Blades Failures

Blade Root Connection Failures

Most modern wind turbines use root inserts or a T-bolt connection to join the blade to the pitch bearing. The root insert is a metal bushing imbedded within the laminate. These bushings can separate from the laminate which may lead to catastrophic liberation of the blade.

Also referred to as:
Root Inserts, Bolts, Joint, Blade Gap

Introduction to blade root connection failures

Modern wind turbine blade manufacturers often use the root insert architecture because higher bolt counts are possible for a given root diameter, meaning blades may get longer without necessarily increasing the root diameter. This not only reduces weight and cost but also keeps larger blades within shipping constraints.

While the bolt count is higher, the root insert technology is not impervious to manufacturing defects, operational fatigue, and damage progression even to the point of catastrophic blade liberation. There are two main subtypes of root inserts: bonded and co-infused. The bonded subtype of root insert has a drilling operation followed by application of adhesive and inserting the bushing into the blade laminate body. The co-infused bushing is placed during the infusion of the blade in the manufacturing process.

Both Root insert types (bonded or embedded) facilitate load transfer from the blade composite laminates to the internal bushing via shear. The size and geometry of the bushing are designed to keep these shear stresses at acceptable limits for the materials at the interface of the bushing and the laminate: either an adhesive or the infusion resin for the different insert types.

What causes blade root connection failures?

Manufacturing:
Defects in the composite and resin or adhesive interface to the bushing are a potential source of initiation of cracks and delamination which progress with continued loading. High porosity, poor resin/adhesive cure, and contamination are all possible sources of defects in the manufacturing process.

In both design subtypes the bushing surface must be clear of debris and uncontaminated for proper adhesion to the composite laminates of the blade.

Design:
Material strengths of adhesives and resins are critical in safely transferring loads from the blade laminates to the bushing. Inadequate accounting of stress concentrations and combined loads in operation may also lead to premature failures.

Operation / Environment:
Improper sealing of the blade flange and the bearing may allow water ingress and subsequent rust formation. Inadequate bolt pretension may result in higher than expected stresses on neighbouring bolts prompting crack/delamination initiation.

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Appearance

The root insert failure is internal to the laminate and is not typically visible directly from the inside or outside of the blade.

Secondary effects of the failure such as rusting or metallic particles may be visible with advanced cases. Physical gaps between the blade and bearing may be visible if multiple root inserts have delaminated or cracked away from the composite laminate of the blade root. In many cases, visual detection is not possible.

The most prone locations for damage presence are within a 30 degree sector of the root circle near to the leading edge or trailing edge.

How blade root failures progress

Once initiated, delamination damage between the root insert and the blade laminate is unlikely to arrest. Repeated cyclic loads (mostly gravity cycles as the rotor spins) will continue loading damaged areas and opening the delamination crack to neighbouring root inserts.

The delamination may continue to grow even without visual indications and can lead to catastrophic failure or liberation of the blade from the hub. In some cases, the imbalance load created during a full blade liberation may be sufficient to buckle the tower and cause complete loss of the wind turbine.

Progression depends on the loading and extent of damage and may reach a severe state within 3 months from initiation.

Webinar: blade root failures in conversation with EDP Renewables

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How to detect blade root failures

Method

Detection Efficiency

Notes

Root insert monitoring with ecoPITCH

Easy

ONYX Insight offers a permanent installation or a temporary measurement with ecoPITCH to determine the health of the root joint connection.

Gap measurement with dial indicators

Medium

Discrete, one-off measurements have been used to assess blades with root insert failures. The process requires trained personnel, a turbine climb and hub entry, and several rotor repositions to characterise each blade. Repeated inspections require repeating this costly process.

Ultrasonic inspection

Hard

Ultrasonic inspections have been used to find delaminate areas in composite laminates however they require highly specialised and experienced technicians to assess the results. UT inspections also have difficulty if the gap area is in contact with the underlying laminate it has separated from and thus detection misses are possible.

SCADA analysis

Hard

This damage will be very difficult to detect at all stages of progression until immediate collapse occurs. SCADA monitoring is often too slow and insensitive to capture this damage.

Visual inspection

Hard

The root insert delamination may not be visible through the laminate of the blade both externally and internally. In some cases, rust and metallic dust have been found near the damaged areas but it is commonly masked by pitch bearing grease leakages.

Case Study: identifying a 20% blade root failure rate

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Discussion

Root insert failures have become more prevalent as blades have grown in length, but shipping constraints and material costs have applied pressure to keep root diameters small. They provide an efficient means of transmitting loads from the blade to the hub while holding the bolt stresses within specifications.

The root insert technology has evolved over the last 15+ years, but like other blade components, it is susceptible to defects in manufacturing and is also stressed to higher levels as blades grow larger.

Severity rating

Rank 1 N/A

Detection:

N/A

Recommended action:

N/A

Rank 2 N/A

Detection:

N/A

Recommended action:

N/A

Rank 3 Gap exists between blade and laminate.

Detection:

Measurement of gap between blade and bearing.

Recommended action:

Consider continuous monitoring system like ecoPITCH to monitor progression.

Rank 4 Gap and rust or metallic debris present

Detection:

Measurement of gap between blade and bearing.

Recommended action:

Shut down wind turbine for blade replacement.

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