These long period fluctuations drive the TDP fatigue. The other is the relatively long period (~10-30 minutes) fluctuations that occur due to global weight variations, which are due to spatial integration of density time series over the lower catenary. One is the relatively short period (~20-60 seconds) fluctuations that occur because of local weight variations as a slug-bubble passes a riser node. The new method shows that the cyclic riser response at the touchdown point (TDP) is composed of two parts. As the Orcaflex slug form approach was computationally expensive, we also developed an extrenal slug loader, which provides same level of accuracy while being computationally fast and full automated. Therefore, full time series of density at each node was input into the riser analysis using the existing capabilities of Orcaflex software. Statistical and spectral analysis of the Olga output showed that assumptions of constant slug-bubble density, and of slug behavior being uniform over long segments of riser are too simplistic. To model a more accurate and realistic representation of slugging behavior, the time series of density was extracted at each node from the multiphase flow simulator Olga. A separate slug train was propagated in five sub-segments of the riser. The slugging fatigue of a steel lazy wave riser was analyzed in Orcaflex software by modeling a repeating pair of slug and bubble with constant slug and bubble densities and associated lengths over the 3-hour simulation time. By explicitly modeling full spatial and temporal variability and randomness of slugs, the new analysis method is significantly more accurate than other methods and it captures physics of riser's slugging response. Due to local and global weight variations, the riser undergoes cycles of bending which cause fatigue.

The variation of slug and bubble density at a location with time is random, and slug characteristics can also change significantly along the riser length. Under certain flow regimes, a multiphase (oil-gas-water) flow can result in slug flow, in which a sequence of relatively high density slugs and relatively low density bubbles propagate along the flowline and the riser. floating production storage and offloading (8)Ībstract The objective of this paper is to introduce a new analysis methodology for assessment of riser fatigue due to slugging.

floating production storage & offloading (8).

Orcaflex pipeline stability example