A modelled wind turbine generator subjected to combined ice- and aerodynamic loading is analyzed ... more A modelled wind turbine generator subjected to combined ice- and aerodynamic loading is analyzed with the focus on its fatigue lifetime. A comparison is made between the prediction of a combined analysis, taking both ice- and wind loads into account simultaneously, and a superposition analysis, computing the response of the structure as a result of ice and wind loading separately. The accumulated fatigue damage is computed considering different descriptions of the ice load. Prescribed ice load curves from current design standards, as well as phenomenological models for the prediction of dynamic ice-structure interaction are employed. Results show that the superposition method underpredicts the accumulated fatigue damage in the range of frequency lock-in, but only when phenomenological models, which are more advanced than those recommended by the design standards, are used to model the ice load. Furthermore the predicted fatigue damage computed using the design standards for the desc...
Fixed offshore wind turbines continue to be developed for high latitude areas where not only wind... more Fixed offshore wind turbines continue to be developed for high latitude areas where not only wind and wave loads need to be considered but also moving sea ice. Current rules and regulations for the design of fixed offshore structures in ice-covered waters do not adequately consider the effects of ice loading and its stochastic nature on the fatigue life of the structure. Ice crushing on such structures results in ice-induced vibrations, which can be represented by loading the structure using a variable-amplitude loading (VAL) sequence. Typical offshore load spectra are developed for wave and wind loading. Thus, a combined VAL spectrum is developed for wind, wave, and ice action. To this goal, numerical models are used to simulate the dynamic ice-, wind-, and wave-structure interaction. The stress time-history at an exemplarily selected critical point in an offshore wind energy monopile support structure is extracted from the model and translated into a VAL sequence, which can then b...
In this paper, the efficacy of an effective fluid model (EFM) is studied for replicating the effe... more In this paper, the efficacy of an effective fluid model (EFM) is studied for replicating the effects that hydrodynamics has on the interaction between level ice, modeled as a semi-infinite Kirchhoff-Love plate, and a downward sloping structure, modeled as a rigid and immobile body. The proposed EFM is based on a distributed frequency-independent added mass and damping coefficient, as well as a damper located at the point of contact with the structure. The optimal value of the three coefficients of the EFM is obtained by minimizing the error of the predicted breaking length and maximum contact force over a range of ice velocities when compared to a true hydrodynamics ISI model that is based on incompressible potential flow. The resulting effective hydrodynamic ISI model has greatly improved performance compared to an ISI model that only accounts for hydrostatics, even when the parameters of the system are changed. Moreover, it is much easier to implement and has significantly faster ...
Offshore wind turbines at locations where sea or lake ice is present need to be designed to withs... more Offshore wind turbines at locations where sea or lake ice is present need to be designed to withstand ice-induced loading. For vertical-sided support structures, such as monopiles, the effects of ice-induced vibrations need to be considered in the design. Current practice is either to use approaches provided in design standards, or for example to apply pre-generated ice load time series in the wind turbine aeroelastic model. These approaches have the drawback that the coupling between ice failure behavior and structural motion is not included. The effect of omitting this coupling on predictions for fatigue and ultimate limit states is currently not known. To enable fully coupled simulations in the design of offshore wind turbines, an existing simulation model for ice crushing has been recently coupled (“VANILLA”) to the in-house aeroelastic software package BHawC. In this paper this fully coupled model is applied to simulate ultimate limit state design load cases (DLCs) for a recent...
Author remarks This implementation does not contain pre-processing tools for defining the input p... more Author remarks This implementation does not contain pre-processing tools for defining the input parameters of the model based on full-scale data (Hendrikse, unpublished), the drift model in Hendrikse and Nord (2018), the option to include ice buckling (Hendrikse and Metrikine, 2016), and multi-degree-of-freedom structures, which are available at Delft University of Technology. For more information or possibilities to collaborate please send an e-mail to [email protected].
M. Maattanen, S. Loset, A. Metrikine, K.-U. Evers, H. Hendrikse, C. Lonoy, I. Metrikin, T. Nord, ... more M. Maattanen, S. Loset, A. Metrikine, K.-U. Evers, H. Hendrikse, C. Lonoy, I. Metrikin, T. Nord, and S. Sukhorukov 1. Sustainable Arctic Marine and Coastal Technology (SAMCoT), Centre for Research-based Innovation (CRI), Norwegian University of Science and Technology, Trondheim, Norway 2. Delft University of Technology, Delft, The Netherlands 3. Hamburg Ship Model Basin (HSVA), Hamburg, Germany 4. DNV, Nora, Norway * [email protected]
Due to increasing trend of building offshore wind turbines (OWTs) in seas at high latitudes where... more Due to increasing trend of building offshore wind turbines (OWTs) in seas at high latitudes where seasonal sea ice occurs, novel methods for design of such structures are needed. Specifically, the effect of ice-induced vibrations (IIVs) on fatigue life of the structures is currently poorly understood. Therefore, the goal of this paper is to analyze the current state-of-the-art approach for estimating the ice loads contribution to the fatigue life of OWTs and identify the current knowledge gaps. Moreover, the paper proposes a methodology for developing a combined load spectrum of wind, waves and ice using numerical simulations, with an ultimate goal to develop applicable Gasner curves characterizing the variable-amplitude nature of the loading pattern. Finally, the use of small-scale fatigue tests under sub-zero temperatures in order to develop the appropriate S-N and Gasner curves is discussed.
For the design of offshore structures in regions with ice-infested waters, the prediction of inte... more For the design of offshore structures in regions with ice-infested waters, the prediction of interaction between ice floe and support structure is essential. If the structure is vertically sided at the ice-structure interface, then ice-induced vibrations can develop. Recently, a dynamic icestructure interaction model has been developed and validation has been attempted based on dedicated experiments. This study extends the validation by investigating the capabilities of the analytical model in predicting the indentation speed at which transition from the intermittent crushing to frequency lock-in regime of ice-induced vibrations occurs with various input parameters. Implementation of these various input parameters seeks to address the challenge of adapting the analytical model from the reference input parameters to scenarios with other structural properties. Using these various input parameters, the analytical model can demonstrate accurate prediction of the transition ice speed fro...
Ice-induced vibrations have to be considered in the design of vertically sided offshore structure... more Ice-induced vibrations have to be considered in the design of vertically sided offshore structures which may encounter drifting sea or lake ice during their lifetime. One particular aspect is the contribution of ice-induced vibrations to the fatigue of such structures. Estimation of the duration of events is often difficult, due to limited available data on ice drift, leading to conservative assumptions. In this paper, the approach followed for assessing the fatigue resulting from frequency lock-in vibrations in the design stage of a recent offshore wind project is presented. The project concerned offshore wind turbines with jacket support structures consisting partly of vertical structural members. The severity of ice-induced vibrations for the structures is first assessed using a simulation model. Following this, ice drift is included in the assessment to obtain an estimate of the number of cycles of frequency lock-in over the lifetime of the structure. Results show that sitespeci...
Ice-induced vibrations have to be considered in design of vertically sided offshore structures su... more Ice-induced vibrations have to be considered in design of vertically sided offshore structures subjected to loading by sea ice, such as offshore wind turbines and oil- and gas platforms. The interaction between ice and structure may result in high global peak loads and the occurring structural vibrations can contribute significantly to the overall fatigue of the structure. A regime of particular interest is the frequency lock-in regime in which the interaction causes the structure to oscillate at high amplitude with a frequency close to one of its natural frequencies. Assessment of frequency lock-in in the design phase can be done based on simple approaches once for given ice conditions the natural modes to experience frequency lock-in and the range of ice drift velocities for which lock-in develops are known. Determining those modes and velocities is however challenging due to the nonlinear nature of the interaction between ice and structure and limited available reference data. In...
The second edition of the ISO19906 design standard contains changes in the informative appendix c... more The second edition of the ISO19906 design standard contains changes in the informative appendix concerning dynamic ice actions. Challenges in application of the previous approach and advances in understanding of the dynamic ice-structure interaction process were the main motivation for the revision. The new guideline provides relevant information for determining dynamic ice actions, but does not yet give a complete simplified design method. In this paper, the changes made in section A.8.2.6.1 covering dynamic ice actions on vertical and near vertical structures are discussed and remaining knowledge gaps are defined. Changes in the sections dealing with sloping structures and fatigue analysis were not made in the revision of ISO19906 and are therefore not considered. An important omission is identified related to the peak loads in intermittent crushing, which are potentially underestimated based on the new description. The main knowledge gap is found to be the definition of the ice d...
Vertically sided offshore structures subjected to level ice are designed to withstand the effects... more Vertically sided offshore structures subjected to level ice are designed to withstand the effects of ice-induced vibrations. Such structures are, for example, offshore wind turbines on monopile foundations, multi-legged oil-and gas platforms or lighthouses. For the prediction of dynamic interaction between ice and structures, several phenomenological models exist. The main challenge with these models is the limited amount of data available for validation, which makes it difficult to determine their applicability. In this study, an attempt is made to validate one of the existing models. First, the parameters which define the ice in the model were derived from new model-scale experiments with a rigid rectangular structure. The model was subsequently applied to simulate the interaction between ice and two compliant rectangular structures with different structural properties. Finally, model-scale experiments were conducted for the two compliant structures. Results of the experiments and model were compared to assess the capability of the model to predict dynamic ice-structure interaction. Results show that the adopted approach allows for a definition of the input parameters of the model and accurate prediction of frequency lock-in and continuous brittle crushing for compliant structures. Intermittent crushing was not observed in the model-scale experiments due to the model-scale ice bending significantly during low ice speeds. As a consequence, the model could not be validated for this regime of interaction. The approach followed-and challenges encountered during its application-are discussed.
With the ongoing development of offshore wind in cold regions where the foundations are exposed t... more With the ongoing development of offshore wind in cold regions where the foundations are exposed to sea ice, there is a strong need for data to validate the numerically predicted dynamic interaction between ice and structure used for design. Full-scale data is non-existent and only a limited number of experimental campaigns in ice tanks have been conducted for this specific problem. When compared to traditional structures subjected to sea ice loading like lighthouses and oil and gas platforms, the motion of the turbines at the ice action point is both in line with the ice drift direction but also significantly across due to the interaction of the turbine with the wind. Furthermore, the structure being slender overall and having a large top mass results in a very particular set of modes of oscillation where at least both the first and second global bending mode are expected to interact with the ice. To capture this complexity, a real-time hybrid test setup has been designed for basin ...
In this paper, an analytical method is presented to assess FLI (Frequency lock-in) of monopile su... more In this paper, an analytical method is presented to assess FLI (Frequency lock-in) of monopile support structures for offshore wind turbines, subjected to loading by floating sea ice. The method is introduced, justified and presented with an example. The main idea of the method is to use existing knowledge regarding structural response velocities at which FLI can occur. Using such velocities, the structural response can be determined easily when single mode response is assumed, which is a conservative choice when analyzing FLI. The method is particularly useful for early stages of the design process, when the implication of ice loads on the design needs to be assessed without time-consuming simulations. Results from the method can also be used to identify structural configurations which are most critical for ice loading.
Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences
Identifying the modal parameters of structures located in ice-infested waters may be challenging ... more Identifying the modal parameters of structures located in ice-infested waters may be challenging due to the interaction between the ice and structure. In this study, both simulated data from a state-of-the-art ice–structure interaction model and measured data of ice–structure interaction were both used in conjunction with a covariance-driven stochastic subspace identification method to identify the modal parameters and their corresponding variances. The variances can be used to assign confidence to the identified eigenfrequencies, and effectively eliminate the eigenfrequencies with large variances. This enables a comparison between the identified eigenfrequencies for different ice conditions. Simulated data were used to assess the accuracy of the identified modal parameters during ice–structure interactions, and they were further used to guide the choice of parameters for the subspace identification when applied to measured data. The measured data consisted of 150 recordings of ice ...
A modelled wind turbine generator subjected to combined ice- and aerodynamic loading is analyzed ... more A modelled wind turbine generator subjected to combined ice- and aerodynamic loading is analyzed with the focus on its fatigue lifetime. A comparison is made between the prediction of a combined analysis, taking both ice- and wind loads into account simultaneously, and a superposition analysis, computing the response of the structure as a result of ice and wind loading separately. The accumulated fatigue damage is computed considering different descriptions of the ice load. Prescribed ice load curves from current design standards, as well as phenomenological models for the prediction of dynamic ice-structure interaction are employed. Results show that the superposition method underpredicts the accumulated fatigue damage in the range of frequency lock-in, but only when phenomenological models, which are more advanced than those recommended by the design standards, are used to model the ice load. Furthermore the predicted fatigue damage computed using the design standards for the desc...
Fixed offshore wind turbines continue to be developed for high latitude areas where not only wind... more Fixed offshore wind turbines continue to be developed for high latitude areas where not only wind and wave loads need to be considered but also moving sea ice. Current rules and regulations for the design of fixed offshore structures in ice-covered waters do not adequately consider the effects of ice loading and its stochastic nature on the fatigue life of the structure. Ice crushing on such structures results in ice-induced vibrations, which can be represented by loading the structure using a variable-amplitude loading (VAL) sequence. Typical offshore load spectra are developed for wave and wind loading. Thus, a combined VAL spectrum is developed for wind, wave, and ice action. To this goal, numerical models are used to simulate the dynamic ice-, wind-, and wave-structure interaction. The stress time-history at an exemplarily selected critical point in an offshore wind energy monopile support structure is extracted from the model and translated into a VAL sequence, which can then b...
In this paper, the efficacy of an effective fluid model (EFM) is studied for replicating the effe... more In this paper, the efficacy of an effective fluid model (EFM) is studied for replicating the effects that hydrodynamics has on the interaction between level ice, modeled as a semi-infinite Kirchhoff-Love plate, and a downward sloping structure, modeled as a rigid and immobile body. The proposed EFM is based on a distributed frequency-independent added mass and damping coefficient, as well as a damper located at the point of contact with the structure. The optimal value of the three coefficients of the EFM is obtained by minimizing the error of the predicted breaking length and maximum contact force over a range of ice velocities when compared to a true hydrodynamics ISI model that is based on incompressible potential flow. The resulting effective hydrodynamic ISI model has greatly improved performance compared to an ISI model that only accounts for hydrostatics, even when the parameters of the system are changed. Moreover, it is much easier to implement and has significantly faster ...
Offshore wind turbines at locations where sea or lake ice is present need to be designed to withs... more Offshore wind turbines at locations where sea or lake ice is present need to be designed to withstand ice-induced loading. For vertical-sided support structures, such as monopiles, the effects of ice-induced vibrations need to be considered in the design. Current practice is either to use approaches provided in design standards, or for example to apply pre-generated ice load time series in the wind turbine aeroelastic model. These approaches have the drawback that the coupling between ice failure behavior and structural motion is not included. The effect of omitting this coupling on predictions for fatigue and ultimate limit states is currently not known. To enable fully coupled simulations in the design of offshore wind turbines, an existing simulation model for ice crushing has been recently coupled (“VANILLA”) to the in-house aeroelastic software package BHawC. In this paper this fully coupled model is applied to simulate ultimate limit state design load cases (DLCs) for a recent...
Author remarks This implementation does not contain pre-processing tools for defining the input p... more Author remarks This implementation does not contain pre-processing tools for defining the input parameters of the model based on full-scale data (Hendrikse, unpublished), the drift model in Hendrikse and Nord (2018), the option to include ice buckling (Hendrikse and Metrikine, 2016), and multi-degree-of-freedom structures, which are available at Delft University of Technology. For more information or possibilities to collaborate please send an e-mail to [email protected].
M. Maattanen, S. Loset, A. Metrikine, K.-U. Evers, H. Hendrikse, C. Lonoy, I. Metrikin, T. Nord, ... more M. Maattanen, S. Loset, A. Metrikine, K.-U. Evers, H. Hendrikse, C. Lonoy, I. Metrikin, T. Nord, and S. Sukhorukov 1. Sustainable Arctic Marine and Coastal Technology (SAMCoT), Centre for Research-based Innovation (CRI), Norwegian University of Science and Technology, Trondheim, Norway 2. Delft University of Technology, Delft, The Netherlands 3. Hamburg Ship Model Basin (HSVA), Hamburg, Germany 4. DNV, Nora, Norway * [email protected]
Due to increasing trend of building offshore wind turbines (OWTs) in seas at high latitudes where... more Due to increasing trend of building offshore wind turbines (OWTs) in seas at high latitudes where seasonal sea ice occurs, novel methods for design of such structures are needed. Specifically, the effect of ice-induced vibrations (IIVs) on fatigue life of the structures is currently poorly understood. Therefore, the goal of this paper is to analyze the current state-of-the-art approach for estimating the ice loads contribution to the fatigue life of OWTs and identify the current knowledge gaps. Moreover, the paper proposes a methodology for developing a combined load spectrum of wind, waves and ice using numerical simulations, with an ultimate goal to develop applicable Gasner curves characterizing the variable-amplitude nature of the loading pattern. Finally, the use of small-scale fatigue tests under sub-zero temperatures in order to develop the appropriate S-N and Gasner curves is discussed.
For the design of offshore structures in regions with ice-infested waters, the prediction of inte... more For the design of offshore structures in regions with ice-infested waters, the prediction of interaction between ice floe and support structure is essential. If the structure is vertically sided at the ice-structure interface, then ice-induced vibrations can develop. Recently, a dynamic icestructure interaction model has been developed and validation has been attempted based on dedicated experiments. This study extends the validation by investigating the capabilities of the analytical model in predicting the indentation speed at which transition from the intermittent crushing to frequency lock-in regime of ice-induced vibrations occurs with various input parameters. Implementation of these various input parameters seeks to address the challenge of adapting the analytical model from the reference input parameters to scenarios with other structural properties. Using these various input parameters, the analytical model can demonstrate accurate prediction of the transition ice speed fro...
Ice-induced vibrations have to be considered in the design of vertically sided offshore structure... more Ice-induced vibrations have to be considered in the design of vertically sided offshore structures which may encounter drifting sea or lake ice during their lifetime. One particular aspect is the contribution of ice-induced vibrations to the fatigue of such structures. Estimation of the duration of events is often difficult, due to limited available data on ice drift, leading to conservative assumptions. In this paper, the approach followed for assessing the fatigue resulting from frequency lock-in vibrations in the design stage of a recent offshore wind project is presented. The project concerned offshore wind turbines with jacket support structures consisting partly of vertical structural members. The severity of ice-induced vibrations for the structures is first assessed using a simulation model. Following this, ice drift is included in the assessment to obtain an estimate of the number of cycles of frequency lock-in over the lifetime of the structure. Results show that sitespeci...
Ice-induced vibrations have to be considered in design of vertically sided offshore structures su... more Ice-induced vibrations have to be considered in design of vertically sided offshore structures subjected to loading by sea ice, such as offshore wind turbines and oil- and gas platforms. The interaction between ice and structure may result in high global peak loads and the occurring structural vibrations can contribute significantly to the overall fatigue of the structure. A regime of particular interest is the frequency lock-in regime in which the interaction causes the structure to oscillate at high amplitude with a frequency close to one of its natural frequencies. Assessment of frequency lock-in in the design phase can be done based on simple approaches once for given ice conditions the natural modes to experience frequency lock-in and the range of ice drift velocities for which lock-in develops are known. Determining those modes and velocities is however challenging due to the nonlinear nature of the interaction between ice and structure and limited available reference data. In...
The second edition of the ISO19906 design standard contains changes in the informative appendix c... more The second edition of the ISO19906 design standard contains changes in the informative appendix concerning dynamic ice actions. Challenges in application of the previous approach and advances in understanding of the dynamic ice-structure interaction process were the main motivation for the revision. The new guideline provides relevant information for determining dynamic ice actions, but does not yet give a complete simplified design method. In this paper, the changes made in section A.8.2.6.1 covering dynamic ice actions on vertical and near vertical structures are discussed and remaining knowledge gaps are defined. Changes in the sections dealing with sloping structures and fatigue analysis were not made in the revision of ISO19906 and are therefore not considered. An important omission is identified related to the peak loads in intermittent crushing, which are potentially underestimated based on the new description. The main knowledge gap is found to be the definition of the ice d...
Vertically sided offshore structures subjected to level ice are designed to withstand the effects... more Vertically sided offshore structures subjected to level ice are designed to withstand the effects of ice-induced vibrations. Such structures are, for example, offshore wind turbines on monopile foundations, multi-legged oil-and gas platforms or lighthouses. For the prediction of dynamic interaction between ice and structures, several phenomenological models exist. The main challenge with these models is the limited amount of data available for validation, which makes it difficult to determine their applicability. In this study, an attempt is made to validate one of the existing models. First, the parameters which define the ice in the model were derived from new model-scale experiments with a rigid rectangular structure. The model was subsequently applied to simulate the interaction between ice and two compliant rectangular structures with different structural properties. Finally, model-scale experiments were conducted for the two compliant structures. Results of the experiments and model were compared to assess the capability of the model to predict dynamic ice-structure interaction. Results show that the adopted approach allows for a definition of the input parameters of the model and accurate prediction of frequency lock-in and continuous brittle crushing for compliant structures. Intermittent crushing was not observed in the model-scale experiments due to the model-scale ice bending significantly during low ice speeds. As a consequence, the model could not be validated for this regime of interaction. The approach followed-and challenges encountered during its application-are discussed.
With the ongoing development of offshore wind in cold regions where the foundations are exposed t... more With the ongoing development of offshore wind in cold regions where the foundations are exposed to sea ice, there is a strong need for data to validate the numerically predicted dynamic interaction between ice and structure used for design. Full-scale data is non-existent and only a limited number of experimental campaigns in ice tanks have been conducted for this specific problem. When compared to traditional structures subjected to sea ice loading like lighthouses and oil and gas platforms, the motion of the turbines at the ice action point is both in line with the ice drift direction but also significantly across due to the interaction of the turbine with the wind. Furthermore, the structure being slender overall and having a large top mass results in a very particular set of modes of oscillation where at least both the first and second global bending mode are expected to interact with the ice. To capture this complexity, a real-time hybrid test setup has been designed for basin ...
In this paper, an analytical method is presented to assess FLI (Frequency lock-in) of monopile su... more In this paper, an analytical method is presented to assess FLI (Frequency lock-in) of monopile support structures for offshore wind turbines, subjected to loading by floating sea ice. The method is introduced, justified and presented with an example. The main idea of the method is to use existing knowledge regarding structural response velocities at which FLI can occur. Using such velocities, the structural response can be determined easily when single mode response is assumed, which is a conservative choice when analyzing FLI. The method is particularly useful for early stages of the design process, when the implication of ice loads on the design needs to be assessed without time-consuming simulations. Results from the method can also be used to identify structural configurations which are most critical for ice loading.
Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences
Identifying the modal parameters of structures located in ice-infested waters may be challenging ... more Identifying the modal parameters of structures located in ice-infested waters may be challenging due to the interaction between the ice and structure. In this study, both simulated data from a state-of-the-art ice–structure interaction model and measured data of ice–structure interaction were both used in conjunction with a covariance-driven stochastic subspace identification method to identify the modal parameters and their corresponding variances. The variances can be used to assign confidence to the identified eigenfrequencies, and effectively eliminate the eigenfrequencies with large variances. This enables a comparison between the identified eigenfrequencies for different ice conditions. Simulated data were used to assess the accuracy of the identified modal parameters during ice–structure interactions, and they were further used to guide the choice of parameters for the subspace identification when applied to measured data. The measured data consisted of 150 recordings of ice ...
Uploads
Papers by Hayo Hendrikse