Fatigue Analysis of Tether Chain in Hybrid Risers

Abstract

Hybrid risers represent an excellent way to isolate the riser from most of the host vessel motions and thereby limit riser fatigue. A common arrangement features the riser supported by a buoyancy can via a tether chain. The tether chain is a cheap simple way to make the connection while providing flexibility for installation. However, in service the tether is under very high tension, and the chain is not really flexible in the face of small amplitude fatigue loads. The friction effectively “welds” the chain together. Moment and torque input to the system by first order vessel motions and vortex induced vibrations are carried through the chain and induce fatigue loading in the links. Analysis of the chain can be problematic because the determination of the detailed stress in the chain requires a refined FEA model with contact element between the links. From the global sense the analysis may require running hundreds of sea-state realizations in the time domain and the vortex induced vibration (VIV) assessment of thousands of current profiles. In this paper an efficient numerical method is described to rigorously determine fatigue damage at locations throughout the chain.

Parametric Analysis of an Energy-Harvesting Device for a Riser Based on Vortex-Induced Vibrations

“…A marine riser is a long thin tubular structure connecting the water surface platform and the subsea wellhead. In ultra-deep water, marine risers often experience a broad range of motion under the action of adverse environmental forces such as vortex shedding, currents, and wave forces [1]. Vortex-induced vibration (VIV) caused by vortex shedding leads to an accumulation of riser fatigue damage.…”

“…Vortex-induced vibration (VIV) caused by vortex shedding leads to an accumulation of riser fatigue damage. With increasing water depth, VIV becomes the main factor in marine riser failure [1,2].…”