DEEP7 2017 - Subsea 7 - page 7

Deeper water production, cost
efficiency, higher pressure and
temperature wells and more
aggressive service environments are
testing the limits of conventional
pipeline material solutions.
The increasing need for improved
performance is driving the selection of
higher specification pipeline materials,
invariably resulting in increased fabrication
and installation costs which can critically
impact the economic feasibility of field
A key driver in Subsea 7’s strategic
technology development programmes
is the need for more cost-efficient
pipeline material solutions and associated
fabrication technologies for the CAPEX
constrained oil price environment.
High-strength steels
Pipeline installation in deeper waters with
higher operating pressures can benefit from
the use of higher-strength steels exceeding
X65. Decreased pipe wall thickness,
derived through higher strength steel, can
contribute to lower material procurement
and fabrication costs. The reduced pipeline
weight decreases the installation top-
tension requirement and therefore extends
the capacity of existing Reel-Lay vessels to
even greater water depths.
Pipeline weight reduction also creates
opportunities for increased flexibility in
riser design options to further optimise
cost efficiencies. For example, a Steel
Lazy-Wave Riser (SLWR) design in X65
could be replaced with a more lightweight
Steel Catenary Riser (SCR) design in
X80. Alternatively, buoyancy module
requirements for an SLWR could be reduced
by the use of high-strength steels.
Subsea 7 has recently performed
qualification programmes for X80 pipe
in collaboration with steel tubular
manufacturer Vallourec for Reel-Lay
installation, using welding solutions
that meet the strength and toughness
requirements of DNV-OS-F101.
This development has demonstrated that
X80 pipe can be fabricated using welding
and inspection technologies similar to those
deployed for X65 pipe but with the benefit
of a reduced wall thickness.
Fatigue testing of reeled full-scale test
samples has exceeded API X’ design
requirements and therefore meets typical
Gulf of Mexico riser design requirements.
Despite the slightly higher hardness
levels which are inevitable with higher
strength steels, sour service corrosion
(SSC) testing has also demonstrated their
capability to meet the needs of mildly sour
environments. Subsea 7 can now offer
pipeline material and fabrication solutions
to support the design of offshore risers and
flowlines in X80 pipe.
Our next focus will be on the use of
even higher-strength steels, exceeding
X80, which offer the prospect of further
reductions in pipeline wall thickness
and weight. These materials bring new
challenges, not the least of which is their
less favourable weldability. In such cases,
alternative solid-phase welding processes
may offer an advantage over conventional
welding solutions.
Welded Pipe
HFW pipe comprises longitudinally welded
pipe manufactured from coiled strip and
cut into individual pipe lengths. The
use of HFW pipe can offer a number of
advantages over other pipe types including
improved surface condition and superior
dimensional tolerances.
However the major driver for the use of
HFW pipe is the reduced cost. Due to the
efficient forming method, the cost of HFW
pipe is typically 40% less than seamless
pipe and 20% less than Submerged Arc
Welding pipe. An analysis of the historic
potential cost savings over the past 20
years indicates that up to 53% of pipelines
installed by Subsea 7 could have been
manufactured as HFW pipe, resulting in
a cost saving of $260m.
We anticipate a growing number of
successful applications of HFW pipe,
including water injection lines with polymer
lining where the improved dimensional
tolerances are an added benefit. HFW pipe
of this type has been installed by both
Reel-Lay and as Bundle fabrications, and
HFW pipe has also been utilised as outer
pipe for Pipe-in-Pipe systems and as sleeve
pipe for Pipeline Bundles.
HFW pipe has some limitations, in
particular the restricted pipe size range
and low temperature toughness. Subsea 7
continues to work with pipe suppliers to
address these manufacturing limitations
and also to extend the range of qualified
applications, including for sour service.
Corrosion-Resistant Alloy
The use of CRA mechanically lined pipe
provides a highly cost-efficient alternative
to the use of metallurgically clad pipe for
many applications requiring improved
corrosion performance.
Working closely with manufacturer Butting,
Subsea 7 has pioneered the Reel-Lay
installation of mechanically lined BuBi®
pipe. Internal hydrostatic pressurisation of
the pipeline during reeling and unreeling
is used to completely eliminate the risk
of liner wrinkling. The use of BuBi® pipe
is now well established for production
flowlines installed by Reel-Lay and for
Bundle fabrication.
BuBi® pipe has also been deployed for
SCR applications where fatigue loading is
not critical. However, recent qualification
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