Innovation enhances deepwater flexible pipe life
Managing riser friction
damage, corrosion cuts repair costs
Judimar Clevelario, Helio Marins
Marcio
Albuquerque, Wellstream International Ltd.
As
flexible pipe applications move into deepwater and ultra deepwater, attention
is focused on enhancing service life, particularly with respect to corrosion
and wearing. In Brazil’s offshore Campos basin, Petrobras and Wellstream
International Ltd. have addressed both issues for unbounded dynamic flexible
risers and static flowlines.
The
first means of enhancing service life is the use of a latch sleeve insert to
address damage in flexible risers caused by friction between the outer polymer
sheath of the riser and the I-tube at the vessel hang-off. The second is the
use of anode clamps to protect flexible pipe endfittings against corrosion.
Petrobras’
original I-tube latch sleeve had a metallic “trumpet” that interfaces directly
with the bend stiffener within the I-tube for a metal-polymer contact between
the flexible riser outer sheath and the latch sleeve.
This
metal-polymer contact, when subjected to cyclic bending, tension, friction, and
pressure loads from the FPU’s dynamic response, resulted in excessive wear and
abrasion damage of the riser polymeric outer sheath. In some cases, the outer
sheath was damaged to the point that the outer tensile wires were exposed to
the marine environment and subject to failure from corrosion and metal-metal
contact. Furthermore, Petrobras found that outer sheath wear damage was
critical on FPSO installations. Damage came from axial displacement of the
riser inside the I-tube causing outer sheath (polymer) friction against the
internal surface of the latch sleeve (metallic).
Wellstream
developed a new latch sleeve design to eliminate excessive wear of the riser
outer sheath material within the contact region. The sleeve was designed for a
20-year dynamic service life in a typical Campos basin application.
The
latch sleeve mechanism has an internal replaceable insert made of similar
materials to those used in typical bend stiffener moldings. The polymer-polymer
contact is similar to that between the riser outer sheath and the bend
stiffener. The new latch sleeve still has a trumpet design for a smooth
transition of the flexible riser within the contact region.
The
design was based on the following:
- In-service survey data
- Petrobras’ requirements
- Bend stiffener supplier design and field experience
- Full-scale dynamic fatigue tests and subsequent detailed dissection.
The
most cost-effective solution was a split polymeric replaceable insert made with
the same polyurethane material as the bending stiffener, assembled on the
internal diameter of the latch sleeve adapter. The polymeric insert prevents
direct metal-polymer contact, which is the major cause of wear.
In
developing the design, the design team analyzed concerns such as design
performance, assembly, installation, and maintenance. Upon completion, the
design was submitted to the customer, and following approval, was installed on
the Marlim and Marlim Sul fields. The latch sleeves were scheduled for
installation on all Wellstream flexible pipes using the I-tube latch sleeve
mechanism on Petrobras’ FPSOs.
The
design team developed a non-linear finite element model that reproduces the
contact among the bend stiffener, latch sleeve, and riser to confirm
qualitatively that the compression load magnitude at the contact surface of the
pipe outer sheath and inner bending stiffener has the same magnitude as the
pipe outer sheath and inner latch sleeve insert.
Analysis
results confirmed that the contact pressure between the pipe outer sheath and
inner bending stiffener, and pipe outer sheath and inner latch sleeve insert
have the same magnitude. The team concluded that using the same bending
stiffener material should avoid excessive wearing damage.
It
is important to guarantee that wearing occurs in the polymer insert before it
does in the outer sheath. An abrasion test results-graph of high and medium
performance polyurethane (in accordance with DIN 53516) shows that
medium-performance polyurethane is more susceptible to wear than
high-performance polyurethane, nylon (PA-11), or HDPE - all typical riser outer
sheath materials. Therefore, medium-performance polyurethanes were used in the
new latch sleeve.
Anode clamp
The
second area addressed by the design team was corrosion. Offshore equipment is
subject to severe corrosion. Although equipment often is manufactured of
special materials to prevent or minimize corrosion, cathodic protection still
can be required.
Petrobras
expressed concern about ancillaries on several installed flexible pipe. One of
the most critical ancillaries is the endfitting, a metallic device that
connects the flexible pipe to a pipeline end termination (PLET) or a pipeline
end manifold (PLEM).
The
starting point of the project was the development of cathodic protection that
did not require removal of the endfitting. Removal would increase the
installation cost and possibly halt production.
Wellstream
developed a metallic structure with clamps that can be installed by an ROV
without removing the endfitting or stopping production.
The
result was an anode clamp consisting of a metallic structure with clamps and
clamp grooves for surface cleaning to guarantee a contact between clamp surface
and surface to be protected, with aluminum anodes attached.
In
June 2003, the first anode clamp was installed on the subsea endfitting
abandoned on Barracuda field, near RJS-458, in 800 m (2.625 ft) water depth. In
May 2004, Petrobras requested a new anode clamp design for installation on an
endfitting near the RO-09 well on the Roncador field in 1,800 m (5,905 ft)
water depth. The second anode clamp was installed in August 2004.
A
major concern during the anode clamp design was guaranteeing electrical contact
between the clamp and the endfitting. Primarily, the electrical contact is made
through the clutches. Internal grooves were designed to aid the surface
cleaning and to enhance contact capability. Additionally, contact pins on the
equipment were designed to give extra contact between the anode clamp and
endfitting.
Another
issue was to make installation easy. The design team created a clamp that could
be installed by an ROV; no special vessel is required. Furthermore, production
need not stop during installation. Before the actual clamp placement, an ROV
cleans the endfitting surface with a steel brush for better electrical contact.
Then, the anode is set over the fitting and clamped into place. Then, the
contact pin is set.
Wellstream
carried out testing that led to the conclusion that the system could be used on
flexible pipes. Inquiries revealed that all of the materials required were
available locally.
To
confirm the function of the anode, the electrical potential of the endfitting
was measured before and after the clamp was installed. The measure indicated
the cathodic protection was in place.
Acknowledgement
The
authors thank Petrobras S.A. for contributions to both projects.
11/01/2007
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