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OFFSHORE E-NEWS
May 2000
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NEW DESIGN HYBRIDIZES FPSO WITH DRILLING
CAPABILITY – CREATING AN FPDSO
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The
Floating Production, Drilling and Storage Offshore (FPSDO)
design, when outfitted with a tension-leg deck (TLD) system,
can provide maximum flexibility while substantially reducing
or eliminating the need for subsea trees, manifolds and risers.
The ABS Surveyor magazine, in its most recent issue, covered
the discussions surrounding this emerging technology. Here
is the reprinted article.
In the
changing world of FPSOs, D stands for "drilling."
One of the latest developments in FPSO technology is a proposal
for the FPDSO, an FPSO that incorporates drilling capability
through a moon pool in its ship-shaped hull.
Brazilian
national oil company Petrobras has begun studying this new
technology in its PROCAP 3000 R&D program.
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The company is
widely respected for its advances in offshore technology. Its most
recent innovations include development of synthetic mooring lines,
development of the DICAS system mooring system (see Surveyor
March 1998), and deployment of what is currently the deepest FPSO
project, in 1,853 m water depths in Brasil’s Campos Basin. Petrobras
has selected FPSOs for eight of its last ten development projects.
The next four fields are planned for development with three FPSOs.
As these projects are to begin in the near future, Petrobras is not
considering an FPDSO; the technology is still under study.
Even though
the FPDSO concept has not yet been fully developed, in the fast-moving
world of offshore technology, a next step in its evolution has already
been proposed. In a paper delivered to the October 1999 Deep Oil
Technology conference in Stavanger, Norway, Monaco-based Single
Buoy Moorings (SBM) proposed an FPDSO unit incorporating an innovation
called the tension-leg deck (TLD) system.
In the FPDSO
with TLD, drilling facilities are mounted above the moon pool on
the FPDSO, which also includes process, oil storage, offloading
equipment, and accommodations. Mounted in the moon pool, above the
water line, would be the tension-leg deck, which is a flat deck
holding dry production trees, the blowout protector and other equipment.
The TLD is fixed
to the seabed by tendons that restrain the deck against heave. Deck
load is counterbalanced by a system of weights.
Unlike for a
TLP, tension on the tendons would not result from restrained hull
buoyancy. The TLD system counters its deck load using weights that
are hung over the side of the unit, attached to wire and chain that
run through sheaves mounted on the FPDSO deck. The weights are suspended
approximately 100 meters below sea level, to avoid the effects of
wave action and to damp pendulum motions. Telescopic and flexible
joints would account for the relative motions between the FPDSO
and the TLD.
The TLD concept
was originally proposed for development of a West African field,
using a ULCC as the FPDSO hull. It sounds like a complex system,
but according to SBM, will substantially reduce or eliminate the
need for subsea trees, manifold and risers. The question with this,
as with any other new proposal for offshore development is: does
anybody want to be the first?
To
see other stories from the most recent issue of ABS Surveyor magazine,
click here.
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