Loading and Response of Offshore Wind Turbine Support Structures: Prediction with Comparison to Measured Data

Abstract

The effectiveness of numerical prediction of structural dynamic response is assessed for a wind mast support structure typical of wind turbines, but without the turbine itself. This effectiveness is assessed by comparison of predicted mechanical strain below the waterline of the structure with measured field data. Particular attention is paid to the effects of wave directional spreading. In the model, wave loads are generated from a series of simulated irregular time-histories associated a single significant wave height and period, but two distinct spreading parameters. Load predictions are based the Morrison equation and the structural response is computed by solving a finite element model of the structure in the time domain. Comparisons are based on the variances of the response process. Historically, design of offshore turbine structures has evolved from design of two wellknown structural types: onshore turbines and offshore oil production platforms. Wave loading is a principal difference between onshore and offshore structures, and the techniques to predict these loads are based on existing methodologies proven to be effective for offshore oil platforms. However, offshore wind support platforms differ from oil platforms is several important ways: First, wind platforms generally have less area exposed to wave-loading and are considerably taller than oil platforms. This combination makes structural dynamics relatively more important because the natural frequency of the platform is often closer to frequencies at which there is meaningful wave energy. Second, wind farms often include large groups of platforms with very little variation between designs. Applying one design to multiple turbines, combined with the relatively slender profit margins in the offshore wind business, makes cost-optimization of the structural design important. Finally, wind turbines routinely reduce structural loading at times of maximum wind by operating at lower speed or shutting down the turbine completely. This ongoing work is to quantify the effectiveness of wave load and response predictions through critical comparison between numerically predicted strain response and measured strain response on a wind-turbine support structure in the German Bight. Special emphasis is placed on quantifying the relative importance of including wave directional spreading as opposed to the more conventional unidirectional (long-crested) approach.