INCORPORATING RELIABILITY ASSESSMENT IN THE DESIGN DEVELOPMENT & OPTIMIZATION OF FLOATING STRUCTURES

Offshore wind turbines are exposed to fluctuating environmental loads and have to deal with aero-hydro-servo-elastic coupled dynamics. Such complex engineering systems need thorough design processes, as well as sophisticated monitoring and maintenance approaches. The current development trend towards floating support structures for offshore wind turbines makes maintenance and repair work more difficult. This is, on the one hand, as access to, transfer of personnel to, and work of technicians on floating systems are complicated and entail additional hazards. On the other hand, floating wind turbines can be located further from the shore, which significantly reduces the allowable weather windows for offshore work. Thus, researchers and industry must not only focus on efficient maintenance strategies but also put more emphasis on the design process of such offshore structures, focusing on reliable systems ab initio. For reliability assessments of highly complex engineering systems, the combination of different techniques-building on approaches for creating approximate system representations and subsequent reliability analysis and calculation methods-is most promising¹ . The development and assessment of floating wind turbine systems, however, also requires numerical modeling to correctly represent and simulate the fully coupled dynamics. The Modelica^® library for Wind Turbines MoWiT (www.mowit.info), developed at Fraunhofer Institute for Wind Energy Systems IWES, allows for component-based modeling and can be coupled to a framework, programmed in Python, for automated simulation and optimization² . To incorporate the computationally intensive reliability assessment within the highly iterative optimization process, a methodology is developed, by which means approximate models in form of response surfaces for a few potential system geometries out of the entire optimization design space are created ahead of the optimization, corresponding response surfaces for any other system designs are derived based on an interpolation approach, and, finally, the reliability is determined time-efficiently within the optimization procedure, using Monte-Carlo simulation³ . This methodology is applied to a floating wind turbine to obtain a reliability-based optimized support structure, accounting for uncertainties in environmental conditions directly within the design development and ensuring that the structure, including the mooring lines, fulfills certain reliability constraints³ . Furthermore, the numerical framework allows developing digital twins by optimizing the numerical model based on measurements. Such digital twins are highly suitable to assess the system condition and estimate, e.g., the damage or remaining lifetime.