Sectional Blades

Introduction

Access restrictions to some wind farm sites may, in future, limit the size of blades and hence the rating of the turbines that can be used. The concept of constructing blades in two or more sections, which are then assembled on site, is assessed here through a comparison of two conceptual designs. The first is a tubular connection, which has been tested at scale model size; the second is a bolted connection, which has been manufactured and tested as a prototype sectional blade. The results are compared with the equivalent conventional blade design and an economic assessment of the viability of the sectional blade concept is presented.

A public report is available as a download.

Sectional blade with T-bolt connection

This concept was implemented at prototype scale (23.3m blade). It was an existing LM23.3 blade without tip-brake and the connection was at 7.3m.

Design modifications

  • Thickening of basic UD laminate at joint by a factor of about 3,
  • Incorporating UD and ± 45° fabrics, to achieve more isotropic properties about the holes,
  • Drilling of circular and elliptical bores for mounting the cross-nuts and pre-tensioning the bolts,
  • Drilling of holes parallel to blade skin for the bolts themselves,
  • Separation of shear webs and reinforcement using bulkheads

Structural testing

  • Flapwise and two edgewise static load tests
  • 5 x 106 cycles edgewise + 2.5 x 106 cycles flapwise fatigue test
  • Repeated static load test
  • No observed damage or stiffness degradation, BUT
  • Some bolts exceeded their allowable design lives and bolt load factors need to be reduced

Economic assesment

Step 1: Measure increase in manufacturing costs for prototype (23.3m) sectional blade compared to standard blade

Result: Cost increase (23.3m) 68%

Step 2: Estimate increase in manufacturing costs for likely full-scale implementation (60m) - these are proportionately much less since the relative proportion of the blade which must be strengthened is less

Result: Manufacturing cost increase (60m) ~19%

Step 3: Estimate saving in transport costs for typical journey (LM Denmark to Hannover, Germany)

Table showing comparison between cost price of 60m blade and transport costs (for access to a typical wind farm site)

Result: Transport cost saving ~5% (more for a less accessible site)

Conclusion: Likely cost increase for 60m sectional blade ~ 14%

Blade testing

In order to get a deeper understanding of the blade to blade variations and to determine the statistical distribution of the fatigue strength of rotor blades, 37 small rotor blades have been tested in static and fatigue loading. Besides these blades, a number of coupons of identical material as that of the blades has been tested statically and under fatigue loading.

Testing

Static and fatigue tests on rotor blades:

  • Flapwise prismatic tip section
  • Flapwise root section
  • Edgewise root section

Static and fatigue tests on coupons:

  • Fatigue R= 0.1
  • Fatigue R= -1
  • Static tensile
  • Static compression

Analysis

  • Fatigue formulation with Goodman modification has been used.
  • Comparison coupon data with existing data from literature.
  • Non-linear regression analysis of coupon results based on maximum stress for the whole population R=0.1 and R=-1; slope and UTS value are the results.
  • Non-linear regression analysis of blade data with UTS value found from coupons substituted; slopes of each test series are the results.

Conclusions

  • Fatigue formulation with Goodman modification describes influence of stress ratio very well.
  • PROFAR test results are in good agreement with FACT results when divided by the UTS-value of the relevant material.
  • Slopes of regression lines are close to 10 for blade and coupon tests as well.
  • Scatter found from blade tests are comparable with that from coupon tests.

Contact

For inquiries please contact:

Knowledge Centre WMC
Kluisgat 5
1771 MV Wieringerwerf
the Netherlands
phone +31(0)227-50 49 49
info@wmc.eu