Frequency domain analysis for floating offshore wind substructure design

Using frequency domain analysis for design optimisation of your floating offshore wind turbine support structures.

The floating offshore wind industry is still in its early stages with a focus on achieving cost effectiveness by using larger turbines and by designing floaters at minimal cost based on simulations assessing motions, loads and structural responses.

Today, the industry is characterised by prototype development and R&D, while the first (pre)commercial wind farms are coming up too. The prototypes built may not be fully optimised structures as in a commercial wind farm but are rather used for full-scale testing and learning purposes.

The industry is in a development phase with unsettled workflows and analysis methods. Various methods in time domain (TD) and frequency domain (FD) are being explored and customised to different floater types. Rapid redesign loops are crucial in early project stages for analysing the large number of design load cases required. FD methods are therefore being investigated by the industry and academia as a quick and viable methodology for certain stages of the design.

What we offer

DNV meets industry demands with various approaches in both FD and TD. In TD, fully-coupled analysis of the support structure, turbine and mooring lines is possible using Bladed and Sesam’s Sima, while structural analysis can be performed in Sesam using either TD or FD. High fidelity TD workflows for floating wind substructure design are available for FLS and currently being developed for ULS.

In FD, DNV provides powerful tools for engineers in FOWT analysis. The benefits and key features are:

Offers fast and efficient design iterations with low demands for computational power and data storage.
Optimisation of design parameters.
Improvement of stability.
Performs wave load analysis efficiently in FD.
The wind turbine is represented as a point mass, and the wind turbine loads are accounted for as extreme loads (for ULS) or as damage equivalent loads (for FLS).
Identification of critical locations in the structure.

Contact us

Ole Jan Nekstad

Product Director - Sesam

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Workflow

FD methods play a vital role in early design and prototyping. This approach involves a hydrodynamic analysis of wave-structure response in the FD, representing the wind turbine as a point mass and including the effect of moorings. Wind turbine loads are represented by loads obtained from the turbine manufacturer, including damage equivalent loads (DEL) for fatigue analysis (FLS) and extreme loads for ultimate load analysis (ULS).

For FLS, wave and wind turbine loads are considered independently, and the respective fatigue damages may be combined or assessed separately. For ULS, the wind turbine extreme loading is considered along with mooring forces and a design wave approach for wave loading, followed by a code check on the total response.

This FD workflow is based on well-established methodology from the offshore oil and gas industry. While offering computational efficiency for fast FOWT design iterations with low computational resource demands, this method also introduces assumptions and limitations. These assumptions include the decoupling of the wave and wind turbine effects and it is limited to linear hydrodynamic effects. Including the coupling effects of the wave and wind turbine through FD is being investigated by JIPs and in various research studies. Despite these considerations FD methods offer interesting benefits and possibilities to floating offshore wind substructure designers.