Access Platforms for Offshore Wind Turbines Using Gratings
ACE2008, Famagusta, Northern Cyprus
Lykke Andersen, T., Aalborg University, Denmark
Rasmussen, M. R., Aalborg University, Denmark
ACCESS PLATFORMS FOR OFFSHORE WIND TURBINES USING GRATINGS
Lykke Andersen & Rasmussen
ACE2008, Famagusta, Northern Cyprus, Sep., 2008
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The Problem
Damage 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?
ACCESS PLATFORMS FOR OFFSHORE WIND TURBINES USING GRATINGS
Lykke Andersen & Rasmussen
ACE2008, Famagusta, Northern Cyprus, Sep., 2008
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Previous Investigations for Solid Plates
by Lykke Andersen & Brorsen (2006)
Calibrate a three step slamming load model:
1. Estimation of maximum run-up height without platform
Ru  max  m 
u2
2g
2. Calculate velocity at platform level from the run-up height
calculated in item 1
v( z )  2 g  Ru  z 
3. Slamming force model gives the slamming pressure on the platform
ps  12  Cs    v 2
m and Cs were calibrated against a large number of scale model test
data.
ACCESS PLATFORMS FOR OFFSHORE WIND TURBINES USING GRATINGS
Lykke Andersen & Rasmussen
ACE2008, Famagusta, Northern Cyprus, Sep., 2008
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Evaluation of Load Model for Solid Plate Platforms
Lykke Andersen & Brorsen (2006) Model
Ru  max  m 
u2
2g
v( z )  2 g  Ru  z 
ps  12  Cs    v 2
u and η 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)
ACCESS PLATFORMS FOR OFFSHORE WIND TURBINES USING GRATINGS
Lykke Andersen & Rasmussen
ACE2008, Famagusta, Northern Cyprus, Sep., 2008
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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
β = 0.70
Fiberline 50
β = 0.72
Weland H4
β = 0.72
Weland J9
β = 0.87
ACCESS PLATFORMS FOR OFFSHORE WIND TURBINES USING GRATINGS
Lykke Andersen & Rasmussen
ACE2008, Famagusta, Northern Cyprus, Sep., 2008
5 of 12
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.
ACCESS PLATFORMS FOR OFFSHORE WIND TURBINES USING GRATINGS
Lykke Andersen & Rasmussen
ACE2008, Famagusta, Northern Cyprus, Sep., 2008
6 of 12
Verification of Flow Transducer
Flow measured by clamp on ultrasonic flow transducer.
Solid wall results for the two
different sets of electrodes
120000
B3 Transducers
C3 Transducers
100000
CD = 2.14
CD = 1.94
F / A [Pa]
80000
60000
40000
20000
0
0
2
4
6
Velocity [m/s]
8
10
Agrees well with theory (CD = 2)
ACCESS PLATFORMS FOR OFFSHORE WIND TURBINES USING GRATINGS
Lykke Andersen & Rasmussen
ACE2008, Famagusta, Northern Cyprus, Sep., 2008
7 of 12
Video - Solid Plate (1-β = 1)
Measured drag coefficient: CD = 1.96
ACCESS PLATFORMS FOR OFFSHORE WIND TURBINES USING GRATINGS
Lykke Andersen & Rasmussen
ACE2008, Famagusta, Northern Cyprus, Sep., 2008
8 of 12
Video – Fiberline 40 (1-β = 0.30)
Measured drag coefficient: CD = 0.47, i.e. 24% of solid plate force.
ACCESS PLATFORMS FOR OFFSHORE WIND TURBINES USING GRATINGS
Lykke Andersen & Rasmussen
ACE2008, Famagusta, Northern Cyprus, Sep., 2008
9 of 12
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.
ACCESS PLATFORMS FOR OFFSHORE WIND TURBINES USING GRATINGS
Lykke Andersen & Rasmussen
ACE2008, Famagusta, Northern Cyprus, Sep., 2008
10 of 12
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
ACCESS PLATFORMS FOR OFFSHORE WIND TURBINES USING GRATINGS
Lykke Andersen & Rasmussen
ACE2008, Famagusta, Northern Cyprus, Sep., 2008
11 of 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
ACCESS PLATFORMS FOR OFFSHORE WIND TURBINES USING GRATINGS
Lykke Andersen & Rasmussen
ACE2008, Famagusta, Northern Cyprus, Sep., 2008
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