THESIS
2019
xvi, 169 pages : illustrations (some color) ; 30 cm
Abstract
Offshore wind turbine foundations are usually located in shallow depth of sea (<50m).
Conventionally, to evaluate the deformation response of wind turbine foundations, centrifuge
tests are performed by applying equivalent magnitude of wind and wave load on the model
foundations. However, equalizing the wave loading to act similar to wind load, grossly
ignores the bed shear and rotation of principle stresses in the soil caused by a progressive sea
waves. This aspect of wave loading on soil as well as on structure is pronounced in the nearshore
sea. The aim of this research is to evaluate the effect of wave loading on settlement
behavior of offshore wind turbine foundation under prototype stress condition. Centrifuge
modeling was chosen as an appropriate tool for this purpose.
To apply fluid-wave loading on offshore foundation in Geotechnical centrifuge, an
Environmental Hydraulic Loading System (EHLS) was envisaged. A piston type wave
paddle, driven by a hydraulic actuator, was utilized to mimic the near shore wave motion.
The EHLS was manufactured and tested under high-g field, to evaluate its performance
envelop. Optimization and improvements of various components of the EHLS were carried
out to add robustness and versatility to the system. A wave frequency up to 15Hz can be
effectively generated with a maximum paddle displacement of 60mm at 100g by the EHLS.
The characteristic of the generated waves (such as, wave pressure, wavelength etc) were
measured at high-g. A m...[
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Offshore wind turbine foundations are usually located in shallow depth of sea (<50m).
Conventionally, to evaluate the deformation response of wind turbine foundations, centrifuge
tests are performed by applying equivalent magnitude of wind and wave load on the model
foundations. However, equalizing the wave loading to act similar to wind load, grossly
ignores the bed shear and rotation of principle stresses in the soil caused by a progressive sea
waves. This aspect of wave loading on soil as well as on structure is pronounced in the nearshore
sea. The aim of this research is to evaluate the effect of wave loading on settlement
behavior of offshore wind turbine foundation under prototype stress condition. Centrifuge
modeling was chosen as an appropriate tool for this purpose.
To apply fluid-wave loading on offshore foundation in Geotechnical centrifuge, an
Environmental Hydraulic Loading System (EHLS) was envisaged. A piston type wave
paddle, driven by a hydraulic actuator, was utilized to mimic the near shore wave motion.
The EHLS was manufactured and tested under high-g field, to evaluate its performance
envelop. Optimization and improvements of various components of the EHLS were carried
out to add robustness and versatility to the system. A wave frequency up to 15Hz can be
effectively generated with a maximum paddle displacement of 60mm at 100g by the EHLS.
The characteristic of the generated waves (such as, wave pressure, wavelength etc) were
measured at high-g. A maximum wave pressure of 13kPa could be applied on the soil bed at a
wave frequency of 5Hz at 50g. To obtain a progressive wave train, an efficient mesh-type
wave absorber was used at the reflecting end. The reflection coefficient was obtained to be
less than 0.38 for all paddle displacement and wave frequencies.
Being a newly manufactured system, the performance of EHLS was required to be
evaluated theoretically to verify the measured wave characteristics. For this purpose, the
concept of modeling of models was employed. Two different centrifuge models (representing
the same prototype), with scaled water depth and wave frequency were tested at 50g and
100g. The time series of wave pressure exerted on sand bed in each test was compared (in
prototype scale) and was observed to match, thereby validating the scaling law for wave
frequency in EHLS. Further, a study with incremental wave loading on settlement of sand
bed was conducted at 50g, to identify wave parameter causing highest settlement in sand bed.
Two wave frequencies, 2Hz and 5Hz (model scale) were chosen, that resulted in minimum
and maximum settlement of sand bed, respectively. The obtained wave frequencies were then
utilized as test parameter for evaluating the behavior of skirted foundation, tested in EHLS.
An in-flight installed model skirted caisson foundation (embedded in sand (D
R=45%))
under compressive vertical load was exposed to wave loading at 50g. Two centrifuge tests, St
and Cy were conducted with static and cyclic vertical loading, applied respectively on the
caisson foundation. Each test was divided in three loading stages (with constant average
compressive load). In load stage 1 (LS1) static and cyclic vertical load was applied in test St
and Cy, respectively. Stage 2 and 3 loading consisted of loading from LS1 along with wave
loading of 2Hz and 5Hz (model scale) frequency, respectively. From the tests, it was
observed that the skirted caisson foundation settled up to twice the skirt length under 5Hz
wave loading. Wave loading induced an excess pore water pressure inside the soil-plug of
caisson, thereby creating a hydraulic gradient across the skirt. Dimensionless unloading
stiffness of the foundation was observed to reduce by a magnitude, after application of 5Hz
wave loading.
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