Query on Dominant Physical Frequencies for Preserving Platform–Mooring Dynamics

Wind & Water Computer-Aided Engineering Software Tools
1

Dear all,

I am currently working on the analysis and data-driven modeling of floating offshore wind turbine (FOWT) systems, with particular focus on the coupled dynamics between platform motions and mooring line tensions.

As part of preprocessing and model development, I am seeking guidance on identifying the dominant physical frequency content of platform responses (surge, sway, heave, roll, pitch, and yaw) and corresponding mooring tensions, such that the essential structural and hydrodynamic dynamics of the system are preserved.

Specifically, I would appreciate insights on the following aspects:

  • The typical frequency ranges that govern the primary physical behavior of floating platforms (e.g., low-frequency surge/sway, wave-frequency responses, and any higher-frequency contributions of practical importance).

  • How these dominant frequencies translate into mooring line tension responses, particularly in terms of low-frequency drift, wave-frequency excitation, and potential coupling effects.

  • Recommended cutoff frequencies or filtering strategies (if any) that are commonly adopted to retain physically meaningful dynamics while suppressing numerical noise or non-physical components.

  • Any references, guidelines, or best practices—especially from OpenFAST or NREL-supported studies—that discuss frequency-domain characteristics of platform–mooring interaction.

Thanking you. I would be grateful for any recommendations or references you could share.

Best regards,
Kajal Thakur

2

Dear @Kajal.Thakur,

I won’t answer all of your questions, but certainly tools like OpenFAST can assist you in answer these questions.

Just a few comments:

  • The dominant wave-excitation periods are in first-order wave-energy range (e.g., 2-20 s), so, floater natural periods are typically placed outside of this range (lower or higher periods)
  • Rotor excitation from wind turbulence happens at low frequency and is sampled by the rotor at harmonics of the rotor speed (1P, 2P, 3P, etc. in the rotating frame of reference, which translates–for a 3-bladed rotor–to 3P, 6P, 9P, etc. in the fixed frame of reference), so, turbine natural frequencies are typically placed to avoid this excitation wherever possible. Mean loads are also important.
  • Certainly you’ll see the floater motion frequencies show up in the mooring line tension, but you’ll also see the natural frequencies of the mooring lines themselves show up in the mooring line tension.

Best regards,

3

Given your interest in platform-mooring dynamics, the relevant frequencies will also depend somewhat on the design, especially if the mooring system is very stiff or taut. For whatever design(s) you’re interested in, a good way to answer the questions is to run some simulations of relevant load cases and then look at PSD plots of the platform motions and mooring tensions to see which frequencies are excited.