## 1 Introduction  
Many control devices have been proposed and successfully applied to control onshore civil structures in past decades [1; 2; 3; 4; 5; 6; 7]. Nowadays, thousands of kilometers of pipelines have been installed on the seabed to transport oil and gas. Undesirable vibrations of subsea pipelines subjected to the ocean current may cause the fatigue of pipeline structures and shorten their lifecycle [8]. The control devices used to suppress vibration in onshore structures may be unsuitable or may be too hard to implement in a seawater environment. Therefore, an effective control device which can be employed in seawater environments for vibration suppression of subsea pipelines should be developed. Tuned mass damper (TMD) [9] is a traditional passive control device that has been successfully applied in vibration control of civil structures [10; 11; 12; 13; 14; 15] and machining processes [16; 17; 18], which consists of a spring-masssystem and a damper. When the frequency of the TMD is tuned closely to that of the primary structure, the vibration energy of the primary structure will be transferred to the TMD through the stiffness force and partly dissipated by the damping force [19; 20]. In the previous studies, conventional fluid dampers such as viscous oil dampers [21], Magneto rheological (MR) dampers [22] and impact dampers [23; 24] are used to dissipate vibration energy in a TMD system. It should be noted that MR dampers are also employed to control the frequency of a semi-active TMD system in real-time [25; 26]. Since the friction force always exists in the relative movement between the TMD and the fluid damper, the conventional TMD equipped with a fluid damper is only activated when the acceleration of the controlled structure exceeds a certain value [14; 15]. Because the damping force generated by eddy currents is a non-contact force, no friction force exists in the relative movement between the TMD and eddy current damper. Moreover, compared with conventional oil damper, the eddy current damper is insensitive to the change of temperature. In the regard of those points, eddy current dampers have been proposed to provide the additional damping for a TMD system [27; 28]. The damping force generated by the eddy currents is proportional to the relative velocity, which can be regarded as a linear viscous damping [27; 28; 29; 30; 31]. Some works on the vibration suppression using eddy current tuned mass damper (ECTMD) have been presented. [PERSON] et al. [32] investigated the vibration control of a wind tunnel bridge model using an ECTMD. In their device, the damping ratio of the ECTMD can be adjusted by changing the clearance between the magnet and the aluminum conductive plate. [PERSON] et al. [33] designed an ECTMD to control the vibration of a cantilever beam and proposed a relatively simple method to calculate the damping force provided by eddy currents numerically. For verifying that an ECTMD is an effective way to control a large-scale structure, a large scale ECTMD was designed and constructed to control a large beam structure [34]. [PERSON] et al. [35] installed an ECTMD system on a footbridge to control the human-induced vibration, resulting in the large-amplitude vibrations being suppressed. [PERSON] et al. [36] proposed two ECTMDs to control the first two modes of a bridge mock-up and found that the maximum achievable damping is obtained when a desirable viscous damping level is selected. However, the feasibility of using ECTMDs for vibration control in a seawater environment has not been studied before. In this paper, an underwater experiment is conducted to validate the feasibility of eddy current damping in a seawater environment. A submerged pipeline is employed as the controlled structure to experimentally study the effectiveness of an ECTMD and the dynamic properties of the pipeline are obtained from dynamic tests and the finite element method (FEM).