Subsea Safety Systems Enable Rapid Disconnection from Well Following Unexpected Rig Power Outage

Operating environment complicates deepwater well test

During a deepwater exploration well test, where the water depth exceeded 1,500 m and currents exceeded 3.5 m/s, an operator encountered challenges aboard a dynamically positioned rig. With the gas well live and flowing hydrocarbons during the cleanup phase, the rig experienced an initial partial power blackout, and later suffered a full power blackout. For a reservoir where pressures exceeded 6,500 psi, the operator knew it was important to rapidly control and shut in the well as safely and efficiently as possible as the rig began to drift.

Subsea test tree enhances safety during critical period

As the rig began to lose power during the initial partial blackout, the rig’s dynamic positioning (DP) system continued to operate. As a precaution, the operator decided to close the test valve within the well’s IRDV dual valve to bleed off pressure. Approximately 15 min later, power returned, but the IRDV valve was kept closed, and roughly 4 min later, the rig lost power completely and started to drift.

Following the loss of DP functionality, the second emergency shutdown system (ESD-2) was activated and the SenTREE 3 test tree with an electrohydraulic deepwater control system (DWCS) disconnected the landing string in 11 s. The rig had drifted 150 m off the well center to a riser angle of more than 5.7° when the LMRP disconnected.

Landing string relatched on first attempt following rapid disconnection

In an operating environment with high surface currents it was unknown if the latch skirt had been damaged due to the angle of disconnection; it was later determined that the latch skirt had not been damaged. The operator ran the original tool following a surface inspection and resumed operations after approximately 5 days of being off the well.

Enabled by local technical support, the relatch was successful on the first attempt. All samples were collected before the well was killed and testing resumed seamlessly after having been suspended, without environmental incident.