  Floating offshore wind turbine (FOWT) systems involve several coupled
physical analysis disciplines, including aeroelasticity, multi-body structural
dynamics, hydrodynamics, and controls. Conventionally, physical structure
(plant) and control design decisions are treated as two separate problems, and
generally, control design is performed after the plant design is complete.
However, this sequential design approach cannot fully capitalize upon the
synergy between plant and control design decisions. These conventional design
practices produce suboptimal designs, especially in cases with strong coupling
between plant and control design decisions. Control co-design (CCD) is a
holistic design approach that accounts fully for plant-control design coupling
by optimizing these decisions simultaneously. CCD is especially advantageous
for system design problems with complex interactions between physics
disciplines, which is the case for FOWT systems. This paper presents and
demonstrates a nested CCD approach using open-loop optimal control (OLOC) for a
simplified reduced-order model that simulates FOWT dynamic behavior. This
simplified model is helpful for optimization studies due to its computational
efficiency, but is still sufficiently rich enough to capture important
multidisciplinary physics couplings and plant-control design coupling
associated with a horizontal-axis FOWT system with a spar buoy floating
platform. The CCD result shows an improvement in the objective function, annual
energy production (AEP), compared to the baseline design by more than eleven
percent. Optimization studies at this fidelity level can provide system design
engineers with insights into design directions that leverage design coupling to
improve performance. These studies also provide a template for future more
detailed turbine CCD optimization studies that utilize higher fidelity models
and design representations.
