An unbonded flexible pipe is one of the most important equipment in offshore engineering, transporting oil and gas between the floater on the sea and the well located on the seabed. Flexible pipes consist of several metallic helical-reinforced layers and internal and external polymer sheaths, and relative slip between the layers is allowed, so that the structure show high axial stiffness and radial stiffness associated with relatively low bending stiffness. During the operation and installation, the flexible pipe will be subjected to complex and coupled loads such as tension, internal pressure, external pressure, torsion and bending, which lead to multiple structural failures. This chapter will present the current theoretical models and research progress to effectively evaluate the response of such composite structure, providing reference ideas for the engineering design of the flexible pipes.

Flexible pipe is one of the most important types of equipment applied in the deep-water development of oil and gas and deep-sea metal mining. The carcass of an unbonded flexible pipe with a typical interlocked structure prevents buckling failure under external hydrostatic pressure. The process and principle of carcass layer deformation are described, and a three-dimensional finite element model with solid-shell elements is developed to simulate the cold forming process of a metal strap subjected to a series of rollers. The deflection and deformation behavior in the bend-rolling and interlocking process are investigated, and the residual stress due to deformation is calculated. Taking the carcass layer of a 4-inch internal diameter flexible pipe as an example, a three-dimensional finite element model of the carcass layer loaded with external hydrostatic pressure is developed. The buckling collapse of the carcass layer is evaluated considering different initial imperfections, including residual stress. The results show that the critical pressure can be 60% less than under ideal conditions when the geometric imperfection, material nonlinearity and residual stress due to deformation are considered, which indicates that the effect of residual stress on buckling collapse cannot be ignored. The numerical model and results provide an efficient method and basis for nonlinear buckling analysis and a collapse-resistant unbonded flexible pipe design for industry.

In this study, the Mo/C276 coating was deposited on Q690E steel by high velocity oxy-fuel spraying (HVOF), and the coating was treated with an organosilicon sealer. Further, the corrosion behaviors of the coating in the simulated deep-sea cold spring environment with hydrogen sulfide and high pressure were studied. The results show that when the oxygen flow rate is 220 NL/min, the coating has the lowest porosity of 1.71% and excellent mechanical properties. Combined with the micromorphology and elemental analysis of the coating, it was assumed that the Fe element generated by the corrosion of the Q690E substrate migrates to the surface of the coating. The corrosion tests in the simulated deep-sea cold showed that before the failure of the coating in the edge and corner areas, the corrosion rate of the coating was less than 0.002 mm/a, which could meet the long-term use requirements in the real cold spring environment.

In deep-sea oil and gas development scenarios, deep-sea dual-channel connectors often face the risk of seal failure due to internal and external temperature difference loads. To address this issue, this paper systematically establishes equivalent heat transfer models for the key parts of the connector based on the third-type boundary condition. On this basis, the quantitative correlation between the equivalent thermal conductivity, composite heat transfer coefficient and temperature of each part is explored. Using the finite element numerical simulation method, the transient temperature field of the connector under three working conditions (heating, cooling and temperature shock) is simulated and analyzed, revealing the temperature distribution characteristics and temperature change trends of the maximum temperature difference of each key component of the connector; combined with thermal–structural coupling simulation, the temperature field is converted into static load, to determine the behavior of the contact stress on the sealing surface under different temperature–pressure coupling working conditions; in addition, by placing the test prototype in a high-low temperature cycle chamber, the seal performance tests under pressurized and non-pressurized working conditions are carried out to verify the reliable sealing performance of the connector under variable temperature conditions. The results of this paper provide comprehensive theoretical support and an experimental basis for the thermodynamic optimization design of deep-sea connectors and the improvement of the reliability of the sealing system.

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