In an article for Star Trek : The Magazine ("Designing the Ti'Mur"), Doug Drexler; Senior Production Artist for ST: Enterprise described the circular hoop feature as warp nacelles:
'Ah,' I thought, as I mulled over the Vulcan ship design question for
Enterprise. 'This is the perfect place to fit the hoop ship.' "The
script stated that Trip would be ga-ga over this Surak-class starship.
After laying eyes on it, there was no question in my mind that he
[Matt Jeffries] went to bed that night puzzling out the exotic shape.
The other change involved eliminating any physical connection between
the main body of the ship and the hoop, so they are actually separate
elements. "We liked the defiance of conventional structural
support," Drexler explains. "It makes the Vulcans look like they
control powers beyond human ken. This was true of the original TV
Enterprise. Those struts that support the nacelles defy what we
understand today. It says that these people are masters of
technologies that we don't yet understand. It speaks volumes for the
technology at play."
- JULY 2002 ISSUE 39 STAR TREK: THE
MAGAZINE
Rick Sternbach; Senior Production Illustrator for ST: Enterprise described the circular hoop feature as an annular warp ring:
Abandoning the preliminary design lines which echoed the design of the
long range shuttle, Sternbach arrived at a final version in September
1991 and his notes on the final design read, "Vulcan Ship V Variant of
Annular; No windows or other details; basic body shape." Later he
recalled, "The commandeered Vulcan ships in "Unification" followed a
pretty familiar approvals flow of initial idea, producer changes, and
final concept to go to the model maker, in this case Greg Jein. Since
we hadn't seen much in the way of Vulcan ship technology, beyond the
motion picture shuttle, it was a bit daunting to home in on a true
Vulcan style, and I can't say I'm terribly happy with the final
result. Hindsight always invokes a desire for more design time, which
might have helped. Perhaps different proportions on the annular warp
ring, more curves, and more positive-negative surface detailing." - Star Trek: The Magazine Volume 3, Issue 8, page 104
Michael Okuda, Art Supervisor for ST: Enterprise has offered an in-depth treknobabble explanation of how the rings create a warp field:
One of the most radical experiments in early Earth starship design was
the Enterprise XCV. Unlike the traditional nacelle-and-saucer
configuration, the XCV uses an annular propulsion system, based on
Vulcan vehicle designs. This ship however, employed cyclotron
accelerators to create a high-energy proton flux. The protons circled
through the massive outer rings of verterium gallenide segments,
generating a symmetrical subspace field. Each of the two coleopter
ring structures contained two counter-rotating cyclotrons. The
cyclotrons in each ring operated slightly out of phase with each
other, generating the propulsive field imbalance that carried the ship
through subspace at warp speeds.