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Access Platforms for Offshore Wind Turbines Using Gratings

Investigating the damage to access platforms at Horns Reef due to run-up forces and the benefits of using gratings instead of solid plates. Results from static tests show significant force reduction with grates. Findings suggest reduction factors derived from static tests can be applied to slamming scenarios as well. Study includes detailed drag coefficient comparisons for different grate materials.

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Access Platforms for Offshore Wind Turbines Using Gratings

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  1. ACE2008, Famagusta, Northern Cyprus Lykke Andersen, T., Aalborg University, Denmark Rasmussen, M. R., Aalborg University, Denmark Access Platforms for Offshore Wind Turbines Using Gratings

  2. 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?

  3. 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.

  4. Evaluation of Load Model for Solid Plate PlatformsLykke Andersen & Brorsen (2006) Model u and ηcalculated from streamfunctiontheory. 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)

  5. 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 • β = 0.70 β = 0.72β = 0.72β = 0.87

  6. 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.

  7. 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-β = 1) Measured drag coefficient: CD = 1.96

  9. Video – Fiberline 40 (1-β = 0.30) Measured drag coefficient: CD = 0.47, i.e. 24% of solid plate force.

  10. Video – Weland J9 (1-β = 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.

  11. 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

  12. 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

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