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

to provide for barrier integrity as well as well control. (Ref. the Facilities Regulations,
Section 83, No. 2 relating to entry into force.)
• The Activity Regulations, Section 27 relating to Planning, which require that the
planning shall take into account the status <strong>of</strong> important contributors to risk and to the
change in risk.
• Odfjell Drilling's ”Golden Rules”
5.1.24 Non-conformity 24. Inadequate well barriers when pulling scab-liner through
BOP
On 28 November 2004, just after 0500 hours, the scab-liner was pulled through the BOP.
When a 7 5/8” scab-liner is pulled through the <strong>blowout</strong> preventer, the function <strong>of</strong> the upper
and lower pipe ram are primarily affected, while a secondary effect is in relation to the
function <strong>of</strong> the shear ram. There is no possibility <strong>of</strong> cutting or holding the scab-liner.
Evidence:
It emerged from interviews that the scab-liner was pulled through the BOP without Statoil
ensuring that the risk was evaluated and compensatory measures were established.
Requirement:
• The Activity Regulations, Section 76 relating to Well barriers requires that, during
drilling and well activities, there shall at all times be at least two independent and
tested well barriers after the surface casing is in place.
5.1.25 Non-conformity 25. Inadequate risk assessments in connection with swabbing
On 27 November 2004 at night, during pulling <strong>of</strong> the scab-liner, the crew observed swabbing
for the first time. Several known factors could lead to and reinforce swabbing, which is
regarded as being an important contributing factor for risk. Among other things, swabbing
could occur as a consequence <strong>of</strong> minimal clearance between the scab-liner and 9 5/8” casing,
and as a consequence <strong>of</strong> ”gelling”.
In the event <strong>of</strong> communication with hydrocarbon-bearing zones, swabbing will lead to <strong>gas</strong>
and other formation liquids being sucked into the well. The mixture <strong>of</strong> <strong>gas</strong> and drilling mud
reduces the specific gravity <strong>of</strong> the drilling fluid and the barrier function <strong>of</strong> the mud is affected.
Failure <strong>of</strong> the primary barrier (the drilling mud) after swabbing is one <strong>of</strong> the causes that
triggers the incident on P-31A. The crew were unable to restore the primary barrier after
swabbing.
Evidence:
In interviews and on the daily drilling report, it emerges that the drilling management was
uncertain <strong>of</strong> the status <strong>of</strong> the primary barrier. They implemented counter-measures
(flowchecks) that proved to be insufficient.
Other activities were conducted in the well, other than those aimed at restoring barriers.
The drilling management was aware <strong>of</strong> the effect <strong>of</strong> ”gelling” from previous incidents 34 . The
detailed plans showed that previous incidents were not taken into consideration, and swabbing
was not listed as a risk factor.
Requirement:
34 Ref. Incident on well P-02 in September 2003.
35