Geneva, Switzerland [Renewable Energy World Magazine] The wind turbine sector is growing rapidly both with regards to the number of installed wind turbines, and the capacity of newly developed and manufactured machines. The importance of considering all stages of a wind turbine project as a life cycle circle, together with the need to provide appropriate services throughout the life cycle, cannot be underestimated, Torsten Muuss explains.
Wind power accounts for the dominant share of global investment in renewable energy – total wind power capacity grew by 27% worldwide in 2007 to reach an estimated 94 GW. Today, wind power produces around 1% of global electricity production, but this is projected to increase to 2% in 2010 and to 6% in 2017, corresponding to an installed wind capacity of 270 GW in 2010, and 700 GW in 2017. The annual investment value for this level of development is estimated at €58 billion in 2010, and €118 billion from 2017 and onwards. This compares with a unit cost for a state-of-the-art onshore wind turbine of €1380 per kW installed in 2007.
Given such a large level of investment, the absolute necessity of operating wind turbines on an economically sound footing means that down-time must be minimized. Consequently, operators and manufacturers of wind turbines try to obtain operational availability of more than the 97%, which is often contractually established and guaranteed by the manufacturers.
However, the wind energy market is very demanding – not only for the manufacturers but also for the project developers and owners – and there is a pressing requirement to plan bigger projects and to increase the energy output per site. This trend has driven demand for machines with a larger rated capacity and, as Table 1, opposite, shows, that the rated power of turbines has increased exponentially over the last 25 years.
Wind speed generally increases exponentially with hub height, and larger machines are therefore exposed to higher wind speeds. Together with the larger rotor diameter seen in larger machines, these factors have a direct influence on the power output. But growth in the size and capacity of wind turbines has also resulted in the loads on individual components increasing dramatically.
Due to this rapid market development, companies are under pressure to start building new, larger designs as quickly as possible. Consequently, some turbines have been developed or older designs up-scaled over such a short period that the results from measurements and field data could not be considered in the new design.
Inevitably, a lot of lessons have been learned by all involved parties. The testing period has been prolonged before putting newly developed wind turbine designs into serial production. Nonetheless, it is essential to recognize that overall performance of a turbine not only depends on the design stage, but also on each stage of its complete life time. Most of the wind turbines types are certified to international or national standards and requirements. But none of these Type Certificates available in the market today will give the turbine owner any warranties. The certificate does only state that the design is in conformity with given requirements, if the turbine is manufactured, installed and operated in accordance with the design documentation.
According to the certification standard IEC WT 01 manufacturing evaluation has to be carried out for the type certification. But this general inspection is normally carried out only on the prototype. Manufacturing evaluation activities will have much higher value when the focus is on specific deliveries. 
The wind turbine life can only be fulfilled if certification requirements are sufficient, the turbine is manufactured and installed in accordance with specifications and it is operated and maintained as specified. This clearly emphasises the need for inspection on each unit, especially in case of local manufacturing.
A full life cycle circle services strategy for wind turbines is therefore indispensable in order to predict future damage, to prevent secondary damage to other components and in order to schedule maintenance requirements and therefore keep downtime to a minimum.
The six stages of life cycle services
The complete lifetime of a wind turbine or a wind turbine project may be broken down into six phases, each with its associated life cycle services, please see Figure 1 (below). Each phase may further be broken down into separate components. The six phases are:
- Phase I: Verification of the design basis
- Phase II: Verification of the detailed design
- Phase III: Manufacturing survey
- Phase IV: Monitoring and supervision of transportation & installation
- Phase V: Survey of the commissioning process
- Phase VI: Performing of different periodical in-service inspections.
Phase I – Verification of design basis
Site conditions: Evaluation of the site conditions verifies that the wind, environmental and geotechnical conditions are in compliance with the relevant guideline and standard.
Codes, standards and requirements: The codes and standards which form the basis for the project should be listed. For the site in question, relevant statutory requirements also need to be listed. Such requirements could be safety-related issues such as embarkation, rescue and decommissioning.
Design: The design of the integrated structure, comprising wind turbine and tower and foundation, should be based on the specific site conditions. This includes the review of wind turbine approval and grid connection.
Transport, installation, commissioning, O&M: It is necessary that requirements and manuals for transport, installation, commissioning, operation and maintenance should be reviewed for assessment at a later stage.
Phase II – Verification of design
Based on the approved design basis for the project, the second phase should start with verification of the detailed design for the specific project.
Verification of load and response: The loads should be verified based on documentation review and independent analyses for compliance with the approved design basics. In order to verify the site-specific loads for the turbine or turbines in question, an independent, full dynamic load modelling of the whole integrated system consisting of wind turbine and tower and foundation should be carried out. Critical load combinations should be analysed in order to verify the loads.
Verification of the wind turbine: The design evaluation should be carried out to an extent that is sufficient to establish whether a component, load assumptions, etc. comply with the design basis. This practice should also cover tower and foundation. The verification of the wind turbine should consider the most aggressive environmental conditions.
Environmental (climatic) conditions other than wind can also affect the integrity and safety of the wind turbine. These conditions include, but are not limited to, thermal, photochemical, corrosive, mechanical, electrical issues and other physical actions.
Moreover, combinations of the given climatic parameters may increase their effects. Hence, the documentation for utilization ratios should be subject to special considerations.
The resultant site-specific loads for all components should be evaluated with respect to the loads used in the type approval. Any increases in load level as well as any changes in vibration modes/natural frequencies need to be considered. This evaluation should consider the relevance and validity of load measurements, functional testing and component tests such as blade tests.
Verification of electrical systems: The electrical system comprises electrical design not covered by the wind turbine type approval in question. The design should be verified for compliance with the appropriate standards focusing on the safety of the installations defined in the approved design basis.
Verification of installation and commissioning procedures: The wind turbine should have an installation and commissioning manual, which as a minimum consists of the installation and commissioning procedures, and emergency procedures specified by the wind turbine manufacturer. The manual should also include contingency procedures.
Verification of operation and maintenance: User and service and maintenance manuals, which at a minimum contain the service and maintenance requirements and emergency procedures specified by the manufacturer, should be supplied with the wind turbine. The manuals should also provide for unscheduled maintenance.
Phase III – Manufacturing survey
For this phase the information from Phase II is essential to check if the approved design requirements are implemented into the manufacturing phase.
The evaluation of quality control mainly relies on the presence of a certified ISO 9001 system. In addition, a manufacturing survey is necessary to include inspection/audit activities to verify that the manufacture of the wind turbines for the specific project is carried out according to the approved design and quality.
The extent of inspection and audits to be carried out should be evaluated for each single project and wind turbine type. The following items may be used as basis for this evaluation:
- Blades: Inspection, typically on a spot check basis. Visual inspection of a minimum of one blade before it is shipped. Review of final manufacturing documentation.
- Assembly of the turbine machinery: Inspection of the assembled machinery in the nacelle and review of final documentation.
- Main gear: Audit of the manufacturing process and/or assembly process. Witness of the final testing and review of the final manufacturing documentation.
- Hub, cast iron: Inspection on a spot check basis or audit of the manufacturing process. Visual inspection of a minimum of one hub before it is shipped. Review of final documentation.
- Main shaft, forged, rolled or cast: Inspection on a spot check basis or audit of the manufacturing process. Visual inspection of a minimum of one shaft before it is shipped. Review of final documentation.
- Machine frame structure and main bearing housing, welded or cast: Inspection on a spot check basis. Visual inspection of a minimum of one machine frame before it is shipped. Review of final documentation.
- Generator, transformer and converter: Normal type tests and individual tests shall be carried out by the manufacturer. Attend the type tests and review the documentation.
- Other components such as slew ring and special bearings, cranes, elevators/lifts, couplings, pitch and yaw drives, controller, hydraulic units, power cables, and electrical panels may also be subject to surveillance.
A scope of work for inspection service has to be developed/specified for each of these items. This scope should include utilization of standards together with input from the design evaluation. Such input from the design evaluation may contain critical items/processes identified during the design evaluation, test programmes/procedures for serial production, approved design documentation such as drawings and specifications, and details from prototype testing.
For the tower and foundations, the extent of inspections and audits to be carried out will be evaluated for each single project. Depending on the type of structure and evaluation of the manufacturing of steel plates, primary and secondary steel structure and the manufacturing of concrete structures may be required.
The following considerations should also typically influence the detailed scope for the manufacturing survey:
- Manufacturer’s experience, with respect to delivery of the specific item to wind turbines
- Experience with the manufacturer
- Time schedule and number of items for the specific delivery
- Type of manufacturing process used, e.g. hand lay-up, pre-preg or vacuum infusion of laminates for blades, nacelles, spinners; manual or automatic welding for towers, etc.
- Type of quality control, for example, NDT or visual inspection, statistical methods or testing each item
- Appropriateness of the manufacturer’s quality system, in relation to the specific manufacturing process and control activities
- Extent of inspection by the purchaser, e.g. wind turbine manufacturer’s inspection of sub-suppliers
- Availability of certified documents specifying the quality requirements
- National or international manufacturing codes and standards applied
- Availability of relevant quality control documents, such as requirements for final manufacturing documentation, test programmes, acceptance test procedures, NDT procedures, weld procedures, corrosion protection, handling, curing, heat treatment, mechanical testing requirements, etc.
- Access to the manufacturing facilities, sub-suppliers, and manufacturing documents
- Procedures for handling of deviations to requirements, e.g. waiver procedures.
Phase IV – Installation survey
The transportation and installation of wind turbine components are crucial phases for a wind farm project. Monitoring and supervision at these phases is important to detect defects before the commencement of installation since it may not be possible to correct deviations after installation. For example failures during the installation of foundation could result in foundation cracks during the operation of the turbine which could not be detected after the foundation is set in the ground.
Phase V – Commissioning survey
During commissioning, all systems and equipment should be checked for compliance with approved documentation and commissioning procedures. The relevant systems need to be functionally tested as practicable, in accordance with approved procedures. An independent third party inspection company should witness commissioning of at least one wind turbine at the site in order to verify that the site works involving assembly and erection, and commissioning is performed according to the approved wind turbine design and installation manual. The survey should be concluded with reports that describe the activities carried out and detail the observations made during the course of the audit. For the commissioning survey a scope of work will be tailored.
Phase VI – In-service
In order to secure that high standards are maintained throughout the lifetime of the wind farm, in-service inspections are critical over the lifetime of a turbine. The in-service phase implies an activity in which the wind turbine, tower and foundation are regularly surveyed. Different inspection types exist, for example, regular inspections, gearbox analysis including oil analysis, and inspection of rotor blades and coatings. In addition, condition monitoring supported inspections should be applied, extended by vibration measurement, endoscopy of the gearbox, thermography of electrical components, and such like.
Inspections on a regular basis can also ensure both public safety as well as the safety of the wind turbine. It cannot replace the necessary maintenance of the machine, but it is an additional measure to evaluate the condition.
The strictest consideration must be given to the detection of defects at an early stage, with the subsequent avoidance of consequential damages and unscheduled downtime.
In any case it is important that the individual demands and requirements of the inspection are considered in choosing individual and applied inspection techniques and sequences, together with the history of wind turbines, including operational data and condition monitoring.
The detailed inspection plan will identify the survey activities required as well as instructions for the reporting. Reports should highlight any findings or deviations during the annual survey. Major findings and deviations should be reported as recommendations, for the owner for example.
As a part of the inspection, records of maintenance, repairs and inspections carried out have to be reviewed and verified against the approved programme. During the inspection the follow-up on outstanding items from the previous annual survey and status on recommendations should be conducted together with a review of revised procedures, maintenance documentation, and maintenance history.
The inspection should cover the relevant systems of the wind turbine installations, such as the rotor – including blades and hub assembly – mechanical transmission including gear boxes; nacelle structure and connections; generators, converters and transformers; control and protection systems; electrical systems; lifting appliances; and personnel safety installations. And the inspection of these systems should further focus on fatigue cracks; dents and deformation(s); bolt pre-tension; status on outstanding points from previous surveys; settings and parameters used by the control system; cooling media for transformer and generator, if applicable; lubrication where applicable; testing of the control and protection system (witness tests carried out by the operator); the condition monitoring system; and, additional surveys identified based on findings and deviations, for example following witnessing of tests and inspections in order to distinguish between random and systematic failures.
Additional measurement and analysis
It may be helpful and/or necessary to extend the normal inspection scope by other techniques. These include vibration measurement. For the vibration measurement sensors are installed (by magnets or gluing) on the main bearing, gearbox and generator. With a measurement that takes approximately five minutes during operation of the turbine, it is possible to detect failures at bearings and within the gearbox. Furthermore it is possible to pin-point the defect or the damaged part of a bearing. Using vibration monitoring it is also possible to detect a misalignment of the drive train between the generator and gearbox.
However, due to the fact that analysis of vibration measurement has its limits – for example vibration measurement has difficulty in revealing damage to the planetary stage of the gearbox because of overlapping frequencies from several parts – it is necessary to complement this by oil analysis and endoscopy inspection.
For the wind turbine gearbox, wear debris particle analysis is a powerful tool for predicting mechanical failures. The technique isolates wear particles from machinery, and classifies the debris by both microscopic and chemical analyses. Testing of used oil is a preventative maintenance tool that operates as an early warning system.
Using this technique it is possible to detect failures at the planetary stage of the gearbox, for example. An investigation of the shape, surface, size and colours of the material can identify the alloy, and the frequency of its occurrence may be used to determine if the wear is severe. With this examination, it is possible to identify if particles come from cutting wear, severe sliding or fatigue. Their origin might also be as a result of ingress from the environment, like sand or dust. The particles will be classified into different sizes and numbers to be able to identify if the level is changing during the on-going operation of the wind turbine.
Based on the results of the vibration measurement and/or oil analysis, a video-endoscopy inspection of the accessible gearbox toothing and bearings can be carried out. A visual inspection of the damaged parts gives a clear picture of their condition.
Based on the scope of a general inspection it is also useful to extend the inspection at the end of warranty period by vibration measurement of the drive train (main bearing, gearbox, generator bearings) as well as a gear oil analysis.
Live long and prosper
In the last 25 years the size of wind turbines has increased very rapidly – rotor diameters have increased eight-fold and the rated power more than 150-fold. Due to this rapid growth, wind turbine manufacturers have expanded research and development activities, and sometimes resorted to insufficient and relatively short testing periods before newly-designed turbines are launched commercially. This has contributed to some serial failures that occurred shortly after commissioning of the wind turbines.
Consequently, it is necessary to adopt a strategy to carry out life cycle services, not only to assure the financially viable operation of the wind turbines, but also to reduce down-times due to detectable failures. Life cycle services rely on the adequate certification, verification, testing and inspection of a wind turbine by using the information gathered in each stage of a wind farm project phase.
Starting at the design phase, the life cycle continues with the different manufacturing processes, the installation phase, the commissioning phase and the in-service stage – the longest period of all.
A number of critical factors must be considered, most importantly are the detection of defects at an early stage, and the avoidance of consequential damages and unscheduled downtime. Choosing the right individual and applied inspection techniques and inspection sequences are therefore crucial.
