Ultra-high pressure risers for deepwater drilling

Deepwater development requires increasing attention to riser system design, given the challenges of water and well depth.

Offshore staff

MONACO -- Deepwater development requires increasing attention to riser system design, given the challenges of water and well depth. According to a presentation by Karim Jan, Simon Luffrum, John Shields, and Paul Brett of SRP, one solution is a technique of fabrication using shrink-fitting that allows higher grade materials to be used and eliminates the problems associated with welding thick walled pipes. For many years shrink-fitting has been a standard connection method whereby a shaft is inserted into a pre-heated female component which is then left to cool. Although common in many industries, its use in the offshore oil and gas industry is limited. Welding and threading are the connection methods of choice to this day. The company's DOT presentation covered the development, testing, and manufacture of a full-bore (19-in. internal diameter) shrink-fit riser connection for a high pressure drilling system with a surface BOP. 

The team described the methodologies employed and the value to be gained from such a system. They also discussed the applicability of the system already constructed for a North Sea application to deepwater systems in which high strength steels of 110 ksi and beyond might be employed. The shrink-fit connection and riser system was developed in response to the challenge of drilling a shallow water, sour, high pressure subsea tieback in the most cost-effective manner. A key aspect of this challenge was the maximum predicted shut-in tubing head pressure of the development: approximately 11,000 psi. Ultra high pressure wells have typically been drilled using a subsea BOP, low pressure marine drilling riser, and semisubmersible drilling rig However in this instance a jackup drilling rig was selected and with a strong preference to maintain a full bore riser with surface BOP. This arrangement allowed cost benefits to be achieved on the rig rates but placed a number of challenges on the riser system.

The preliminary design and analysis of the riser system indicated that a solution could be achieved with a 19.5-in. (50-cm) bore riser with a 2 in. (5 cm) wall thickness using a relatively high strength steel (80 ksi). However, it was recognized that manufacturing such pipe and fabricating riser joints would be a significant challenge if NACE requirements are to be achieved and this represented a significant commercial and schedule risk. Therefore the option to use a new shrink fit solution was seriously considered by the project and ultimately qualified and selected in preference to the welded route.

The authors found that the excellent fatigue performance of the shrink-fit connection lends itself well to riser joints, bend stiffeners, and other fatigue critical services. SRP is currently developing a low-profile shrink-fit sleeve to enable construction of joints up to 30 m (98 ft) long for deepwater drilling applications. Extensive FE analysis of a sleeve design has already been carried out, and the results show that a sleeve coupling with a wall thickness comparable to the pipe would be feasible. In deepwater applications high strength steels of 125 ksi minimum yield strength or greater may be used with the shrink-fit technique to minimise riser weight.

In critical locations along the riser length forged taper joints are often required to control bending and fatigue. The use of shrink-fit flanges or connections allows equivalent fatigue performance to be achieved using ‘fabricated’ joints. This greatly simplifies the procurement process reducing cost and shortening schedule. Of particular interest is the possibility of shrink fitting a steel flange onto a titanium pipe. This allows the designer to take benefit from the titanium properties without the cost and complexity of the titanium connector that must either be forged integrally or connected through a welding process that degrades the fatigue performance, the presenters said.

11/04/2009

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