the prototype stage towards commercial deployment. Whereas prototypes require the demonstration of e.g. fundamental working principles, conversion efficiency and survivability; commercial deployment is driven by plant-performance indicators.
Reliability, availability and maintainability heavily influence cost and revenue and therefore govern the viability of wave energy devices. As wave energy competes with other forms of electricity generation ‘there is very little margin between the technological development phase and the high reliability demonstration’ [1].
At this stage of early industrial development the reliability assessment of MECs is a challenging task due to an omnipresent lack of applicable failure rate data, leading to rather unfavourable and highly uncertain results. A common issue is the application of proven technologies (e.g. hydraulics) in this new marine environment and how this impacts on the component/system reliability.
Fatigue is conceived as a governing failure mechanism for MECs [2, 3]. However, accurate predictions on fatigue life are very difficult with available methodologies; hence the best information on fatigue properties come from realistic service-simulation fatigue tests [4]. Reliability testing is essential for any engineering development programme with high risks involved [5]. The necessity to engage in component testing at this stage of marine energy development is reasoned by three aspects:
- The need to reduce costly safety factors
- The development towards commercial deployments
- The dependency of stakeholders on reliability test results
The intention of the proposed paper is to review the means of component testing and service-simulation testing, and to provide details how the PRIMaRE group at University Exeter is actively engaging in the establishment of failure rate data for the marine renewable energy industry. The capability of two facilities, namely i) the South Western Mooring Test Facility (SWMTF) and ii) the Dynamic Marine Component Test facility (DMaC), to perform specimen and accelerated component testing for marine renewable energy components will be discussed. It will be discussed how loads that are experienced in the field through prototype testing at SWMTF, or through information from WEC developers, will be used to accurately replicate load conditions for accelerated testing at the DMaC facility. Component lifetimes and failures can be simulated and subsequently optimised and mitigated.
The appeal of such testing is not only to obtain meaningful and necessary data for marine energy applications but to gain valuable insight into the physics of failure. Components tested under service simulated conditions could be evaluated regarding performance, expected lifetime and subsequently be
(cost-)optimised.
References
[1] Bechou, L., Dantoa, Y., Deletagea, J.Y., Verdiera, F., Deshayesa, Y., Fregonsea, S., Maneuxa, C., Zimmera, T. & Laffitte, D. (2008). Challenges and potential of new approaches for reliability assessment of nanotechnologies. In: Comptes Rendus Physique, 9, 95-109.
[2]Hudson, J.A., Phillips, D.C. & Wilkins, N.J.M. (1980). Material aspects of wave energy converters. Journal of materials science, 15, 1337-1363.
[3]Det Norske Veritas DNV (2005). Guidelines on design and operation of wave energy converters. Technical report, Carbon Trust.
[4] Schijve, J. (2003). Fatigue of structures and materials in the 20th century and the state of the art. In: International Journal of Fatigue, 25, 679-702.
[5] O'Connor, P.D.T. (2008). Practical reliability engineering. Wiley, 4th edn.
Type testing of high capacity wind turbine gearboxes - are we doing enough?
Dave Smurthwaite, Prof. Jarek Rosinski, Transmission Dynamics
Demand for the supply of high capacity wind turbines imposes an unprecedented strain on designers. Five megawatts gearboxes have just
been deployed in routine operation and 10MW units are now leaving drawing boards and will be ready for offshore installations in 2015.
With the increasing size of the new gearboxes designers worldwide are entering entirely new challenges. The weight of 5 MW gearboxes exceeds
70T and the weight of modular gearboxes for 10MW units will exceed 400T with input bearing diameter exceeding 3 metres. The weight of planet
carriers in epicyclic stages is such that mesh alignment changes dynamically and large mesh misalignment cannot be eliminated with any of
the known methods of supporting the planets, planet carriers and the sun gears. There are no known or proven designs of these large dimensions
which may offer feedback on their long-term reliability or failures modes. With the new designs which break new ground in almost every
aspect of technical implementation, technological risks are high and the cost of repeated failures will be impossible to accommodate by any of
the existing globally operating suppliers.
The new challenges in gearbox design require high attention to type testing, diagnostics and condition monitoring. This paper outlines the
need for comprehensive type testing, supported by implementation of modern wireless instrumentation techniques for deployment on rotating
parts inside the gearboxes. The paper describes the challenges and outlines the best practices for type testing in the new demanding environment of power transmission in renewable energy applications.
Designing structures for Wind and Wave Loading
Dr Lewis Lack, M.I.Mech.E., C.Eng, Managing Director, Xanthus Energy Ltd
Wind and waves contain energy and capturing that energy is the prime focus for engineers in the renewable energy market. Dealing with the effects of Newton’s 3rd law of motion is a major design issue for wind, wave and tidal structures. Offshore wind turbines need to cope with all these forces simultaneously. So how can such structures be constructed to avoid fatigue issues? What are the key requirements for the design and how can engineers meet these? What design process should be used and how can the latter be improved to achieve better designs? What lessons can we learn from other industries and is this knowledge directly transferable or is another approach needed? Some answers to these questions will be presented and discussed in this paper.
Alleviation of fatigue in offshore wind support structures
Prof Feargal Brennan, Head of Offshore, Process & Energy Engineering, Cranfield University
Offshore wind energy is experiencing an explosion of activity in response to ambitious renewable energy targets, however the drive to increase turbine size in deeper water whilst at the same time to reduce capex and installation costs in addition to the speed of development means there is a danger that structures may be designed and deployed that are inherently prone to fatigue.
Offshore structures have come a long way since the pioneering early Oil & Gas jackets in the 1960s and 1970s. In forty years of designing and operating large Oil & Gas structures in the North Sea tremendous changes have occurred in development of advanced numerical modelling of stress, fatigue and loading in addition to vast improvements in steel quality/strength, manufacturing processes and inspection, monitoring and quality control.
This presentation will address some of the fundamental areas where current design standards may not be appropriate for offshore wind turbine structures in this new era of advanced sensors and information systems. It will also discuss advanced fatigue alleviation techniques.
Tidal Stream Turbine Blades – Improving Performance and Reducing Cost
Paul Harper, Stephen Hallett (University of Bristol)
Tidal Stream turbine blades are required to operate in harsh marine environments for lifetimes of 20-25 years, where the high fatigue loads demand the use of carbon and glass fibre composites. Although these can provide improved performance relative to metals and have become widely used in the wind turbine industry, their use for tidal stream turbine blades has posed a number of new challenges:
- Very limited understanding exists of the long-term fatigue degradation of composite resins exposed to sea-water. This also applies to the adhesive used to bond the turbine blades to the metallic hub.
- Even when using high-strength carbon-fibre composites, the extreme bending loads on tidal turbine blades demand the use of very thick composite sections at the blade root. Such sections experience high through-thickness loading and are highly susceptible to crack propagation both between composite plies and along adhesive bond-lines.
Since July 2008, 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 been addressing these challenges. Principal outcomes from the work at the University of Bristol have been the development of modelling techniques for predicting the nature of fatigue damage progression within design features of a blade.
This presentation will provide a general overview of work being conducted within the research programme, focusing on the University of Bristol’s fatigue modelling technique. Using standard finite element software and a bespoke ‘interface element’ developed at the University, the fatigue model enables fatigue crack propagation to be simulated both between composite plies and along adhesive bond-lines. The model will be demonstrated on critical failure regions within the blade such as composite ply-drops, which are used to taper the blade thickness from root to tip. The model 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. This gives potential both to reduce fatigue design margins and provide design guidance on issues such as ply lay-up sequence and ply-drop geometry. The modelling technique is also applicable to composite wind turbine blades and any other applications involving delamination and/or bond-line failure.
Acknowledgements
The authors gratefully acknowledge the support of the consortium; Aviation Enterprises Ltd, ACG, and MERL (as well as the University of Bristol), under the Technology Strategy Board funded NEW-MMEETT project
“Structural Capacity” – and scaling up renewables.
Professor S. D. Garvey, University of Nottingham.
Some schoolchildren wonder why it is that renewable energy is expensive – given that the fuel itself is absolutely free. It is straightforward enough to explain why our existing wind, wave and tidal machines cost what they do but that does not answer the important question. In fact, there is no law of physics or engineering that dictates that renewables must be more expensive than fossil-fuelled generation. Thinking only about the “mechanical renewables” (wind/wave/tidal power), we can set aside any specific preconceptions about how the machines must look and focus instead on what the machines do. If we then think about what is needed in each machine to discharge that function, we discover that we really need a measure of the total amount of structurally-active material. This measure is “structural capacity” and it has the same units (Nm) as both torque and work but it is neither of those.
Using the “lens” provided by this measure, we can do some accounting on various mechanical devices but we focus on horizontal-axis wind turbines here. The results prompt us to consider some major changes to offshore wind turbine designs: cable-braced rotors to reduce bending moments in the blades, a floating framework to replace a tall slender tower, fluid-film bearings to replace rolling-element main bearings and (most controversially) a power conversion mechanism that operates within the rotor itself such that all of the power gathered from the wind is delivered in the (storable) form of compressed air.
This talk paints a credible scenario wherein the UK could source ALL of its energy requirements from offshore wind at a cost of ~£150Bn for the primary energy collectors, a further ~£70Bn for expander-generator sets and transmission elements and an additional expenditure of ~£50Bn to provide more than one full week of energy storage for the entire country at present consumption levels. Moreover, we could conceivably do this by 2030 and the vast majority of all of the hardware might be developed, built and serviced by businesses in the UK. The numbers seem astronomical – until they are compared with the ~£100Bn that we presently envisage spending with foreign companies by 2020 on offshore wind turbines having no associated energy storage and the capacity to supply only around 25% of our present electrical energy usage.
Why is there such diversity amongst Wave Energy Converters concepts?
Dr Jamie Grimwade, National Renewable Energy Centre (Narec)
The desire to harness wave energy is a long standing ambition as illustrated by the first patent relating to a wave power concept dating back to 1799. However since this time the efforts to both technically and economically harness this renewable energy source have largely been frustrated. In presenting this topic the author will explore the reasons and rational behind the engineering design decisions that have led to the current crop of very functionally diverse wave energy extraction concepts. This will involve considering;
- The fundamental different ways in which the wave energy exhibits itself in different spatial locations
- The nature of extreme storm events
- The interface systems adopted to convert high force-low frequency wave energy input into low force-high frequency power, typically suited for electrical power generation
Avoiding Fatigue In The Tidal Race
Ian Godfrey, IT Power Ltd
As one of the longest standing, consultant engineering firms to the marine energy sector, IT Power has had a unique history of exposure to this emerging sector. Until now, the sector has been driven by the need to demonstrate the fundamental principles of energy extraction and survival rather than the commercial cost of energy. As a result, engineering design has appropriately accepted the material cost and simplicity of over-engineering in demonstrator machinery rather than the design cost and complexity of designing for fatigue and durability in commercial machinery.
The sector is however beginning to change as investor interest grows, and technology moves from research and development to commercialisation, the need to deliver against forecast cost of energy figures through sophisticated design is becoming more pertinent.
The presentation will look at IT Power's exposure to the changing requirements of engineering in the sector using the design of the Pulse Tidal ltd energy converter as a case study. In this case study I will examine the challenges associated with limited environmental loading data available for design of the 100 kW prototype Pulse machine. I will also look at the complexity and expense associated with improving and rationalising environmental loading data for design at the pre-commercial level required for the current Pulse Tidal machine.
The presentation will also look at a secondary market technology under development by a consortium including IT Power: the use of long range ultrasonic technology (LRUT) to remotely monitor the health of structures used in marine energy conversion.
Enhancing Wind Turbine Condition Monitoring through Blade Load Measurements
Mark Osborne, Moog
The reliability of wind turbines is a key factor in the value proposition for the use of wind power to generate electricity. At this time, turbines are growing in size, while at the same time as designers are under immense pressure to reduce the weight in all areas and increase reliability. For offshore wind turbines, the costs of maintenance, especially unscheduled maintenance are an order of magnitude higher than for the onshore turbines, meaning that effective condition monitoring strategies are needed in order to maximize revenue. The majority of condition monitoring systems on the market are reactive, i.e. they diagnose faults after they have occurred, but what is really needed are pro-active condition monitoring systems, i.e. that monitor the CAUSE of the faults rather than the EFFECTS.
It is possible to derive a reliable, live measurement of the bending moment in the flapwise and edgewise planes at the root of each blade using optical strain guages. This bending moment can be used to individually control each blade to reduce peak to peak loads on the blades and rotor (Individual Pitch Control), enabling blades to be lighter or enabling the rotor diameter to be increased by fitting larger blades without changing any of the other key turbine components. In addition, these loads can be used as an input to the condition monitoring system, making it possible to detect, directly, the build-up of ice on the blades, mass imbalance and aerodynamic imbalance on the blades, monitor the performance of each blade relative to each other, detect pitch misalignments or indications of blade damage and calculate torque at the main shaft about three axes, in order to understand the loads that are going into the gearbox.
In this presentation, we will first ask why use optical strain sensors? and show why optical FBG based strain sensors are an effective solution for long term turbine monitoring. Following on from this, we will focus on some of the key applications related to fatigue and reliability, namely gearbox health monitoring and predictive life calculations, monitoring of turbine power directly from the blade sensors and monitoring of mass imbalance and aerodynamic imbalance on the rotor and show how an effective holistic condition monitoring strategy can improve turbine reliability.
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