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

clay. The foundations and the facility are tied together with four steel pipes (tension legs) in
each corner. The weight <strong>of</strong> the concrete foundations and ballast is about 7000 tonnes. The
tension from the legs is about 6500 tonnes - when there are no waves. If there is a storm, the
extra tension will largely be absorbed by friction between the walls <strong>of</strong> the skirts and the clay.
As a point <strong>of</strong> departure, the foundation has had a safety factor against failure <strong>of</strong> about 2.
Statoil has performed analyses in which they have removed all friction from one <strong>of</strong> the three
skirt cells. This is an extremely conservative assumption - based on the observed craters.
This resulted in a safety factor <strong>of</strong> 1.4 in a 100-year storm. NORSOK requires a safety factor
<strong>of</strong> at least 1.25.
Figure 10. Craters on the seabed
Statoil has analyzed several potential explanations <strong>of</strong> the changes in the tension. There are
several explanations that, viewed independently, could have contributed to increased tension
balance error, but not to the extent observed. Therefore, we cannot rule out that a movement
<strong>of</strong> the foundations on the northwestern corner have contributed to the tension balance error.
A detailed inspection has been conducted <strong>of</strong> the transition between the foundation and the
seabed. Based on this inspection, one cannot see signs <strong>of</strong> movement or rotation <strong>of</strong> the
concrete foundation. However, it is uncertain whether it is possible to observe movements in
a centimeter range. About three cubic meters <strong>of</strong> <strong>gas</strong> were drained out <strong>of</strong> the bucket
foundation in the northwestern corner after the <strong>blowout</strong>. A subsequent bleed showed that no
more <strong>gas</strong> had entered the foundations. The <strong>gas</strong> in the bucket foundations indicates that there
may be an internal crater. The volume <strong>of</strong> <strong>gas</strong> would indicate that the crater is probably not
very big. However, we cannot rule out that there was higher overpressure during the <strong>blowout</strong>
than what was drained out on 6 December 2004. No cracks have been observed in the
foundations. A drainage through cracks must have come from below ground level - perhaps
in connection with one <strong>of</strong> the craters.
When we have an unexplained change <strong>of</strong> the tension in the tension legs, this means that we
cannot be completely certain as to what has transpired. Therefore, it is also impossible to say
anything about the safety <strong>of</strong> the foundations or the tension legs during the incident, or what
would have happened in the event <strong>of</strong> a full <strong>blowout</strong>.
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