The installation of a subsea tree from a floating drilling rig is one of the most complex and time-consuming portions of a deepwater subsea well completion. Removing subsea tree installation from the rig "critical path" would yield a step-change reduction in the time and cost to complete a subsea well.
Disappearing plug technology holds the key to achieving this improvement. Regulations (and good practice) dictate the use of a plug set in the tubing hanger of a completed well to provide a barrier to well flow. Until recently, most subsea trees were installed from the drilling rig on a completion riser, after the BOP stack and marine riser were removed. The completion riser also provided the conduit to remove the tubing hanger plug. "Disappearing plug" technology eliminates the need for the completion riser and drilling rig during subsea tree installation because it acts as a well barrier, performing the same function as the tubing hanger plug, until it is opened remotely from the host facility.
This paper discusses the application of disappearing plug technology to a subsea project, the synergies created between downhole completion operations and subsea installation activities, and the resultant time and cost savings.
One of the largest components of the cost of a deepwater subsea development is the day-rate of the drilling rig and equipment spread used to drill and complete the wells. Roughly two thirds of completion costs are directly related to the time it takes to complete a well. Therefore, any significant activities that can be removed from the rig "critical path" will generate a substantial reduction in the capital cost of completion.
Subsea well completion operations conducted from a rig include not only the downhole completion work, but the installation of subsea trees and other equipment.Subsea trees are normally run on some type of completion riser, which is run from the main derrick. Running a tree in this manner requires that the marine riser and BOP stack be removed, a time-consuming and costly operation.
Other subsea equipment installation techniques, such as running on wire from an installation vessel or a rig-based winch, could be made suitable for installation of subsea trees. However, a completion riser system must still be run at some point to remove the required tubing hanger plug that blocks well flow until the subsea tree is installed.
The key to enabling non-rig based tree installation is to replace the tubing hanger plug with an "alternate barrier system" to prevent well flow when the BOP stack is removed, but permit opening of the well remotely when required, without well intervention.
Such an alternate barrier, or "disappearing plug" system, has now been employed successfully in the three-well Crosby (MC-899) development in the GOM. Use of an alternate barrier system downhole enabled subsea tree installation and activation without the need for a completion riser. Time savings associated with not having to trip the BOP stack multiple times resulted in the completion of the Crosby wells in less than 60% of the expected time and 20% below budget.