## 1 Introduction  
The consumption of oil and gas is exponentially increasing throughout the world. The limitation of crude resources of oil and gas on land makes the resources on the ocean floor more and more attractive. In order to achieve the offshore oil and natural gas extraction, many different types of production systems including conventional fixed platforms, tension-leg platforms, compliant towers, semi-submersibles and floating production facilities, have been employed. While these constructions vary mainly in the ways in which their platforms are positioned over sub-sea wells, the risers, both drilling risers which are used to guide a drilling stem and conduct the drilling fluid upwards, and production risers which raise the extracted oil or natural gas to a floating platform [1; 2], are indispensable. In offshore engineering, the material usually employed for structural components is high-grade steel. Nowadays, the utilization of fiber reinforced polymer (FRP) composites is gradually increasing due to their superior mechanical properties, low density and other desirable properties, such as better thermal insulation, and corrosion and fatigue resistance [3; 4]. Therefore, it has been widely recognizedthat using FRP composites instead of steel for risers will lead to the significant reduction of operational costs and maintenance costs of the whole offshore production platform systems [5; 6; 7]. In order to apply FRP composites in offshore risers successfully, their durability in sea water is crucial. [PERSON] et al. [6] found that none of the long-term properties of carbon fiber reinforced polymer (CFRP) composites experienced any significant variations after exposure to pure water and seawater at different temperatures. On the other hand, many researchers [3; 4; 8; 9] found that the long-term tensile strength of a CFRP reduced to about 80-95% of its short-term values. For thermoplastic composite carbon/polyether ether ketone (PEEK), the transverse strength decreased obviously in boiling water [10] and therefore, the maximum service temperature of thermoplastic FRP composites has to be well below the glass transition temperatures of their polymer matrices. Moreover, in order to obtain reliable results using a numerical model, constitutive laws and damage models for FRP components have to be selected to optimize the accuracy of the numerical predictions [11]. Besides the research of mechanical properties of FRP composite materials in seawater, the responses and performances, including load distributions, fatigue, and vortex-induced vibrations (VIVs) of composite risers under environmental and functional loads have also been investigated [12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25]. The improvement of composite risers in performance over a pure metallic mandrel had been proved through a scaled-down test [23]. For the global response, the tension force decreases with an increasing water depth and the maximum bending moment occurs on the bottom, followed by the joints at the sea surface in general [12; 13; 14; 16; 20; 22]. The axial tension and the bending moment throughout an entire composite riser are smaller due to its lower overall weight compared with steel risers [14; 16]. The excellent fatigue resistance of FRP composites contributes to the better fatigue durability of composite risers, which was confirmed by previous studies [13; 14; 16]. More specifically, the structural composite body of the composite riser joint is likely to have an infinite fatigue life [13; 14; 16], but still may vary depending on the choice of constituent materials and the manufacturing process. In terms of the VIV of composite risers, it is found that the fundamental frequency of a composite riser with the same boundary condition and geometry was higher than that of a steel riser [18] since the composite riser was much lighter and the value was relatively small [13; 14].