Operability of WTG Blade Installation on a Semi-submersible FOWT
M.J.W.M. Hoogeslag (TU Delft - Offshore and Dredging Engineering)
B.C. Ummels – Mentor (TU Delft - Offshore Engineering)
Marilena Greco – Mentor (Norwegian University of Science and Technology (NTNU))
Rogier de Bruijn – Mentor (GustoMSC)
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Abstract
The global concern over climate change has accelerated the shift towards renewable energy sources, in which floating offshore wind energy is expected to play an important role. A floating wind turbine could be installed in a harbour using a quayside crane. Various types of floating foundations are available, where a semi-submersible floater has the advantage of versatility in different water depths due to its small draft. However, the continuous upscaling of wind turbine sizes introduces new challenges for the installation process. Installing these larger turbines requires the use of the world’s largest cranes and large quaysides. The operability of the wind turbine generator (WTG) installation has a significant impact on the planning and feasibility of a floating wind farm project. Therefore, this research is aimed at studying and understanding the factors that influence the operability of the WTG blade installation on a semi-submersible floating wind foundation using a quayside crane.
To investigate how the dynamics of the floater and blade are affected by wind and wind conditions, a numerical model is developed. This is done in frequency domain which allows for fast calculations to gain insight into a variety of parameters. The floater model includes a 3D wave diffraction-radiation analysis, as well as a procedure for including viscous damping. Moreover, a response-based approach is applied to obtain the characteristic displacement and velocity of the turbine hub, which correspond to specific sea states. The blade is modelled as a pendulum with two degrees of freedom in axial and radial directions. Taglines are modelled as spring dampers that reduce the blade motions. A similar response-based method is used to obtain the characteristic motions that correspond to specific wind speeds and directions. During the critical alignment and mating phases, the relative displacement and velocity between the blade root and turbine hub are the limiting factors for installation. These relative motions, that follow from the floater and blade model, are then used to calculate the allowable environmental conditions for the wind turbine blade installation. There after, these allowable conditions are combined with metocean data to obtain the operability.
The model is used to study the sensitivities of parameters for the blade and floater motions. Aligning the blade with the wind is the most effective way to reduce blade motions because the motions in the axial direction are significantly lower compared to the radial direction. The influence of the distance between the quayside and the floater is also studied. For specific sea states, there are beneficial distances. However, when considering a large range of sea states, there is no significant advantage for certain distances. An important parameter affecting the relative motions between the blade and hub is the nacelle and blade heading. Optimising this orientation significantly increases the allowable environmental conditions. For low wind speeds, the optimal relative nacelle heading with respect to the wind and wave directions is dependent on these directions. For high wind speeds, above approximately 6.5 m/s, the blade should be aligned with the wind. Additionally, the lowest motions occur when the nacelle is rotated inward, closer to the floater’s centre of gravity.
Five different blade installation strategies are studied to identify one that yields the highest operability. A method where the nacelle and blade heading is adjusted to the optimal position for each change in environmental conditions yields the highest theoretical operability. However, in practice, a better solution is to select a single nacelle heading that is optimised for high wind speeds from a dominating wind direction. Utilising this strategy, it was found that a yearly operability of approximately 65% and 45% could be achieved for installing the turbine blade in a sheltered and exposed harbour, respectively. During the summer months, this operability increases to approximately 80% and 58%, respectively. The operability is mainly driven by wind-induced motions, so a location where wind speeds are low is favourable for achieving high operability.
In conclusion, this research contributes to the understanding of the critical factors that influence the op- erability of wind turbine blade installation on floating foundations, which is a crucial step in the upscaling of floating offshore wind technology.