Even though the PTMD has been demonstrated to share the advantages of the impact damper, such as conceptual simplicity, moderate cost [30], high vibration control efficiency [35; 41; 47; 49] and robustness to the detuning effect [50; 51], it may also suffer from the defects of the impact damper. One major shortcoming of the impact damper is that the impacts between rigid mass and boundaries may increase the acceleration and cause a high level of noise [54; 55]. Hence, subsequent researchers  
Figure 1: Schematic of the PTMD. have proposed many solutions to address this issue. One group of solutions involve replacing the single mass with a number of smaller masses [30]. This idea has evolved to be used in new types of dampers, such as multi-unit damper [56; 57], particle damper [58; 59] and multi-unit particle dampers [60]. These dampers have been used in applications in many fields. [PERSON] et al. [61; 62] used the particle damper to enhance the seismic performance of classical columns at historical monuments. [PERSON] et al. [63] proposed a modified approach based on the discrete element method to simulate the behavior of the particle damper system and compared its vibration control effectiveness with a suspended TMD [64]. Another method to reduce the acceleration and noise problem is to add a flexible buffer zone between the mass and the boundary. [PERSON] and [PERSON] [54; 55] proposed to use sponge, rubber and plastic as the buffer material. The experimental results revealed that not only the accelerations, contact forces and the associated noise generated by collision were reduced by the buffer but also the vibration control effectiveness was also enhanced. Moreover, the buffer type design is also introduced to the particle damper. [PERSON] and [PERSON] [65] put the particles in a container covered with rubber and named this damper as the buffered particle damper. The experimental results indicated that the buffered container enhanced the energy dissipation level and reduced the impact force and noise. Due to the soft buffer zone being able to increase the energy dissipation and reduce the impact force and noise, the PTMD adapted this design. However, the viscoelastic material used in PTMD is a polymer composite and its damping ability may deteriorate in the subsea environment [66]. Therefore, it is necessary to investigate the influence of the seawater on the viscoelastic material before the PTMD is applied for subsea structures. Over the past decades, the viscoelastic material has been intensively studied. [PERSON] et al. [67] studied the crack propagation in viscoelastic materials. [PERSON] et al. [68] investigated the contact mechanism between rough surfaces of the viscoelastic material. [PERSON] et al. [69] studied the fatigue behavior of the viscoelastic material that was subjected to continuous bounding. The effects of moisture and solvent on the viscoelastic foams have also been intensively studied. In the 1990s, [PERSON] [66] found that environmental stress due to real ocean conditions have clearly been shown to degrade polymer composite materials. Later on, [PERSON] and [PERSON] [70] reviewed the effects of fluids on the deformation and durability of polymeric composites. Recently, [PERSON] et al. [71] studied the effect of seawater absorption on the viscoelastic composites. It is well known the chloride diffusion will cause damages to various materials [72; 73]. [PERSON] et al. [74] investigated the mechanical property of closed cell polyurethane foam immersed in salt water and the experimental results indicated that the flexural modulus and strength decreased by 8.9% and 13%, respectively, after 150 days of exposure to salt water. [PERSON] et al. [75] tested halloysite nanotubes-polyester nanocomposites and found that the storage modulus, tensile properties and flexural properties of polyester and its nanocomposites decreased after seawater exposure. [PERSON] et al. [76] investigated the aging of the polymethacrylimide (PMI) foam submerged in deionized and seawater.