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Experiments by Invited Researchers

 

Design of entrance platforms for offshore wind turbines

Project acronym: HyIII-FZK-02
Name of Group Leader: Peter Frigaard, Aalborg University
User-Project Title: Design of entrance platforms for offshore wind turbines
Facility: Large Wave Flume, GWK
Proceedings TA Project: Loads on entrance platforms for offshore wind turbines
Data Management Report: There is no Data Management Report available for this project

User-Project Objectives

Summary:

If wind turbines are placed in an area with risk of sea ice, an ice cone is typically applied to break the ice by bending which significantly reduce the horizontal force from the ice. The ice cone provides also a platform with access to the wind turbine. In case there is no risk of sea ice a platform consisting of closed plates or gratings has typically been applied. Actual run-up values are much higher than expected or accounted for in the design.

The aims of the large scale tests were to:

  • Study scale effects on run-up on slender piles by comparing results from GWK with smallscale test results;
  • Study scale effects related to impact pressures and forces on a closed plate platform by comparing results from GWK with small-scale test results;
  • Study slamming forces on grates which is impossible to study in small scale wave flume tests. Compare slamming reduction factors with drag coefficient reduction factors found in experiments at Aalborg University;
  • Study scale effects related to the time scale of the back-filling of an initial deep scour hole;
  • Study soil strength of back-filling which is of great important for fatigue calculation of piles designed without scour protection;
  • Give design guidelines for above topics.

Freak waves and irregular waves were tested for run-up and load measurements with identical paddle steering signals in the two cases. Thus interrelated values of run-up and loads are available.

The run-up height and velocity was measured by use of high speed video recordings supplemented by some wave gauges mounted at the pile. Hereafter, the run-up generated impact forces were measured on two types of grates and a solid plate (Porosity varies from 80%, 60% and 0%). The pressure distribution was also measured for the solid plate. In addition to this the wave generated backfilling of an initial scour hole and the strength of the backfilling soil was studied.

The observation from small scale tests, that breaking waves give much higher run-ups and higher loads than non-breaking waves, were verified in large scale. Actually the process is very sensitive to the breaking point. To get maximum run-up and force on the platform the wave needs to break just before the pile or directly on the pile. This also shows that the irregular wave kinematics are extremely important in order to correctly estimate wave run-up.

It is evident that the solid plate experiences significantly larger forces than the porous grates. The vertical loads seem to scale approximately with the solidity (1 – porosity). However, the vertical loads for all three platform types are very large, corresponding to approximately 0.5 to 2.5 MN in prototype. The loads are typical church roof shaped with a short impulse followed by a longer uration quasi-static load and some vibrations of the structure. Therefore, the maximum force is not the only parameter to describe the loads, but also the time history is very important. Moreover, the response of the structure for such types of loads should be determined for design purposes in orders to know if dynamic amplification or dampening occurs. Here it should be noted that the Eigen frequency of the platform was 50-60 Hz in the model, which is expected to correlate quite well with prototype platforms of such types.

Design guidelines are currently based on small scale test results, which are not significantly different from the results of large scale tests. However, it is expected that the design guidelines can be improved by more detailed consideration of the wave kinematics in both breaking and close to breaking irregular waves.

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