Comparison of Coupled and Uncoupled Load Simulations on a Jacket Support Structure

Abstract

In this article, a comparison of the moments and forces at the joints of a jacket structure is made between fully coupled aero-hydro-elastic simulations in HAWC2 and uncoupled load predictions in the finite element software Abaqus. The jacket sub structure is modelled in moderate deep waters of 50m and designed for the 5MW NREL baseline wind turbine. External conditions are based on wind and wave joint distribution for a site in the North Sea. The turbulent wind field in HAWC2 is generated by random values, defined by the Mann Turbulence model, for each operational mean wind speed. A four-legged jacket structure similar to the Upwind reference jacket is developed in the Abaqus environment, to which is added the transition piece and tower. The aeroelastic loads determined in normal operating conditions of the turbine is integrated and centralized as nodal forces and moments acting at the tower top of the finite element model. Hydrodynamic loads from the incoming waves are computed using the Morison equation and based on a nonlinear irregular wave field. Velocities, accelerations and amplitudes of the wave field as well as tower top forces and moments are used as inputs for the structural analysis in Abaqus. The fully coupled simulation is implemented and performed in HAWC2. In the uncoupled case, the loads (wave loads and tower base loads) are analysed by an implicit structural Finite Element Analysis (Abaqus 6.11-1). A subroutine is used as a preprocessor generating a beam element model and linking the loads to the components as nodal forces. In both simulation cases, the integrated loads acting on the jacket legs are computed as time series and as damage equivalent loading. The analysis and comparison of the fully coupled and decoupled simulation method show that the results vary depending on the structural stiffness and the applied wave loads. Variation in the amplitudes of the moments and forces on the jacket legs up to 25% was observed between the results obtained from coupled and uncoupled simulations.