Wind turbines are mainly classified into horizontal axis wind turbines (HAWTs) and vertical axis wind turbines (VAWTs) in terms of the orientation of rotating axis.
As the aerodynamic efficiency of HAWTs is generally better than VAWTs, nowadays the application of HAWTs with higher commercial values are the main stream in the world. Since larger wind resource and potential could be explored in deeper seas, wind farms are moving towards deep water in recent years.
Crucial dynamic responses
The floating offshore wind turbines have become the available solution which could be widely used in deep water. The spar, semi-submersible and tension-leg platform (TLP), which have been utilized in oil and gas industry for a long time, are three primary types of floating structures for offshore wind turbines.
The dynamic responses of these floating structures in the presence of the marine environment are crucial for a design purpose.
Several prototypes of floating HAWTs are developed, such as a catenary moored spar of the Hywind project in Norway, a semi-submersible of the WindFloat demo in Portugal, and a spar-type floating wind turbine of MOE project at Kabashima in Japan.
The commercial floating wind farm, i.e. the Hywind Scotland by Equinor, started its production in 2017.
UiS researchers have performed dynamic analysis of both semi-submersible floating HAWTs and VAWTs as well as spar-type VAWTs. Currently, UiS researchers are working on the dynamic responses of a spar-type VAWT integrated several wave-energy converters.
Aerodynamic Loads on Large Wind Turbines:
Increasing wind turbine size combined with necessary design improvements is the prevailing approach to reduce cost of the wind power production.
Large rotor blades (e.g. 178 m and 252 m rotor diameter for a 10 MW and a 20 MW turbine respectively) will operate higher up in the marine atmospheric boundary layer (MABL) compared to the wind farms currently in operation, and often protrude above the so-called surface layer.
The impact of such complex wind conditions for the design of wind turbines will be addresses by performing the full-scale wind measurements, as well as the advanced numerical simulations.
The offshore wind field at a site in the North Sea will be surveyed by optic wind sensors (so-called lidars), in collaboration with the University of Bergen (UiB), Christian Mikkelsen Research (CMR) and Equinor.
The activity will build on the pioneering work performed by UiS, UiB and CMR in 2014. The focus will be on capturing the spatio- temporal structure of turbulence, in particular the decreasing synchronization of the wind velocity fluctuations for increasing lateral distances in the rotor plane and increasing frequencies.
The impact of the variable spatial structure of the wind velocity field on the fatigue and the extreme loading of relevant wind turbine designs will be studied by numerically simulating the representative wind fields and performing the related structural response analyses in time-domain.
Such analyses are computationally demanding, as they require a multi-body modeling of the wind turbine and its moving rotor, immersed in a simulated three-dimensional wind (and wave) field, and the response computations for a number of time-steps. This response analyses will be performed in collaboration with Statkraft.
Another relevant topic is the effect of wave-wind interaction on the aerodynamic loads and the power production. This will be studied utilizing the relevant full-scale measurement data, as well as performing the CFD simulations.
Numerical Scour Prediction Model for Offshore Wind Turbine Foundations:
Many offshore sites for wind farms are located on seabeds of mobile sediments. For such cases, the interaction of the sediments with the turbine support structure must be taken into consideration.
Scour is the erosion of sediment in the vicinity of a structure, leading to a lowering of the seabed surrounding the structure. This can potentially be detrimental to the stability of the structure and its fatigue life.
Offshore turbine foundations are subject to high Reynolds number seabed boundary layer flows, which are induced by current and waves. To date, very few numerical studies have been performed to predict high Reynolds number flows around wind turbine substructures due to the complexity of the flow and the high demand on computational resource.
Currently, UiS professors and PhD students are developing a scour prediction CFD model.
Modeling of Violent Breaking Wave Loads on Support Structures of Offshore Wind Turbines:
Accurate prediction of breaking wave impact loads is a key factor in the design of maritime structures. The loading caused by steep storm waves impacting coastal and offshore structures has caused significant damage to vertical sea walls, caisson breakwaters, coastal bridges, oil platforms, FPSOs and LNG carriers.
Impulsive wave loadings are particularly difficult to predict, hence marine structures are often studied using hydraulic models and this generally introduces a number of ‘model’ and ‘scale’ effects. These combined effects result in deviation between loadings recorded in physical model tests and their real-world prototype equivalent hence in deviations between the up-scaled model and prototype observations.
A recent collaborative project between UIS and NTNU called WaveSlam aimed to investigate the wave slamming forces from plunging breaking waves on a truss structure in shallow water. A large-scale (1:8) jacket model was tested for plunging breaking waves in the Large Wave Facility in Germany. During the experiments, unique data sets were collected and recorded.
This valuable data set will be used to develop new load models for jacket structures and further validate CFD models developed in a previous PhD project for breaking wave loads on monopiles. A new study is also planned in cooperation with HR Wallingford in the UK to develop a compressible multiphase flow solver based on OpenFOAM capable of generating stable, coherent and statistically meaningful wave time histories of breaking wave loads on marine structures.
It is expected that results from this research will be used to shed light on the complex physics that stands behind wave-structure interaction, improve modeling capabilities and give recommendations to improve empirical formula for practical design purposes.
Installations of offshore wind turbines:
The installation of offshore wind farms presents great challenges as the industry move farther offshore and into deeper waters, and the turbines and foundations are getting larger and heavier.
Current installation methods are all sensitive to weather conditions: lifting the foundations using floating crane vessels, deploying and retrieving jack-ups' legs, and lifting turbine nacelles and rotors at a large lift height. Careful studies on these critical installation scenarios during the planning phase of the installation are important.
The research at UiS focuses on numerical analysis of various marine operation and installation activities for offshore wind turbine foundations. An example is the use of numerical study on lifting operation for a tripod foundation. Complicated numerical models are established, and critical responses which limit the weather windows are analyzed.
The simulations serve as important input for planning and execution of marine operations. Furthermore, measures to increase operability for such weather-sensitive operations will be proposed, which may reduce the installation cost.