Engineering Structures 31 (2009) 358–368 Contents lists available at ScienceDirect Engineering Structures journal homepage: www.elsevier.com/locate/engstruct Tuned liquid column dampers in offshore wind turbines for structural control Shane Colwell, Biswajit Basu ∗ Department of Civil, Structural and Environmental Engineering, Trinity College Dublin, Ireland article info Article history: Received 19 June 2007 Received in revised form 20 July 2008 Accepted 1 September 2008 Available online 1 October 2008 Keywords: TLCD Offshore wind turbine Vibration control Wind loading Wave loading Correlation abstract With offshore wind turbines becoming larger, being moved out further at sea and subjected to ever greater wind and wave forces, it is necessary to analyse the dynamics and minimise the responses of these structures. In this paper, the structural responses of offshore wind turbines are simulated with an attached damper (Tuned Liquid Column Damper (TLCD)) for controlling the vibrations induced within the structure. This requires a realistic simulation of the forces that these tall, flexible and slender structures are subjected to, and consequently the implementation of a damper to control the resulting undesirable vibrations that are induced within the structure. Since sea waves are caused by wind blowing for a sufficiently long time, the state of the sea is related to wind parameters and there exists the possibility of correlating wind and wave loading conditions on structures. The Kaimal spectrum for wind loading is combined with the JONSWAP wave spectrum to formulate correlated wind and wave loadings. The offshore turbine tower is modelled as a Multi-Degree-of-Freedom (MDOF) structure. Cases for flat sea conditions, with which parallels to onshore wind turbines may be drawn, are first simulated. Simulations are presented for the MDOF structure subjected to both ‘moderate’ and ‘strong’ wind and wave loadings. Cases of the blades lumped at the nacelle along with rotating blades are investigated. The reduction in bending moments and structural displacement response with TLCDs for each case are examined. A fatigue analysis is carried out and the implementation of TLCDs is seen to enhance the fatigue life of the structure. An analysis, taking into account the extended fatigue life and reduced bending moments on the structure- TLCD system, is presented. © 2008 Elsevier Ltd. All rights reserved. 1. Introduction With the present day energy crisis becoming more pronounced, due to depletion of fossil fuel stocks, offshore wind turbines are becoming a viable and attractive means of producing electricity. An offshore wind turbine harnesses the wind energy out at sea to produce electrical energy. To dynamically examine these slender structures in an offshore environment, it is necessary to correlate wind and wave loadings, and to investigate the force-structure interaction arising from such a correlation. Due to the slenderness of offshore wind turbines, the combination of wind and wave forces may produce excessive vibrations that will inhibit the mechanical system in the nacelle from converting wind energy to electrical energy. Reductions in fatigue life, and higher foundation and tower construction costs will also arise from uncontrolled vibrations in the offshore wind turbine structural system. Turbulence from offshore wind produces small capillary waves at the sea surface, with similarly small wavelengths in the range of centimetres. The wind acts on the tiny walls that these ripples create, causing them to become larger. Wind blowing over the ∗ Corresponding author. E-mail address: basub@tcd.ie (B. Basu). wave produces pressure differences along the wave profile, causing the wave to grow. The process is unstable because, as the wave gets bigger, the pressure differences get bigger, and the wave grows exponentially. Finally, the waves begin to interact among themselves to produce longer waves [1]. The interaction transfers wave energy from short waves generated by the Miles mechanism, to waves with frequencies slightly lower than the frequency of waves at the peak of the spectrum. Eventually, this leads to waves going faster than the wind, as noted by [2]. Davenport [3] proposed an expression for the velocity spectrum for the distribution of energy within turbulent wind flow, at a height related to the size of gusts at that height. However, this spectrum was independent of height, and was also seen to overestimate the energy in the higher frequency range. Harris [4] proposed a velocity spectrum which was independent of height, and that guaranteed a non-zero integral length scale of turbulence. Harris [5] subsequently provided a wind spectrum based on a modified version of the Von-Kàrmàn spectrum, which included the variation of spectral energy with height. Kaimal et al. [6] developed the first expression for the variation of spectral energy with height, which includes eddy currents of varying size acting between the structural nodes. Despite the irregular random nature of wind inducing seem- ingly random wave heights and wave periods in a typical offshore 0141-0296/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.engstruct.2008.09.001