Kjell Edvard Apeland, Jan Olav Berge,
Richard Verley - Statoil ASA
Michael Armstrong, Neil Woodward - Isotek Electronics Ltd.
The Pipeline Repair System pool (PRS
pool) is a joint development between Statoil and Hydro to provide repair and
construction support for the large oil and gas pipeline transportation system
on and from the Norwegian Continental Shelf in the North Sea.
The development is funded by a
consortium of companies sharing costs in exchange for access to the equipment.
In 1987 Statoil was appointed to manage and operate the system and since then a
continuous development has been undertaken. Currently PRS is the main repair
contingency for approximately 10,000 km of subsea pipelines with dimensions
ranging from 8 to 44 in. and water depths down to 600 m. This coverage is now
being extended to water depths of 1,000 m as new pipelines come onstream.
The PRS is a comprehensive suite of subsea
pipeline construction and repair tools, from isolation plugs and cleaning tools
to large manipulation and installation frames, and welding habitat enclosures.
The repair methods range from applying support clamps to weakened sections to
cutting away damaged sections and replacing with new pipe, joining to the old
by either mechanical connections or hyperbaric welding.
The PRS pool has over the last few
years also invested in technology for remote hot-tapping into subsea pipelines,
the objective being to provide technology for development projects which the
commercial supplier market does not provide on short notice.
In order to achieve this, new unique
equipment and welding technology has been developed and qualified with the
objective to provide a fully remote operated system without the need for
diver-assisted tasks.
Pipeline repair by
welded sleeve technique
Traditional hyperbaric welding
techniques involve the use of precision machining of the pipe ends and
performing butt welds using the GTAW (gas tungsten arc welding) process. This
involves precision alignment that can be very demanding (particularly on the
second end and especially for large-diameter pipes).
The new approach avoids the need to
achieve butt to butt closure and limits the requirement on precision alignment
by threading a sleeve (slightly oversized to the pipe) over one end and drawing
it back over the two pipe ends to be joined and making the welded join between
the end of the sleeve and the pipe using a GMAW (gas metal arc welding) fillet
weld. This technique is used on relatively small-diameter onshore pipelines and
is part of the tools of the plumbing trade, but it has not been deployed subsea
for production pipeline repair.
The development described in this
paper is intended for use for repair of up to 44-in. pipelines down to depths
in excess of 1,000 m.
Such a method is not covered directly
in the existing regulations and codes of practice, although some work has been
performed to establish fitness for purpose assessment criteria for sleeve
welds, and as a result the project has been working in conjunction with Det
Norske Veritas to establish criteria that could eventually form a code of
practice.
The authors discuss next the
structural design of the welded sleeve against all relevant limit states for
maximum loads that can occur and with a safety margin dictated by the use of
appropriate safety factors.
The relevant limit states are
bursting, global yielding (including buckling), local
overstressing/overstraining, unstable fracture (including possible lifetime
crack growth) and fatigue. The relevant load cases are pressure testing (after
repair), maximum loading during operation and fatigue during operation. It is
necessary to consider axial loads that are both tensile-dominated (e.g., for
unrestrained pipe segments) and compressive-dominated (e.g., for partially or
fully restrained segments). Generally the design is governed by the
tensile-dominated maximum loading case in operation.
Remote hot-tapping
into subsea pipelines
The basic principle of hot-tapping is
to establish a new branch pipeline connection to an existing (mother) pipeline
while under full pressure. This involves connecting the branch pipe, including
a valve, to the mother pipeline, usually by means of welding or mechanical
clamp connections, cutting a hole in the pipe wall by a machine attached to the
valve, retracting the cutting head, closing the valve, and disconnecting and
recovering the cutting machine. The pipe branch may now be extended by spools
and tied-in to a new pipeline in the usual manner. This strategy has been shown
to be very cost-effective compared to alternative methods, including shutdown
and tie-in at ambient pressure.
So far, divers have been used to weld
the branch pipe to the mother pipeline and for all installation and cutting
operations.
The primary focus of the remote
hot-tap project is the development of a novel design combining the use of a
remotely installed mechanical clamp (the retrofit tee), providing the necessary
structural strength as well as interfaces toward the isolation valve module and
the hot-tap cutting tool, and a saddle-formed “seal weld” made by remotely
operated hyperbaric GMA welding inside the branch pipe.
The authors continue to provide a
comprehensive overview of the structural design of the hot-tap tee, the
hyperbaric GMAW process, welding qualifications, experimental equipment,
procedural development, and installation of the welded sleeve and hot-tap tee.
Dry hyperbaric GMAW technology has
been formally qualified for water depths down to 1,000 m and demonstrated and
verified to a water depth down to 2,500 m.
The offshore systems and welding
technology is part of the PRS pool in Norway and is ready for real applications
offshore.
Source:
http://www.offshore-mag.com/articles/print/volume-66/issue-11/dot-technical-preview/deepwater-remote-welding-technology-for-pipeline-repair-and-hot-tapping.html. Accessed by 22-1-2014
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