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.
