Some Problems Of Fatigue Resistance Design Of Flexible Risers Used In Deep Water

An unbounded flexible riser is the key equipment in oil and gas production in deep water, connecting the floater with wells located on the seabed. When the flexible riser is subjected to periodic and alternative loads, it is the metal armored layers that easy to take place fatigue failure. The bending behavior shows high nonlinearity due to contact and friction between different metal armored layers. Because of interlayer contact and friction, the fatigue-resistant design and analysis involve many problems. The fatigue life of the flexible riser is the key index to assess fatigue property, which determines directly the safety and reliability of the flexible riser in the operation. The fatigue-resistant design of flexible risers is based on the fatigue analysis. Based on the finite-life design criteria associated with nominal stress method, the fatigue calculation model and process are proposed to predict fatigue life of the unbounded flexible riser. In order to better design the structure of flexible riser with longer service life, some problems is as follows:It is impossible to apply the classical beam theory to describe the nonlinear bending response of flexible riser due to its complex structure and severe environment. The analytical and numerical models for nonlinear bending stiffness of flexible riser are established and compared with testing result, verifying the proposed FE model is validated. Based on the result obtained from the numerical model, a fatigue loading analysis model is developed with consideration of nonlinear relationship between bending moment and curvature.Aim at limitations of the current theoretical models for bending stress of a helical wire, a three-dimensional finite element model of a helical wire wound around a cylindrical core is proposed and to determine the local stress of wire under specific curvatures. The published analytical models including seven species are classified into three kinds. The results predicted by the analytical models are compared with them obtained from the numerical model, and to determine the validity of the analytical models.And the influence of width-thickness ratio and lay angle of wire on the validity of the analytical models is discussed.This research depends on the "Key techniques research of offshore flexible pipes" project supported by National High Technology Research and Development Program of China (863) in the twelfth five-year plan. Taking a flexible riser with 8 inches inner diameter used in 1500m deep-water as an example, the fatigue life is calculated. Considering the influence factors for the fatigue strength of flexible riser, a modified S-N curve for fatigue-resistant is established. The influence of various nonlinear factors and mean stress on the fatigue property is discussed emphatically.