For the evaluation of extreme loads and the assessment of the fatigue life of flex risers, use is made of discrete element models. Most of the models describe the fluid forces by means of empirical formulations using coefficients from 2-D cylinder tests. Recent research has shown that the fluid force modelling should incorporate lift forces perpendicular to the incoming flow. Moreover, model tests on typical flex riser sections have proven that the 2-D cylinder formulations are not applicable and may lead to significant errors. In this paper a new fluid model formulation is presented. This formulation is derived from both model tests and theoretical vortex simulations. Results of systematic drag tests for riser sections are discussed. Capabilities of vortex simulations are evaluated.
Flexible risers have proven to be a key item for cost effective floating production systems. Most problems involved with the application of rigid risers, such as restricted horizontal floater motions and heavy and costly heave compensators, can be eliminated with the application of flexibles. Many flow lines have already been used in mild environments and for limited exposure times. In harsh environments such as the North Sea, the large diameter and relatively stiff flexible risers for permanent floating production (Fig. 1), require extensive design and engineering analysis. Riser systems have to be evaluated for extreme conditions as well as fatigue life. To this end analysis of the dynamic behaviour of the riser system is of prime importance. Typical items of investigation are:
interference with mooring or other risers;
minimum bending radii;
flow induced vibrations.