First-rate repair: blade maintenance round table

Mark Brierley: What consideration is given to the ease of maintenance of wind blades, either during the blade design process or when choosing a wind turbine manufacturer?

Thomas Schlenzig: Siemens developed the IntegralBlade process in 2000 with the goal of increasing reliability. Siemens is the only wind turbine manufacturer to use blades cast in one piece in a closed process that eliminates glue joints, providing blades with optimum quality, strength and reliability.

Furthermore, in 2011, we introduced groundbreaking Quantum Blade technology. Quantum Blades are lighter than previous models, and the structural load on blade and nacelle is significantly reduced. In 2012, the aeroelastically tailored second-generation Quantum Blade followed, which features bend-twist-coupling within the blade to lighten structural loads even further.

We perform thorough blade testing to maximise quality and lifetime at our R&D facilities in Aalborg, Denmark. It is our aim to change industrial standards for component testing by running every component through a simulated 25 years of highly accelerated lifetime testing (HALT). For blades, Siemens runs statics and fatigue tests.

Chris Smith: I find that ease of maintenance is not really a big factor in the selection of turbines. Turbines are selected based on their suitability for a project in terms of wind resource, expected yield and return on investment, with some peripheral considerations given to supplier relationship, quality and technical performance. All modern, large-scale turbines have similar blade management requirements.

The only time blades would form a part of turbine manufacturer decisions for new projects is if there were known issues in regard to blade quality. For instance, if a particular blade had previously resulted in significant defects and failures; I have seen this happen on rare occasions.

TS: Siemens' turbine condition monitoring (TCM) carries out precise online vibration diagnostics that can aid early detection of irregularities before they potentially turn into fatal breakdowns and consequential downstream damage. These vibration measurements are performed not only on main components, such as the gearbox, generator and main shaft bearings, but also on blades. The service diagnostic team applies analytic tools to compare vibration patterns and relevant data with historical records and the performance of other plants with the same technology. The resultant data analysis goes hand in hand with predictive methods, allowing the optimisation of service planning and preventive repair of components during and beyond the warranty period, depending on the duration of the service contract. TCM systems are installed on all multimegawatt-class Siemens wind turbines - more than 6,000 worldwide.

How does the maintenance regime for blades differ on and offshore?

Charles Awbery-Maskel: The main issue associated with offshore wind farms is accessibility when it comes to performing necessary repairs. Therefore, time on site is a valuable commodity and wind repair solutions must be tailored to allow the repair to be completed in the time available. Repair durability is also a key factor as the costs associated with repairing offshore wind turbines are significantly higher than those of onshore turbines.

CS: Previously, working offshore involved less frequent inspection periods as the risks were considered lower. Findings over the last seven years, however, suggest that, although public risk remains much lower offshore, the risk of blade damage is equal to onshore, which suggests that maintenance regimes must be the same in both environments.

Blade inspections are not easily managed because they are expensive and time-consuming, and result in lost production. Such issues become more significant offshore due to the logistics of transporting blade inspection teams, moving equipment between turbines and setting up emergency escape plans.

TS: When servicing wind turbine blades onshore and offshore, we normally send rope access technicians to inspect and correct smaller findings. Onshore, we also focus on visual inspections using advanced photographic equipment on the ground. This photographic equipment cannot be used offshore due to the movement of service vessels. And, as I mentioned before, newer-generation Siemens turbines are equipped with TCM systems, whether on or offshore.

What dialogue exists between stakeholders involved in blade repair?

CS: Basic blade inspections form a part of the normal scheduled services and the owner would be provided with evidence of service reports, which highlight any blade defects. The turbine manufacturer would also be expected to notify the owner of any know blade issues. As an operator, RES also includes a visual inspection of the blades within its regular site inspections.

As a minimum, a detailed blade inspection is performed in year four in preparation for the end of the defect liability period. Based on the risk profile of the turbines and the site, close-up blade inspections may also be performed at set intervals throughout the farm's lifetime using specialist contractors. Such inspections and reports are provided to operator requirements. Defects are then discussed by the operator and blade inspector in order to understand their severity and agree on the appropriate action required - no action, monitor, repair in time or repair urgently.

TS: We consider wind turbines and blades as one unit when providing services. Siemens offers regular blade inspections during the two to five-year warranty period in certain service contracts. During the warranty and service period, our service expert team supports the operator or owner of the wind farm on everything related to our products, ranging from service planning and scheduled/unscheduled service through to monitoring and diagnostics, and technical on-site follow-up. Accordingly, our blade service dialogue involves multiple stakeholders and is an integral part of our regular customer communication.

CA-M: Sika actively canvasses the opinions of repair companies and OEMs' repair divisions to ensure that our products work well in the maintenance environment. For example, based on dialogue with major repair stakeholders, Sika has changed to pre-measured-only packaging for repair products in order to remove the need for on-site component measuring; this ensures a more accurate mix ratio control and higher final product quality.

Do you see improvements in wind blade design having an impact on the service life of blades in future?

CA-M: Wind blades are now being designed with leading edge protection applied in the factory, which will majorly benefit the service life of blades that are subject to leading-edge erosion. However, a workable solution for repair and replacement of such factory-applied leading edge protection in the field is an area that needs to be addressed.

TS: To date, Siemens has introduced three major innovations in blade technology - the IntegralBlade, the Quantum Blade and the next-generation aeroelastic Quantum Blade. Over the past 30 years, our blades have grown 15-fold. They now harvest ever more energy and have become stronger, and more reliable - in part, thanks to 21st-century blade tests such as HALT.

At the same time, we could achieve higher robustness by casting blades in one piece in a closed process, and reduced loads by using a smarter, lighter blade profile with aeroelastic features.

CS: There have been blade design improvements: some for performance reasons, and some for defect and wear protection. It would certainly be valuable for wind farm owners to see further progress in blade design in areas such as improved lightning protection, gel coat protection, leading-edge protection and blade condition monitoring. It is hoped that improvements in design will result in improved longevity and fewer serial defects, which some owners have experienced, but the technology is still evolving. At the current time, MW-class turbines in the five to ten-year range are not showing significantly fewer defects than kW-class turbines.

The key requirement is to make blade inspections quicker, easier, cheaper, less time-consuming and with less lost production; operational costs must come down, but inspection regimes must be maintained.

Given the extreme weather in which blades must operate, are you consistently finding similar repairs are needed and tailoring your repair techniques, designs or products accordingly?

TS: Based on the service experience we gain in the field, we give regular feedback to our design engineers of wind turbines and blades to allow for implementation of improvements. In other words, we are modelling our products based on what we learn from our customers.

We're also doing all we can to prevent maintenance work, such as through the IntegralBlade process - a closed, one-piece, blade-casting process that eliminates glue joints and optimises blade quality, strength and reliability. Siemens' new Quantum Blade technology also reduces structural loads while retaining the strength of earlier generations.

In addition, aerodynamic accessories, known as DinoTails, DinoShells and vortex generators, installed on the blades can maximise a turbine's aerodynamic performance and robustness with regards to roughness sensitivity. This so-called power curve upgrade increases a wind turbine's energy output. Newer-generation blades are already delivered with these aerodynamic improvements, but Siemens also offers this upgrade to existing blades.

CA-M: Wind turbines are typically positioned in areas that have some of the harshest environmental working conditions. Leading edge erosion and lightning strikes are the most common forms of repair seen in the market, and Sika has addressed the need for rapid structural laminate repairs by developing high-performance Sikadur Blade Repair Kits that cure in two hours at 60°C to Germanischer Lloyd performance levels. This is a step change in what is currently available for structural laminate repairs and offers maintenance companies huge advantages.

All Sika's two-component repair products are available in either pre-measured MixPax packaging or cartridges, ensuring accurate mix ratio control. Sika follows the 'one gun' approach whereby both one-component sealants and two-component cartridges can be used in a standard caulking gun. Its lightweight, heavy-duty application gun - the Sika Gun HD - has a carabiner clip that attaches it to the operator, preventing loss from a great height.

CS: Blade defects follow similar patterns and categories; however, they do vary based on blade type and site location. Many sites will experience leading-edge erosion over the longer term, along with minor gel coat defects, and splits and tip damage from lightning strikes. Several factors determine blade defects, including initial factory quality assurance, site installation, early defect identification and remedy, local air quality and weather, and severity and occurrence of lightning.