A marine riser that includes active boundary-layer control ("BLC") for
reducing hydrodynamic drag and vortex-induced-vibration ("VIV") caused by
currents. The marine riser includes at least one riser pipe surrounded by
a BLC sheath. Extending through the sheath is at least one nozzle,
preferably at least one pair of slot-nozzles, for discharging water into
the sea in which the marine riser is deployed. The BLC sheath may be
rotate-able to align a pair of slit-nozzles with the current direction,
or in a fixed BLC sheath, pairs of slot-nozzles may be selectively
activated based on the current direction. The BLC sheath may also include
a tail-jet or retractable tail-fin. The BLC sheath may be formed from
telescoping sections that decrease in diameter toward the bottom of the
riser and that are nested when the telescoping sheath is retracted.
Finally, a BLC system may also be implemented in a production riser
system.