With the drastic reduction of China's land and shallow sea oil and natural gas resources, offshore oil and gas field exploration has gradually developed into the deep sea of 1500-300 m. Traditional steel materials are no longer applicable, and fibre composite materials have become the most exploited in subsea oilfields. One of the good materials is widely used in all aspects of deep-sea oil field equipment, providing a reliable guarantee for the development of deep-sea oil and gas fields. Based on this research background, the paper discusses the characteristics of carbon fibre composite materials (light weight, high specific strength, high specific modulus, fatigue resistance, corrosion resistance, and small thermal expansion coefficient), and discusses its application in the field of subsea oilfields.

The cyclic plastic characteristics of metal materials are different from the deformation characteristics under monotonic loading, which has an important effect on the safety of structures in service under cyclic loading. However, GS-20Mn5, which is commonly used in large hydraulic machine beams, offshore platforms and large Bridges, is still lacking the studies of mechanical response characteristics under cyclic loading. In this study, the cyclic softening/hardening characteristics of GS-20Mn5 are studied by a series of cyclic loading tests under uniaxial strain control. Combined with transmission electron microscope (TEM) analysis of cyclic loading tests under typical strain levels, the microscopic mechanism of cyclic softening/hardening is discussed. The results show that the cyclic softening/hardening properties of GS-20Mn5 cast steel are sensitive to amplitudes and cycles. At smaller strain amplitudes (0.16%,0.2% and 0.3%), the cyclic hardening properties of GS-20Mn5 cast steel are rapid at the beginning of the cycle, followed by cyclic softening and then slow secondary cyclic hardening at the end. However, under larger strain amplitudes (0.4% and 0.5%), the cyclic hardening continues during the cyclic loading, and the hardening rate is bigger at the beginning of the cyclic loading and smaller at the later cyclic stage. The cyclic softening/hardening characteristics of GS-20Mn5 cast steel are related to the dislocation structure of ferrite and pearlite. Taking the strain amplitude of 0.2% as an example, the initial cyclic hardening is mainly caused by the proliferation and interaction of dislocations in ferrite. Dislocation spots and cell walls in ferrite grains are mainly caused cyclic softening at the initial stage, the secondary cyclic hardening is directly related to dislocation proliferation and entanglement in pearlite.