On the Comparison of the Dynamic Response of an Offshore Floating VAWT System When Adopting Two Different Mooring System Model of Dynamics: Quasi-Static vs Lumped Mass Approach

Abstract

The interest in floating offshore wind turbines (FOWT) has been growing substantially over the last decade and, after a number of prototypes deployed [1], the first offshore floating wind farms have been approved and are being developed. While a number of international research activities have been conducted on the dynamics of offshore floating HAWT systems (e.g. OC3-Phase IV2, OC4-Phase II3), relatively few studies have been conducted on floating VAWT systems, despite their potential advantages [2]. Due to the substantial differences between HAWT and VAWT aerodynamics, the analyses on floating HAWT cannot be extended to floating VAWT systems. The main aim of the present work is to compare the dynamic response of the FOWT system adopting two different mooring dynamics approaches. Two version of the in-house aero-hydro-mooring coupled model of dynamics for VAWT “FloVAWT” [3] are used: one which adopts a mooring quasi-static model, and solves the equations using an energetic approach [4], and a modified version of FloVAWT, which uses instead the lumped-mass mooring line model “MoorDyn” [5]. The floating VAWT system considered is based on a 5MW Darrieus type rotor supported by the OC4-Phase II3 semi-submersible. The results for the considered metocean conditions show that MoorDyn approach estimate larger translational displacements of the platform, compared to the quasi-static rigid approach previously implemented in FloVAWT. As expected, the magnitudes of the forces along the lines are lower, being part of the energy employed for the elastic deformation of the cables. A systematic comparison of the differences between the two approaches is presented.

On mooring line tension and fatigue prediction for offshore vertical axis wind turbines: A comparison of lumped mass and quasi-static approaches

“…After having shown that for worsening metocean conditions, MD dynamic approach is recommended if the interest is in modelling the translational displacements of the platform (Cevasco et al, 2017), a further analysis is here developed to analyse this behaviour, understanding its physical causes and to better analyse which phenomena play an important role in the estimation of the mooring line tension.…”

“…Oppositely, with the energy moving towards higher frequencies for worsening metocean condition, the MD modelled line shows a higher response to the first-order wave excitation, in the range of frequencies between the 0.1 and 0.08 Hz (Figures 6 and 7, top-right). This behaviour can be explained by the lower stiffness of the MD modelled line, which cause larger motions of the platform (Cevasco et al, 2017).…”

“…Starting from a systematic comparative analysis of platform motions and mooring loading conditions for worsening metocean conditions (Cevasco et al, 2017), a comprehensive study on the fatigue assessment and phenomena playing an important role in the tension of the mooring chain is carried out. Therefore, the purpose of this article is to support and complete the recommendations on when it is necessary to adopt a more accurate dynamic modelling approach than a static one for catenary mooring lines of floating VAWTs, aiming to reach a good compromise between computational time and accuracy.…”

“…This premise motivates the study conducted in this work, where the low order lumped mass model of MoorDyn (MD) (Hall, 2015), integrated into FloVAWT (Cevasco et al, 2017), is used to capture more accurately the system dynamics of a VAWT system. Although MD does not account for wave kinematics and seabed friction interaction, it has been validated in the FAST fully coupled simulations, showing a good agreement with experimental data for the 5 MW DeepCwind reference model (Hall and Goupee, 2015).…”

“…This work presents the response of a 5 MW Darrieus type rotor (Vita, 2011), supported by the OC4-DeepCwind semisubmersible, rearranging the ballast to achieve the same draft of the floating HAWT configuration (as described by Wang et al (2013)). The mooring line layout and characteristics are the one suggested by the OC4 phase II and already used by Cevasco et al (2017): three catenary lines spread symmetrically about the vertical axis, each connected to one column of the support structure ( Figure 1).…”