The global shift to renewable energy and expanded use of wind power has significantly increased the number of wind turbines, both onshore and offshore. The demand for wind turbine maintenance is also increasing, especially for critical components such as gearboxes. Borescope inspections of gearboxes are a cost-effective way to shorten the downtime of wind turbines and prevent early detection of defects leading to gearbox replacement.
Wind turbine gearboxes, generators and blades require intensive maintenance because wind turbines face significant stress and wear. Maintaining these parts helps prevent expensive repairs and downtime. Gearbox failures are relatively rare (once every 10 years on average), but wind turbine downtime can last as long as six months while waiting for this part to be repaired.
A typical 2.4-MW wind turbine generates about $1,000 worth of electricity per day, so a several-month downtime can be costly in terms of lost revenue. Gearbox failures can also cause catastrophic damage to the gearbox, such as a fire due to overheating, which could cause a turbine to permanently go out of service.
Advantages of using borescopes
The condition of wind turbines is typically monitored using supervisory control and data acquisition (SCADA) systems and condition monitoring systems (CMS) before and after inspections are performed at the top of the tower for oil sampling and noise checks. SCADA or CMS equipment can collect inline vibration and oil data from wind turbines to predict or detect blade, main bearing and gearbox failures up to 30 days in advance of the potential failure date.
However, SCADA and CMS error information does not indicate the exact location of the defect or specify the failure conditions. Even if you receive a 30-day warning about a gearbox failure, your wind turbine could be out of service for weeks until the parts arrive. Complementing preventive maintenance strategies with remote visual inspection (RVI) equipment that looks inside the gearbox can help identify failing components more quickly and accurately.
With a nearly six-month delay in the delivery and replacement of certain gearbox components, the sooner the required part is determined, the less downtime the wind turbine will have. In addition, knowing the status of potential failures lets you make proactive and informed decisions about parts procurement and maintenance planning. For example, regular borescope inspections during low wind season can help monitor internal gearbox deterioration and possibly prevent equipment failure.
Observation images taken with a borescope or videoscope (an advanced borescope with video imaging) help identify deterioration and defects in specific locations at an early stage. Operators can use this data to direct maintenance activities and part replacement plans.
Where remote visual inspection is required
The interior of the gearbox consists of a series of transmissions that convert the low-speed rotation driven by the blades into a high-speed rotation that drives the generator. Components can be damaged by vibration, foreign matter mixed in lubricating oil or excessive stress. A visual check of defects, such as fatigue, wear, corrosion and fractures, is recommended.
The planetary stage bearings and planetary gear provide support to the low-speed shaft. The planetary stage absorbs high stress from the slow stage in variable weather conditions. The structure of the planetary gear teeth is complicated and its location within the gearbox makes it difficult to access with inspection equipment.
Intermediary stage bearings support the intermediary shaft and are located at the front and the back — directly below the other shafts. Intermediary bearings are difficult to inspect, mainly due to their location.
High-speed stage bearings are easier to access during an inspection but are more likely to be damaged due to their high speed of between 1,500-1,800 rpm. This high speed also means that damage to the high-speed bearings is more likely to affect other parts of the gearbox.
The planetary stage has two planetary carrier bearings and nine cylindrical planetary bearings. You can access the planetary stage through small hatches on the rotor side of the gearbox, usually located at around the 2 o’clock and 10 o’clock positions.
Ring gear, planetary gears and sun gear: These gears can be easily inspected with a borescope through the rotor-side hatches. You will want to check the surface condition of their gear teeth, so a near-focus, forward-viewing videoscope is recommended.
Planetary bearings: Thread the scope through the space between the planetary carrier and planetary wheel to reach the planetary bearings. Using a bent guide tube will help you prevent the lens from touching the oily surfaces inside the gearbox, and an oil-clearing lens adapter can also improve your view. Since there are three planetary wheels, you must rotate the rotor to access the two other sets of planetary bearings.
Rotor side carrier bearings: Insert the scope between the planetary carrier and the gearbox’s rotor side wall to inspect the rotor side carrier bearings. The generator side carrier bearing inspection is sometimes difficult as it is located far from the front hatch. Try the parallel stage hatch on top of the gearbox, as some gearboxes have a large opening between the planetary and parallel stages. If that is impossible, use a long guide tube from the front hatch to reach the generator side carrier bearing.
The two parallel stages have three shafts, each with one cylindrical roller bearing and two tapered roller bearings for shaft axial load and guidance. You can access the parallel stages through the large hatch on top of the gearbox.
High, intermediate and low-speed gears, pinions and bearings: The access hatch on top of the gearbox is large enough to use a regular digital camera to take photos of these gears and pinions. However, you need a borescope to check the condition of the bearings.
High-speed shaft (HSS) bearings: These bearings are usually located near the access hatch on top of the gearbox. You can hold the borescope with your hand to insert the scope head between the roller bearings.
Intermediate-speed shaft (IMS) and low-speed shaft (LSS) bearings: Use the access hatch on the top. You will need a guide tube to insert the borescope far enough to reach them.
To perform a gearbox inspection accurately and efficiently, select a borescope with the performance and functions that match the inspection environment and purpose. Consider the following points when selecting a borescope.
- Clear observation even in oily environments
The equipment inside the gearbox is covered with lubricating oil. When observing with a borescope, lubricant often gets inside the optical adapter attached to the borescope tip and makes the view unclear. Inspectors can waste half of their gearbox inspections cleaning and repositioning the scope lens. Even with a borescope that does not have an interchangeable optical adapter, lubricating oil adhering to the lens at the tip of the insertion tube causes the lens to become colored, impairing the observation performance of the borescope. As a result, inspectors will need to repair or replace the borescope.
Therefore, it is essential that the borescope used for gearbox inspection is resistant to lubricating oil and can prevent oil from adhering to and entering the lens surface and inside of the borescope tip.
- Image quality that enables detection of small defects
The gearbox inspection requires checks for defects such as scratches, flaking and dents on the bearing surfaces, as well as defects such as cracks, pitting and discoloration of the gear teeth.
Near-focus observation is important for detecting initial defects on bearing surfaces such as micro-pitting. Deep observation of the entire gear is important for gear teeth inspection. Choose a borescope that meets both observation needs to perform inspections efficiently.
- Durability suitable for bearing inspection
Borescope durability is important, as damage of inspection tools during gearbox inspection can undermine your wind turbine inspection plan. In particular, when inspecting bearings, the borescope is placed in the gap between the bearings to observe their surfaces. Physical stress tends to be applied to the scope insertion tube during this inspection, posing a risk of damage to the scope articulation section. It is recommended to choose a borescope with a durable scope bend.
- Designed to work inside the confined nacelle
The space inside the nacelle of a wind turbine is narrow, so there is limited space for equipment placement and movement during inspections. Choose a handheld borescope that is highly portable to reduce stress during work and shorten the inspection time.
Regular gearbox inspections are a cost-effective way to shorten the downtime of wind turbines and prevent early detection of defects leading to gearbox replacement.
The scale of wind power generation is expected to continue expanding as demand for renewable energy increases worldwide due to climate change issues, the finite nature of fossil fuels and geopolitical risks. The importance of borescopes and videoscopes will continue to grow for the stable operation of wind power generation facilities.
Junko Uehara has held various positions providing remote visual inspection solutions since joining the Evident Scientific in 2002. She has experience in global marketing, marketing communications, and videoscopes and their applications.
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