Vortex-Induced Vibrations of Steel Catenary Risers and Steel Offloading Lines due to Platform Heave Motions

Abstract

Abstract This paper deals with fatigue of Steel Catenary Risers (SCR) and Steel Offloading Lines (SOL) caused by heave-induced intermittent Vortex-Induced Vibration (VIV). The frequency domain models in current use have considerable limitations for such systems and currently available time domain models are computationally expensive. The paper explores alternative time-domain analysis techniques using wake oscillator models and a vortex tracking model which gives much of the realism of Computational Fluid Dynamics (CFD) techniques without the associated extravagant runtimes. Results are compared for deep water systems in GOM and Brazilian conditions Introduction SCRs and SOLs are increasingly favored for deep water developments because of reduced cost compared with flexibles. Current applications are either in relatively mild conditions (e.g. West Africa) or hung off platforms at Spars, TLPs or semisubs. However a recent study showed that it is feasible to connect SCRs to a spread-moored FPU monohull in a field offshore Brazil [1], and we may expect similar proposals to become increasingly prevalent. Heave-induced VIV was an unexpected finding in the model test in a 1998 JIP of the Highly Compliant Risers by PMB [2], and since then it has gained increasing attention but remains poorly understood. Our aim in this paper has been to investigate advanced techniques to capture the heave-induced VIV. A number of candidate analysis techniques have recently become available. These techniques, which are as yet not fully proven, are outlined in the next section of the paper. We then consider the behavior of two example systems - one SOL and one SCR. We should point out that no attempt has been made to optimise either system, and the design conditions considered are only loosely based on previous experience and have been simplified for convenience in this study. The issue is not whether fatigue damage from heaveinduced VIV is acceptable for these two particular systems, but rather, how fatigue damage from this loading regime compares with that from others. Fatigue damage results are therefore presented in normalised form. Heave-induced VIV - Flow Characteristics When a steel pipe is suspended in a catenary configuration from a floating body in waves, the near-horizontal parts of the pipe will be forced to oscillate vertically in phase with the floating vessel and at much the same amplitude. Typical oscillations may be +/- 1m amplitude at 10s period. As the line is dragged upwards through the water, a wake is generated beneath it, vortices are shed and the line will tend to oscillate normal to its axis and to the direction of motion. The motion then reverses and the line begins to fall. The wake collapses and re-forms above it, again generating oscillatory forces. Important characteristics of the phenomenon are that the flow never achieves a steady state, and the pipe is continually swept back through its own turbulent wake. This means that we can immediately rule out analysis by means of frequency domain packages such as SHEAR7 or VIVA, since these are applicable only to steady flow conditions in which the wake is continually carried away. A time-domain approach is essential. Time Domain Modeling Techniques The analysis package used for the present work was OrcaFlex, a time-domain package widely used in the offshore industry for modeling the behavior of risers, cables and the like under wave loading and forced motion from ships, buoys, TLPs, etc. A 'VIV Toolbox' has recently been released which adds VIV analysis capability. The toolbox includes links to existing frequency domain packages together with a number of time domain options which we now disc