MCS Risers

First, it may be helpful to highlight a 1996 article by P.F Pickering, G.F. Hewitt, et.al. entitled "The Prediction of Flows in Production Risers - Truth & Myth?"  See "Reference" page for a link to the full article.  The following is a quote from its conclusion:

"For multi-phase flow in risers, it is known that the vast majority of experimental data have been collected in vertical air-water systems with pipes less than 2 inches in diameter, although there are some exceptions.  Current design practice for larger diameters (such as those proposed for deepwater risers) relies on the extrapolation of the methods developed from the data gathered in the small diameter tests.  The reliability of this extrapolation is extremely doubtful and it is highly likely that the characteristics of multiphase flows are markedly different in larger diameters.  In particular, there is evidence that suggests that classical hydrodynamic slugging in larger diameters may not occur due to instabilities in the Taylor bubble.  Recent tests by a major oil company in a 12 inch diameter air-water system also support this view."

In essence, the authors question the predictability of multi-phase flow performance when extrapolating from existing data (i.e., 2-inch diameter risers) to 12-inch risers.  They point to differences in the character of the flow regimes expressed, explaining that differences in flow performance are a direct consequence of the different flow mechanics of varying underlying flow regimes.  

We agree with their fundamental premise, that tubing diameter greatly influences which gas-liquid flow regime 
is expressed, and therefore the mechanics of the system.
We suggest that as a general rule, gas-liquid flow regimes become more "coherent" as the flow diameter decreases,
and therefore more predictable.  This is largely because,          1) the Reynolds number (turbulence) decreases linearly 
with diameter, 2) the effects of surface tension increase with declining diameter, and 3) the flow becomes more axially oriented given lateral confinement.  And as is usually the case, greater coherence leads to higher efficiency (reduced gas slippage in this case).

Using an MCS riser would result in greater flow coherence within each flow passageway.  In addition, given multiple individual flow passageways, output is further smoothed out
in the aggregate, effectively eliminating severe slugging.  
And the number of open passageways could be actively managed at surface to optimize flow and to assist in kickoff.

As a general rule, while dividing the flow volume into more and more individual flows results in improved flow performance (less gas slippage and lower Turner critical rate), even dividing the flow into three or four discrete flows provides significant benefits.  For example, converting a 4-inch diameter riser into an MCS riser having four passageways of 2-inch diameter each would reduce the minimum flow velocity to maintain steady state by almost half, together with reducing the probability and extent of severe slugging.  

Given issues with downhole access, we propose a dual-riser concept that would be especially economic for offshore use.
Patching in a secondary MCS riser at the ocean floor would permit optimization of riser flow performance, while at the same time ensuring downhole access.  Even a 100-foot MCS riser section to the surface would provide significant benefits.
Offshore application provides the benefit of not having to excavate, but some onshore wells having dual completions may be reconfigured to efficiently tap the best remaining reservoir.

Other Applications