Abstract
Global efforts to combat climate change involve increasing renewableenergy use, with a focus on wind energy. While onshore wind is reaching
its limits, offshore wind, and particularly Floating Offshore Wind (FOW),
is gaining huge interest. Moreover, FOW gives access to deeper waters,
unlocking 80% of marine wind resources, leading to install larger and more
powerful wind turbines. Among the floating substructures used to support
the wind turbines, semi-submersible configuration offers several advantages,
including access to various water depths, easy onshore turbine assembly and
tow-out, and simple installation. While it is still an emerging technology,
there is growing interest regarding this platform type, and a large variety
of platform concepts are currently in design stage, driving the need for optimisation.
The behaviour of Floating Offshore Wind Turbines (FOWT) is influenced by structural dynamics, aerodynamic, hydrodynamic, and mooring
dynamics, with control systems managing their interactions. Modelling
these complex coupling effects is challenging, so fully-coupled time-domain
numerical tools are commonly utilised to address both linear and non-linear
loads. However, due to the high computational effort required for these type
of analyses, in the preliminary design stages frequency domain based tools
are often employed.
In this Thesis, an efficient frequency domain numerical tool for the
preliminary design of the floating offshore wind substructures has been developed, investigated and validated against a time domain state-of-the-art
method. Since it is focused on the design of the floating substructures, and
particularly applied to semi-submersible platforms, the acceleration of the
design process is sought. The large number of platform assessment requires
high computational cost, and therefore, a novel method to obtain the hydrodynamic coefficients is here presented. This method has demonstrated
to estimate the hydrodynamic coefficients with reasonable accuracy and, as
the main advantage, it performs the calculation at least five times faster
than the conventional methods.
Additionally, another novel method has been developed, that predicts
the second-order hydrodynamic loads at linear cost, in computational terms.
These loads are not commonly considered in the initial stages of design,
however, it is known they are significant for the mooring design, especially when catenary mooring systems are used. This method has shown
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to provide accurate results, despite the simplifications that have been adopted. Similarly to the first method, the main advantage this one presents
is the important reduction of the computational cost achieved, making it
suitable for the preliminary design stage of FOW substructures. Both developed methods have been validated against radiation-diffraction analysis
and have been integrated into the frequency domain simplified response
model. The proposed tool has shown to provide similar results regarding
the dynamic behaviour of FOWT structures and enables to identify the
most suitable platform designs to be assessed in a more advance phase of
design, narrowing down the number of potential platform solutions.
Moreover, the notable efficiency of the proposed tool makes it suitable
for sensitivity analyses, in order to have a better understanding of the
FOWT dynamic behaviour and quantify the influence of the different parameters on the platform design. For instance, in this Thesis, two sensitivity
studies have been carried out. On one hand, how the platform dimensions
change when a larger wind turbine aims to be installed has been discussed,
resulting that the diameter of the columns is the design parameter that
most changes. And, on the other hand, how the mooring system affects to
the platform design, as well as the importance of including the second-order
loads is highlighted.
| Date of Award | 2023 |
|---|---|
| Original language | English |
| Awarding Institution |
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| Supervisor | Victor Petuya Arcocha (Supervisor) & Nava (Supervisor) |