Research On The Behavior Of Lazy-wave Flexible Riser In Catenary Anchor Leg Mooring System

Catenary anchor leg mooring system has a lot of advantages, such as a simple structure, good direction and low-cost. It not only can be used for the development of marginal oilfield and trial production of early oilfield but also be used as an oil transfer station at sea and oil unloading terminal. Riser is an essential component to transport oil in the system, Which determines the system running or not. In this paper, unbonded flexible risers are applied to catenary anchor leg mooring system and a coupling analysis model is established. Mechanical properties of flexible risers, layout of the lazy-wave type and the sensitivity of factors in this system are researched. Effects of motion of floating bodies on the responses of risers are analyzed. Both Strength evaluation and fatigue analysis are also finished by Orcaflex software.The axial stiffness curve and bending stiffness curve are obtained by the analysis of the mechanical properties of the riser using ABAQUS software.At the same time, hydrodynamic data of the buoy and oil tanker, such as RAO, the second-order drift forces, added mass and added damping are obtained by hydrodynamic analysis using SESAM software. After that, lazy-wave style risers are designed according to the theory of catenary and the coupling model of catenary anchor leg mooring system is finished using Orcaflex software.Based on this model, some researches are carried out. Some conclusions are shown:Components subjected to axial loading are mainly two tensile layers of flexible riser.Some factors such as the hand-off angle, the floats arrangement and the fluid properties in the riser have effects on effective tension and curvature of the riser. Under normal operating conditions, the case of ship ballasts is more dangerous than that of a fully loaded. Effective tension and curvature of the riser are seriously affected by buoy swaying, heaving and rolling as well as tanker surging and second-order motion. Through analysis and discussion, the riser’s tension, curvature and stress meet the specification requirements. The maximum effective tension occurs at the far site and maximum curvature occurs at the near site. Fatigue life of flexible risers in this system is determined by the inner tensile layer. The point of maximum fatigue damage is closed to the location at which the riser is connected to the end of bend stiffener. And the effect of the tension stress on fatigue damage is more serious than that of bending stress for the flexible riser in this system.