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The production casing runs from the bottom of the well up to the
wellhead. There are two general design options for production casings – a long
string design or a liner design. A long string casing design consists of a
production casing that extends from the bottom of the well to the top of the
wellbore. A liner casing design consists of a casing that is anchored or
suspended from inside the bottom of the previous casing string and does not
extend to the top of the wellbore. The liner design has the option of being tied
back to the top of the wellhead.

In general, both long string and liner production casings have two
annulus barriers: cement across the hydrocarbon zones and the mechanical seal
at the top of each string. The mechanical seal in a long string casing includes the
wellhead casing hanger seal assembly. A liner design uses a liner top packer
assembly. The liner tie‐back provides two additional barriers: the liner tie‐back
cement and the tie‐back seal assembly. However, the use of a liner tie‐back
design also involves risks associated with the possibility of mechanical integrity
failure at the tie‐back junction, as well as the potential for increased annular
pressure build‐up, which could occur as a result of annular fluid expansion
caused by heat transfer during the well’s production phase. Annular pressure
build‐up increases the risk that the production casing or tie‐back string will
collapse if annulus fluids or pressure become trapped by the assembly with no
outlet for bleed‐off.

BP’s well engineering team prepared an undated “Forward Plan Review”
addressing production casing and temporary abandonment options for the
Macondo well and circulated it within BP in April 2010. The forward plan
recommended against using the long string design because of the risks described
above that are associated with that type of design. This document stated that,
while the long string of 9‐7/8 x 7 inch casing “was the primary option,” the use of
a 7 inch liner was “now the recommended option” for the following reasons:
cement simulations suggested that cementing the long string was unlikely to be
successful due to formation breakdown; using a long string would prevent BP
from meeting regulatory requirements of 500 feet of cement above the top
hydrocarbon zone; the long string would result in an open annulus to the
wellhead, with hydrocarbon zones(s) open to 9‐7/8 inch seal assembly as the only
barrier; and the potential need to verify the cement job with a bond log and to
perform one or more remedial cement jobs prior to the temporary abandonment
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of the well.

81 BP‐HZN‐MBI00020910.

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