Abstract
To meet the growing demand for renewable energy and net zero carbon emission targets set by respective governments, offshore wind is looking to move into deeper waters using floating offshore wind turbines (FOWTs) to capture the more consistent winds and expand the area available for energy capture. This has presented the challenge of being able to accurately model FOWTs allowing for the design of cost-effective and reliable structures. In recent years, code comparison studies have revealed that current modelling tools and design techniques used in the industry yield a significant underprediction in low frequency forces and responses with respect to the surge and pitch degrees of freedom. This has been attributed to platforms being excited by nonlinear hydrodynamic forces outside of the linear wave excitation region which are not being properly captured by current market available modelling tools. However, other studies have revealed that aerodynamic loading can produce additional low frequency forces which could in combination give dangerously underpredicted structural loads.
This paper looks to examine the underpredicted forces and responses through use of the in-house developed program ‘DARwind’ and the commercial software OrcaFlex, with an emphasis on the surge degree of freedom motion. The calculations are conducted for a range of different analysis settings and loading conditions in order to compare the contribution of low-frequency loads and the responses of FOWT. In addition, wind load in the simulation will also be divided into two conditions, one is for constant wind speed, and the other is for wind spectrum which considers the low-frequency contribution. It was found that whilst both programs used during this study underpredicted low frequency motion, incorporating various theories and semi-empirical corrections helped reduce this underprediction. This work has allowed for a clearer view of the issues surrounding the underprediction of low frequency loads and responses and will assist in furthering research in this area.
