Advanced Wave and Wind Load Models for Floating Wind Turbine Mooring System Design

Offshore wind farms are an important and growing part of the renewable energy mix. The industry is experiencing fast development - moving from shallow to deeper water - with an increasing need for floating technology. However, floating wind turbines (FWTs) still cost more than bottom-fixed wind turbines; the FWT industry needs further cost reductions and design improvements in order to grow from its present nascent stage. The hull and mooring system are the enablers of floating wind, and the parts that truly differentiate floating wind from traditional bottom-fixed wind turbines. Therefore, it is important to examine the potential for cost reduction through mooring system optimization and lower uncertainty in the loads on the hull. Designing cost-effective mooring systems for FWTs is challenging for shallow water - where hundreds of meters of chain on the seabed might be required in order to survive storms, and strong lines, connectors and anchors might be needed to keep the platform in place to protect the power cable - but also for very deep water, where the cost of material becomes high. Understanding the design drivers for mooring systems is crucial. Assessing novel mooring designs, including new materials, innovative methods of sharing anchors, or even floater-to-floater moorings, requires reliable analysis methods, considering complex material behaviour, accurate wave load models and wind field models, and aerodynamic interactions between FWTs in a farm. The WINDMOOR project aims to better understand the forces from wind, waves and current - all of them being significant drivers for the mooring system design - as well as assessing novel mooring systems for wind farms in shallow and deep water. To achieve these goals the project will combine advanced computational methods, experiments, and simplified methods which can be used in design. A floating wind turbine reference design as basis for case studies was established early in project. This reference case was established in close collaboration with the WINDMOOR industry partners. Equinor and Inocean contributed with a floating substructure design that can be openly used in the project. This concept, named INO WINDMOOR FWT, is a triangular shaped semisubmersible adapted to a 12MW wind turbine. The turbine, WINDMOOR 12MW, is upscaled from IEA's 10MW turbine. The INO WINDMOOR concept was tested in SINTEF's Ocean Basin early 2020. These model tests were important to determine the platforms performance when it is exposed to waves and wind. Analysis of the experimental data have provided empirical data for slowly varying wave loads on the platform hull, which are important for mooring system design. To reduce costs in FWT mooring systems, novel concepts using synthetic fibre ropes or shared mooring components were explored. Optimization algorithms were used for mooring system design, but they required faster models than traditional aero-elastic load simulation tools. A simplified model was developed and used in optimization, and the recently developed Syrope model, which describes the stiffness of synthetic fibre ropes more accurately, was applied in the analysis of the INO WINDMOOR FWT. Improvements in the prediction of wave-induced low-frequency loads and responses were identified as a need in state-of-the-art design practice. Hydrodynamic load effects were identified from model tests, and empirical results were used to assess theoretical and semi-empirical formulations. The analysis showed the importance of considering full Quadratic Transfer Functions (QTFs) for wave drift loads, the accuracy of the 2nd order solution without free surface integral, and the limitations of Pinkster approximation. Wave-current effects and viscous wave drift effects were also found to increase wave loads in steep sea states. Semi-empirical methods were implemented to improve the prediction of wave drift loads. Comprehensive numerical analyses using CFD (Computational Fluid Dynamics) have also been conducted to identify local and global hydrodynamic forces on the hull of the INO WINDMOOR platform. These data have been used to calibrate empirical coefficients in the Morison's load model, which is a semi-empirical model for calculating hydrodynamic forces on a structure in oscillating flow. The low-frequency motions of the floater induced by wind were studied using high-fidelity wind fields and synthetic wind fields generated based on measurements. Atmospheric stability, turbulence intensity, wind shear, and coherence were identified as important parameters for wind turbine responses. Different models were compared, and coherence was found to be critical for wake meandering. The findings from this project can contribute to the advancement of FWT technology and help reduce costs in the design and operation of floating offshore wind farms.

WINDMOOR prosjekt har resultert i publiseringen av forskningsdata, som har blitt mye brukt i ulike forskningsprosjekter. Prosjektet har bidratt til fremskritt innen forankringsoptimalisering, forbedret forståelse av hydrodynamikk, økt forståelsen av vindfelt til havs med påvirkning fra turbinvaker, og utvikling av nye metoder for å forutsi belastninger og respons for flytende offshore vindturbiner. Resultatene fra prosjektet har potensial til å redusere usikkerheter i design, forbedre pålitelighet og redusere kostnader i havvindindustrien, samtidig som de bidrar til å redusere CO2-utslipp gjennom økt bruk av fornybar energi. Prosjektet har også generert et unikt datagrunnlag som kan brukes til validering i videre forskningsprosjekter, og erfaringene fra prosjektet forventes å påvirke fremtidige praksiser for modelltesting av flytende offshore vindturbiner.

In order to grow from its present nascent stage – several demonstration projects and one small commercial farm – the floating wind turbine (FWT) industry needs further cost reductions and design improvements, especially related to the hull and mooring system. This project aims to enable more efficient design of FWT farms by addressing several limitations of today’s global aero-hydro-servo-elastic analysis tools: - Synthetic fiber rope mooring components require adapted material models and analysis methods. Relevant material models have been implemented, but their consequences for design have not yet been studied, and a systematic investigation of design drivers for the mooring system at different depths is proposed. - The prediction of low-freq. hydrodynamics responses due to waves and currents, especially for semi-submersible type FWTs, using state-of-the-art tools, is not satisfactory. Alternative load and wave kinematic models, including application of computational fluid dynamics, will be studied and validated against new experimental tests. The new experimental tests will include quantification of the repeatability of low-frequency responses, systematic investigations of the effect of changing the platform pitch angle, and simultaneously inclusion of realistic aerodynamic loads. - The incoming wind field applied to a turbine is usually described based on spectral parameters for neutral atmospheric conditions, and only a single turbine is typically considered. The present project will address the consequences of accounting for atmospheric stability and dynamic wake meandering effects. - Simplified simulation methods which can account for shared mooring components in a farm configuration will be developed. This project aims to produce and disseminate results of academic and practical importance, leading to better understanding of the dynamics of FWT farms, and reduced uncertainties in design and analysis of the hull and mooring components is anticipated.