Prestige: The "second curtain"
One of the lessons learned during the Erika spill and that was used to refashion the Polmar plan was the new twin curtain or two phase sea response concept. Here is how it works: inshore response teams busy themselves with the task of recovering as much oil as possible along the coastal fringes before the oil actually manages to reach the beach once the high sea response teams have done their utmost. The response plan that was enacted to save Arcachon Bay is a good case in point and an excellent example of how to implement the new concept.
On 3 January 2003, the arrival of patchy oil off Arcachon Bay led authorities to implement a precautionary and temporary ban on the sales of shellfish in addition to reinforcing standing Polmar plan measures for the bay which is a sensitive area ecologically and that many professionals depend on for their livelihood. A technical committee drafted an eight item response plan that could be adapted to the ever changing situation as wind, current and tide regimes changed constantly in the area. The spill was a good example of how a wide range of resources has to be provided when seeking to protect sensitive areas such as the Bay of Arcachon which covers a vast surface area.
Location of the response and protection systems used in Arcachon
Bay (click to enlarge)
1.
Situational monitoring near the bay entrance and along the seaboard
Two trawlers were requisitioned and engaged in seabed
tar ball collection just offshore near Cape Ferret between Lacanau
and Biscarosse. They were only able to located sheens and small
thin patches of oil in sedimentary layers 0.3 to 1 nautical
mile offshore.
Inspecting the seabed (bottom trawl) (Source: Cedre)
2.
Situational monitoring on the surface and in the water column
off shore
Two trawlers with a deep-sea trawl net were engaged
to monitor the area between the Arcachon landfall buoy and Lacanau
extending 20 nm offshore with depths ranging from 0 to 40 metres.
As a result, strings of tarballs were observed visually on the
surface and oil was seen on trawl nets. A makeshift, home-made
filtering system on a marine engine water intake pipe also attested
to the presence of oil in the water column near the surface.
Inspecting the water column (pelagic trawl)
(Source: Cedre)
3.
Deployment of oil recovery nets off the entrance to Arcachon
Bay
Polmar stockpile oil recovery nets (Seynip) designed
along the lines of standard fishing nets and geared to oil recovery
were implemented on 30 January in the northern channel of the
Bay. As the oil dispersed naturally, ten tonnes of emulsion
were recovered that would otherwise have been a potential threat
to the ecosystem.
Implementing pollution response trawls (Seynip) (Source: Cedre)
4.
Oil deflection used at the entrance of the bay
Inspired by the Spanish response procedure implemented
in rias, this system was used to trap crude oil reaching the
main inland water systems in the bay. Several dozen small boats
were tied to a 500 metre hawser and positioned at 90° to
the main current. During trials, 15 tonnes of emulsion were
recovered. The system was subsequently redesigned to improve
safety for responders and management in addition to the actual
techniques used but that were never tested again on a real live
situation involving massive arrivals of oils.
Fixed deflection booms (Source: Cedre)
5.
Dynamic surface recovery in the bay
Implementing surface trawls (Source: Cedre)
This technique involved disposable purse seine nets
changed into small size surface trawl nets that were more functional
and partially disposable, easy to handle and well suited to
recovering large quantities of patchy and mobile oil. The technique
performed well but was never actually used on massive arrivals.
Implementing surface trawls (Source: Cedre)
6.
Standing protective systems
These permanent systems were used to stop oil floating
inland down the south channel which is far harder to reach for
small boats. They were comprised of three inflatable barges
set at an angle of 45° in relation to the banks at strategically
located downstream positions in the south channel and were fitted
with hand-woven, ballasted polyethylene nets that were laid
out on the barges and deployed in the event of an alert then
replaced if damaged or clogged. The system when tested worked
very well but was not used on massive arrivals.
7.
Mobile protective systems
These were used mainly at the beginning of January
and were very efficient in providing quick protection for sensitive
and strategic parts of the bay. They comprised a 50 metre 10mm
mesh net with a draught of 3 metres and a free board of 30 cm
laid at an angle of 20° in relation to the channel banks.
As the systems proved to be maintenance intensive, less robust
and efficient than other systems, the trials were discontinued
and the nets stowed away.
8.
Trials using nets to protect the oyster beds
Protective nets were used in the bay to keep oil out
of the oyster beds and enhance recovery in the beds that were
the most westward facing. Trials were conducted in February
in an oyster bed that was located to the north-east of Arcachon
harbour and turned out to be conclusive from the safety point
of view, ease and quickness of deployment and weather resistant.
The net was orange in colour and connected to an inflatable
chamber but could not be used because of colour and cost considerations.
The system was not used in other oyster beds as there was less
and less oil to contend with.
Oyster bed trials (Source: Cedre)
9.
Protecting water intakes
Filtration systems were put in place to prevent water
intakes from being polluted in the fishfarms and oysterbeds
located towards the top end of the bay. The filtration systems
comprised a metal collar fitted to the intakes that would be
fitted with filter cartridges in the event of arrivals of oil.
However, they were never used because oil hardly ever reached
the top end of the bay.
Protecting water intakes (Source: Cedre)
Last update: April
2004