Abstract
Offshore wind power is becoming an affordable form of clean energy that will positively
affect the environment. Modern-day wind farm is a collection of Wind Turbine Generators
(WTG), and the majority of currently installed are supported on a single large diameter pile
(commonly known as monopile). Foundations for an offshore wind turbine is one of the most
expensive components (typically costs between 16 and 34% of the overall cost). The analysis,
design and construction of the foundation of these structures require multidisciplinary
knowledge. In order to reduce the cost of wind power, there has been extensive research to
find new types of foundations. Naturally, for such novel/new foundations, codes of practice
for design will not be fully developed. Due to technological innovations, the sizes of turbines
(rated power and RNA mass) are increasing. As offshore wind turbines are being planned to
be installed in deeper waters, customised or non-standard foundations are necessary.
Furthermore, offshore wind turbines are also sited not only in deeper waters but also in
seismic areas and disaster-prone areas (typhoons and hurricanes). To satisfy the industry
requirements and de-risk the installation and operation, any new foundation must be validated
through a structured study of the Technology Readiness Level (TRL). This thesis developed
techniques to study different aspects of TRL studies for new foundations with a super focus
on the long-term performance of foundations.
TRL studies have been carried out for gravity-based foundations and hybrid foundations. By
hybrid foundations, it is meant that foundations have aspects of shallow and deep foundations,
i.e., where loads will be transferred to shallow layers (shallow foundations) and deeper layers
(deep foundations). Hybrid foundations can be attractive in many situations: ground having
shallow rock, to reduce the diameters of monopile before it becomes too large to handle and
finally to avoid tilting. Different forms of hybrid foundations are studied using a numerical
and physical modelling approach. To understand the effectiveness of a hybrid foundation, the
comparison is made with monopile alone. One particular hybrid foundation is studied in more
detail: Combination of a caisson and a monopile named Collared Monopile. It was found that
the stiffness of a Collar Monopile foundation depends on the stiffness of the collar and
monopile and the surrounding soil. Due to the addition of the collar, the reserve strength and
stiffness of the overall hybrid foundation increases; therefore, the risk of long-term tilt can be
better engineered. A novel approach is proposed to analyse the concept of a hybrid
foundation.
Gravity-based foundations are becoming an attractive option for shallower water depths. A
simple integrated method within the so-called “10 Step method” is developed to find the initial
size of the foundation.
Monopile design is dominated by the pile head stiffness and is required for SLS
(Serviceability Limit State), natural frequency estimation and long-term tilt calculations.
Estimating load carrying capacity (combined effect of moment and lateral load) is needed to
optimise monopile and long-term tilt calculation. Long term tilt needs a LU (Load Utilisation)
ratio, which is the ratio of applied load to the load-carrying capacity. The thesis also analyses
the LU ratio for many European Wind Farms based on minimum data. It is hoped that these
calculations will support a quick preliminary design process. Finally, future suggested works
are also provided.