The Low Riser Return System (LRRS) is a method used for managing wellbore pressure during offshore drilling operations by adjusting the mud level in the marine riser by returning mud and cuttings to surface via a subsea pump in a separate conduit. It is a single mud gradient, open MPD system particularly designed for subsea drilling.
The LRRS can be used in two application modes. The first has a full riser during static conditions using conventional mud weights and the capability to lower the fluid level to compensate for the ECD effect during circulation and drilling as needed. With this technique, conventional well control procedures are used. The second technique involves using higher than conventional mud weights and lower fluid levels for both static and dynamic (circulating) operations. With this technique, modified well control procedures must be used. Both of these methods improve safety margins, allow for better pressure control, and increase efficiency for most well operations.
This paper focuses on the well control issues of drilling with a partially evacuated marine drilling riser. Two cases using data from deepwater US GoM wells illustrate how the LRRS gradient fits well inside the drilling window. Kick margins and kick detection are improved compared to conventional drilling operations, and riser margin can be achieved even for relatively deep water, which allows for safer disconnections. Improved cementing by compensating for density and ECD effects is another important result of applying this system.
We conclude that this technology can greatly improve safety in drilling of deepwater exploration and production wells as well as for infill drilling in depleted fields; enabling safe access to more reserves and improving recovery.