Experimental and Numerical Study of a Combined Offshore Wind and Wave Energy Converter Concept
Doctoral thesis
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Date
2016Metadata
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- Institutt for marin teknikk [3469]
Abstract
Clean and renewable energy is increasingly emphasized with the growing public attention
to the environmental pollution problem. Among renewable energy resources, both
offshore wind and wave energy are resources with great potential. However, the stages of
technological development are quite different for wind and wave energy: wind energy
technology has already been commercialized, while wave energy technology is still
immature. To address the integration of wind and wave energy devices in one single
offshore floating platform, a combined wind and wave energy converter concept named
the spar torus combination (STC) concept was proposed under the European Commission
FP7 Marine Renewable Integrated Application (MARINA) Platform project. The STC
concept is composed of a 5 MW spar-type floating wind turbine and a torus-shaped wave
energy converter. As a consequence, the investment cost can be reduced due to the
common infrastructure, the augmentation of produced power, and the positive synergy
between the spar and the torus regarding dynamic responses and power production. In the
farm configuration, ocean space and energy can be better utilized.
For the design of the STC concept, it is critical to have a feasible spar-torus interface
including power take off (PTO) system under operational environmental conditions.
However, it is challenging to ensure the structural integrity of the STC concept under
extreme environmental conditions due to large wave forces on the torus. Numerical
simulations considering linear hydrodynamic forces, aerodynamic forces, spar-torus
interface, mooring system and PTO system were performed to predict the integrated
dynamic responses of the STC concept under different working modes. Furthermore,
model tests were performed with the focus on functionality and survivability, respectively,
to validate the numerical model. However, strongly nonlinear phenomena, i.e., water
entry and exit of the torus in the survivability model test was observed. A nonlinear
numerical model based on the nonlinear potential flow solver with a local impact solution
for bottom slamming events and an approximated model for the water shipped on the
deck was used to investigate the water entry and exit processes.
For the functionality model test, a roller-guide system was designed to model the spartorus
interface; and the PTO system was modelled by two pneumatic dampers. However,
with large PTO damping levels, strong air compressibility was observed. The numerical
model for the functionality test provides satisfactory results compared with the model test
when there is no strong air compressibility in the pneumatic dampers. In cases with
strong air compressibility, an additional linear stiffness term in the PTO model was
considered; and this additional factor improved comparisons between the numerical and experimental models. Wind effects were modelled by a simplified drag disc without
considering centrifugal forces or gyroscopic effects. Wind forces significantly affect
surge and pitch motions, but are only marginally important for the heave motion and
wave power absorption. The wave power absorption is significantly affected by the PTO
parameters. Reasonable PTO damping can boost the wave power absorption, while PTO
with too high damping will restrain the wave power absorption because it reduces the
relative heave motions.
Survivability model tests were designed and performed with the focus on two survival
modes of the torus: the Mean Water Level (MWL) survival mode and the Submerged
(SUB) survival mode. For the MWL mode, the torus is locked to the spar, and the whole
structure floats at the mean water level. For the SUB mode, the torus is locked to the spar
at the mean water level and then is totally submerged to a specified position by additional
ballast. The numerical model considering linear hydrodynamic properties predicts the
dynamic responses well in cases with no water entering or exiting of the torus. The
performance of the SUB survival mode is much better than the MWL mode in terms of
both motions and interface forces, but complex mechanisms and remote operation
procedures should be deployed to implement the SUB mode in the prototype. A wind
drag disc was also used in the survivability model test, and the wind-induced mean drift
for the SUB mode is dominant because the wave drift forces for the SUB mode are rather
small.
Two survivability model tests were performed respectively in the towing tanks of
MARINTEK, Norway, and of INSEAN, Italy. The purpose of these two tests was to
address the uncertainties and performances of different testing facilities. The physical
models used in the two towing tanks have similar properties with slight differences.
Consistent comparisons were achieved between the model tests in the two testing
facilities. The differences and performances of the two models were also documented.
In the MWL survivability test, water entering and exiting of the torus was observed to be
accompanied by bottom slamming and water shipped on deck in cases with a wave height
of 9m and an incident wave period close to 13s, which is the heave resonant period. In
these strongly nonlinear cases, nonlinear hydrodynamic properties should be considered.
A nonlinear numerical solver was used, and satisfactory comparisons were achieved
between the solver and the model test. With water entry and exit, the spar-torus interface
forces are strongly modified. Further nonlinear simulations showed that the slamming
forces have insignificant effects on motions compared with the water on deck forces. The
water on deck effects on heave motions are more significant than its effects on surge and
pitch motions, and the effects on motions increase with increasing wave steepness and
wave length due to the larger amount of water shipped on deck.