The world’s highest-pressure large-bore drilling riser
2H and SRP have undertaken an exhaustive development and testing programme to qualify this technology, which is likely to find broad application for both shallow and deepwater riser systems in the future. (For an explanation of this aspect of the project, see the attached technical note.)
Dannie Claxton, engineering director, Claxton Engineering, said, “We have worked closely with our Acteon colleagues to offer Venture this very practical solution to the challenges of drilling high-pressure wells. The first of its type in the world, the riser is important because it gives Venture the opportunity to make a cost-effective step-out in drilling practice – one we expect others to follow.”
As well as acting as the lead contractor, equipment integrator and offshore service supplier, Claxton will provide a range of ancillary equipment, including an umbilical, wellhead and BOP connectors, a tensioning ring, and a hydraulic power and control system. A team from Claxton will be responsible for running and pulling the riser on the rig, and for its inspection and maintenance.
2H carried out the initial riser design and analysis work, and SRP has led the development of the new shrink-fit technology. SRP is ultimately responsible for supplying the riser, which has 13 main sections plus fatigue-critical, tapered stress and tension joints.
Forging the main pipe sections has already commenced at two plants in France and Italy. The flanges will be forged once the main pipes are finished, and then extensive machining will be required before the flanges are shrink fitted. Once the riser is complete, a detailed testing programme will be carried out before delivery to Venture in September 2009.
Venture expects to begin using the riser immediately thereafter to drill HP/HT development and appraisal wells in several of its Central North Sea assets in water depths to 120 m.
Shrink fit technology
As far as the riser is concerned, Venture has two key requirements for its 2009 HP/HT drilling campaign. The company needs a full-bore (18¾-in.) riser with a pressure rating to 12,200 psi. Individually, these are not uncommon requirements. However, when taken together they pose a considerable manufacturing challenge.
If a weldable grade, say 65 ksi, of steel were to be used for the riser, a wall thickness of up to 75 mm would be required. Apart from the problems of manufacturing pipes of this type, the weight of the riser string would be enormous. Moreover, welding connectors of any type to pipe of this thickness is not straightforward and would probably result in a poor weld-fatigue performance classification. Selecting a high-strength steel would reduce the wall thickness needed (using 110-ksi steel would mean a wall thickness of about 30 mm), but such steels cannot be welded successfully; it is difficult to achieve the required physical properties in the weld.
This problem has been resolved by using a shrink-fit process to attach the flange connectors to the pipe sections. It was Steve Hatton, founder of 2H Offshore and a vice-president of parent company Acteon, who had the idea of shrink fitting to avoid the problems typically encountered with high-integrity riser fabrication. Shrink fitting is not an unusual process in other industries but it had never been proposed for an application such as this.
Hatton said, “The industry has been seeking a solution to the problem of welding high-strength riser joints for years, and with the current increase in HP/HT applications, the problem has become critical. Shrink fitting is an excellent solution, we believe, and this has been well understood by Venture; the company has supported the technology and fast-tracked our qualification process. There is significant potential in this technology to enable the manufacture of higher-strength, lighter-weight risers with improved fatigue performance.”
Development and testing
Shrink fitting is a simple process in principle. However, there are still several issues that have to be understood in order to guarantee repeatable performance to the level demanded by the offshore industry, particularly for critical HP/HT riser applications. In view of this, a thorough testing programme has been conducted in Sheffield, UK, to prove the process and confirm the earlier, extensive finite element analysis work.
When making the joints, the machining of component profiles and finishes is tightly controlled, as is the heating of the flange body during the assembly process. Mating the two components is a further practical challenge requ
iring precise alignment at the instant the two are stabbed together. Simply allowing the mated assembly to cool generates a high-quality structural connection and a gas-tight seal to complete the assembly process.
Rigorous load and pressure testing, witnessed by Bureau Veritas, has been conducted on a series of joints made using the 80-ksi steel chosen for this project. (There is no reason, however, why the same process cannot be used for steels in excess of 110 ksi.)
The joints have successfully completed hydrostatic pressure testing up to 13,500 psi (equal to 90% of pipe body yield strength) under various external tension and bending loads. In addition, gas testing has been conducted to 12,200 psi.
The integrity of a shrink-fit joint relies primarily on the radial contact force generated as the connection cools and shrinks onto the pipe end. The friction generated at the interface is further enhanced by geometrical and mechanical features that 2H has incorporated into the design. The resulting connection capacity is demonstrated to be as strong as the pipe.
2H believes that shrink fitting will become increasingly used in the riser industry to provide an effective alternative to welding. The process constitutes an important enabling technology that opens up the use of high-quality and ultra-high-strength steels, which previously could not be used owing to welding limitations.
The company is already focused on HP/HT applications in deep water in the Gulf of Mexico, such as the BP Kaskida development, which may require risers capable of working at even higher pressures than the Venture system. Shrink-fit technology can be easily extrapolated for use at much higher pressures than the current system and is only limited by the availability of suitable pipe. It is believed that configurations up to 18¾-in. bore and 20,000 psi are possible through this approach.
The technology, on which 2H has patents pending, has wider applications. SRP is proposing the process as a solution for reducing the cost of manufacturing forged taper joints and riser joints with complex non-ferrous metallurgy such as titanium and aluminium.
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