Wind towers are getting taller as the industry moves toward larger, heavier and more efficient nacelles. However, the foundation connections have not changed. Over the past 40 years, the base dimensions for the previous generation of towers (i.e., 40-, 60- and 80-meter hub heights) have all used the same diameter: approximately 15 feet. Why?
It is because the tubular steel base tower dimensions cannot exceed approximately 15 feet in the U.S. due to shipping restrictions for trucking/bridge clearance. This creates severe limitations on the base tower dimensions from conventional tower design (i.e., steel tubes). As towers get taller, the loads get higher. However, the foundation dimensions remain the same – creating the biggest impact on the tower foundation bolts and ground anchors.
As the tower base diameter gets larger, the resisting force demand on the bolted connections lowers. This is based on distributing the wind tower’s overturning moment over a larger-diameter cross section. Inversely, the smaller the diameter, the larger the forces and greater demand on the tower bolts. So, you have two options: increase the total number of bolts, or increase the strength of each individual bolt. Most tower designers choose the latter.
The end result is you have the same tower base flange with approximately 80 bolt pairs (160 bolts) that was used for the 80-meter tower now being used for the 90-meter to 100-meter tower with a 3 MW wind turbine generator. The towers are getting taller, but the base is not. The bolt loads are increasing, which creates more design issues in the foundation connection. So, what issues does this raise?
The tower foundation bolts must have greater embedment depth and more steel in the base footing to distribute these loads. This is true for both spread footings and drilled piers. The issues of foundation cracking, long-term fatigue, corrosion and relaxation of the post-tension forces are the usual suspects, but they are exaggerated because the load demand is higher. In most cases, the foundation designer has no option regarding the selection of the base tower bolt pattern because this is dictated by the tower designer. Towers are exclusively designed by the turbine manufacturer engineer, and the engineer specifies the base bolt connection to be used for the tower with additional specifications on the base foundation’s performance. The foundation engineer is then charged with accommodating the tower designer’s specifications. There continues to be issues with these bolted base connections when they are not properly designed into the base footing:
Corrosion: This occurs from the intrusion of water into the bolts and causes loss of steel material that will weaken the connection.
Post-Tension Loss/Relaxation: Loss of post-tension is expected but can accelerate if not checked periodically and maintained properly.
Fatigue Cracking: This is due to excessive cyclic loading that will start with small cracks and then propagate through the base tower flange.
Base Footing Cracking: This is due to fatigue loads and poor detailing in the base footing design.
Most of these issues are isolated and not a recurring problem. But when they do occur, they cause temporary chaos for a wind farm manager because each wind tower and foundation is one typical design that is replicated across several wind farms in many states. It is usually one design used in hundreds of tower applications. Therefore, should one or a dozen tower connections become problematic, the question then arises as to how many others are affected. This can lead to potentially big numbers when you consider that most wind turbine designers are using the same base tower design on many site applications worldwide. Fixing a tower base connection is not easy either. It is very difficult to replace a tower bolted connection that is improperly embedded in concrete or has poor detailing. In many cases, the design engineer will have the retrofit option, or in extreme circumstances, the foundation is abandoned completely.
Foundation anchorage is a critical part of the load path for a wind tower. It is the junction from wherein the tower loads are officially transferred into the foundation and further into the soil. At this point, the structure experiences the highest bending stresses, shear forces and vertical strain demand than the rest of the tower. The base connection determines the performance of the top of the structure because a minor rotation/deflection at the base is geometrically magnified due to the height.
The bolt loads are increasing, which creates more design issues in the foundation connection. So, what issues does this raise?
For example, a 0.05˚ (i.e., 5/100 of one degree) rotation at the base of a 100-meter tower will cause 0.0873 meters (i.e., 87 mm or 3.4 inches) of deflection at the top. It is not possible to visually see a 0.05˚ rotation, but if you have this over a 20-year lifetime, it will cause cracking in the tower and foundation at some point. This is why the foundation is designed to strict deflection limitations and extremely high stiffness requirements.
These design issues and long-term maintenance are all solvable problems. Part of these are resolved by having a good solid design with a contractor that knows how to build wind towers and foundations, as well as a good service and maintenance team that is on top of the project regularly. The use of high-strength tower bolts is becoming more imperative given the direction of the industry toward taller wind towers. As we get more innovative with tower designs and on-site fabrication of assembled towers, this may change in the coming decade because there are other new tower designs that relieve the foundation engineer from these restrictive constraints.
Because many wind power projects are continuing to service beyond their expected 20-year service life, the long-term effects must be taken into account. Although this is exciting in one sense, it also poses challenges to the industry to keep a good maintenance log and performance database on these “older” wind farms, as the machines can perform so long as they are maintained. It is necessary to keep a good watch on the infrastructure that supports your wind power generating system so that it may continue to serve the public and be a profit generator. w
Marketplace: Bolts, Anchors & Tensioning
Understanding Foundation Bolts And Ground Anchors
By Dilip Khatri
Foundation bolts and ground anchors are essential components in effectively distributing load.
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