Question about the natural frequencies constraints utilized for upscaling the IEA 15 MW to the 22 MW

[fnaos]

Hi!

In the report (page 40), it is stated: "The natural period constraints were placed to avoid excitation in severe sea states. Increasing the heave natural period led to infeasible designs"

I have been wondering, why exactly is the heave natural period a constraint added to the floater optimization problem. I can see that the heave natural frequency is the highest one and therefore the most likely to be excited directly by the sea state. However, given that the heave consists only of up and down motion, then the questions comes up on how does it impacts the structural loads and the aerodynamics up to the point it is worth being utilized as a constraint in the optimization problem. So, my question is: what was the reasoning for adding the heave motion as a constraint for the design optimization? From a loads perspective, is it relevant (perhaps it can couple with other degrees of freedom)?

Cheers.

[ptrbortolotti]

@dzalkind

[dzalkind]

Hi,

The heave period constraint was included because it is standard practice to avoid resonance of the substructure motion with wave periods.

Excessive heave motion can lead to large nacelle accelerations or mooring line fatigue. These dynamic loads may be better constraints, but the natural periods are helpful for guiding the design, as well.

I hope this helps.

Best, Dan