Other Planning LL with Wellesley

Hemispheres (36/1-4) – Surface section

What happened:

Ahead of a connection at 1146m (105m below the 13 3/8″ shoe), a blockage occurred in the cuttings chute between the cuttings screw conveyor in the shaker house and the CCBs on the main deck. The Driller picked up off bottom and continued circulating with 3000 lpm for 10 minutes while the crew attempted to clear the blockage without success. To try to assist the crew, the Driller reduced the mud flow to 1000 lpm for 8 mins, during which time he saw the torque increase. Recognising the danger, he increased the flow again to 3000 lpm during which time the torque disappeared. But the cuttings chute continued to fill as the blockage was still not cleared, so he reduced the flow again, this time to 500 lpm.
The string packed off and became stuck with the ability to circulate lost and the elevators close to the rig floor with 4.5m rig heave.
The string was freed by jarring down once, working torque into the string (max 30 kNm) while applying 20-25 bar reference pressure to the string. After an hour circulation was restored with 200 lpm, allowing the flow to be staged up, the well cleaned up. The string was backreamed into the shoe and a longer drilling pup installed on the drilling stand to assist with drilling the stand with high rig heave.
T

What was planned:

Drill 12 1/4″ hole to section TD with OBM and cuttings skip/ship.

What was learned:

The screw conveyor and cuttings chute are single point failure items. If either fails there is no back-up. But if we drop the flowrate too much if either fails, we risk getting irretrievably stuck with an expensive BHA which is the worst outcome.
Mitigation since implemented is (with the mud sheared) to dress the lower shaker screens with 170s – not only does this take more solids from the mud but it also means the cuttings run slightly wetter off the screens, improving the flow through the chute.

What actions are required:

Identify this risk in the Risk Register.
Unless an engineering change to improve the current system is considered viable, the default solution has to be to specify a minimum flowrate (eg 2500 lpm) and allow the shakers to overflow onto the shaker room floor while clearing any blockage.