Well control with Low Riser Return System

Abstract

A response to the increasingly more challenging prospects to drill, due to depletion of old reservoirs and increasing water depths, is the Low Riser Return System (LRRS). The LRRS is a system designed to manage the wellbore pressure in offshore drilling by riser level regulations. A subsea pump module is connected to the riser and enables cuttings and mud return to surface in a separate conduit. One of the systems applications, which will be discussed throughout this report, involves operating with a partially evacuated riser together with heavier than conventional mud. With this technique, new well control procedures have to be established.

Due to the utilization of a lowered mud level in the riser with corresponding heavy mud, various new elements and procedures compared to conventional well control are introduced, which subsequently raises additional challenges and possible problems. This report will first introduce the LRRS system with its equipment, before describing the general principles of conventional well control. Thereafter well control procedure based on published material will be presented and the different elements that constitute LRRS well control will be described. Discussions regarding the well control implementation, the reliability of the well control procedures, contingencies and alternative procedures will be conducted at the end.

Procedures and well control elements that are enlighten and discussed in this report are the effect of a quick BOP closure, probability and contingency procedures for a plugged subsea choke, riser level regulation for maintaining choke opening within optimal operational area, procedures and potential problems related to circulating gas into the riser, gas whereabouts during circulation and the influence LRRS could have on the drilling and completion of Macondo well.

As a supplement to the discussion, a simulation in the program Drillbench Kick has been conducted to reveal the friction pressure loss in choke line for conventional and LRRS well control, demonstrate the gas behavior and emphasize the choke response during circulation. Since Kick is designed for conventional systems, manual calculation had to be included as well.