20
through the ocean water column. During DWH, point releases occurred at the end
of the broken riser pipe, from holes that formed in the kink above the blowout preventer (BOP) and finally directly from the open pipe above the BOP after the riser
was removed in preparation for attaching a containment system.
During the early days of DWH, British Petroleum (BP) attempted to control the
oil plume by confining it in a cofferdam, a large dome placed over the broken riser.
The attempt failed because the cofferdam rapidly filled with gas hydrates. These
hydrates increased the buoyancy of the structure to such an extent that it became
unstable and had to be removed. Natural gas hydrates, discovered by Sir Humphrey
Davy in 1810, are formed when water molecules are linked through hydrogen bonding and create cavities that enclose hydrocarbon gas molecules. However, no chemical bonding takes place between the host water molecules and the enclosed
hydrocarbon. The resulting solid has a snow-like appearance with density less than
ice. The amount of naturally occurring hydrates, 3000 trillion cubic meters according to Chong et al. (2016), dwarfs known traditional gas reserves. Hydrates first
became of interest to the hydrocarbon industry in 1934, when they were observed
blocking pipelines. Blockage in offshore production has become more severe with
the increase of the water depth (Sinquin et al. 2004). Hydrate formation is favored
by low temperature and high pressure such as found at the sea floor.
If hydrocarbons escape from the reservoir through a point release, the transport of
gas and liquid petroleum occurs in four distinct stages, each governed by slightly different physics and each occurring at later times such that the gas and liquid phases may
have different composition and physical/chemical characteristics. These stages are (1)
the initial jet breakup, where the petroleum fluids are the most fresh; (2) the ensuing
buoyant plume, where much of the dissolution of the soluble compounds occurs; (3) a
region where the density stratification of the ocean arrests the upward buoyant plume,
forming a horizontal intrusion layer rich in dissolved petroleum compounds; and (4)
the final transport phase, where gas bubbles and oil droplets rise independently, moved
by their own buoyancy and by the currents and turbulence of the ocean.
The jet break-up region is strongly turbulent, dominated by momentum coming from
the pipeline fluids, and occurs immediately downstream of the spill orifice. Seawater
rapidly mixes into the gas and liquid petroleum stream, breaking the immiscible oil up
into individual gas bubbles and oil droplets. The velocity of the oil and gas in this region
is similar to that in the leaking pipeline at the orifice, and the strong turbulence is responsible for creating the initial size distribution of gas bubbles and oil droplets.
Shortly after release, the strong buoyancy of the light gas and liquid petroleum
fluids dominates the dynamics, and a vertically rising buoyant plume forms. The
strength of this plume is characterized by the initial buoyancy flux coming from the
release, given
B g Q
= ′ 0
(2.6)
where g
′
is the reduced gravitational acceleration of the oil through the water and Q 0
is the initial gas/oil volume flow rate from the release point. Because of its net positive buoyancy flux, the plume lifts the gas and liquid droplets faster through the
water than individual bubbles or droplets would rise on their own, similar to the
W. Lehr and S. A. Socolofsky
through the ocean water column. During DWH, point releases occurred at the end
of the broken riser pipe, from holes that formed in the kink above the blowout preventer (BOP) and finally directly from the open pipe above the BOP after the riser
was removed in preparation for attaching a containment system.
During the early days of DWH, British Petroleum (BP) attempted to control the
oil plume by confining it in a cofferdam, a large dome placed over the broken riser.
The attempt failed because the cofferdam rapidly filled with gas hydrates. These
hydrates increased the buoyancy of the structure to such an extent that it became
unstable and had to be removed. Natural gas hydrates, discovered by Sir Humphrey
Davy in 1810, are formed when water molecules are linked through hydrogen bonding and create cavities that enclose hydrocarbon gas molecules. However, no chemical bonding takes place between the host water molecules and the enclosed
hydrocarbon. The resulting solid has a snow-like appearance with density less than
ice. The amount of naturally occurring hydrates, 3000 trillion cubic meters according to Chong et al. (2016), dwarfs known traditional gas reserves. Hydrates first
became of interest to the hydrocarbon industry in 1934, when they were observed
blocking pipelines. Blockage in offshore production has become more severe with
the increase of the water depth (Sinquin et al. 2004). Hydrate formation is favored
by low temperature and high pressure such as found at the sea floor.
If hydrocarbons escape from the reservoir through a point release, the transport of
gas and liquid petroleum occurs in four distinct stages, each governed by slightly different physics and each occurring at later times such that the gas and liquid phases may
have different composition and physical/chemical characteristics. These stages are (1)
the initial jet breakup, where the petroleum fluids are the most fresh; (2) the ensuing
buoyant plume, where much of the dissolution of the soluble compounds occurs; (3) a
region where the density stratification of the ocean arrests the upward buoyant plume,
forming a horizontal intrusion layer rich in dissolved petroleum compounds; and (4)
the final transport phase, where gas bubbles and oil droplets rise independently, moved
by their own buoyancy and by the currents and turbulence of the ocean.
The jet break-up region is strongly turbulent, dominated by momentum coming from
the pipeline fluids, and occurs immediately downstream of the spill orifice. Seawater
rapidly mixes into the gas and liquid petroleum stream, breaking the immiscible oil up
into individual gas bubbles and oil droplets. The velocity of the oil and gas in this region
is similar to that in the leaking pipeline at the orifice, and the strong turbulence is responsible for creating the initial size distribution of gas bubbles and oil droplets.
Shortly after release, the strong buoyancy of the light gas and liquid petroleum
fluids dominates the dynamics, and a vertically rising buoyant plume forms. The
strength of this plume is characterized by the initial buoyancy flux coming from the
release, given
B g Q
= ′ 0
(2.6)
where g
′
is the reduced gravitational acceleration of the oil through the water and Q 0
is the initial gas/oil volume flow rate from the release point. Because of its net positive buoyancy flux, the plume lifts the gas and liquid droplets faster through the
water than individual bubbles or droplets would rise on their own, similar to the
W. Lehr and S. A. Socolofsky