I. LITERATURE REVIEW OF OFFSHORE OIL AND GAS DRILLING
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The accidentology analysis reveals that a well blowout is the type of accident most damaging to
the environment in the case of offshore hydrocarbon exploration-production (EP) activities. A
"blowout" is an uncontrolled release of effluent from a well. The effluent exit point can be at the
wellhead or along the well, in line with vulnerable underground formations.
This imbalance can be linked to a set of reasons, of which the most frequently encountered in
accidents are below.
- Shallow gas: located in geological layers at shallow depths (between a few dozen and a few
hundred meters below the seabed, in the form of hydrates (shallow gas). When drilling through
these layers, the gas is released, leading to a gassing of the mud. Gas blowout is then almost
inevitable and difficult to control, as the well does not have a blowout preventer (BOP). This
scenario is one of the most common causes of blowouts in the offshore.
- Insufficient mud density: due to human or instrumental error in the formulation, manufacture or
testing of the mud before it is pumped into the well.
- A too rapid manoeuvre of the packer: a lightening of the mud column and an imbalance of
pressure at the bottom of the well is due to a too rapid rise of the drill packer during a manoeuvre
can lead, by piston effect, to a loss of pressure.
- Unexpected encounter with an over pressured formation: a formation containing fluids under
pressure was not identified and taken into account in the drilling programme.
- A loss of mud circulation, i.e. a decrease in the height of the mud column in the well due to
losses in the ground.
- Uncontrolled emptying or tearing of the extension tube (only in deep water): the extension tube
is a particularly stressed element during drilling, particularly due to marine currents or swell. It
can therefore be weakened, in particular by fatigue effects, and be subject to the rupture of one of
its elements. In the event of a rupture, the extension tube can be partially emptied of its mud,
which leads to a pressure imbalance at the bottom of the well.
- The intersection of an adjacent well: a pressure imbalance at the bottom of the well caused by
human or instrumental error in the control of the drilling direction.
I.4.2
Pollution from well drilling and drilling waste
The development of offshore drilling must be accompanied by an increase in support vessel and
tanker traffic. In general, the impacts associated with this industrial activity are of several types:
- Construction noise causes marine wildlife such as fish and marine mammals to avoid the area.
- Vibrations and the presence of erected equipment: noise pollution.
30
The accidentology analysis reveals that a well blowout is the type of accident most damaging to
the environment in the case of offshore hydrocarbon exploration-production (EP) activities. A
"blowout" is an uncontrolled release of effluent from a well. The effluent exit point can be at the
wellhead or along the well, in line with vulnerable underground formations.
This imbalance can be linked to a set of reasons, of which the most frequently encountered in
accidents are below.
- Shallow gas: located in geological layers at shallow depths (between a few dozen and a few
hundred meters below the seabed, in the form of hydrates (shallow gas). When drilling through
these layers, the gas is released, leading to a gassing of the mud. Gas blowout is then almost
inevitable and difficult to control, as the well does not have a blowout preventer (BOP). This
scenario is one of the most common causes of blowouts in the offshore.
- Insufficient mud density: due to human or instrumental error in the formulation, manufacture or
testing of the mud before it is pumped into the well.
- A too rapid manoeuvre of the packer: a lightening of the mud column and an imbalance of
pressure at the bottom of the well is due to a too rapid rise of the drill packer during a manoeuvre
can lead, by piston effect, to a loss of pressure.
- Unexpected encounter with an over pressured formation: a formation containing fluids under
pressure was not identified and taken into account in the drilling programme.
- A loss of mud circulation, i.e. a decrease in the height of the mud column in the well due to
losses in the ground.
- Uncontrolled emptying or tearing of the extension tube (only in deep water): the extension tube
is a particularly stressed element during drilling, particularly due to marine currents or swell. It
can therefore be weakened, in particular by fatigue effects, and be subject to the rupture of one of
its elements. In the event of a rupture, the extension tube can be partially emptied of its mud,
which leads to a pressure imbalance at the bottom of the well.
- The intersection of an adjacent well: a pressure imbalance at the bottom of the well caused by
human or instrumental error in the control of the drilling direction.
I.4.2
Pollution from well drilling and drilling waste
The development of offshore drilling must be accompanied by an increase in support vessel and
tanker traffic. In general, the impacts associated with this industrial activity are of several types:
- Construction noise causes marine wildlife such as fish and marine mammals to avoid the area.
- Vibrations and the presence of erected equipment: noise pollution.