Operational Modal Analysis of a Spar-Type Floating Platform Using Frequency Domain Decomposition Method

Ruzzo, Carlo; Failla, Giuseppe; Collu, Maurizio; Nava, Vincenzo; Fiamma, Vincenzo; Arena, Felice

doi:10.3390/en9110870

Cited by 16 publications

(10 citation statements)

References 13 publications

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“…Given the findings of Ruzzo et al [18], it is part of the present study to investigate how well the damping detected with OMA can lead to a good match between the measurements and the model output. For this reason, a calibrated modal damping ratio is also found for each sea state.…”

Section: Linear Damping Matrixmentioning

confidence: 96%

“…The application of OMA techniques to the rigid-body motion of floating wind turbines was first done by Ruzzo et al [18]. In their work, which was purely numerical, a numerical model was set up for a spar floating wind turbine, with Morison-based hydrodynamics and linear waves from a JONSWAP spectrum with significant wave height of 2 m and two different peak periods.…”

Section: Introductionmentioning

confidence: 99%

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Reproduction of slow-drift motions of a floating wind turbine using second-order hydrodynamics and Operational Modal Analysis

Pegalajar‐Jurado

Bredmose

2019

Marine Structures

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Slow-drift forcing and damping are important for an accurate reproduction of the low-frequency motion of floating offshore wind turbines. In this study we present a numerical model that includes both inviscid slow-drift forcing through full quadratic transfer functions (QTFs) and viscous forcing. To accommodate a linear damping matrix that represents all the damping effects on the floater, the classical Morison formulation of the drag forcing with relative velocity is simplified into a pure forcing term and a linear constant damping term. The frequency-dependent radiation properties are also replaced by constant matrices. We investigate the application of Operational Modal Analysis (OMA) to estimate the damping ratios from the test data, and also pursue a calibration method where the linear damping is calibrated in the modal space and subsequently transformed back to the physical space. The model is compared to wave basin data for four environmental conditions including the 50-year sea state. The damping ratios estimated with OMA generally increase with the sea state. The measured response can be generally well reproduced by the model when the damping is calibrated for each sea state. Due to the neglection of firstorder motion effects in the applied QTFs and to the damping introduced by the mooring lines, however, the surge response is underpredicted for the severe sea states, and the yaw motion is always underpredicted. The commonly used approach where the damping is calibrated to the decay tests is found to provide less accurate results than the approach of calibrating the modal damping ratios

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Section: Linear Damping Matrixmentioning

confidence: 96%

Section: Introductionmentioning

confidence: 99%

Reproduction of slow-drift motions of a floating wind turbine using second-order hydrodynamics and Operational Modal Analysis

Pegalajar‐Jurado

Bredmose

2019

Marine Structures

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“…Although the analysis of response spectra gives some clear information about the dynamic behaviour of the model in surge and sway degrees of freedom, it could not be used for direct estimation of the corresponding linear damping coefficients necessary for the calibration of the numerical model. To this aim, the Authors proposed an alternative identification method [43], based on the adoption of output-only identification techniques, borrowed from Operational Modal Analysis. This approach has already proved successful for the dynamic identification of a numerical model of a spar structure, similar to the present one, and may be hence used in future studies to complement the identification process of the intermediate-scale models of offshore structures installed at sea.…”

Section: Irregular Wave Tests: Raos and Damping Estimationsmentioning

confidence: 99%

“…Spectral analysis of surge and sway motions has proved to be suitable for identifying the corresponding natural frequencies and the most relevant dynamic characteristics. Finally, output-only identification techniques typical of Operational Modal Analysis [43] have been suggested as potential alternative/complementary methods for the full identification of intermediate-scale models of floating structures installed at sea.…”

Section: Irregular Wave Tests: Raos and Damping Estimationsmentioning

confidence: 99%

On intermediate-scale open-sea experiments on floating offshore structures: Feasibility and application on a spar support for offshore wind turbines

et al. 2018

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Experimental investigation of floating structures represents the most direct way for achieving their dynamic identification and it is particularly valuable for relatively new concepts, such as floating supports for offshore wind turbines, in order to fully understand their dynamic behaviour. Traditional experimental campaigns on floating structures are carried out at small scale, in indoor laboratories, equipped with wave and wind generation facilities. This article presents the results of a 1:30 open-sea experimental activity on a scale model of the OC3-Hywind spar, in parked rotor conditions, carried out at the Natural Ocean Engineering Laboratory (NOEL) of Reggio Calabria (Italy). The aim of the experiment is twofold. Firstly, it aims to assess the feasibility of low-cost, intermediate-scale, open-sea activities on offshore structures, which are proposed to substitute or complement the traditional indoor activities in ocean basins. Secondly, it provides useful experimental data on damping properties of spar support structures for offshore wind turbines, with respect to heave, roll and pitch degrees of freedom. It has been proven that the proposed approach may overcome some limitations of traditional small-scale activities, namely high costs and small scale, and allows to enhance the fidelity of the experimental data currently available in literature for spar floating supports for offshore wind turbines. Keywords 1. Offshore structures 2. Spar 3. Floating wind turbines 4. Physical model 5. Heave damping 6. Roll damping development of reliable dynamic models, able to represent the coupled behaviour of the floating wind turbines [2-3]. While such models are usually implemented by means of numerical codes [4-5], experimental activities play a crucial role for their validation, as well as for the system identification. The experimental activities on floating offshore wind turbines may be classified in two groups, namely small-scale and large-scale ones. Traditional small-scale activities (1:50-1:100) are carried out in controlled environment such as wave tanks and ocean basins, where the desired wind-wave conditions can be reproduced, to measure the dynamic response of the structure and to calibrate opportunely the numerical codes [6-8]. Although the controlled environment allows to achieve very precise and reliable results, these activities have some relevant disadvantages, namely high rental fees of the basins, limited duration of the experiments, and limitations in representing all the relevant physical phenomena at scale level, which may alter significantly the dynamic behaviour of the model with respect to the full-scale structure. On the opposite side, large-scale activities (1:1-1:10) are carried out in open-sea and allow to represent all the relevant features of the offshore wind turbines, including turbine-support interaction, mooring system and grid connection, in relevant operational conditions [9-11]. Clearly, such projects are very expensive and usually represent pilot activities, which are carried out...

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“…Bajrić et al [6] compared the damping ratio for offshore wind turbine structures by different modal identification techniques validated by real vibration measurements of an offshore wind turbine under nonoperating conditions. Ruzzo et al [7] proposed the identification of the rigid body motions of a spar floating support for offshore wind turbines through OMA. e method applied to a numerical model based on a linear equation of motion has proven to be a viable method for output-only identification of floating structures.…”

Section: Introductionmentioning

confidence: 99%

Fatigue Stress Estimation of an Offshore Jacket Structure Based on Operational Modal Analysis

Nabuco

Tarpø

Tygesen

et al. 2020

Shock and Vibration

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Fatigue life assessment currently recommended by offshore standards is associated with a large number of uncertainties mainly related to the environmental loads and the numerical model. Recently, for economic reasons, the need for extending the lifetime of existing offshore structures led to the necessity of developing more accurate and realistic predicting models so that damage detection and maintenance can be optimized. This paper proposes the implementation of Structural Health Monitoring Systems in order to extract modal properties—such as mode shapes, natural frequencies, and damping ratios—throughout Operational Modal Analysis (OMA), which is the engineering field that studies the modal properties of systems under ambient vibrations or normal operating conditions. The identified modal properties of the structural system are the fundamental information to update a finite element model by means of an expansion technique. Then, the virtual sensing technique—modal expansion—is used to estimate the stress in the entire structure. Though existing models depend on the load estimation, the model based on OMA-assisted virtual sensing depends on the measured responses and assumes that the loads act as random vibrations. A case study using data from a real offshore structure is presented based on measurements recorded during normal operation conditions of an offshore tripod jacket. From strains estimated using OMA and virtual sensing, fatigue stresses are predicted and verified by applying the concept of equivalent stress range. Both estimated and measured strains are given as input data to evaluate the equivalent stress range and compared with each other. Based on this study, structural health monitoring estimates the fatigue stresses with high precision. As conclusion, this study describes how the fatigue can be assessed based on a more accurate value of stress and less uncertainties, which may allow extending the fatigue life of offshore platforms.

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Operational Modal Analysis of a Spar-Type Floating Platform Using Frequency Domain Decomposition Method

Cited by 16 publications

References 13 publications

Reproduction of slow-drift motions of a floating wind turbine using second-order hydrodynamics and Operational Modal Analysis

Reproduction of slow-drift motions of a floating wind turbine using second-order hydrodynamics and Operational Modal Analysis

On intermediate-scale open-sea experiments on floating offshore structures: Feasibility and application on a spar support for offshore wind turbines

Fatigue Stress Estimation of an Offshore Jacket Structure Based on Operational Modal Analysis

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