456
Hjalmar THIEL
the sea by the discharges of (1) drill-cuttings, and
(2) production water. Discharged drill-cuttings consist
of material separated from the drilling fluids which
have been pumped through the drill-string to lubricate
the drilling bit, to carry drillings to the surface and
to control the reservoir pressure. This drilling fluid
contains water, bentonite, barium sulphate, cellulose
compounds and occasionally salt and biocides. In
the past, drilling fluids also contained oil. This fluid
is recycled after separating out the drill-cuttings, to
minimize the escape of pollutants.
Kingston (1992) studied the benthos around drilling
platforms in the North Sea (to a depth of about 150 m),
after surveying sampling transects of 5 km length
following the prevailing bottom currents. The results
may be summarized by characterizing four zones at
increasing distances from the platform:
(1) 0–750 m: oil level 1000 times background value,
disrupted community, low diversity;
(2) 750–1500 m: oil level 20–100 times background
value, transition from low to background diversity;
(3) 1500–3000 m: oil levels 5–20 times background
value, diversity appears normal, effects on indicator species may be detected;
(4) >3000 m: oil level elevated above background
value at distances up to 10 km, no confirmed
evidence of any effect on the benthos.
Rather similar results were obtained by Olsgard
and Gray (1995), reporting on studies around 14 oil
and gas fields; but in some fields contamination was
much less. After 6–9 years, contamination had spread
to distances of 2 to 6 km from the platform. The
relationship between fauna and environmental variables
indicated that the main components of drill cuttings
impacting the benthic fauna were total hydrocarbons,
barium and strontium, and also metals such as zinc,
copper, cadmium and lead. Impacts by water-based
mud clearly indicated less effect on the environment
and on organisms than those of oil-based fluids. The
replacement of larger epifauna by smaller infauna may
have had an influence on food for fishes.
Drilling in deeper waters would probably produce
similar effects to those observed around these relatively
shallow (<400 m) North Sea platforms. Discharged
drilling muds would immediately sink to the seafloor,
although currents may separate out clay and silt
components, and they may settle at greater distances
from the platfom, presumably at lower concentrations.
If a riser tube were used, resulting in the mud
being released close to the seafloor, impacts could
be restricted to a smaller field. Whereas water-based
drilling fluids are allowed to be released into the sea,
oil-based materials should be disposed of on the land.
Oil companies are aware of the environmental
problems introduced into the ocean by drilling actvities,
and a more responsible approach has developed during
the decades of drilling in shallow seas. Clean drilling
and exploitation are possible, as demonstrated by the
Deutsche Erd¨ ol-Aktiengesellschft with their platform
in the tidal area of the German Wadden Sea. There
is no contamination, and drilling muds are transported
back to the coast for terrestrial disposal (Mittelplate
Konsortium, 1999).
Methane hydrate
This gas was discovered only recently, and it may
become an important resource. Methane occurs in the
oceans as a compound with water, and in the deep
sea, due to the high pressure and low temperature,
this compound exists in the form of a solid “ice”.
Techniques for the extraction of methane hydrates have
not yet been developed, but potential environmental
hazards have been suggested. Extraction of methane
“ice” may destabilize sloping areas and trigger mud
slides down into greater depth, similar to those
occurring naturally (see Gas Technology Information,
2000).
Living resources and fishing
Some deep-water fish stocks have been exploited for
many years, and the long-line technique employed for
catching black scabbardfish (Aphanopus carbo) off
Madeira and the Azores is a positive example of the
sustainable use of deep-living fish species. However,
during recent decades industrial fishing activities have
shifted to greater depth, following the construction
of more powerful trawlers, the development of highly
sophisticated methods for the detection of and trawling
for aggregated fish schools, and also because of the
decline of many fish stocks in shelf areas due to
overfishing. In recent publications, deep-water fishing
is defined as occurring below 400 m or 500 m, and
fishing activities have been recorded to a depth of
nearly 2000 m.
The inherent problems of deep-water fishery are
basically understood, and are well summarized in a
number of review papers [e.g., those of Merrett and
Haedrich (1997) and Jennings and Kaiser (1998), and
Hjalmar THIEL
the sea by the discharges of (1) drill-cuttings, and
(2) production water. Discharged drill-cuttings consist
of material separated from the drilling fluids which
have been pumped through the drill-string to lubricate
the drilling bit, to carry drillings to the surface and
to control the reservoir pressure. This drilling fluid
contains water, bentonite, barium sulphate, cellulose
compounds and occasionally salt and biocides. In
the past, drilling fluids also contained oil. This fluid
is recycled after separating out the drill-cuttings, to
minimize the escape of pollutants.
Kingston (1992) studied the benthos around drilling
platforms in the North Sea (to a depth of about 150 m),
after surveying sampling transects of 5 km length
following the prevailing bottom currents. The results
may be summarized by characterizing four zones at
increasing distances from the platform:
(1) 0–750 m: oil level 1000 times background value,
disrupted community, low diversity;
(2) 750–1500 m: oil level 20–100 times background
value, transition from low to background diversity;
(3) 1500–3000 m: oil levels 5–20 times background
value, diversity appears normal, effects on indicator species may be detected;
(4) >3000 m: oil level elevated above background
value at distances up to 10 km, no confirmed
evidence of any effect on the benthos.
Rather similar results were obtained by Olsgard
and Gray (1995), reporting on studies around 14 oil
and gas fields; but in some fields contamination was
much less. After 6–9 years, contamination had spread
to distances of 2 to 6 km from the platform. The
relationship between fauna and environmental variables
indicated that the main components of drill cuttings
impacting the benthic fauna were total hydrocarbons,
barium and strontium, and also metals such as zinc,
copper, cadmium and lead. Impacts by water-based
mud clearly indicated less effect on the environment
and on organisms than those of oil-based fluids. The
replacement of larger epifauna by smaller infauna may
have had an influence on food for fishes.
Drilling in deeper waters would probably produce
similar effects to those observed around these relatively
shallow (<400 m) North Sea platforms. Discharged
drilling muds would immediately sink to the seafloor,
although currents may separate out clay and silt
components, and they may settle at greater distances
from the platfom, presumably at lower concentrations.
If a riser tube were used, resulting in the mud
being released close to the seafloor, impacts could
be restricted to a smaller field. Whereas water-based
drilling fluids are allowed to be released into the sea,
oil-based materials should be disposed of on the land.
Oil companies are aware of the environmental
problems introduced into the ocean by drilling actvities,
and a more responsible approach has developed during
the decades of drilling in shallow seas. Clean drilling
and exploitation are possible, as demonstrated by the
Deutsche Erd¨ ol-Aktiengesellschft with their platform
in the tidal area of the German Wadden Sea. There
is no contamination, and drilling muds are transported
back to the coast for terrestrial disposal (Mittelplate
Konsortium, 1999).
Methane hydrate
This gas was discovered only recently, and it may
become an important resource. Methane occurs in the
oceans as a compound with water, and in the deep
sea, due to the high pressure and low temperature,
this compound exists in the form of a solid “ice”.
Techniques for the extraction of methane hydrates have
not yet been developed, but potential environmental
hazards have been suggested. Extraction of methane
“ice” may destabilize sloping areas and trigger mud
slides down into greater depth, similar to those
occurring naturally (see Gas Technology Information,
2000).
Living resources and fishing
Some deep-water fish stocks have been exploited for
many years, and the long-line technique employed for
catching black scabbardfish (Aphanopus carbo) off
Madeira and the Azores is a positive example of the
sustainable use of deep-living fish species. However,
during recent decades industrial fishing activities have
shifted to greater depth, following the construction
of more powerful trawlers, the development of highly
sophisticated methods for the detection of and trawling
for aggregated fish schools, and also because of the
decline of many fish stocks in shelf areas due to
overfishing. In recent publications, deep-water fishing
is defined as occurring below 400 m or 500 m, and
fishing activities have been recorded to a depth of
nearly 2000 m.
The inherent problems of deep-water fishery are
basically understood, and are well summarized in a
number of review papers [e.g., those of Merrett and
Haedrich (1997) and Jennings and Kaiser (1998), and