Computational Tool for the Dynamic Analysis of Flexible Risers Incorporating Bending Hysteresis

Non-bonded flexible-pipe risers provide a structurally compliant solution in offshore floating production systems for the recovery of oil & gas. The bending stiffness of the flexible pipe is an important property in designing the riser system to safely withstand extreme and fatigue loading conditions. These risers have two fundamentally different bending stiffness properties that depend on if the riser system is pressurized or depressurized. A depressurized riser has a comparatively small linear bending stiffness. Most riser designs apply this stiffness as its produces conservative (large) bending responses. In recent years, the bending response predicted from the depressurized bending stiffness has proven overly conservative and there has been an increasing demand to consider the larger hysteretic bending stiffness of the pressurized riser. The objective is to reduce the conservatism and achieve an approved safe design. Recent developments have advanced the modeling of flexible riser bending with hysteresis and this capability has now been incorporated into an industry standard finite-element riser analysis tool. This paper describes the background of hysteresis in relation to non-bonded flexible pipes and outlines the methodology of the riser motions software that incorporates bending stiffness with hysteresis. Riser systems where the dynamic bending response is critical to the success of the design are the main applications that will benefit from this new technology. Examples include: i.) The dynamic bending response at the seabed touchdown of a deepwater catenary riser. ii.) Bending at an interface with the riser hang-off or subsea tie-in.