Speaker
Description
Offshore platforms and in particular floating offshore platforms have a long history of use in the oil & gas industry. The first tension leg platform (TLP) was installed in the early 1980’s in the North Sea, at a depth of 147m. Since then the application of TLPs has grown significantly, with 30 platforms installed or decommissioned globally as of 2020 at depths of up to 1615m. While the fundamentals of the TLP remain the same when applied to wind energy, the dynamics of the system are quite different. The floating wind turbine becomes a complex problem coupling the system motion, hydrodynamics, aerodynamics, aeroelasticity, controller operation, and mooring reactions. Although there are no commercially operating wind turbines with a TLP support structure, it is a promising area of development and an active area of research. The main goal is to study the influence of mooring depth on the system dynamics, and to investigate advantages, challenges, and relevant design considerations for adapting the TLP model to deep water. The IEA 15MW RWT [1] with the TLP, which consist of a concrete floater with eight steel rod tendons anchored to the seabed with suction buckets (developed as collaboration between BlueNewables, DTU, Tubacex, and GMC deepwater), is considered. Hydrodynamic loads are calculated in WAMIT using first order potential flow theory. HAWC2, an Aeroelastic modelling tool developed by DTU, is used to solve the entire coupled system in the time domain. In this study, three different water depths, 120m, 250m, and 1000m, are considered. To provide a clear comparison of system dynamics for the different mooring depths, the same platform/turbine model and external conditions are used for all depths. Decay tests are first run in heave, surge, pitch and yaw to investigate the system natural frequencies including the effect of mooring depth and tower stiffness. This is followed by a number of steady state simulations and dynamic simulations of selected DLCs (1.1, 1.6, and 6.1) [2], which are based on metocean conditions at Buchan Deep [3]. Increasing the mooring depth had only a minor impact on the turbine and structural loads during the operation. Furthermore, the aerodynamic performance of the turbine is not sensitive to mooring depth. At larger water depths, we observed a decrease of the platform’s natural frequency especially in surge and sway. More detailed numerical results will be discussed and presented at the conference.
References
[1] Gaertner Evan et al. Definition of the IEA 15Megawatt Offshore Reference Wind Turbine. Tech. rep. NREL/TP500075698. Boulder, CO: NREL, 2020.
[2] IEC 6140031(EN). Wind Energy Generation Systems Part 31: Design requirements for fixed offshore wind turbines. Standard. Geneva, Switzerland: International Electrotechnical Commission, 2019.
[3] Martin Mathiesen et al. Hywind Buchan Deep Metocean Design Basis. Tech. rep. RE2014002. Statoil, 2014.
| Keywords | Floating Wind Turbine System, TLP, Aero-servo-hydro-elastic analysis |
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