Today, offshore floating wind turbines are a viable long-term source of renewable energy and are a particular focus for Naval Energies. However, they present engineers with an especially tricky design process for two reasons.
The first is the fundamental differences in the design approach between the two primary structures involved, the ‘sea-going’ floater and the traditionally ‘bottom-fixed’ wind turbine. Where wind turbine design modelling relies on a highly non-linear ‘temporal’ design analysis to simulate the real-time behaviour of the structure submitted to wind, offshore structure design is generally achieved using a linearized design model that relies more on frequency analysis to calculate the motions due to the relevant external forces coming from waves at hand. These two very different schools for design render the simulation of the structure’s global behaviour very tricky.
The second reason is also related to the composite behaviour of the structure as it deals with the multiple actions of the various structural and digital components of the turbine, and how they impact one another in time. As such, the design process must cater for a number of ‘coupling’ effects involving aerodynamics, hydrodynamics, computational reactions of the wind turbine controller and structural behaviours related to the various physical components.
A wind of change
Back when Naval Group’s design experience revolved primarily around the design of naval vessels, there was an obvious need to develop tailored design tools specifically adapted to the design constraints and criteria of floating offshore wind turbines.
In 2010, Naval Group and its subsidiary Sirehna used FAST, a pre-existing wind turbine design software, to develop their own FASTHYDRO, a similar software with hydrodynamic and mooring components enabling the simulation of the overall turbine structure. In 2011, Naval Group and Sirehna managed to further validate this software, based on innovative aero-hydrodynamics tests performed with IFREMER in a wave tank with a wind tunnel. This enabled to validate the coupled simulation of the wind turbine’s aerodynamics with the floater’s hydrodynamics.
In 2014 Naval Group turned its attention to project Seareed, which is the company’s current research & development effort in all matters relating to the integrated design of floating wind turbines. The Genesea software platform was thus developed: this new-and-improved integrated simulation & design software performs 4 major tasks that are 1) the creation of a fully parametric model, 2) calculations of the structure’s general behaviour using FASTHYDRO, 3) calculations of the stress to be borne by the structure, 4) the automated post-treatment of all the data, thereby enabling the simple application of the design criteria to verify the mechanical integrity of the global structure with regard to strength, stability and fatigue.
Beyond being 100% automated, Genesea presents a number of additional advantages. The software reduces risk by eliminating any potential for human error, represents considerable resource savings and can process thousands of loading scenarios, or ‘load cases’, simulating a vast array of weather patterns that include wind strength & orientation, currents, waves, swell and so on. “Where we would previously take three months, using old tools, to design an offshore floating wind turbine structure, today it takes us just a few days, or a week tops!”
Genesea was first used in the summer of 2015 and is under on-going development. One of its latest developments is the new concrete design feature that enables the modelling of hybrid floats made of concrete and steel, as opposed to just steel. The software now also includes an added visualisation feature that is extremely helpful in illustrating the moving float and its mooring.
The digital edge
In a word, the use of digital tools in the design of offshore floating wind turbines allows Naval Energies to:
- Calculate the structure’s global behaviour according to site-specific meteorological conditions,
- Calculate the internal loading to verify the design’s structural integrity,
- Calculate the wind turbines power production as a result of all these parameters.
Beyond determining whether or not a floating offshore wind turbine structure will resist the elements, this integrated digital design tool presents a rapid and effective method of automating calculations to optimising the design, thereby giving Naval Energies a significant competitive edge on the market.
So thorough is the Genesea tool that the world leader in Testing, Inspection and Certification (TIC) services, Bureau Veritas, has certified the design principles and design methodologies upon which the design software is based.
Naval Group leads the way in all matters relating to digital innovations applicable to the naval defence and naval energy sectors, providing its clients with competitive state-of-the-art solutions across the board. It’s what we do!