Effect of Scour on the Natural Frequency Responses of the Meteorological Mast in the Taiwan Strait

Abstract

The meteorological mast (met mast) for the Taiwan Power Company's offshore wind farm is located in Taiwan Strait near Changhua County. The p-y curve method recommended in the current offshore foundation design codes does not account for the local scour around the pile foundation; it overestimates the lateral pile deformation and underestimates the foundation stiffness. This paper presents a method to correct the initial modulus of subgrade reaction and modify the ultimate lateral resistance caused by the local scour. The natural frequency of the met mast structure is also determined by a numerical model and verified with the measured data in situ. A comprehensive parameter study is performed to analyze the effect of scour on the dynamic responses of the met mast. Two types of foundation model, a coupled-springs foundation model and a distributed-springs foundation model, are considered in the dynamic analysis of the met mast. The results demonstrate that using a distributed-springs foundation model provides a relatively accurate estimate of the natural frequencies of the met mast structure. Furthermore, the scour exerted significant effects on certain modes of the vibration responses. The natural frequencies of the met mast structure can be reduced by approximately 14% due to scour, particularly in the horizontal bending modes. This paper also provides a preliminary strategy for structural monitoring and analysis to detect scour damage on offshore wind turbines with monopile foundations.

Evaluation of vertical effective stress and pile tension capacity in sands considering scour-hole dimensions

“…Compared with the pre-scour values (i.e., Sdl=0), the first fore-aft bending mode of natural frequency (f1) decreased by 12%-30% when Sdl=1.3Dop-2.5Dop. This amount of frequency reduction was greater than that in sands (6%-9% when Sdl=1.3Dop-2.5Dop) (Li et al, 2018;Prendergast et al, 2015;Tseng et al, 2018), indicating that the increase of Sdl in soft clays is more likely to shift f1 into the range of the rotor frequency of the turbine (f1P), causing resonance. In this study, post-scour f1 values (0.25-0.20 Hz) fell into the typical f1P range of 0.14-0.30 Hz (Bhattacharya et al, 2011) when Sdl=0.5Dop-3.0Dop.…”

“…Previous research on the effects of scour-hole dimensions and soil stress history has mainly focused on the static responses of OWTs or piles (Achmus et al, 2010;He et al, 2019;Li et al, 2020;Tseng et al, 2018); however, there is almost no study on their effects on the dynamic responses of OWTs or piles. The evaluation of scour-hole dimensions was highlighted by a recent centrifuge test to investigate the effects of both local and general scour on the monotonic lateral monopile response.…”

“…Contrarily,He et al (2019) considered and highlighted the effects of soil stress-history changes on the structural responses. Recent studies(Tseng et al, 2018(Tseng et al, , 2017 adopted the post-scour p-y springs(Lin et al, 2014a) to account for the scour-hole dimensions, but no soil stress history was included. Overall, the above literature review indicates two research gaps: (1) lack of understanding of the combined effects of earthquakes and scour where multiple recorded earthquakes are considered, and (2) lack of research on the effects of scour-hole dimensions and soil stress history on seismic responses of OWTs.This study aimed to propose a simple method for investigating the combined effects of earthquakes and scour on OWTs in soft clays, in which the effects of both scour-hole dimensions and soil stress history were incorporated into the framework of dynamic soil-structure interaction [i.e., DBNWF(Boulanger et al, 1999)].…”

A Fluid-Structure Interaction Model for Bridge Safety Assessment under Scour Conditions

“…A lower dominant frequency was considered to imply a lower constraint, suggesting a weakened structure. Prendergast et al [9] studied the natural frequency response of structure, and Tseng et al [10] performed numerical simulations on the piers of o shore wind turbines, concluding that the natural frequency decreased, and the deformation increased with the loss of riverbed materials near the piers.…”

Visible Light Communication System for Offshore Wind Turbine Foundation Scour Early Warning Monitoring

“…The Keulegan-Carpenter number (KC) was applied to realize the foundation uncertainties of marine wind farm structures scour [18]. There were many works focused on the vibration-based approaches to monitoring the structural health status of the wind turbine [19][20][21]. For example, a distributed-spring foundation model to estimate the variation of natural frequencies and provide a strategy for addressing scour-induced damage around monopile foundations has used [22].…”