In this paper, the new procedures to assess the bending performance of offshore unbonded flexible risers have been proposed. The multilayer bending responses derived from time-domain global dynamic analysis, instead of the traditional local model analysis, may be adopted in the assessment. This can help to save the computation time and increase the accuracy. In addition, the effects of bending hysteresis behavior are considered in the flexible risers' global dynamic analysis and nonlinear collapsed hypothesis loop model is assumed in order to calculate the bending damage. Further, the sensitivity analysis is also carried out to examine the influence of various design parameters, such as critical curvature, slip stiffness and etc. The purpose of this research is to tentatively propose new assessment procedures to standardize the dynamic response analysis of flexible pipes in offshore engineering, which could be part of guidelines accepted by various class societies.
The offshore steel risers' in-service may undergo severe corrosions. An effective way to alleviate the problem is to use unbounded flexible risers (Kyriakides and Corona, 2007). Due to the harsh environment, the safety issue seems to hinder its wide application in offshore engineering (Kardomateas and Simitses, 2005). Unbonded flexible pipes have been employed since the 1970s by the offshore oil and gas industry to transfer oil and gas from offshore wells to floating units (or between floating units), inject water or gas in offshore wells, or control and monitor them. When these pipes are used to transport fluids from the seafloor to production or drilling facilities (or from these facilities to the seafloor), they are called flexible risers. As illustrated in Fig. 1, the typical structure of unbounded flexible risers is mainly made of several steel and plastic concentric layers with low bending stiffness but relatively high radial and longitudinal stiffness. Three types of metallic layers are turned out to mainly withstand the imposed structural loads (Berge et al., 1992): inner carcass mainly provides strength against external hydrostatic pressure and crushing loads during installation operations; pressure sheath provides resistance against the hoop stress caused by internal pressure; tensile armors provide strength against the axial stress caused by internal pressure and by external load. Feret and Bournazel (1987) based on experimental tests, proposed the first model that presented the bending behavior of unbonded flexible risers.