Guide to wind turbine blade root insert failures.

An introduction to blade root failures

In the last decade, wind turbine blades have generally been configured with one of two different types of bolted joint architectures to connect the turbine blade to the pitch bearing.

Of these two types, the root insert style derives its name from having an embedded metallic bushing in the blade composite with internal threads to engage the bolt stud. This bolted joint configuration is popular with modern designs because it allows a higher bolt count and smaller root diameters on blades; namely due to lower overall system cost.

Despite the higher bolt count, the root insert architecture is not immune to failures. In recent years, blade losses have occurred due to the failure of this jointed connection and in some cases have led to the collapse of the entire wind turbine.

Modern wind turbines have not only grown to enormous scales but have also evolved to stay cost-competitive; a consequence of this mixture is the higher imbalance loads created when large blades are detached from the rotor in an instant, high enough to buckle towers. A growing sense of urgency for solutions among owners and operators has spurred root cause investigations in OEMs, recommendations for inspections, and monitoring solutions to provide effective containment and advanced warning of progressive failures.

The true cost of a blade root failure

During the operation of the wind turbine, there is a power-producing state often referred to in design as maximum thrust. At this point in operation, the blades collectively create enough thrust loading to rival and even surpass modern jet engines for commercial airlines. The blades also have enough mass, and their centers of gravity are far enough away from the hub center, and spin at a rate that produces centrifugal forces high enough to match the maximum thrust forces of the wind turbine.

When a blade is thrown (or most of it) aerodynamic thrust immediately is reduced and the tower accelerates forward. Simultaneously, the imbalance created on the rotor causes the tower to bend laterally as the remaining two blades continue to spin around until settling. These combined forces have grown as rotors have grown and tower designs have consequently become more susceptible to buckling in these extreme conditions. This is the worst possible scenario but in some cases, it has been a reality for owner-operators.

Diagnosing the causes of blade root failures

Should a Root Cause Analysis (RCA) conclude a turbine collapse is due to a blade root insert failure, the next logical step is to diagnose the cause of the blade root failure.

Let’s examine the available evidence of a blade root failure:

1. The damage is progressive and continues to worsen.
2. The damage is often found biased predominately near the leading and trailing edge of the blade.
3. Rust and metallic debris are often found in the blade near the interface to the bearing.
4. Root insert bushings appear often completely loose in damaged zones.

These observations point to a few focus points to continue an RCA: since the damage is progressive and predominately located near the leading / trailing edge, fatigue loading may play a central role.

Based on the appearance of rust, water ingress and therefore joint gapping may be occurring. The interface of the bushing to the composite having a smooth surface is also indicative of poor bonding strength or adhesion of the resin-metal interface and possibly contamination of the bonding surface in manufacturing.

The presence of cracks in the laminate near the termination of the metal bushing may indicate stresses being too high and thereby either a design or loads estimation error. My experience in design and RCAs of failures of wind blades reinforces conclusions derived long ago – it’s never just one cause but most likely the combination of at least 2 or 3.

I've detected a potential failure, what next?

1. Get to know the failure

The first step to planning a course of action is to familiarize yourself with the issue.

Our deep-dive webinar ‘The Impact of Blade Root Failure and How to Monitor Them’ will provide you with key in-depth insights into tried and tested monitoring solutions available for these failures.

Watch the webinar

2. Contain the problem

The next course of action should be to contain the problem.

Inspections and measurements to determine which units have the issue is a critical component of the containment action. The tell-tale signs of this issue however mean some technologies are ineffective at finding this issue (namely external drone blade inspections or external visual inspections). These damages exist internal to the blade and are not visible.

3. Explore Available Technologies

SCADA Analysis

SCADA analysis focused on searching for this failure mode is unlikely to capture the damage when it is still small. By the time statistical summarizations of turbine operation data are analyzed and a change is identified, it is likely already large enough that collapse is imminent.

Portable Health Sweep

Using a portable health sweep, a snapshot of your blade root connections’ health can be taken using a portable condition monitoring system. Whilst this still includes the additional costs of needing an engineer to climb the turbine each time an inspection is needed, it has the additional benefit of providing clear, comprehensive, and trustworthy data on the health of your asset.

Permanent Monitoring System

A permanent blade root connection monitoring system offers in-depth, continuous monitoring of your asset. Using finely placed sensors within the turbine, condition monitoring hardware can transmit round-the-clock insights and data analytics, reducing the need for engineers to climb turbines.

Having access to on-demand data means failures can be predicted and detected long before catastrophic failure occurs, and as a result, reduces downtime and allows maintenance to be planned in advance.