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Access Platforms for Offshore Wind Turbines Using
Gratings

• ACE2008, Famagusta, Northern Cyprus
• Lykke Andersen, T., Aalborg University, Denmark
• Rasmussen, M. R., Aalborg University, Denmark

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The ProblemDamage to platforms at Horns Reef due
to run-up generated forces

• Access platforms on the offshore wind turbine
  park Horns Reef in Denmark has observed severe
  damage with dislodgement of grates.
• The damage is expected to be due to significant
  larger run-up generated forces than accounted for
  in the design.
• Run-up generated slamming forces on platforms
  with solid plates has previously been
  investigated in the model test study by Lykke
  Andersen Brorsen (2007).
• The question was then how large is the reduction
  in the forces on the platforms if we use gratings
  instead of solid plates?

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Previous Investigations for Solid Platesby Lykke
Andersen Brorsen (2006)

• Calibrate a three step slamming load model
• Estimation of maximum run-up height without
  platform
• Calculate velocity at platform level from the
  run-up height calculated in item 1
• Slamming force model gives the slamming pressure
  on the platform
• m and Cs were calibrated against a large number
  of scale model test data.

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Evaluation of Load Model for Solid Plate
PlatformsLykke Andersen Brorsen (2006) Model
u and ? calculated from stream function
theory. s0p 0.020 m 5.6 s0p 0.035 m
4.2 Cs 10 for maximum pressure Cs 1.5 for
spatial averaged pressure
Max. local pressures (model scale) Max.
average pressures (model scale)
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The New Investigations on Grates

• Clients Vattenfall and Dong Energy (Horns Reef 1
  Windfarm repair project)
• Due to the grate structure it is not possible to
  study the problem with run-up generated forces on
  grates in a small scale wave flume. Large scale
  tests are needed for such a study.
• Therefore, only the static situation was
  considered in this study (slamming not included).
  Reduction factors for grates compared to solid
  plates that are derived from static tests are
  expected conservatively also to be applicable for
  slamming.
• The size of the jet is assumed small compared to
  the size of the platform.
• Fiberline 40 Fiberline 50
  Weland H4 Weland J9
• ß 0.70 ß 0.72
  ß 0.72 ß
  0.87

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Test Set-Up
Flow generated by two large pumps to obtain flow
velocities corresponding to prototype conditions
(6.1 and 9.7 m/s tested). Diameter of jet 0.081 m.
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Verification of Flow Transducer
Flow measured by clamp on ultrasonic flow
transducer.
Solid wall results for the two different sets of
electrodes
Agrees well with theory (CD 2)

8
Video - Solid Plate (1-ß 1)
Measured drag coefficient CD 1.96
9
Video Fiberline 40 (1-ß 0.30)
Measured drag coefficient CD 0.47, i.e. 24
of solid plate force.
10
Video Weland J9 (1-ß 0.17)
Tested at two positions due to the size of the
grating structure compared to the size of the
jet. This test is for highest solidity which gave
slightly larger forces.
Measured drag coefficient CD 0.12, i.e. 6 of
solid plate force.
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Summary of Results
Drag coefficients as function of solidity and
comparison with literature values for grates

• Drag coefficients are in most cases lower than
  literature values. Can be explained by
• Well ventilated rear side which means present
  tests are much different from resistance tests
  where the grate is submerged in a stationary
  flow.
• The very large Reynolds numbers tested

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Overall Conclusions

• The jet generated forces on 4 different grates
  and a solid wall have been measured.
• The results show that the use of grates instead
  of solid plates will reduce the forces on wind
  turbine access platforms very significantly.
• The main parameter describing the reduction is
  the opening percentage of the grate, but also the
  geometry of the grate is important. For very
  typical grate geometries the force is reduced by
  a factor 4.
• For local loads (one grate) the use of the
  stationary reduction factors might be unsafe as
  only the case where the size of the exposed area
  is small compared to the grate have been tested.
• The use of the stationary reduction factors also
  for slamming is expected to be conservative when
  the total force on the platform is considered.

Thank you for your attention