## 6 Vibration-Based Damage Detection on Offshore Structures
An experiment-based dynamic test in a water tank using triaxial capacitive accelerometers was conducted to evaluate the proposed method's efficiency. It was found that the method can accurately localize the damage between two adjacent measurements, even with spatially incomplete data. Research on damage detection based on laboratory tests considering semi-real conditions with different boundary conditions is scarce. [PERSON] et al. [111] proposed an experimental test of a scaled six-legged jacket-type offshore platform in hinge-based conditions and pile-supported conditions, performed using a small and limited number of sensors to detect the damage location. The rate of signal energy (RSE) using wavelet packet transform to detect the damage location was introduced. In this work, the dynamic responses of the real offshore platform from forced vibration tests could be used as inputs. Furthermore, at present, two crucial challenges in damage detection for situ platforms are spatial incompleteness of data and noisy data measurement. In 2016, [PERSON] and [PERSON] [112] developed experimental and numerical-based damage detection in a fixed offshore platform using a combination of the Cross Model Cross Mode (CMCM) method and the Pseudo Simplified (PS) model. The major problems in detecting damage in an offshore platform, such as the process being time-consuming and expensive and involving a high computational burden, are solved using this methodology. Furthermore, ocean wind turbines have attracted increasing interest due to their potential energy resources. However, several technological issues, such as damage detection, still require further development for their successful application. [PERSON] et al. [113] addressed crack detection and localization based on strain measurements in an offshore tripod-supported ocean wind turbine structure using experimental and numerical tests. An FBG sensor was used in the experimental test to obtain strain measurement data. The proposed approach can be applied in frames and truss structures. Such research was also performed by [PERSON] and [PERSON] [114]. In this work, an experimental laboratory investigation in a water basin using FBG sensors installed on the underwater level was conducted to monitor crack occurrence and location in an offshore wind turbine support structure. The structure was excited by both artificial irregular waves and wind conditions. The result verified the convenience of FBG strain sensors permanently placed on the underwater element of a wind turbine tripod. To establish an automated methodology for crack propagation detection in operational conditions of a wind turbine in the Polish Baltic sea, [PERSON] et al. [115] compared the result of acceleration-based damage detection between experimental modal analysis (EMA) in dry conditions and operational modal analysis (OMA) in a towing tank with different real operational conditions. Piezoelectric tri-axial accelerometers were deployed to measure the acceleration signal. The developed method revealed consistent results with either the EMA or OMA method, and its applicability in different operational conditions. In real applications, when monitoring a jacket platform, it is necessary to consider unstable wave excitations that influence the structural response. To eliminate the influence of environmental effects, [PERSON] et al. [116] proposed combining the cross-correlation function and principal component analysis (PCA) method using both experimental and numerical tests. The result showed that the proposed method has good accuracy in different damage locations and severities under random wave excitations. However, complicated noises and operational effects still existed in the practical test. [PERSON] et al. [117] proposed a Bayesian approach for damage severity assessment for the first time in offshore structures. A 2D fixed offshore platform with three damage scenarios was developed, and an experimental test using uni-axial accelerometers was carried out to validate the method.