And with the maturity of the market has come an increased focus from turbine manufacturers, wind farm operators and regulators alike on the safety of construction crews, service technicians and other field service providers.
Now, as the industry has entered what is likely to be a period of lower growth rates, these factors have been cast in a different light, as excess turbine supply alongside reduced governmental support add pressure to the turbine manufacturers and wind farm operators to improve profit on existing operating wind assets. One key element in this respect is the cost associated with maintaining a staff of qualified service technicians while ensuring their health and safety, maximising their productivity and eliminating wasted labour.
For example, time spent by technicians travelling up to and down from the nacelle is unproductive time and can increase turbine downtime by as much as 30 minutes during a maintenance intervention.
Climbing 80 metre-plus ladders causes strains over time, and some surveys suggest that service technician turnover may approach 25% annually in some regions, with climbing and fatigue-related injuries the leading cause of missed work days. As significant initial investments are made in hiring, training and equipment, replacing technicians can become an expensive drain on a wind project operator.
Moving technicians to and from the work safely and efficiently, whether it is inside the nacelle or outside on the tower or blade, is therefore a key focus for development. And, in recent years, three powered access methods have moved forward, and are now becoming standard in turbine maintenance: service lifts, climb assist tools and blade-access platforms.
The first turbine service lifts - elevators that run inside the tower - were introduced more than a decade ago as tower height began to reach beyond 65 metres. Originally developed in Europe, the earliest service lifts were adapted from suspended access platforms used in building construction and industrial applications. Designed to carry two technicians at a time, these lifts were powered by a traction hoist and guided up the tower through intermediate platform levels by tensioned guide wires. Suspension and guiding wires were rigged to a suspension beam attached to the tower walls just below the yaw deck.
Movement of the service lift is controlled inside the cabin by constant pressure, or 'deadman' switches, where motion stops if the operator releases the switch. Sensors above and below the cabin stop the lift if obstruction is sensed, and overspeed and slack rope safeties in the traction hoist lock the lift onto the safety cable if the primary suspension cable fails. Overload sensors prevent movement if the rated load is exceeded.
Subsequent generations of service lifts operate according to these same fundamental principles, while continuing to evolve in accordance with the needs and constraints of each turbine manufacturer's tower configuration.
Today, fully enclosed cabins are standard, increasing the safety of operators. As well as the internal deadman switches, many lifts now also include external automatic controls, allowing them to be used to transport tools and equipment in the tower. Ladder-guided lifts are available, as are higher-capacity lifts, carrying up to four technicians, for larger towers.
Additionally, increasingly stringent turbine manufacturer safety requirements continue to drive innovation in service lifts. In Europe, a regulation in force since 2009 - Machinery Directive 2006/42/EC - requires additional measures to prevent accidental contact between personnel and the service lift anywhere in the tower. In North America the development of the ASME A22.214.171.124 Wind Turbine Elevator (CSA B126.96.36.199) standard, to be published by the end of 2012, will for the first time provide a national standard in the US and Canada. This will place significant additional requirements on service lift manufacturers and turbine makers, while potentially reducing variations in requirements and interpretation among states that regulate service lifts as elevators.
Climb-assist systems are mounted to the ladder in a turbine tower and relieve some of the climber's bodyweight when ascending and descending, used by one climber at a time. The first climb-assist systems were introduced nearly a decade ago, and were intended primarily to be retrofitted into turbines which had a bare ladder inside, and no service lift.
The first such system was designed using counterweights attached to steel cables running up and down the back of the ladder. As the climber ascends the ladder, the counterweights descend, relieving the climber by the amount of the weights, typically around 32-36 kilogrammes. While straightforward and functional, this early system has several disadvantages, one being that the weights must be returned to the top of the ladder between climbers, increasing the time it takes for two technicians to reach the nacelle.
The next generation of climb assists introduced a continuous loop of rope driven by a motor that allows a second climber to mount and use the system immediately after the first climber detached from the rope. Replacing counterweights with a motor to drive the rope simplified installation and eliminated the inherent risks associated with heavy counterweights moving all the way up and down the tower.
Today, leading climb-assist systems feature wireless remote controls, with stop/start buttons, that attach the climber's harness to the moving rope or polymer belt. Some contemporary climb-assist products offer portable components, such as electronic control boxes or motors, for clients who are prepared to trade the increased risk of damage to such transferrable components for a lower overall system cost.
With no specific standards for climb-assist systems, manufacturers have designed to generic national standards, such as the general Machinery Directive, or Occupational Safety and Health Administration (OSHA) standards in the US.
Emerging products that are now coming onto the market, again without specific standards, integrate the climb-assist function with a fall-arrest mechanism, and some can even offer emergency rescue capabilities. While not yet broadly adopted by the industry, such products may offer the potential for increased comfort and safety for users, and lower overall cost to turbine manufacturer and wind farm operator.
External tower or blade access
The exterior surfaces of wind turbines and blades have many maintenance needs, such as cleaning of oil leaks and dirt, welding, painting and many types of structural and composite repairs on the blades. All these repairs require the contractor to put workers safely at elevation or to bring the work down tower, for example, by using cranes to lower blades to the ground for repairs.
Most common solutions for work in-situ use rope access, ultra-high reach aerial lifts, or the newer suspended access platforms specifically designed for blade or tower access. Leveraging decades of experience in other industries, these suspended access platforms can mobilise quickly in small delivery trucks, take just an hour or two to install and can offer significant savings over other methods. Steel suspension wires are attached inside the nacelle or at the main shaft. The technician operates the equipment and, with modular platforms, onsite decisions can be made to add to the scope of work if necessary.
Ground-based access provides easy transfer from one turbine to another, so ideal for moving around on a site. But, while fast once in place, both cranes and high reach or truck-mounted lifts can be expensive to mobilise and operate and have wheel loads that may not be suitable for every site.
Cranes carrying man-baskets are now seen as risky for personnel, as the employee has no direct control over movement in the basket. The crane operator and technician in the crane basket must work well together to ensure the dangling worker's safety and productivity. Industry groups representing crane manufacturers state that they are not to be used for lifting people, and in the US cranes are prohibited from carrying suspended workers unless there is no other feasible means of access and only then, with extensive equipment improvements and pre-work planning.
Rope access or absailing is also used, more commonly on smaller repair projects. Limited by the high level of technical skill required to perform the work, rope-access technicians not only need training to perform inspections and repairs on turbine blades, they also need many hours of practical experience in rope work.
Rope access is limited by the amount of materials and tools that can be easily lifted to the repair, as well as by the demanding nature of the process. Few individuals can perform this physically challenging and repetitive work on a sustained basis.
Efforts to improve wind technician safety, retention, and productivity will continue to grow in importance in coming years. Powered access solutions inside and outside the tower are being increasingly adopted as part of such efforts. Service lifts are standard in most new turbines built in Europe, and have growing acceptance in other regions. Climb assist products offer an effective retrofit solution for existing turbines. Suspended-access platforms for tower and blade maintenance allow easier transportation, not only between turbines but also over longer distances as well. Equipment suppliers continue to work closely with turbine makers, operators, and safety professionals to develop cutting-edge solutions to the challenges facing the industry. One recent appearance is remote-controlled robots that can be sent to the top of a turbine tower to take images. While the rate of adoption of these innovations varies greatly by geography, project size and the maintenance technician population, a constant is the desire held by project owners, health and safety authorities and the support industry for safer workplaces with access solutions that deliver more productive work from the uptower professionals.
Gregory Crew is director, global wind product management with Power Climber Wind, a division of SafeWorks LLC, which provides access solutions for work at height.