Maximum Loads on a One Degree of Freedom Model-scale Offshore Wind Turbine

AbstractThis paper presents the results of an experiment carried out in a wave flume aiming at reproducing a 50-year wave condition on an extra-large bottom-fixed offshore wind turbine mounted on a monopile. The model is a stiff cylinder mounted on a spring allowing rotation of the system around its base only in the wave propagation direction. Under these conditions, the turbine is assumed to be idling, and the damping ratio of the system is 2.4%. The overturning moment at the base of the cylinder is measured, and it is found that the maximum responses are recorded when long steep breaking or near-breaking waves hit the cylinder and excite the first eigenperiod of the structure. For a selected event involving a breaking wave, the response of the system is compared to numerical simulations using the FNV method. The higher order excitation loads from the FNV are approximated as sinusoid pulse loads, and it is shown that since the duration of these pulses lies close to the eigenperiod of the structure, they suffice to trigger the first mode motion, without the need for a slamming model. A consequence of the low damping is that if the structure has been previously excited at its 1st mode (linearly or by higher order phenomena such as springing), the structure can already have a motion that adds up to the transient response to the pulse loads. The findings of this study also challenge some of the load models currently used by the industry to estimate the response of offshore wind turbines during extreme events

OC6 Phase II: Integration and verification of a new soil–structure interaction model for offshore wind design

This paper provides a summary of the work done within the OC6 Phase II project, which was focused on the implementation and verification of an advanced soil–structure interaction model for offshore wind system design and analysis. The soil–structure interaction model comes from the REDWIN project and uses an elastoplastic, macroelement model with kinematic hardening, which captures the stiffness and damping characteristics of offshore wind foundations more accurately than more traditional and simplified soil–structure interaction modeling approaches. Participants in the OC6 project integrated this macroelement capability to coupled aero-hydro-servo-elastic offshore wind turbine modeling tools and verified the implementation by comparing simulation results across the modeling tools for an example monopile design. The simulation results were also compared to more traditional soil–structure interaction modeling approaches like apparent fixity, coupled springs, and distributed springs models. The macroelement approach resulted in smaller overall loading in the system due to both shifts in the system frequencies and increased energy dissipation. No validation work was performed, but the macroelement approach has shown increased accuracy within the REDWIN project, resulting in decreased uncertainty in the design. For the monopile design investigated here, that implies a less conservative and thus more cost-effective offshore wind design.US Department of Energy Office of Energy Efficiency and Renewable Energy Wind Energy Technologies Office, Grant/Award Number: DE-AC36-08GO2830

Response of Monopile Wind Turbines to Higher Order Wave Loads

Monopile bottom-fixed offshore wind turbines have been in operation for around 30 years, and more are being planned for the next decades. The location and wave conditions of the planned wind farms make them susceptible to encounter steep and /or breaking waves during extreme weather events, typically storms with a return period of 50 years. These waves can produce large dynamic responses that threaten the structural integrity of the turbine. It is therefore necessary to accurately model the loads produced by these steep waves and the responses of the structure during the design stage of offshore wind turbines. In this thesis, measurements from three experimental campaigns with model-scale wind turbines subjected to extreme sea states are presented. In all three campaigns, the largest responses were measured when a steep and breaking wave was passing through the turbine. The response was a combination of a ringing type of response (i.e. transient excitation of the first eigenmode of the structure) and excitation of the second mode of the structure. Ringing responses were attributed to second and third order hydrodynamic loads, while second mode response was triggered by so-called slamming loads (i.e. impulse loads produced by a wave breaking at the cylinder). The response was decomposed and for extreme events, quasi-static response accounted for 40 to 50% of the total response, the first mode accounted for 30 to 40% and the second mode for up to 20%. Higher modes were not analyzed. Three design standards typically used for wind turbines in the North Sea were followed in this thesis. Different models suggested in the standards were implemented to simulate the responses measured during the experimental campaigns. Consistently with previous research, it was found that models that did not include non-linear kinematics did not match the measured first mode response, and models that did not include slamming loads did not produce significant second mode response. One of the commonly used models, the Morison equation with stream function wave kinematics and Wienke’s slamming load model, was found to have the capability of exciting both the first and second mode of the turbine, but generally missed the balance between the contribution of these two modes. The model recently developed by Kristiansen and Faltinsen (2017), hereafter referred to as KF was implemented for irregular waves, and it was found that this model has the capability to produce ringing responses, but generally overestimates the first mode response for very steep waves. The Rainey model, which in the present implementation only differs from the KF model in their respective point loads, produced responses very similar to those calculated with the KF model. This was shown to be due to the point loads producing similar forces for steep waves. The original contributions of this work to the offshore wind research community include the quantification of the contributions of different eigenmodes in the response to steep and breaking waves, the assessment of different hydrodynamic load models compared to experimental results, and a suggestion for the implementation of the KF model in irregular waves with fully non-linear kinematics

Experimental results of a multimode monopile offshore wind turbine support structure subjected to steep and breaking irregular waves

We present experimental data from MARIN on a bottom-fixed offshore wind turbine mounted on a monopile in intermediate water depth subjected to severe irregular wave conditions. Two models are analysed: the first model is fully flexible and its 1st and 2nd eigenfrequencies and 1st mode shape are representative of those of a full-scale turbine. This model is used to study the structural response with special focus on ringing and response to breaking wave events. The second model is stiff and is used to analyse the hydrodynamic excitation loads, in particular the so-called secondary load cycle. The largest responses are registered when the second mode of the structure is triggered by a breaking wave on top of a ringing response. In such events, the quasi-static response accounts for between 40 and 50% of the total load, the 1st mode response between 30 and 40%, and the 2nd mode response up to 20%. A statistical analysis on the occurrences and characteristics of the secondary load cycle shows that this phenomenon is not directly linked to ringing

Critical assessment of hydrodynamic load models for a monopile structure in finite water depth

The response from steep and breaking waves for a monopile structure is investigated by analysis of experimental results and by application of numerical models for six irregular sea states. The experimental monopile was designed to reproduce the first and second natural frequencies of the NREL 5 MW reference monopile wind turbine. The measured response is reproduced in a finite element model using the Morison equation extended to full Lagrangian acceleration with second-order wave kinematics, and with fully non-linear kinematics and the axial divergence term. For the fully nonlinear wave kinematics, the additional point forces of Rainey and Kristiansen & Faltinsen (KF) are further added. In the latter model, the kinematics at the still water level are obtained by Taylor expansion of the kinematics from the free surface. The shear force at the sea bed and the structural accelerations are next compared between the force models and the experimental data. Among the findings are that the extreme force events are generally smallest for the second-order Morison approach, followed by the extended Morison model, and then the Rainey and KF model which produce similar results. While the total accelerations are found to generally match the measurements with fair accuracy, a modal decomposition shows that all models overpredict the response at the first eigenfrequency and underpredict it at the second eigenfrequency for extreme events. The latter is linked to the missing description of slamming loads in the modelling approach. The point force models of Rainey and KF are found to give quite similar results for the extreme events, the reasons for which are demonstrated by regular wave analysis

Validation of Numerical Models of the Offshore Wind Turbine From the Alpha Ventus Wind Farm Against Full-Scale Measurements Within OC5 Phase III

The main objective of the Offshore Code Comparison Collaboration Continuation, with Correlation (OC5) project is validation of aero-hydro-servo-elastic simulation tools for offshore wind turbines (OWTs) through comparison of simulated results to the response data of physical systems. Phase III of the OC5 project validates OWT models against the measurements recorded on a Senvion 5M wind turbine supported by the OWEC Quattropod from the alpha ventus offshore wind farm. The following operating conditions of the wind turbine were chosen for the validation: (1) Idling below the cut-in wind speed; (2) Rotor-nacelle assembly (RNA) rotation maneuver below the cut-in wind speed; (3) Power production below and above the rated wind speed; and (4) Shutdown. A number of validation load cases were defined based on these operating conditions. The following measurements were used for validation: (1) Strains and accelerations recorded on the support structure; (2) Pitch, yaw, and azimuth angles, generator speed, and electrical power recorded from the RNA. Strains were not directly available from the majority of the OWT simulation tools. Therefore, strains were calculated based on out-of-plane bending moments, axial forces, and cross-sectional properties of the structural members. Also, a number of issues arose during the validation: (1) The need for a thorough quality check of sensor measurements; (2) The sensitivity of the turbine loads to the controller and airfoil properties, which were only approximated in the modeling approach; (3) The importance of estimating and applying an appropriate damping value for the structure; and (4) The importance of wind characteristics beyond turbulence on the loads. The simulation results and measurements were compared in terms of time series, discrete Fourier transforms, power spectral densities, probability density functions of strains and accelerometers. A good match was achieved between the measurements and models set up by OC5 Phase III participants