There is a trend churning in the wind power industry; it involves upgrading wind turbines with newer, more powerful generation systems. Thanks to higher-capacity turbines and more efficient components, repowered turbines can produce up to 20% more energy annually. At a time when future tax credits for wind power are uncertain, choosing to repower fleets is an efficient way to increase wind energy productivity and help ensure maintenance costs don’t exceed profits.

Although beneficial, repowering is not as straightforward as it may sound. It requires bigger, more exacting equipment, and may result in larger nuts and bolts that are placed closer together than in the past. Whether you are considering upgrading your wind turbines or looking for safer and more efficient tools for your current fleet, now is a great time to review wind turbine maintenance and bolting tool options.

A wind turbine can contain somewhere around 25,000 bolts, each of them under precise bolt load and up against a variety of forces in nature. Once a wind turbine is commissioned for service, a maintenance schedule dictated by the original equipment manufacturer (OEM) must be maintained to have the tower covered by the warranty. One of those regular maintenance tasks is checking that bolt loads are within the variation specified by the OEM. In a worst-case scenario, if too many bolts become loose, a catastrophic failure could occur.

Technicians must check the load on a certain percentage of bolts, as directed by the OEM’s specifications. One example might be a 10% check: Check 10% of the bolts for 10% of variance. Over time, small variances can occur because things shift and push. Or a technician could accidentally overstress a bolt and stretch the metal beyond a yield point.

During maintenance, the technician will check whether a bolt is still as tight as it was a year ago, and either tighten it or replace it if it went past its yield. If too many variations are found, the OEM specification may require that 100% of the bolts must be tested.

Maintenance Frequency

Maintenance schedules are dictated by the turbine OEM, and must be maintained and documented for the warranty. The schedule varies based on the age of the wind tower (Figure 1) or number of hours in use. For bolting, maintenance may need to occur every three months, six months, or annually. If there was zero variation in the bolted joints from the last maintenance check, the wind tower operator can ask for an exemption on the next service, which saves time and money.

1. Maintenance schedules for wind turbines are dictated by the turbine’s original equipment manufacturer. This turbine, on an installation in the North Sea, northwest of the German island of Borkum, is among those using Enerpac systems and equipment. Courtesy: Enerpac

Controlled bolting is the process of applying an accurate bolt load to a mechanical joint to meet design specifications. Bolt load can be measured directly using hydraulic bolt tensioners, ultrasonics, or calculating torque values determined by the bolt material, size, and friction of the stud and nut. Having the means to measure bolt load during or immediately following the bolting process is essential for joint integrity and safety.

Torque wrenches apply rotational force onto a nut to stretch, not yield, the bolt to achieve bolt load. A hydraulic torque wrench, when coupled with a specially designed torque pump, creates the torque output, or torque load, necessary to create the clamping force to secure bolted-joint integrity.

While torque wrenches and bolt tensioners work to accomplish the same objective—to clamp nuts and bolts together for joint integrity—tensioning directly stretches the bolt against the flange rather than nut rotation from the torque wrench. Tensioning typically creates a more accurate load often required on larger bolts. The decision of which to use is driven by the engineer. As wind towers get repowered with bigger equipment, more exacting and reliable tools are required.

New features are available on torque wrenches that set a higher standard in safety, versatility, simplicity, and performance. For example, when it comes to safety, a built-in, work-at-height safety tether connection, such as that on the Enerpac DSX-Series aluminum torque wrenches, helps prevent injuries to workers below. Meanwhile, a fully enclosed square drive keeps technicians’ hands protected from moving parts, and its optimized weight-to-output ratio and slim design help prevent operator fatigue.

Versatility is a benefit of this modular torque wrench. The module drive units and hex cassettes are interchangeable with a wide range of hex- and square-drive cassettes from several manufacturers, while the combination of superior alloys makes it one of the most durable wrenches in its class.

Items to consider when selecting torqueing tools include handle options, size, and material strength. Some designs allow handles to mount on both sides of a square-drive torque wrench, which can provide safer maneuverability. Push-button square drives and secured reaction arms are features that allow fast changes and adjustments, even with gloves on. Compact designs can offer a smaller operating radius than larger options. Other things to evaluate are the wrench’s strength-to-size ratio and flexibility of use, which can be enhanced in some cases with an optional tilt-and-swivel manifold for the torque hose.

Bolt Tensioners

Specialized bolt tensioners (Figure 2) are often used for controlled bolting during installation and maintenance checks on wind turbines, including the foundations. Critical fastening applications for wind and other power generation applications are best served by single-stage and double-deck tensioners, which provide the speed and accuracy required for safe operation at the wind site.

2. Bolt tensioners are typically used for controlled bolting during installation and maintenance checks on wind turbines, including the tower foundations. Enerpac has several tensioners in use on wind installations, including from its PGT-Series, FTR-Series, and FTE-Series. Courtesy: Enerpac

There are tensioners designed specifically for wind tower foundations or base bolts, such as Enerpac’s FTR-Series and FTE-Series tensioners. These elliptical and round tensioners provide a fastening solution on wind tower foundations where long bolt stretch is required, and limited space between the stud and wall prevent the use of standard tensioners. They feature an elliptical geometry, which fits in narrow foundation applications, and exceeds bolt load design requirements.

The torque wrench or tensioner itself does not do the work alone—it is part of a system that also includes a hydraulic pump, hose, and accessories. Designing a solution with components all ergonomically devised to work together enhances operator safety and productivity. A complete system from a single brand means a functional design that is intuitive to use through common fittings, safety handles, and on/off pendant controls, all while ensuring hand comfort when repeatedly operating the tools.

Torque Wrench Pumps and Accessories

Given the number of bolts on a wind tower, bolting pumps should have high flow, fast performance, and portability as key advantages. Cordless torque wrench pumps are available and can work well for wind tower maintenance. Some are also designed for ergonomic, portable, and safe use where weight and power are critical factors.

3. Torque wrenches enable faster, easier, and safer maintenance on wind turbines. New technology is converting some square drive torque wrenches into hands-free tools. Courtesy: Enerpac

Accessories for torque wrenches (Figure 3) and tensioners can make the job easier and safer. Options are available today that enhance overall safety using a patented technology that converts square drive torque wrenches into hands-free tools. With one twist of the locking collar, the accessory minimizes exposure to pinch points and high-pressure hydraulic connections. There are also methods available to instantly validate calibration of a fully connected torque system on the job prior to use. This can eliminate the need for calibration certificates onsite, and reduces time and expense of sending tools out for calibration confirmation.

As for tensioners, like torque wrenches, pairing your hydraulic bolt tensioner with a compatible bolt tensioner pump and hydraulic hoses will ensure optimal productivity. Operators can choose from air-driven, electric, or manual tensioner pumps. Electric tensioner pumps can provide high efficiency with a compact and lightweight design. For example, a two-stage pump can provide high flow at low pressure for fast system fills and controlled flow at high pressure for safe and accurate operation.

Bolting Tool Preparation

Operating high-pressure hydraulics requires sound knowledge of how they work and how they should be maintained. The correct use and maintenance of these tools increases safety and reduces risk—both for the operator and for the environment in which the tools are used. The Occupational Safety and Health Administration does not provide standards for high-pressure hydraulics. A good tools partner can help improve the safety of your wind turbine maintenance program. Training can show technicians how to use tools properly and minimize risk.

As for tool maintenance, safety checks can be performed by tool OEMs. Also known as a tool crib audit, this is typically a visual inspection of all high-pressure hydraulic tools and a detailed report summarizing the condition of the tools and suggested improvements. This can minimize downtime and ensure that tools are ready to perform.

Often, loosening and tightening of bolted joints is a critical path item to larger maintenance outages. A tool that fails during an outage can cause a tremendous headache when it comes to workflow and keeping a project on schedule. Because work must stop until the tool is repaired or replaced, you could easily incur a $50,000 delay due to downtime. A good tool supplier will be your partner for site maintenance and help make expert decisions about the right bolting tool that will perform reliably, every time.

Maintenance will be an ongoing need in the wind power industry. As wind energy continues to rise in popularity, the technology, tools, and equipment will continue to advance as well. Streamlining an operations and maintenance strategy will keep a wind farm operating efficiently, and the right tools will enable safe performance of critical jobs.

Jon Bice ( is director of bolting at Enerpac Americas.