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Bolt Tensioning on Wind Turbines - Considerations when choosing bolt tensioning tools

tentecBolting on Wind Turbine Towers

Bolt tensioning is vital to the integrity of wind turbines. As well as being extremely important for applying accurate bolt loads, efficiently and safely, it is also a consideration both for the builders and installers of the turbines, as well as the maintenance engineers who are required to carry out regular bolt load checks.

This article will look at why accurate bolt tensioning is important, particularly for the foundation bolting in USA and Canada and what aspects are important when choosing the right equipment for the job.

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Where Bolt Tensioning is Required

There are 8 main areas where bolt tensioning is needed on a wind turbine. As illustrated in Photo1, from the foundations upwards, different parts of the wind turbines have to be safely put together.

Due to their size, the turbine towers are transported to the installation site in pieces and then assembled. Throughout Europe and the rest of the world, the bottom section of the tower is cast into the concrete foundation and the lowest section is then welded on site. However, North America and Canada have a unique system. Here, a foundation laying company lays the foundations, concreting the bolts into the ground. After the concrete is set, the turbine towers arrive on site and are simply lowered over the bolts.

 

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Hydraulic Bolt Tensioning for Foundations

Hydraulic bolt tensioning is now recognized as the standard and preferred method for securing wind tower sections, particularly the foundations. This is due to the fact that foundation bolts can be up to 20ft long and it is not feasible to use hydraulic torque wrenches as this would add twist to the bolt rather than the elongation required. Elongation on these larger bolts can be up to 1” and therefore tensioners have to be used to stretch the bolt.

Installing Wind Turbine Tower Foundations in the USA and Canada

A reinforcing steel bar (rebar) bolt is used in foundation bolting applications, as the threads used in the rest of the world would get damaged during this process of concreting them into the ground. These bolts are similar to those used throughout the construction industry and manufactured to tolerances and standards required to meet the exacting safety requirements of the industry. The use of these rebar bolts means that foundation bolting applications have different requirements from, for example, blade bolting. As the rebar bolts are manufactured in softer material than the blade and tower bolts, this means that they will stretch more and the extensions will be longer.

In light of this unique foundation system, specialist hydraulic bolt tensioning equipment is needed. So what should companies look for when evaluating such equipment?

Choosing the Right Bolt Tensioning Equipment for Foundations

In general, two designs of bolt tensioners are available and the choice depends on two factors. The first is dependent on the length of the bolt, which determines the amount of elongation needed and the other deciding factor is the amount of stud protrusion above the tower base flange. Due to the fact that the foundation bolts are concreted into the ground, the protrusion cannot be precisely controlled.

Therefore, contractors need to determine which is the most appropriate tool for the job. A short stroke tool, as the name suggests, is used when the stretch needed is up to 3/8” and the protrusion is a minimum of 7.5”, whereas a long stroke tensioner can achieve an elongation of up to 1” on a minimum protrusion of 10.5”. For speed and efficiency, combined with ease of use for the engineer, these tensioning tools should also allow for “one pull” uninterrupted tensioning. This enables the longest of bolts to be stretched in a single operation. If only a short stroke tool was available for one of the larger bolts, then the tensioning would have to be carried out in two or three different steps. Photo 3 shows an example of a long and short stroke tensioner.

As the amount of thread protruding from a foundation bolt can vary enormously from turbine to turbine, many contractors will carry both tools to ensure they have the correct equipment for each and every job.

Safety Considerations

Safety is paramount in all construction jobs and when looking for foundation bolt tensioning equipment, safety features must be properly assessed. One of the most important features to be considered is to ensure that there is overstroke protection as an integral part of the tool. Without this feature, engineers are prone to keep stretching and the seals will fail. Once this occurs, the operation has to be stopped as the seals have to be replaced. This results in costly downtime that could be crucial for a profitable on-site job.

Avoiding Overstroke

Manufacturers of bolt tensioning tools are mindful of the overstroke problem and have incorporated some form of protection in their tool sets. The latest technical advance in this area is a unique positive mechanical stop system. This failsafe system has been designed so that just fractions of an inch over the required extension and engineers will hit a mechanical stop and stretching will cease.

Versatility for all types of bolts

Another important aspect to consider is the different types of rebar bolts that are used throughout the industry. In the past, the standard construction bolt was the Grade 75 ksi, accounting for approximately 90% of applications. Now, a 150ksi rebar is becoming increasingly common and recently, a new Grade 90 ksi bolt has been developed for the wind industry. This new all-thread rebar allows for higher levels of design stress or designs with less anchors for critical applications such as wind energy foundations than other conventional types of rebar. Tensioning tools must be versatile enough to handle all three types of rebar.

Space Restrictions and Accessibility

In addition to the long and short stroke bolt tensioners, contractors will also need to bear in mind space restrictions and accessibility. Elliptical tensioners have been developed that help overcome these problems. Most have been designed with a width reduction, so they can fit and operate in situations where space is limited, often found while tensioning inner bolts when they are close to the tower wall.

Streamlining Operations

Also, engineers should look for other features and benefits that can streamline operations. To increase speed, some tensioners are fitted with a spring mechanism that automatically resets the tensioner once the pressure has been released to zero. This means the tensioner is then ready to tension the next bolt, with no operator intervention required.

In order to further streamline operations, multiple tensioners can be linked together with high pressure, flexible link hoses to give simultaneous bolt loading. Most contactors only use one tensioner at a time, but by using 2 tensioners, 180º apart and attached to one pump unit, more even tensioning can be achieved and when there can be up to 100 bolts to tension, this technique will also save a considerable amount of time. In an industry where time is money, payback on the original purchase price can be achieved quickly.

Conclusion

Due to the unique foundation system in the USA and Canada, particular considerations have to be taken into account when choosing bolt tensioning equipment. Specialist tensioning tools are available that will ensure the integrity of the tower and make installation and ongoing maintenance an efficient and safe process.

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