Wide Frequency Range Damping Solutions for Offshore Wind Turbines

Wind turbine substructures are subject to wide band frequency loading. Turbine towers and substructure do have a number of eigenfrequencies. Floating foundations have lower eigenfrequencies due to the motion of the floater in water. This results in design challenges with possible resonances. Undamped resonances result in increased extreme- and fatigue response. The most used strategy is to design the structures so that eigenfrequencies does not match the loading frequencies. This is governing for the size, materials and costs. It is challenging/impossible to design all the eigenfrequencies not to coincide with the loading frequencies. Another design strategy to avoid that resonances results in increased response is to add structural and hydrodynamic damping. Tuned mass damping has been used for Wind turbine towers. This may be successful if there is only one governing frequency. But the system is sensitive to shift in frequency and need to be combined with other damping. Hydrodynamic damping has previously been obtained by use of large plates or slender elements that give drag damping. But large elements are needed to obtain significant damping. In this project two technologies of damping have been used, known from other industries, and that so far have not been used for Wind turbines. Several ways of introducing damping in the tower bending modes by use of viscoelastic materials that have broad band damping, have been assessed. This is widely used in the car and aerospace industry, for tall buildings and for bridges. Aibel has previously used the technology with success for machine and piping vibrations. A practical solution has been found, by using damping cassettes eccentric along and over a length of the tower wall in the region where the tower modes have the highest strain energy. A specific viscoelastic material has been found and used in numerical non-linear FE analyses to quantify the damping effect. For floating turbine foundations, a method of using hydraulic damping, using adapted hydraulic shock absorbers mounted between parts with relative motion, has been developed. It has been shown in numerical simulations that this add a sufficient degree of damping to the heave and pitch hydrodynamic modes, to allow a design where the eigenperiods in heave and pitch may be in the frequency range of environmental and turbine loads. A physical design of a hydraulic damper with the required properties has been developed. The tower damping may be used for fixed and floating towers. A previously developed spreadsheet for optimization of turbine floater has been updated and the effect of relaxing the criteria for the hydrodynamic eigenperiods has been assessed. Parameter studies have shown that there are significant potential to reduce size and weight. This will have a direct impact on the construction cost, and indirect savings in transport, assembly and installation, and on the mooring system. The damping mechanisms are implemented in a simulation software that simulates the behaviour of fixed and floating turbine foundations. For the floating turbine foundation, a systematic approach has been used when investigating the individual effects and the interactions between the loadings and the responses. E.g. by first running decay tests to assess modal damping, add waves only, add wind only, combine and lastly combine also with a full turbine model with control system. This in order to have full overview of the individual contributions and on the various interactions. To arrive at a design to a specific case, an optimization using site specific metocean data and a specific turbine must be done. This project has shown that significant savings in size, weight and cost may be achieved by use of designed damping. Through the systematic approach to understand both the individual effects and the interaction, and through the development of practical design solutions, it is now possible to optimize a wind turbine floater design, and offer cost savings to developers of fixed and floating wind parks.

Alle partnere har utviklet sin generelle kunnskap om og forståelse av flytende Vindturbiner. Nye løsninger er funnet ved å bringe inn dempning som designfaktor. Aibel har videreutviklet generell erfaring og kunnskap om flytende vindturbiner. Prosjektet har styrket IFEs posisjon innen design av flytende vindturbiner. 3DFloat har styrket sin posisjon. Q-ring fått øket innsikt i og trening anvendelse av teknologi fra bil og aerospace til nye områder innen vind. Equinor har deltatt i teknologiutviklingen og fått demonstrert teknologi som kan gjøre fremtidige vindpark utbygginger mer kosteffektive. Servi har gjennom arbeidet fått innsikt i og erfaring med generell demper design ved bruk av hydrauliske dempere for å skape strukturell og hydrodynamisk dempning. Partnerne har utviklet samarbeid som vil være konkurransedyktig i fremtidige prosjekter. Prosjektet er med på å gjøre flytende vind konkurransedyktig og bidrar til økt norsk konkurranseevne innen flytende vind.

The objective of the project is to overcome the current design practice, consisting in designing the supporting structure so that its first natural frequencies are placed away from the wide range of excitation frequencies present in the system. To address this issue, two devices with the purpose of adding designed damping to offshore wind turbines are evaluated, designed and numerically tested. The first innovation is a transition piece designed tuned in order to add damping to a wide range of tower bending modes, the second one is a damped hydrodynamic flap designed to reduce the platform heave and pitch motion. The reduction in energy unit cost resulting from the innovations will be evaluated on a bottom-fixed turbine and on a new, ad-hoc floater concept designed using the aero-servo-hydro-elastic tool 3DFloat, developed internally at IFE. Four companies will be involved in the project: Aibel, Equinor Energy , Servi and IFE. Two possible ways ahead are identified to proceed towards the realization of the innovation. In the first, a follow-up proposal is written by the project participants, where an experimental test is performed. Alternatively, clients can take over the concept evaluation of the proposed innovations.