## 6 Vibration-Based Damage Detection on Offshore Structures  
Offshore platforms are enormous, complex structures that cover a large span of the ocean for various functions and are repeatedly endangered by harsh environmental loads. Fixed offshore platforms including jacket platforms, wind turbines (MTs), marine current turbines (CTs), etc., are the most common types, which suffer severe unforeseen damage during their service life. Therefore, permanent, continuous, and automatic monitoring of the platforms' dynamic behavior needs to be developed to evaluate their safety and durability for a long-term operation. Figure 20: Positions of the deployed “spotter” buoys to measure wave characteristics [107]. Most recently developed offshore platforms are designed to be self-operating and self-producing (unmanned conditions). They have been synchronously equipped with structural health monitoring (SHM) systems to investigate their structural status and assess their functional fitness and performance levels. SHM techniques consist of either global or local damage detection. Local damage techniques, as the most used measuring techniques today, involve non-destructive testing (NDT) methods. The most frequently used NDT tests are visual, magnetic field, acoustic, eddy current, strain measurement, etc. However, detecting damage using local damage inspection is not possible when the structure is enormous and partially submerged in the water. SHM using vibration-based damage detection techniques can identify damage that affects the overall structure and examine changes in global structural vibration characteristics. Damage identification in vibration-based damage techniques is classified into four levels: (i) Level 1, determination of damage occurrence; (ii) Level 2, detection of damage location; (iii) Level 3, measurement of damage severity; and (iv) Level 4, prediction of the remaining service life of the structure. In recent work, considerable new developments in vibration-based damage detection have emerged, particularly the application of advanced sensing technology, signal processing techniques, machine learning (ML), and artificial intelligence (AI). This chapter will review the development of vibration-based damage detection techniques in offshore platforms between 2010 and 2021, classified by various application areas, types of damage, damage tools and instruments, and damage detection strategies. The recent developments and challenges are also summarized to help researchers and practitioners to develop more reliable, practical methods for developing offshore structures in the future. In 2010, [PERSON] et al. [108] performed an in-air laboratory experimental test to detect damage occurrence and location in a four-legged scaled jacket platform using two different random loadings. Accelerometers and strain gauges were used to measure the responses. The combination of the random decrement technique and a neural network can be applied to detect damage magnitude and location. However, it is not recommended that the methodology replaces the traditional inspection method. Damage detection in another offshore structure was performed by [PERSON] and [PERSON] [109] in 2014. The propagating notch of the offshore submerged pipe's girth weld was detected using numerical simulation and experiment-based tests. An experiment using a piezoelectric sensor was conducted using impact and chirp excitations to verify the correctness of the finite element model. The approach selected empirical mode decomposition (EMD) to create energy-based damage indices (EMD_EDIs). The results confirmed that the proposed method can identify the existence and progression of the propagating notch. In the same year, [PERSON] et al. [110] introduced a new modal strain energy change ratio (MSEC) damage indicator for reducing the effects of damage accumulated before the first measurement in an offshore scaled structure. The method is essential because such platforms have seldom been tested for damage detection during their previous lifetime.