When developing shallow water subsea projects, most people in the oil & gas industry will instinctively assume that a semi-submersible rig should be used for the purpose. But this is not the only viable option. In fact, a jack-up using high-pressure risers can be even better suited for the purpose under certain conditions, such as those found in the North Sea.
The main reasons for considering a jack-up rig for subsea drilling are:
- Reduced downtime waiting on weather
A harsh or ultra-harsh environment jack-up with high-pressure riser systems can be used in extreme weather, even in storm conditions you only meet every 50 years in the North Sea. By contrast, a semi-submersible is likely to have significantly more downtime due to harsh weather.
- Improved equipment lifetime
All rig types will transfer motion generated by weather and sea currents to the subsea well. However, a semi-submersible rig will move around more, even during benign conditions, causing wear on critical subsea well equipment. On the other hand, a jack-up with a high-pressure riser provides a more stable system that can significantly reduce fatigue damage to the equipment.
- Additional operational benefits
Using a jack-up rig will typically reduce loading on surface and subsea components, and tree handling and installation can also be completed from a jack-up.
- Potential cost and emissions upsides
Fuel consumption on a jack-up is typically lower than on a semi-submersible rig, leading to a more attractive operational cost picture which also translates into fewer CO2 emissions. In jurisdictions imposing a CO2 tax, this provides an additional cost upside on top of the more responsible emissions profile.
Maersk Drilling has a proven track record of subsea drilling using jack-up rigs, having completed more than 30 jack-up subsea operations in the North Sea.
One example is Maersk Interceptor which in 2018 conducted a highly successful three-well subsea campaign in Norway for Spirit Energy, completing the work 68 days ahead of schedule. Read more about the case and why it was such a success here.