Gas-tight riser connections alternative to welding, flanging
Full scale fatigue tests are shown being conducted on threaded riser joints.
Oil and gas production and export risers are expected to contain high internal production flow pressures as well as resist external deepwater pressures, all the while experiencing millions of cycles in compression, tension, and bending over the life of a producing structure.
Producers expect the connection, long considered the weakest link in risers, to withstand these multiple forces, and so require lots of bench and full scale tests to ensure connectors can deal with the conditions. The riser connections must be gas tight, since leaks or blowouts can result from the production strings contained within production risers.
Traditionally, production and export riser joints are welded or flanged together, since this ensures reasonable integrity and sealing. However, large joints do show some sensitivity to fatigue over time. In addition, they take time to assemble at the site or alternatively, the full riser strings must be assembled ashore and towed out. For shallow and mid-range water depths, this has not been a problem. However, multiple risers for deepwater can require a great deal of time to install. Hence, the research into quickly assembly jointed technologies.
Recently, threaded connections, similar to those used in drilling tubulars, have been used on risers as producers sought to cut down on installation time and improve fatigue resistance in the joints. Also, threaded connections have been accepted for pipeline use but the experience is still low.
North Sea firstHeidrun, Conoco/Statoil's giant concrete tension leg platform in the North Sea, was one of the first to employ new threaded connection joints for risers. Other North Sea producers are examining the technology for future use, but most are awaiting to determine Conoco/Statoil's experience with the connectors. At the end of this year, the risers will have been in place for a period of two years.
Production risers with threaded connections for Heidrun's 44 wells and a number of backup strings were manufactured by Mannesmann of Germany. The riser tubes, with a 10-3/4-in. outside diameter, are virtually standard steel products. The 350-meter long production risers were anchored to the template on the seafloor and suspended at the deck level, where wellheads were attached for well control. In addition, 16-in. outside diameter export riser strings to handle as much as 220,000 b/d of crude from the platform were also manufactured.
The risers manufactured for Heidrun are aluminum coated on the outside to resist corrosion from seawater. The makeup tongs were designed to prevent damage to the aluminum coating. Mannesmann's Dusseldorf-Rath product development team developed the qualification procedures for the premium threaded connections. The connector design selected was based on Mannesmann's MUST thick-walled threaded connectors. The connectors have double shoulders to handle high bending loads. An internal gas-tight metal-to-metal seal contains internal pressures while the external shoulder works as a hydraulic seal against seawater intrusion.
Large upset threadsMannesmann engineers pointed out the connection required high upset thread cross sections, and there was no data on upsets of this type or how the material would behave in thick sections. Development resulted in the creation of a new material labeled X 80 mod., which retained strength and toughness at low temperatures. Toughness had to be retained in both the thick upset and adjacent thin pipe wall. The upset cross-section was 2.6 times greater than the pipe cross section.
Product development and testing also sought to solve other problems related to toughness:
- Ensuring the connection would remain unaffected by the multiple make and break cycles in the event of trouble during running the risers or during maintenance.
- Ensuring the connection would remain gas-tight under the greatest loads, such as occur in storm-driven ocean swells or during full internal pressure loads caused by a damaged production string.
- Ensuring the connection would survive dynamic load cycling caused by ocean swells and during handling operations in the derrick.
There is little question that lab and dynamic testing proved the connections could hold up. Later this year, operators should have sufficient experience with the Heidrun riser joints to gauge the benefits of using premium connection joints in deepwater risers on future production platforms and vessels.
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