PhD defence NJOMO WANDJI Wilfried

Supervisors: Anand Natarajan, DTU Wind Energy - Nikolay K. Dimitrov, DTU Wind Energy - Thomas Buhl, Suzlon Blade Science Center

Examiners: Henrik Bredmose, DTU Wind Energy - Michael Muskulus, NTNU - Subhamoy Bhattacharya, University of Surrey

Title: Probabilistic Design of Wind Turbine Structures: Design Studies and Sensitivities to Model Parameters
Several societies have envisaged for renewable energy sources for ecological and geo-strategical considerations. Amongst others, wind energy has gained considerable interest in the past decades due to its high potential to fulfill the aspirations of the societies that opted for it. However, harnessing offshore wind energy poses some challenges such as cost reduction, issues with large structure size, randomness pertaining to the metocean environment, and better understanding of mechanical behavior of the structure.

This study addresses these challenges sorted in three research areas: (i) Area 1: reduction of conservatism; (ii) Area 2: Lifetime improvement; and (iii) Area 3: Innovative systems. These research areas are differentially implemented through tasks on various wind turbine structures: shaft, jacket substructures, an innovative semi-floater concept, monopile substructure, and conical grouted joint. The ultimate goal of this thesis is to provide methods for the design of robust wind turbine structures that are structurally safe and cost-efficient.

As a result, a method for load exchange is developed to reduce the level of conservatism while being in line with guidelines based on the shaft study. Three methods for fatigue lifetime improvement have been developed for jacket substructures. The first improves the joint design methodology by minimizing stress concentration factors. The second uses magneto-rheological dampers to mitigate the brace vibrations. The third employs an aero-elastically tailored rotor to alleviate fatigue loads on the whole support structure. An innovative semi-floater concept is proposed and is analyzed in detail as an alternative for fixed-bottom substructures of large wind turbines at deep waters with a reduced fatigue level and a moderate cost. The influence of model parameters of monopile substructure is investigated. Recommendations are formulated to achieve the structural integrity and to realize material savings. The key design parameters of conical grouted joints are identified, as well as their respective impacts on the joint behavior associated to extreme events and normal operations.


Wed 07 Mar 18
13:00 - 16:00



Danmarks Tekniske Universitet
Risø Campus, B112 H.H. Koch
4000 Roskilde