PivotBuoy: An Advanced System for Cost-effective and Reliable Mooring, Connection, Installation and Operation of Floating Wind
Creators: name: Casanovas, Carlos
name: Raventos, Alex
name: Verges, María
name: Rocio Torres
Publisher: name: Zenodo
Year: 2019
Subjects: floating offshore wind
offshore wind
LCOE reduction

description: 1. General summary Using adapted Oil&amp;Gas (O&amp;G) platform designs for floating wind is a reliable solution<br> but too expensive for the wind sector. There is a need to redesign floating solutions to<br> reduce the LCOE to competitive levels compared to fixed solutions. X1Wind is<br> developing an innovative weathervaning system to reduce drastically the weight and<br> costs of current solutions: the PivotBuoy. The PivotBuoy is a novel system integrating<br> the mooring, the anchoring and the electrical cable in a single point (single point<br> mooring – SPM) allowing a faster connection of floating platforms.<br> The PivotBuoy brings the stability and light weight of tension-leg systems (TLPs) but<br> can be combined with the simpler installation strategy of ‘onshore assembly and tug to<br> site’ of semi-submersibles. As a result, the PivotBuoy can be easily pre-installed, while<br> the floating platform is towed to site with local vessels (avoiding the need for large<br> expensive vessels) and easily connected to the mini-TLP. By using a downwind<br> configuration, a more efficient structure can be designed, which enables to eliminate<br> the tower and drastically reduce the platform weight, achieving around 260 tons/MW<br> (similar to fixed offshore) while spar and semi-submersibles require between 1.000 and<br> 2.000 tons/MW. Using a truss structure not only allows to reduce platform weight, but<br> also to minimize wave slamming and wind turbulence that traditional tubular towers<br> induce in a downwind rotor.<br> The weight reduction and the installation strategy reduce the capital expenditures<br> (CAPEX) by around 50% compared to the state-of-the-art floating wind. <br> 2. Method<br> After completing the conceptual design phase and simulations with the Orcaflex model,<br> a successful proof of concept was conducted with a 1:64 scale prototype of the X1Wind<br> downwind platform and the PivotBuoy at CIEMLAB wave flume (UPC, Barcelona) to<br> validate the model. Regular wave periods were tested as well as extreme conditions,<br> and up to 90º of wind/waves misalignment, and they confirmed that the stability given<br> by the PivotBuoy is comparable with only a fraction of the weight of semi-submersibles<br> or spar platforms using catenary moorings (much heavier).<br> The next tank tests will be conducted in February through de MariNET2 Programme in<br> the Hydrodynamic and Ocean Engineering tank of ECN (Ecole Centrale Nantes), the<br> results of which will also be presented at the conference. 3. Results<br> Tests showed that the dynamic behaviour of the system matched well the Orcaflex<br> simulation model. Results also showed that while other floating platforms with different<br> mooring systems are 3 to 6 times heavier, PivotBuoy has similar or improved stability<br> performance. While spar and semi-submersibles have a slight improvement in heave<br> and pitch motion (with large weight increase), they have a much larger wave-induced<br> surge motion, that will affect negatively on thrust variation and turbine torque stability.<br> Additionally, results from ongoing projects (e.g. IEA WIND Task 40) show that<br> downwind rotors can bring additional 5-8% reduction of LCOE for large-scale turbines,<br> as they don’t require pre-coning and blade pre-bending (unlike upwind rotors to avoid<br> tower strike), allowing lighter and larger blades, increasing the energy production. <br> 4. Conclusions<br> The PivotBuoy is a new SPM system that can be implemented in deep waters with a<br> cost similar to fixed offshore, making floating wind cost-effective. It has the potential to<br> lower the CAPEX of a downwind floating platform by more than 50%, and the OPEX up<br> to 20%. These significant cost reductions are achieved through mooring and anchoring<br> cost reduction (instead of long and heavy catenary chains), a low-cost and efficient<br> novel connection (avoiding traditional complex turrets), enabling platform weight<br> reduction, and allowing assembly and installation using local infrastructure. <br> 5. Learning Objectives<br> The SPM technology has been widely used in O&amp;G for decades, specifically for the<br> connection of FPSOs (Floating Production, Storage and Offloading), but remains<br> unknown to the majority of the wind industry. A few new entrants in the floating wind<br> sector are proposing to use SPM technology to connect their floating platforms (Eolink,<br> Saitec, X1Wind, Hexicon, and Tetrafloat). Delegates will learn what are the advantages<br> of implementing this technology with floating offshore wind platforms, like the drastic<br> weight and LCOE reduction or the elimination of active systems, as well as the<br> challenges (such as the possible misalignment of wind and waves) in comparison to<br> current mooring solutions.