SERVICEABILITY LIMIT STATE ASSESSMENT OF SEMI-SUBMERSIBLE FLOATING WIND TURBINES

The design of a floating wind turbine (FWT) should satisfy the serviceability limit state (SLS) requirement for an efficient and safe operation throughout the entire work life. The SLS requirements are introduced by the owner/developer of the wind turbine facility to achieve serviceability (production of power) or an efficient operation of the facility or a "first step" towards ensuring safety. However, the owner/developer proposes the limiting values in terms of serviceability mainly according to the experience of onshore wind turbines or offshore oil and gas facilities, which are not well applicable to FWTs due to the different load characteristics and functions. Design standards or guidelines for SLS of FWTs are slowly emerging but are still not mature, and only to some extent refer to serviceability requirements. A good understanding of wind turbine dynamic performance associated with serviceability is essential to facilitate design decision-making. This study deals with an assessment of methods and criteria for the SLS design check with an emphasis on tilt/pitch and nacelle accelerations. Emphasis is placed on assessment from conceptual to detailed design. A 10-MW semi-submersible FWT is used in a case study. The floater geometry is obtained from a conceptual design conducted by Li et al. [1]. Simplified static/dynamic analysis methods for use in the conceptual design phase and high fidelity integrated, dynamic analysis methods for detailed design in terms of serviceability are presented, discussed and applied in the case study. The relative contribution of wind and wave loads to the different SLS criteria is investigated. Finally, the main conclusions are summarized. The study aims at providing a basis for improving design standards and guiding research and engineering practice for the semi-submersible floater design of FWTs.