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

7.6.2 Loss <strong>of</strong> buoyancy <strong>of</strong> the hull in connection with upward flow <strong>of</strong> <strong>gas</strong>
Models <strong>of</strong> <strong>gas</strong> emissions under water show that huge emissions <strong>of</strong> <strong>gas</strong> are required before the
percentage <strong>of</strong> <strong>gas</strong> on the surface becomes dangerous. At water depths <strong>of</strong> 300 meters, the <strong>gas</strong>
will spread out and only parts <strong>of</strong> the <strong>gas</strong> will reach the surface <strong>of</strong> the sea close to the facility.
The extent <strong>of</strong> the <strong>gas</strong> on the surface will depend on factors such as the strength <strong>of</strong> the sea
currents.
As regards the danger <strong>of</strong> loss <strong>of</strong> buoyancy and stability on loosely moored facilities, studies
by Sintef and observations indicate that the flow will result in a "fountain effect" that pushes
facilities/vessels up and away from the area (ref. surveys and experience on Jotun, Åsgard and
West Vanguard). However, this is not so simple for a tension leg facility like the SNA. The
SNA will not be lifted up. If the SNA was pulled out, the tension legs would then help pull
the facility back towards normal position. The facility would then move until equilibrium
was achieved between the forces that would push it out (the <strong>gas</strong> stream) and the tension legs,
which would pull it in again.
In our opinion, increased tension in the tension legs as a result <strong>of</strong> the <strong>gas</strong> flow (when the
dynamic link is greater than the reduction in density) would be just as likely a scenario as loss
<strong>of</strong> buoyancy.
8 OTHER STUDIES
The investigation group's mandate calls for the group to identify the need for assistance,
separate studies and the use <strong>of</strong> consultants.
8.1 Need for additional studies
The following studies are being considered to further elucidate the causal relations <strong>of</strong> the
incident.
• Pressure response, well operations and during well control situation for this incident
• Casing wear / Well design / Well barriers
• Increased buoyancy / fountain effect from the <strong>gas</strong> stream
• Loss <strong>of</strong> / reduced buoyancy due to <strong>gas</strong> bubbles
• Gas under the bucket foundations under various design conditions
• Excavation <strong>of</strong> and craters in the bucket foundations;
• Loss <strong>of</strong> suction under the bucket foundations in the event <strong>of</strong> cracking or damage.
9 ABBREVIATIONS
AR Activity requirement
B&B Drilling and wells
BOP Blow out preventer
CCR Central Control Room
HAZOP hazard and operability study (Risk analysis)
LEL Lower Explosion Limit
mMD meters Measured Depth
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