Deepwater remote welding
technology for pipeline repair and hot-tapping
Kjell Edvard
Apeland, Jan Olav Berge, Richard Verley - Statoil ASA
Michael
Armstrong, Neil Woodward - Isotek Electronics Ltd.
The second paper
highlighted from the subsea/flow assurance track addresses flowline and
pipelines. Remotely operated dry hyperbaric welding technology has been further
developed in recent years and is now becoming the basis for offshore
applications both in subsea pipeline repair and hot-tapping technology. This
paper outlines the welding technology and the operational systems developed and
built to provide an offshore service.
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.
11/01/2006
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