In the years to come an increasing number of offshore wind turbine (OWT) installations are expected in deeper waters. This will require the use of more complex braced support structures such as jackets instead of the more widely used monopole foundations.
For modelling OWT-specific aspects such as rotor aerodynamics and control system actions, designers are required to use a dedicated aero-elastic wind turbine design tool. GL Garrad Hassan has developed a direct link between its own aero-elastic wind turbine design tool Bladed and two industry-leading offshore analysis codes: ANSYS ASAS and Bentley SACS.
This link represents a new approach for the integrated structural analysis of OWT jackets and other space-frame support structures. The Bladed code accurately models the dynamics of the entire offshore wind turbine system, including the turbine, support structure and non-linear pile-soil interaction, using a multi-body approach. Non-linear wind and wave loading on the turbine and support structure is calculated through integrated time-domain modelling of aerodynamic loads, hydrodynamic loads and control system actions. This ensures that the effects of simultaneous wind and wave loading of the turbine and support structure are properly represented. The Bladed output consists of time histories of internal forces and moments at all locations in the wind turbine and support structure.
The ANSYS ASAS and Bentley SACS codes perform fatigue and ultimate strength analysis of tubular joints in offshore structures on the basis of global load time series. Both packages contain built-in code-checking routines which allow the strength of the structure to be checked in accordance with the main design codes, according to joint classification. Stress concentration factors (SCFs) around each weld may be calculated automatically or entered manually by the user.
The Bladed Offshore Code Checking feature creates a user-friendly link between the Bladed output and the ASAS/ SACS code-checking routines. An input file containing the jacket geometry and load time histories in the required format is generated by Bladed. This can be used directly in the ASAS and SACS code checking routines to analyse each joint in the jacket structure and assess the fatigue life and ultimate strength. The Bladed data conversion has been extensively tested and validated. By linking Bladed directly with these general-purpose offshore analysis codes, design iterations of the support structure can be assessed more easily and more accurately, enabling the creation of better optimised design solutions for offshore wind turbine support structures.
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Offshore WIND staff, November 30, 2011; Image: gl-garradhassan