Report Investigation of gas blowout on Snorre A - Well Integrity ...

The log shows confirmation that the flare was extinguished at 0315 hours. 21
At 0330 hours, the wind had turned to south-southwest, and the force increased to 17-19
knots.
At 0400 hours, 80 m 3 <strong>of</strong> new OBM was mixed and ready. Mud was bullheaded into the well
until they ran empty for the second time. After 0530 hours, there was no more OBM on board.
Due to the <strong>gas</strong> on the sea around the facility, there was no way that the supply boat carrying
mud could come alongside the platform. The following pressures were recorded at that time:
32 bar internal pressure in the work string and 55 bar on the annulus side.
At this time, several alternative measures were considered: Cementing, use <strong>of</strong> seawater <strong>of</strong>
mixing <strong>of</strong> emergency mud with the additives that were available on SNA. A decision was
made to mix up water-based mud (WBM) with the following components: water, barite and
bentonite. Between 0400 - 0915 hours, the crew mixed 160 m 3 <strong>of</strong> water-based mud with a
weight <strong>of</strong> 1.8 s.g.
Only monitoring <strong>of</strong> the well took place from 0530 - 0915 hours.
The emergency management on board considered the use <strong>of</strong> WBM to be a final attempt to
stop the influx in the well. The purpose <strong>of</strong> waiting to carry out the bullheading was that the
crew wanted to have sufficient volume available during this final bullheading with WBM.
Before starting bullheading with WBM at 0900 hours, SNA requested elevated evacuation
preparedness in connection with a critical phase; for the third time.
Before bullheading with WBM started, a pressure <strong>of</strong> 72 bar was recorded in the annulus and
156 bar in the work string. The bullheading was started at 0915 hours, and at 1022 hours, 0
bar was recorded on the annulus side and 0 bar in the work string. At this point, the stores <strong>of</strong>
WBM were also empty, only 8-10m 3 remained as residue in the tanks.
4 POTENTIAL OF THE INCIDENT
4.1 Actual consequences
The actual consequences <strong>of</strong> the incident were related to financial loss. The bulk <strong>of</strong> the loss is
related to delayed production on the Snorre field. SNA and Vigdis together produce a total <strong>of</strong>
approx. 200,000 barrels <strong>of</strong> oil per day. The production was shut down when the incident
occurred on 28 November 2004 and, three months later, has still not returned to a normal
level. This is because extensive and costly normalization measures were required before
production could be resumed.
Surveys <strong>of</strong> the seabed show a crater measuring 8 x 3 meters on the west side <strong>of</strong> the well
template, as well as two craters with diameters <strong>of</strong> 2.5 and 3 meters. In addition, several
smaller craters have been noted towards the northwestern anchor point, while the crater with a
21 See Chapter 7.5 Flaring and depressurization
23