| dc.description.abstract | A rapid increase in energy demand and the global warming problem has increased the focus
on renewable energy resources. Among renewable sources, ocean wave and offshore wind
are the topic of this thesis because both resources are considered to have high potential and
they are highly correlated. However, the main challenge is whether they can be exploited to
produce energy at a reasonable cost. To reduce the cost of power from such energy systems,
we need to reduce the cost of the structures and/or increase their power production. The latter
is becoming the focus of many studies of wave energy systems through the use of control.
However, some studies show that this does not necessarily reduce the cost of power due to
the extra costs needed for the control system, especially for wave energy systems. This thesis
focuses on global analyses to estimate the responses during non-operational conditions,
including extreme environmental conditions that will be used for the design of such system.
Additionally, several survival strategies for renewable energy systems were studied in the
present thesis to reduce the structural load during extreme environmental conditions.
As for other floating structures, floating wave energy converters (FWEC) require mooring
systems to remain stationary. In addition to being one of the main cost contributors of a
FWEC project, a catenary mooring system could affect motion and therefore the power.
Different mooring configurations have been considered in one of the case studies in the
present thesis to study the effect of the mooring system on the power capture of a two-body
FWEC. The analyses performed indicate that the influence of the mooring increases as sea
severity increases and as line length decreases. As long as the length of the mooring lines are
sufficient to accommodate the motions of the device due to first order wave forces, especially
in heave, the effect of mooring on the power performance of a two-body axi-symmetric
FWEC will be insignificant. The mooring lines used in the studies were designed to withstand
the relevant extreme environmental conditions (ultimate limit state (ULS) criterion). To
capture the short-term variability of the 3-hour response extreme value, especially due to
different strategies applied for operational and non-operational conditions, the ULS level
response for a two-body FWEC should ideally be estimated by performing the full long term
analysis by taking into account all sea states. However, such an analysis is time consuming;
the simplified contour line method that takes into account only a few sea states along a return
period environmental contour is suggested. Referring to the practices in the oil and gas
industry, the present thesis has explored the feasibility of the contour line method to predict
the ULS response level (100-year) in mooring lines of a two-body FWEC. Commonly, a
short-term extreme is obtained by a high percentile level. In principle however, large numbers of extremes from simulations are required to obtain an adequate asymptotic Gumbel
distribution with a targeted response with a high percentile level. Therefore, an alternative
method using the expected maximum value multiplied by a factor has been explored and was
found to be a more robust method for this application.
To reduce wave loads on the floater in extreme sea states, the over-topping Wave Dragon
FWEC considers lowering the floater to a certain submergence level. A laboratory model test
has shown that this strategy reduces the mooring line tension significantly. The numerical
model developed in the present thesis yields motions and mooring line tensions that are in
good agreement with the model test observations.
Extracting energy from waves and wind simultaneously may be profitable to reduce the cost
of power from the renewable systems. For that reason, a combined concept involving the
combination of a spar-type floating wind turbine (FWT) and a torus-shaped point-absorbertype
FWEC, referred to as the Spar Torus Combination (STC), has been introduced. The
effect of adding a FWEC system onto a spar-type FWT for STC has been investigated in
terms of power production in operational conditions and in terms of extreme responses in
non-operational and extreme environmental conditions. Analyses indicate that the additional
FWEC system on the FWT increases the stability of the spar. It was found that this addition
results in higher wind power production compared to the spar FWT alone for wind speeds
lower than the rated one. However, the additional WEC also increases the structural
responses in extreme conditions. To reduce these responses, some relevant non-operational
modes have been considered and investigated. The analyses performed indicate that the
structural responses of the STC are highly dependent on the applied non-operational mode.
As a very large floater, Wave Dragon could easily support wind turbines (WTs). This
combination is interesting because the power production from wind and waves will be
approximately the same order of magnitude. Simulation results indicate that additional WTs
will not affect the wave power performance of the Wave Dragon. This means that the average
power from a Wave Dragon with 2 WTs could be estimated based on a linear summation of
Wave Dragon wave power and wind turbine wind power. | nb_NO |
