STRUCTURAL REPAIR METHODOLOGY FOR WIND TURBINE BLADES

Transcription

1 STRUCTURAL REPAIR METHODOLOGY FOR WIND TURBINE BLADES Composites Australia and CRC ACS Australian Composite Conference 2015 Gold Coast, Queensland April 2015 Rikard B. Heslehurst, PhD Heslehurst & Associate P/L University of NSW, ADFA Canberra ACT Wind Turbine Environment

2 INTRODUCTION Turbine blade damage requires the application of simple repairs given the physically difficult task The typical operational damage is surface impact and aging. Other damage due to debris of bird and high wind collected object impact, or the propagation of manufacturing anomalies. The repairs need to be simple for ease of installation and enhanced success of retaining the blade structural integrity and operational effectiveness. Aim To provide recommendations for damage repair zoning of the wind turbine blade and Simplifying the installation of repairs to the blade surface. Only considering solid laminate blade surface damage.

3 BACKGROUND Aerodynamic Loading of Wind Turbine Blades Propeller Spanwise Resultant Force Profile at a Moderate Angle of Attach Relative Force per Unit Span 100% 90% 80% 70% 60% 50% 70% 40% 30% 20% 10% 0% 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Percentage Distance Hub-to-Tip of Blade

4 Laminar Flow and Boundary Layer Aeroelasticity of Wind Turbine Blades The interaction between aerodynamic forces structural stiffness. A key parameter is torsional divergence (the more common problem in high aspect ratio aerofoils) The resultant lift force is at the aerodynamic centre (ac) of the blade and is approximately 25% chord length from the blade leading edge. The shear centre is approximately 34.5% of the chord length from the leading edge (for a uniformly distributed material blade cross section of typical aerofoil shape). ac t max Shear Centre 0.25c 0.3c 0.345c 0.345c The importance of not adding significant weight behind the shear centre is crucial.

5 Blade Repair Zoning Blade Repair Zoning Zone 1. For both for aerodynamic and structural purposes. The blade leading edge and requires the mould line (D-nose shape) to be maintained for the laminar boundary layer. Zone 1 is from the 20% to 100% span length and to 25%-30% of the local chordline. Zone 1 will always require a flush repair. Zone 2. For aeroelastic purposes. Not add significant weight to the blade for blade mass balance, but is not a major structural region of the blade. The repair needs to be an aeroelastic semi-structural repair. Zone 3. Primarily for aeroelastic purposes. Not necessarily need to be flush for airflow aerodynamics, but must not add significant weight behind the shear centre position. Trailing edge repair are typically flush for the aeroelastic requirements. Zone 4. Not required to be aerodynamically smooth, but may need to the semi-structural or structural based on the severity of the damage and the location of the damage to the main load bearing region of Zone 4 (i.e. spar cap). Because of the large enclosed area of the blade in Zone 4 the torsional rigidity is much higher than locations in Zone 3, hence aeroelastic requirements are not necessarily critical. However, significant damage to Zone 4 training edge may need a flush semi-structural repair.

6 REPAIR FUNCTIONAL SPECIFICATION The basic functional specification requirements are a combination of the customer requirements and engineering specification. Utilizing the Quality Functional Deployment (QFD) approach develop a functional specification Typical Customer Requirements (not necessarily an exhaustive list) Fast repair application Quick return to operational usage Low cost repair Structurally efficient repair Repair application safety In-situ repair Low impact on operational efficiency Negligible impact on blade aeroelastic balance

7 Notable Engineering Specifications (not necessarily an exhaustive task list) Number of personnel required to undertake the repair. Cure time of the repair resin system Time to undertake the repair process Total downtime of the turbine Repair in precipitation (Yes/No) Allowable maximum wind speed to conduct the repair On-site power (Yes/No) Blade removable (Yes/No) REPAIR METHODOLOGY STEP 1 Damage Assessment The damage is identified (usually visually) Conduct an NDI survey to determine the damage type and extent. The NDI survey will require in-situ positioning of the NDI equipment.

8 REPAIR METHODOLOGY STEP 2 Damage Type/Category The damaged area is zoned and damage type categorized: Intralaminar Matrix Cracks Interlaminar Matrix Cracks (Delaminations) Fibre Fracture (Holes) Cut Fibres Matrix Cracks Delamination REPAIR METHODOLOGY STEP 3 Damage Stress State Engineering evaluation of the repair requirements is based on blade zone and damage type will determine the repair scheme type. Delamination b Nx x a y

9 REPAIR METHODOLOGY STEP 4 Pre-installation Preparation The damaged area is prepared for the repair installation. This is an in-situ activity Remove as little material as possible for both speed and effectiveness of repair REPAIR METHODOLOGY STEP 5 Repair Scheme Fabrication The repair scheme is fabricated at ground level The repair scheme is either pre-cured or prepared as a prepreg type patch ready for installation. If possible the bleed schedule materials and bagging accessories are pre-assembled on the ground ready to be hoisted to in-situ repair technician.

10 REPAIR METHODOLOGY STEP 6 Repair Scheme Installation The repair scheme is installed over the damaged region. This is an in-situ activity. Speed of the repair application is essential when working with uncured resin system. REPAIR METHODOLOGY STEP 7 Repair Scheme Cure The repair is allowed to cured. Under vacuum Compression wrap Long pot life, short duration cure Service return resin green strength

11 REPAIR METHODOLOGY STEP 8 Post-repair Inspection The repaired area is inspected (both visually and approved NDI technique) and blade released for operational requirements. This is an in-situ activity. RECOMMENDED REPAIR SCHEMES The repair schemes can be generalized as follows: Aerodynamic structural Aeroelastic semi-structural Semi-structural Non-aerodynamic structural Non-aerodynamic semi-structural Non-structural

12 Aerodynamic Structural Repairs Aerodynamic structural repairs are basically a scarf repair Cobonded Patch Scarf Angle 3 deg or less Film Adhesive Parent Laminate l l h l damage depth overlap 2 seal edge st tan 1 ply scarf 2 ph Padhesive shear Eallt sin 2 damage depth scarf ( Aeroelastic Semi-Structural Repairs Cobonded Patch Scarf Angle 5-10 degree Film Adhesive Parent Laminate l l h l damage depth overlap 2 seal edge st tan 1 ply scarf 2 ph Padhesive shear Eallt sin 2 damage depth scarf

13 Semi-Structural Repairs Inverse Structural Doubler And Scarf Inlay Film Adhesive Scarf Angle 5-10 degree Damage Removed Low Modulus Plug hscarf depth loverlap 2ndoubler pliesledge overlap lplug tanscarf P t Et 2 e doubler adhesive shear 2 adhesivep max doubler Non-Aerodynamic Structural Repairs Low Modulus Filler Metal Patch Thick Parent Laminate Metal Backing Patch

14 Non-Aerodynamic Semi-Structural Repairs Inverse Structural Doubler Film Adhesive Damage Removed Low Modulus Plug l overlap P 2 FS 2G adhesive p tadhesive Etdoubler Non-Structural Repairs Minor or negligible damage will still need a form of repair. Aeroelastic Semi-Structural Repair Inverse patch lay-up Fill and seal Matrix cracks

15 CONCLUSION This presentation provides guidelines in the development of effective and efficient repair schemes for wind turbine blades. The repair is process includes a thorough determination of the repair requirements and assessment of the damage state. The blades is zoned for specific repair regions. A set of repair schemes are provided for better installation efficiencies whilst maintain the structure effectiveness. CONCLUSION These repair schemes consider: thick composite sections at the wind turbine blade hub aerodynamic contribution of the blade region leading edge. aeroelastic consideration of trailing edge over a wide blade chord region These repair schemes are specifically tailored to solid laminate construction. They are be easily adapted to sandwich structure construction.

16 ACKNOWLEDGEMENT Thanks to the contribution of Louis Dorworth (Abaris Training, Questions