Conventional wisdom says that subsea wells need a semi-submersible rig for drilling, workover and abandonment operations. But, a semi-submersible rig isn’t well suited to the rough environmental conditions and relatively shallow water depths (less than 120 m) found in regions like the North Sea. For these cases, Aquaterra Energy’s high pressure riser systems combined with a jack-up rig can offer a much better solution. But what about the costs? As jack-up rigs are typically cheaper, there can be significant cost savings compared with a semi-submersible rig.
There are also many technical advantages, such as:
- Improved operational window - HP riser systems are designed to withstand 50-year storm conditions even under a well control situation. This is much larger than semi-submersible rig operating envelopes in the North Sea.
- Reduced fatigue loading on subsea well equipment - Low rig motions mean reduced loads on expensive and fatigue-sensitive well equipment.
- Increased BOP reliability - A surface BOP is less complicated to operate and maintain.
- Operational flexibility - Platform, subsea well and exploration operations can be completed by the same rig and drilling team, reducing complexity.
Jack-up rigs offer all of these benefits, along with a cheaper price tag! That’s why we think operators should really consider a jack-up HP riser system before committing to a semi-submersible rig.
Aquaterra Energy has vast experience in designing, optimising and running HP riser systems from jack-up rigs, having previously completed projects for Apache, Maersk and Shell amongst others. So, what factors are important when designing HP riser systems?
- Rig tensioning - is required for the HP riser system to prevent the riser buckling from a combination of the BOP weight, the riser self-weight and the environmental conditions. The tension is typically applied using a tension ring that sits beneath a load shoulder just below the surface BOP. We specify a tension that ensures the riser is within allowable limits for all key scenarios offshore, meaning a more robust system.
- Rig lateral support system - we typically recommend this is used just below the surface BOP. It helps reduce the movement of the HP riser due to environmental conditions and divert loads away from the potentially sensitive diverter and RT equipment. This also minimises loads on critical subsea equipment, improving performance. A push-up style primary tensioning system supplies lateral support to the HP riser, so the rig often needs little modification for the operations. If the rig has a pull-up style tensioning system, we use analysis to optimise requirements and supply a cost-effective lateral tensioning or support system.
- Subsea connections - a solid understanding of the subsea wellhead/tree and associated connections is important. As the full system is only as strong as the weakest component, the riser system and applied tension is optimised to maximise performance from existing subsea components.
- Tension/stress joints - the design of the tension joint just below the surface BOP and the stress joint just above the subsea well equipment are key to success. The tension joint should interface with the tensioning system, connect to the surface BOP and resist the loads from the rig’s lateral support system. The stress joint is located in areas of high loads, so is often tapered to improve structural performance. We use analysis and operational experience to create an optimised solution for each operation. This way, the riser system can stay connected even during extreme storm conditions, minimising rig downtime.
We have been engineering, running and operating HP riser systems since 2010. In every case, we have shown that our system can stay connected and operating in 50-year storm conditions in line with code requirements, including 120m water depths in the North Sea. For shallow water depths, this typically exceeds operating limits for a semi-submersible rig in similar conditions, resulting in less downtime when using a jack-up rig. In addition, as jack-up rig rates are typically lower than semi-subs, considering a HP riser system is a no-brainer for drilling, workover and abandonments of shallow water subsea wells.
Want to know more about our advanced drilling riser systems? Find out more here.