Influence of internal and external hydraulic pressures on torsional resistance of steel-concrete double skin composite tubes

Abstract

In contrast to onshore engineering, the application of steel-concrete double skin composite tubes in deep-sea oil and gas pipelines involves exposure to high external hydrostatic pressure and internal hydrocarbon pressure. The composite effect under dual hydrostatic pressures exhibits unique characteristics distinct from those observed in terrestrial structures. The applicability of traditional land-based engineering design method to deep-water engineering remains uncertain. Therefore, this paper investigated the influence of dual hydraulic pressures on torsional behavior of concrete-filled double skin steel tubular (CFDST) members. The verified finite element (FE) model was employed to reveal the full-range torsional mechanism induced by hydraulic pressures. The mechanism analysis influenced by dual hydraulic pressures indicates that the existing pressures significantly enhance the confinement interactions between the double-skin steel tubes and the concrete, leading to an increased torsional resistance to the entire CFDST member. Subsequently, effects of material strengths of outer tube (f_yo), inner tube (f_yi) and concrete (f_c), diameter-to-thickness ratios of outer (D_o/t_o) and inner tubes (D_i/t_i), hollow ratios (χ), water depth (H) and internal pressure (p₃) are examined through parametric study, which reflects that higher hydraulic pressure will reduce the capacity of CFDST members with large hollow ratios, e.g., the capacity of χ = 0.867 versus that of χ = 0.714 at water depth H = 3200 m, and the internal pressure has a slight effect on the torsional performance. A new calculation approach for evaluating torsional resistance is introduced, integrating the effects of external and internal hydrostatic pressures, thereby informing the application and structural design of steel-concrete double skin composite tubes in subsea engineering.