A novel riser design from Claxton provides operators with a greater choice of rigs when it comes to drilling subsea wells.
The use of subsea facilities tied back to existing infrastructure continues apace in the North Sea. Subsea-based solutions are often considerably more economic than the alternatives, particularly when relatively few subsea Christmas trees are required for a development.
Subsea wells are typically drilled and completed from semisubmersible drilling units using the rig’s subsea blowout preventer (BOP) and drilling riser system. This offers full-bore 18¾-in access to the subsea hardware. Typically, jackup drilling units do not utilise subsea BOPs or drilling risers, or offer such full-bore access to the subsea hardware. Consequently, drilling and completing traditional subsea wells from jackup units can pose unique challenges and difficulties. However, there is a solution – the Claxton high-pressure jackup drilling riser (HPJDR) system.
The Claxton HPJDR system enables traditional subsea wells to be drilled and completed from a jackup unit providing the rig has a full-bore 18¾-in surface BOP system. Claxton offers a full system service, from analysis and specification of the system, through fabrication, equipment rental and writing of operational procedures, for deployment and recovery of the HPJDR system. Full structural, fatigue and vortex-induced-vibration (VIV) assessments can be carried out as necessary, although VIV is rarely an issue because of the tensions employed and the riser system’s diameter.
A typical HPJDR configuration has an 18¾-in hydraulic wellhead connector at the lower end that is suitable for connection to the client’s subsea wellhead system. The riser comprises 40-ft pipe joints of a suitable specification joined by conventional (typically threaded) connectors, with a tension joint and a crossover joint at the upper end. The crossover joint provides the connection to the surface BOP on the rig, and the tension joint interfaces with the rig’s tensioning system, which applies an upward load to the riser system so that it will remain sufficiently stable.
The inside diameter of the HPJDR allows full-bore drift throughout the entire system and into the subsea wellhead, as long as the jackup has an 18¾-in high-pressure surface BOP. This enables traditional 18¾-in subsea wellheads and Christmas trees to be used. Drilling and completion are possible in the same way as if a semisubersible unit had been used.
The Claxton HPJDR system opens up the possibility of using traditional subsea systems from jackups as well as from semisubmersibles.
Four clients have successfully used this approach over the last five years at water depths to 100 m in the southern and central areas of the North Sea. The increasing use of deepwater jackups for development drilling, along with the functionality of the Claxton HPJDR system, offers opportunities to costeffectively drill and complete conventional subsea wells in harsh environments such as the North Sea from jackup as well as semisubmersible units.