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Flexible risers
Flexible dynamic risers are an enabling
technology for producing oil and gas from offshore wells using floating
production systems. Flexibles are manufactured in accordance with API
specification 17J and are produced by three companies today: CSO,
Wellstream, and NKT. A new startup company for flexibles using composite
materials is Deepflex. The main advantages of flexibles are:
- Large experience database (well proven)
- Flexible (robust in high dynamic environments)
- Easy to install and retrieve (low risk)
- Can be pipe-in-pipe or heated in the near future
- Corrosion resistant and reusable
- Can be installed along with the associated flowline.
Flexibles
are designed in approximately 12 different configurations. Eleven of
these are used today. The most common configurations are the "lazy wave"
and "lazy S."
One of the largest deepwater (800-1,000 meter
depths) field developments today using flexible risers is Petrobras'
Barracuda Caratinga project, where Halliburton (Wellstream) will deliver
more than 80 risers in 2003/2004. Experience from previous large riser
projects such as Troll B and Troll C (Coflexip Stena Offshore and
Wellstream) are actively incorporated.
The flexible pipe structure
itself is standardized with minor differences from manufacturer to
manufacturer. At the FPS, the riser is terminated through I/J-tubes, and
protected from overbending by a bend-stiffener. The buoyancy is either
discrete elements of syntactic material or a buoyancy tank (mid-water
arch) of steel or elements of syntactic material.
Near the seabed,
the riser is sometimes protected against impact and abrasion with half
shells of polyurethane. Bend restrictors may be used at the
end-terminations to protect the pipe form over-bending during
installation. Major manufacturers of these ancillary components for
flexibles include CRP, Orcina, and Balmoral.
Remarkable advances
have been made in the development of un-bonded flexible pipe structures
from first use offshore about three decades ago. As early as 1990,
flexible pipe had been installed for dynamic application in water depths
to 1,000 meters. Today, flexible risers are qualified for more than
1,500 meter depths, and are expected to reach 3,000 meters by the year
2002.
Further development programs are in place for reducing
weight (by employing composite tensile armor), increasing the diameter
envelopes for deeper water (targeting 12 in. in 2,500 meter depths by
the year 2002), and improving flow assurance properties (by heating,
advanced insulation materials, and smooth bore design). The price of
flexibles has dropped significantly over the last 10 years due to
increased competition, manufacturer development, and the introduction of
SCRs.
Figure 1: Flexible risers
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